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basalt interactions in the Mururoa Massif
Chemical Geology 158 Ž1999. 21–35
Behaviour of rare earth elements during seawaterrbasalt interactions in the Mururoa Massif C. Guy a
a,)
, V. Daux b, J. Schott
c
Commissariat a` l’Energie Atomique, Departement Analyses SurÕeillance EnÕironnement, serÕice RCE, BP 12, 91680 Bruyeres-Le-Chatel, ´ ` ˆ France b Laboratoire de Geologie des bassins sedimentaires, UniÕersite´ Paris VI, 4 place Jussieu, Paris, France ´ ´ c Laboratoire de Geochimie, 38 rue des trente six ponts, 31400 Toulouse, France ´ Received 13 January 1998; received in revised form 14 December 1998; accepted 14 December 1998
Abstract The rare earth elements ŽREE. were analysed in the interstitial fluids, in the basalts and in the alteration products Žclays and zeolites. of Mururoa atoll volcanic rocks ŽFrench Polynesia. in order to investigate REE behaviour during seawaterrbasalt interactions at low temperature. The REE distribution coefficients between fluids and secondary products were calculated using the EQ3r6 geochemical code. The reversiblerirreversible character of the REE entrapment in secondary products was tested. The REE concentrations in the clays are close to that of basalt. The concentrations in zeolites are 10 times lower. Comparison of measured and modeled concentrations in solutions favours the irreversible incorporation assumption. The affinity of the REE for the secondary products is shown to be high. The calculated distribution coefficients range from 3 = 10 5 for Lu and Eu to 10 7 for Ce in clays and from 10 4 to 7 = 10 5 in zeolites. The distribution coefficients exhibit a progressive decrease from La to Lu. This behaviour is related to the greater stability of HREE complexes relative to LREE complexes in solution. Under reducing conditions in the interstitial fluids ŽEh down to y500 mV., Eu undergoes reduction to the mobile EuŽII.. This enhanced mobility is reflected by Eu distribution coefficient being lower than those of adjacent REE. The distribution coefficient of Ce is higher. As Ce is trivalent in the fluids, fractionation from the other trivalent REE was unexpected. The calculated distribution coefficients of REE are lower in zeolites than in clays and this can be related to the spatial distribution and paragenesis of the secondary phases: the dissolution of the basalt promotes the precipitation of clays at the basalt–fluid interface and of zeolites in the bulk solution. The REE released by dissolution are incorporated into the clays and further uptake occurs in zeolites for the REE left in solution. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Rare earth element; Seawaterrbasalt interactions; Mururoa Massif
1. Introduction The chemical coherence of the rare earth elements ŽREE. makes them useful tracers of a variety of
)
Corresponding author
geochemical processes. For instance, they can be used for understanding low temperature reactions between seawater and the oceanic crust and many studies have documented the distribution and mobility of REE during seawaterrbasalt interactions at high and low temperature. REE have been shown to be more concentrated in hydrothermal fluids than in
0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 0 1 9 - 4
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C. Guy et al.r Chemical Geology 158 (1999) 21–35
seawater ŽKlinkhammer et al., 1983; Michard et al., 1983; Michard and Albarede, ` 1986; Campbell et al., 1988; Michard, 1989.. However, their concentrations in the alteration products were shown to be close to those of the fresh basalt, thus indicating the low mobility of these elements. The overall low abundance of REE in seawater is further evidence for their low solubility in both continent and oceanic crust weathering reactions ŽFrey et al., 1974; Ludden and Thompson, 1979; Menzies and Seyfried, 1979; Bonnot-Courtois, 1980; Desprairies and BonnotCourtois, 1980; Hajash and Chandler, 1984; Schiano et al., 1993; Daux et al., 1994; Ridley et al., 1994.. Experimentally, this poor mobility has been also observed ŽBonnot-Courtois and Jaffrezic-Renault, 1982; Berger, 1992; Berger et al., 1994. and, moreover, it has been demonstrated that the light REE are even less mobile than the heavy REE ŽBruque et al., 1980; Byrne and Kim, 1990.. It has furthermore been emphasised that the behaviour of REE depends heavily on the intensity of weathering Žwaterrrock ratio.. To model the behaviour of REE during basalt alteration, it is necessary to examine the distribution of REE in the secondary phases. Slight changes in the rock composition during alteration may induce dramatic variation in the associated fluid. Thus, very useful information can be obtained from the concomitant study of the fluid. The Mururoa atoll drilling program provides a unique opportunity to document the behaviour of REE during basalt weathering via sampling of altered basalts together with the associated interstitial fluids. This work examines the distribution of REE between solutions and secondary products sampled from two drill-holes ŽNerite in 1986, Exocet in 1989. in the Mururoa Atoll. Using the EQ3r6 geochemical code, this dataset is modelled to understand the behaviour of REE during the low temperature alteration of basalts by seawater. The Mururoa situation in terms of physical properties of the crust is not analogous to ocean-crust: the hydraulic regime Žresidence time and velocity of the fluid in the crust. is probably different, the temperature of the alteration fluid may be higher on average than the low temperature conditions on the seabed. However, from a chemical point of view, we assume that the model presented in this study is relevant to REE behaviour during seawaterrocean-crust interactions.
2. Geological and hydrogeological settings The Mururoa atoll is located in the Tuamotu Archipelago Ž21850X S and 138853X W.. Its formation, between 12 and 9.5 Ma ago, is attributed to hot spot intraplate volcanism that generated the Pitcairn– Gambier chain ŽBardintzeff et al., 1986.. Today, it is covered by a thick Ž180 to 400 m. carbonate layer deposited after subsidence ŽBuigues et al., 1992.. Volcanic rocks are of alkali magmatic series extending from oceanites and Mg-rich basalts to comenditic trachytes. Hawaiites and alkali basalts are the most common rocks among the volcanic series ŽBardintzeff et al., 1986; Dudoignon et al., 1989; Maury et al., 1992; Guille et al., 1993.. In 1986 and 1989, two drill holes were cored under the lagoon ŽFig. 1.: one to a depth of 400 m at the Nerite site and the other to a depth of 690 m at the Exocet site. The sequence found from the top to the bottom of the holes shown in Fig. 1 is: Ž1. limestones, Ž2. basalts erupted in aerial and subaerial conditions, and Ž3. submarine basaltic pillow lavas. Numerous drilling campaigns at Eniwetok, Bikini ŽSwartz, 1958. and Mururoa ŽGuille et al., 1993. and theoretical studies using numerical codes ŽSamaden et al., 1985; Henry et al., 1996., have provided information about the general hydrogeology of atolls. Models of fluid circulation through porous media by diffusion and convection ŽGobelet, 1981; Samaden et al., 1985. show that interstitial water circulates through the atolls due to a thermal gradient between the cold oceanic water and the geothermally heated atoll base. For Mururoa, the reliability of the simulation has been demonstrated by a close agreement between calculated and measured temperatures down to a depth of 1200 m ŽGuille et al., 1993.. The water circulation is described as a centripetal infiltration that starts on the external slope of the atoll and rises to the centre. Circulation modelling suggests that the residence time of the interstitial fluids in the volcanic rocks ranges between 10 4 and 10 5 years ŽHenry et al., 1996.. 3. Fluid sampling and analytical methods Solids and interstitial fluids were sampled during Exocet and Nerite drillings campaigns at Mururoa. Since surface seawater was used as drilling fluid, it
C. Guy et al.r Chemical Geology 158 (1999) 21–35
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Fig. 1. The Mururoa atoll. Southwestrnortheast schematised section and lithology; and location of the Nerite and Exocet drill holes.
was spiked with LiCl Žabout 50 ppm of Li. in order to quantify its mixing with the interstitial fluid. The drilling was generally carried out using a coring tool which enabled the entire core to be withdrawn, but when drilling was destructive, core cuttings were taken. After the sampler tubing had reached the appropriate depth, it was raised several meters. A packer was then brought down, fixed to the sampler tubing and inflated in order to isolate the volume between the packer and the bottom of the hole. A rupture disc was then broken to establish communication between the casing and the core, and the interstitial fluid was pumped from the surrounding rock up to the surface. The Li content of the solution was monitored as a function of time until it reached
a low value, ensuring a minimal contribution of surface seawater to the sampled fluid. Fluid sampling was carried out by a cable-operated borehole sampler. The apparatus included a 1-l vacuum chamber. After sampling, in-situ pressure was maintained as the apparatus was brought to the surface. Temperature was monitored during the drillings. It was constant at about 258C Ž"38C. throughout the two drill cores. The secondary phases in the volcanic rocks were hand-picked under a binocular microscope and analysed for REE. Basalts and interstitial fluids were analysed for major elements and REE. Major elements in the basalts were determined by X-ray fluorescence ŽXRF.. The analytical precision
C. Guy et al.r Chemical Geology 158 (1999) 21–35
24
is 5%. The major element compositions of the interstitial fluids determined by gas chromatography, ion chromatography and spectrophotometry were presented in previous papers. The main results of these studies are summarized in Section 4. REE analyses were performed by inductively coupled plasma mass spectrometry ŽICP-MS.. The interstitial fluids were diluted and acidified with HNO 3 in order to improve the signal stability. For solid samples, 100 mg of powder were attacked using HFrHNO 3 . These solutions were then diluted 10 to 50 times. Indium was added to all solutions as an internal standard. The analytical precision is 5%. Gd analyses are not reported due to analytical interferences.
4. Previous studies The behaviour of the major elements and strontium isotopes during the alteration of Mururoa basalts by seawater has been documented by Destrigneville Ž1991., Guy et al. Ž1992. and Guille et al. Ž1993..
Fig. 2. Si and K concentrations Ž10y3 molrkg. in the interstitial fluids of Mururoa volcanic rocks as a function of the mass of basalt dissolved per volume of interstitial water Ž R r W . Žsee Table 1; data from Guy et al., 1992.. Samples Exocet 1 and Nerite 3 correspond to the lowest and highest R r W ratios, respectively.
The major element composition of the interstitial fluids sampled in the carbonate layer is very close to that of seawater. In contrast, the chemical composi-
Table 1 Major element and REE concentrations in the interstitial fluids of Mururoa volcanic rocks Fluid
Depth
RrW
pH
Si
8.35 8.15 7.52 8.10 9.00 9.00
7 122 132 160 362 331 348
Ce
Pr
Nd
5.7 360 390 330 340 380 290 348 37
3.8 60 71 120 110 160 110 105 36
16.2 265 350 370 540 585 375 414 122
a a a a a a a
Lagoon Exocet 1 Exocet 2 Nerite 1 Nerite 2 Nerite 3 Nerite 4
571–580 613–624 207–223 341–353 390–399 411–423
22 37 52 97 110 94
Fluid
La
b c c c c c c
Seawater Exocet 1 Exocet 2 Nerite 1 Nerite 2 Nerite 3 Nerite 4 Mean SD
25.7 420 520 580 490 550 480 507 56
Mg
Ca
Na
11.4 46.0 82.1 141.0 127.0 99.2 80.7
466 478 450 327 293 264 275
Sm
Eu
Tb
3.0 87 135 280 370 320 230 237 109
0.8 113 174 365 520 485 550 368 186
53.4 34.1 14.4 3.2 32.5 69.4 52.8
0.6 67 39 21 26 30 18 34 18
K 8.7 6.00 3.50 2.20 0.86 0.86 1.10
Sr 0.11 0.31 0.40 0.50 0.72 0.83 0.77
SO4
Cl
Alkalinity
28.2 28.1 19.8 14.1 15.5 13.1 13.4
545 601 609 572 554 579 507
2.38 0.48 0.15 -
Tm
Yb
Lu
4.7 18 29 57 68 52 41 44 19
0.9 8 12 28 43 51 39 30 17
Dy
Ho
Er
5.3 46 80 48 60 76 60 62 14
1.1 11 19 9.5 17 30 17 17 7
5.0 78 57 54 59 54 41 57 12
0.7 6 7 26 29 24 20 19 10
Si concentrations in micromole per kilogram; Mg, Ca, Na, K, Sr, SO4 , Cl in millimole per kilogram and alkalinity in milliequivalent per liter; - for inferior to detection limit; RrW in gram per liter; REE in picomole per kilogram; depth in meter. The major element concentrations in the Mururoa lagoon and the REE concentrations in the Pacific surface seawater are given for comparison. References: Ža. from Guy et al. Ž1992.; Žb. average surface seawater 0–1000 m derived from De Baar et al. Ž1985. and Piepgras and Jacobsen Ž1992.; Žc. this study.
C. Guy et al.r Chemical Geology 158 (1999) 21–35
25
Increasing RrW ratios are related to the following changes in fluid composition ŽTable 1; Fig. 2.: Ž1. Sr and Si are progressively enriched, while K, dissolved carbonates and ÝS are depleted; Ž2. Ca and the pH decrease at first, and then increase as alteration progresses to higher RrW, while Mg exhibits the opposite behaviour; Ž3. the solutions change from oxidising to reducing. The secondary phases formed at Mururoa have already been described in detail ŽDudoignon, 1988; Dudoignon et al., 1989.. Chlorite, aluminium saponite, and calcic zeolites such as chabazite are generally observed from the rim to the centre of veins in the subaerial basalt. Gypsum and apophyllite crystals are observed locally. In the pillow lavas, the secondary minerals assemblages comprised celadonite, iron-rich saponite, magnesium saponite and potassium zeolites Žphillipsite.. The behaviour of major elements during basaltr seawater interactions at Mururoa was modelled by Guy et al. Ž1992. using EQ3r6 code. The paragenesis of secondary minerals calculated as a function of RrW is given in Fig. 3. The results of the geochemical simulations agree with the observations made at Mururoa. Magnesium-rich clays Žsaponites. precipitate at low RrW, yielding a Mg depletion and Ca enrichment in the solution. The precipitation of calcite induces the total removal of dissolved carbonates and a decrease of pH from 8.35 to 7.5. After calcite precipitation, the dissolution process causes
Fig. 3. Sequence of the secondary phases calculated using EQ3r6 as a function of R r W, at 258C. Saponites and zeolites are the most common secondary products. Minors refer to: gibbsite, goethite, pyrite and calcite. After Guy et al. Ž1992..
tion of the interstitial fluids of the basalts exhibits a dramatic change which can be related to the progress of the alteration reaction. The strontium isotope ratios Ž87 Srr86 Sr. of the fluids were used as proxies for the rockrwater ratios Ž RrW: mass of basalt dissolved per volume of interstitial water.. RrW ratios were shown to increase with depth up to 50 grl and 110 grl in Exocet and Nerite cores, respectively. This trend is consistent with seawater percolating from the atoll rim to the centre ŽHenry et al., 1996..
Table 2 Major element Žin %. and REE Žin ppm. concentrations in Mururoa aerial basalts at Exocet and Nerite drilling sites Basalt
SiO 2
TiO 2
Al 2 O 3
3.97 3.35 3.66 0.31
12.16 15.71 13.94 1.78
Fe 2 O 3 13.62 12.50 13.06 0.56
MnO
MgO
0.18 0.18 0.18 0.00
9.86 4.67 7.27 2.60
Exocet Nerite Mean SD
43.9 47.5 45.7 1.8
Basalt
Depth
La
Ce
Pr
Nd
Sm
Exocet Nerite Nerite Nerite Nerite Nerite Mean SD
735 311 346 377 391 407
31.2 29 45 45 51.6 55 42.9 9.5
76.2 71 108 106 115 135 102 22
9.9 r r r 15.5 r 12.7 2.8
42.3 39 64 64 67.9 79 59.4 14.1
8.23 r r r 14.1 r 11.2 2.9
CaO
Na 2 O
K 2O
P2 O5
PF
H 2O
Total
11.29 7.49 9.39 1.90
2.06 3.40 2.73 0.67
0.62 1.90 1.26 0.64
0.50 1.30 0.90 0.40
0.87 1.70 1.29 0.42
0.34 0.35 0.35 0.01
99.37 100.05 99.71 0.34
Eu
Tb
Dy
Ho
Er
Tm
Yb
0.88 r r r 1.53 r 1.2 0.3
4.76 5.1 7.7 7.7 8.13 8.8 7.0 1.5
0.85 r r r 1.43 r 1.1 0.3
2.04 2.4 3.3 3.3 3.4 3.6 3.0 0.6
0.26 r r r 0.44 r 0.35 0.09
2.64 2.7 4.0 4.0 4.9 4.8 3.8 0.9
1.57 1.7 2.5 2.5 2.65 2.8 2.3 0.5
Lu 0.20 r r r 0.35 r 0.27 0.08
Basaltrseawater interactions Žsee Section 6. was modeled using the mean values of the concentrations in the basalts. SD: standard deviation.
C. Guy et al.r Chemical Geology 158 (1999) 21–35
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Table 3 REE concentrations in the saponites and zeolites sampled at Exocet and Nerite drilling site in Mururoa atoll Žppm. Depth
La
Ce
Pr
Nd
Sm
Eu
Tb
Dy
Ho
Er
Tm
Yb
Lu
Saponites 337 386 403 Mean SD
16.3 26.2 20.5 21 4.1
39.1 52.8 65.0 52.3 10.6
4.6 5.7 6.2 5.5 0.7
19.9 23.5 28.0 23.8 3.3
4.2 4.4 5.9 4.8 0.8
1.4 1.3 1.8 1.5 0.2
0.77 0.75 0.77 0.76 0.01
3.0 2.9 3.9 3.3 0.4
0.49 0.45 0.66 0.53 0.09
1.7 1.6 1.8 1.7 0.1
0.17 0.15 0.23 0.18 0.03
1.2 1.0 1.5 1.2 0.2
0.19 0.15 0.20 0.18 0.02
1.9
4.1
0.58
2.5
0.52
1.9
0.06
0.32
0.06
0.14
Zeolites 326
.018
0.11
.015
Depth in meter SD: standard deviation.
proton consumption which results in an increase in pH up to 9. The oxidation of Fe 2q consumes the dissolved oxygen and consequently, Eh decreases from about q350 mV to y500 mV. While RrW increases, ÝS is gradually removed from the solution due to the precipitation of gypsum and pyrite. The increase in Ca concentration and pH in the solution promotes the precipitation of zeolites Ž RrW ) 25 grl.. At higher RrW ratios, the magnesium-rich saponites are destabilised, inducing the formation of Ca- and Na-rich saponites.
5. Results The REE concentrations measured in the interstitial fluids, in the basalts and in the saponites and the zeolites are listed in Tables 1–3.
Fig. 4. Seawater-normalised REE profiles for the mean interstitial fluid of Mururoa volcanic rocks. The error bars correspond to the standard deviation. The seawater composition used for the normalisation correspond to Pacific surface seawater Ždata in Table 1..
5.1. REE concentrations in the interstitial fluids It can be seen in Fig. 4 that the interstitial fluids are significantly enriched in REE with respect to seawater. Hydrothermal solutions also exhibit REE enrichments ŽRefs. in Section 1. which increase with decreasing pH ŽMichard, 1989. and increasing temperature ŽKlinkhammer et al., 1995.. This observation is consistent with Mururoa interstitial fluids being less concentrated Žapproximately tenfold. than the hotter and more acidic fluids of the East Pacific Rise ŽMichard et al., 1983.. The other features illustrated in Fig. 4 are: Ž1. a large Eu positive anomaly, and Ž2. a slight decrease of the enrichment across the REE series. Except for Ce, Tb and Er concentrations which are nearly constant, the REE concentrations in the interstitial fluids increase with increasing RrW ŽTable 1; Fig. 5..
Fig. 5. Nd and Ce concentrations Ž10y1 0 molrkg. in the interstitial fluids of Mururoa volcanic rocks as a function of R r W Ždata in Table 1..
C. Guy et al.r Chemical Geology 158 (1999) 21–35
5.2. REE concentrations in the secondary products Relative to the basalt, the saponites display a slight depletion ŽFig. 6a.. REE concentrations in zeolites are distinctly lower by an order of magnitude. The basalt-normalised REE patterns of the secondary products are almost flat. The main features of the REE normalisation of the secondary products to seawater are ŽFig. 6b.: Ž1. a systematic and nearly tenfold decrease of the normalised concentration across the REE series, and Ž2. a huge positive Ce anomaly. The REE enrichment of the interstitial fluids relative to seawater does not imply that these elements are mobile. The dissolution of 20 g of basalt per litre of solution ŽExocet 1., should result in REE concentrations in solution of ca. 10y7 molrl. The measured concentrations in Exocet 1 do not exceed a few picomoles per litre. These low concentrations result from the incorporation of REE into saponites and, to a lesser extent, into zeolites. The REE content of the clays is close to that of basalts. This low variability of the REE concentrations during basalt to clay transformation has been reported by several authors
Fig. 6. Ža. Basalt-normalised Žlog scale. and Žb. seawater-normalised Žlog scale. REE profiles for the secondary products sampled in Mururoa volcanic rocks. For basalt and saponites, the data used are the mean concentrations. The error bars correspond to the standard deviation Žsee Table 2Table 3..
27
ŽLudden and Thompson, 1979; Jercinovic et al., 1990; Claparols, 1992; Daux et al., 1994..
6. Discussion 6.1. REE in the interstitial fluids: thermodynamic approach The distribution of REE in seawater and in Exocet 1 and Nerite 3 interstitial fluids Žlowest and highest RrW ratios, respectively. and the saturation indexes of these solutions with respect to REE hydroxides, carbonates and phosphates have been calculated using EQ3r6 computer code. 6.1.1. Thermodynamic data EQ3r6 computer code was developed by Wolery Ž1983, 1986.. EQ3 computes the equilibrium distribution of species in the aqueous solution and generates concentrations and activities of ions and complexes. EQ6 calculates chemical equilibrium and mass transfer in aqueous solution–mineral systems using a step-wise procedure involving titration of increments of solid phases into solution. To perform these calculations, thermodynamic parameters for minerals were taken from Helgeson et al. Ž1978. and Bourcier Ž1989., the equation of state of water from Helgeson and Kirkham Ž1974a,b. and the data for aqueous species from Helgeson et al. Ž1981.. The standard molal Gibbs free energies Ž D f G8. and enthalpies Ž D f H8. of formation adopted in this study represent the change in those thermodynamic quantities when 1 mol of a substance in its standard state is formed isothermally from the elements at 1 bar pressure. The standard state for solid phases and H 2 O are unit activity for the pure phases at all temperatures and pressures. For aqueous species, the standard state corresponds to unit activity coefficient for a hypothetical ideal 1 m solution. Molal activity coefficients of neutral aqueous species were assumed to be unity. Activity coefficients of charged species were calculated using the extended Debye–Huckel equation with values of the electro¨ static parameters taken from Helgeson and Kirkham Ž1974a,b.. The thermodynamic parameters for the REE were taken from Drake Ž1975., Sverjensky
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C. Guy et al.r Chemical Geology 158 (1999) 21–35
Ž1984., Brookins Ž1989., Wood Ž1990. and Millero Ž1992.. For saturation index calculations, REE hydroxides REEŽOH. 3 , carbonates REE 2 ŽCO 3 . 3 and phosphates REEŽPO4 . were considered. The thermodynamic constants used in the calculations are given in Appendix A. In the interstitial waters from the volcanic rocks, the concentration of phosphates is inferior to 10y8 molrl ŽDestrigneville, 1991.. We assumed that phosphates were thermodynamically controlled by hydroxyapatite, a basalt constitutive mineral, to calculate the saturation index of REEŽPO4 .. The hydroxyapatite Ca 5 ŽPO4 . 3 OH Žlog K s y11.52 at 258C. was used in the calculation. The calculated ÝPO4 are 3 = 10y1 0 and 1.5 = 10y1 1 molrl for Exocet 1 and Nerite 3 samples, respectively. For seawater, we used the concentration of REE reported in Table 1 and a mean phosphate concentration of 5 = 10y7 molrl measured at Tikehau atoll ŽRougerie et al., 1992..
6.1.2. Speciation and saturation The REE speciation in seawater is dominated by the carbonate complexes ŽFig. 7a.. The most common light REE species are REEŽCO 3 .q and REEqqq. The proportion of REEŽCO 3 .y 2 gradually increases from La to Lu and this complex becomes the most abundant species for the heavy REE. These results are in good agreement with previous studies ŽCantrell and Byrne, 1987; Byrne and Kim, 1990; Lee and Byrne, 1992, 1993; Millero, 1992.. Calculated saturation indexes Ž QrK . reported on Fig. 8a show that seawater is close to equilibrium with respect to light REE phosphates but undersaturated with respect to heavy REE phosphates. Seawater is strongly undersaturated with respect to the REE carbonates and hydroxides. Although dissolved carbonate concentrations in Exocet 1 solution are low Žsee alkalinity in Table 1., REE with the exception of La and Ce are mainly present as carbonate complexes ŽFig. 7b.. The proportion of carbonate complexes increases along the series. For La and Ce, the dominant species are La3q and Ce 3q. REE 3q, REEŽCl. 2q and REEŽCO 3 .q represent 90% of the aqueous species. Saturation indexes calculations ŽFig. 8b. show that these solutions are undersaturated with respect to REE phosphates,
Fig. 7. Trivalent REE speciation in Ža. seawater, Žb. low R r W interstitial fluids ŽExocet 1., Žc. high R r W interstitial fluids ŽNerite 3.. The thermodynamic constants used for the calculation can be found in Appendix A. The percentages of REE 3q and Žor REEŽOH.y . are represented by white areas at REEŽCO 3 .y 2 4 the top and at the bottom of the diagrams, respectively.
carbonates and hydroxides. The saturation indexes of the interstitial fluids with respect to REE 2 ŽCO 3 . 3 are lower than 10y3 0 and are not reported in Fig. 8. Nerite 3 is a carbonate-free solution Žsee alkalinity in Table 1. in which the hydrolysed species predominate ŽFig. 7c.. The proportion of these complexes increases from light to heavy REE Žat the expense of free ions. as does the degree of hydroxylation: La and Ce are present as REE 3q, Nd and Sm as REEŽOH. 2q, Dy, Ho, Er, Yb and Lu as REEŽOH.y 4 . In the same way as for low RrW solutions, the high RrW ones are undersaturated with respect to REE phosphates, carbonates and hydroxides.
C. Guy et al.r Chemical Geology 158 (1999) 21–35
29
phates suggests that the concentrations of REE in the solutions are not controlled by the solubility of REE-phases but, as it is shown in Section 6.2, by REE incorporation in the saponites and zeolites.
6.2. Incorporation of REE in secondary phases The incorporation of REE in the secondary products can be described using two different reactional schemes: Ø Reversible incorporation involving continuous equilibrium between the solution and the secondary phases; Ø Irreversible incorporation: the secondary phases being formed are in equilibrium with the reacting fluid but not with previously formed phases. If reversible incorporation is postulated, the mass conservation for each REE i can be expressed as: n
Fig. 8. Logarithm of the saturation index Ž Qr K . of seawater, and of low and high R r W interstitial fluids of the volcanic rocks ŽExocet 1 and Nerite 3, respectively. with respect to REE hydroxides, carbonates and phosphates. Q is the activity product and K the thermodynamic constant. The K constants used for the saturation calculation can be found in Appendix A. The fluids are undersaturated with respect to REE minerals Žexcept seawater with respect to LaPO4 .. The supersaturation of seawater with respect to CeŽIV.O 2 Žlog Qr K s 4.5. is not reported.
According to our calculation, the percentage of cerium in the form CeŽIV. is negligible in seawater and in the interstitial solutions. In agreement with the thermodynamic calculation reported by De Baar et al. Ž1988., the oxygenated seawater is found to be supersaturated with respect to CeŽIV.O 2 Žlog QrK s 4.5.. In contrast, the reduced interstitial fluids are undersaturated with respect to this oxide. Eu is also present as EuŽIII. in seawater but it is reduced to EuŽII. in the interstitial fluids. The dominant Eu Ž15%. for species are: Eu2q Ž77%. and EuŽCO 3 .y 2 2q Ž . low RrW solutions, Eu 89% and EuŽOH. 2q Ž4%. for high RrW solutions. The undersaturation of the interstitial fluids with respect to the REE hydroxides, carbonates and phos-
R Ž t . C bi s WC wi Ž t . q
Ý M j Ž t . C ji Ž t . ,
Ž 1.
js1
where t is the time, C bi , C wi and C ji refer to element i concentration in the basalt, the solution and the secondary phase j, respectively. R, W and M j stand for the mass of dissolved basalt, the mass of solution involved in the dissolution and the mass of secondary product j. n refers to the number of secondary phases. The distribution coefficient Djirw of the REE i between a secondary phase j and the solution is:
Djir w s
C ji Ž t . C wi Ž t .
.
Ž 2.
Combining Eqs. Ž1. and Ž2. leads to: R Ž t . C bi
C wi Ž t . s
n
Wq
Ý js1
Djir w M j Ž t .
.
Ž 3.
C. Guy et al.r Chemical Geology 158 (1999) 21–35
30
If the incorporation of REE in secondary products is assumed to be irreversible, the Eq. Ž1. applies only during the time frame D t. This can be expressed as: C wi Ž t . W q C bi D R s C wi Ž t q D t . W n
q
Ý C ji Ž t q D t . D M j .
Ž 4.
js1
Combining Eqs. Ž2. and Ž4. leads to: C wi Ž t q D t . s
C wi Ž t . W q D RC bi n
Wq
Ý
.
Ž 5.
Djir w D M j
js1
The incorporation of REE in saponites and zeolites, which are the main secondary phases, has been modelled using EQ3r6 according to the expressions Ž3. and Ž5. Žreversible and irreversible models, respectively.. The distribution coefficients Ž Djirw . were adjusted so that the calculated concentrations fitted the measured REE concentrations. This requires an iterative calculation, which converges after a few cycles. From Eqs. Ž3. and Ž5., the REE concentrations in solution depend on the following parameters. Ž1. The masses of dissolved basalt Ž R . and solution ŽW .. The calculation of REE concentrations in the interstitial fluids was performed as a function of RrW. Ž2. REE concentrations in the basalt Ž C b .. The REE concentrations used in the calculations are the mean values in Table 2. The volcanic rocks in Nerite and Exocet range from basalts to hawaiites; hawaiites are enriched with respect to basalt 1.6 to 1.9 times in REE Ždepending on the element—see Table 2.. This variability was taken into account in the calculation. Ž3. The mass of secondary products Ž M j . can be derived with a good precision from geochemical EQ3r6 simulations constrained by electron microscope observations, laboratory experiments and interstitial fluids sampled in Nerite and Exocet ŽGuy et al., 1992.. The evolution of the amount of secondary products as a function of RrW Žreported in Fig. 3. is used to model the incorporation of the REE in the secondary products. Through the comparison of measured and modeled concentrations in solutions, we can test the
irreversiblerreversible assumptions. Fig. 9 shows that Nd, Eu, and Lu concentrations can be modeled successfully by the irreversible model. The injection of distinct distribution coefficients in the reversible equation is figured by parallel lines in a concentration vs. RrW plot. As can be seen in Fig. 9, translating the reversible lines cannot lead to a satisfactory fit of the measured Nd, Eu and Lu concentrations. The same conclusions are drawn for the other REE. These results corroborate experimental studies from Claparols Ž1992. and Berger et al. Ž1994.. These authors performed dissolution experiments of REE-spiked spherules of basaltic glass. Each individual spherule was doped with a different REE and all the spherules altered together at 3008C in the same solution. It was shown that the altered skin formed on a spherule was enriched in the lanthanide originally present in the spherule. No REE contamination between spherules was observed. If there was a
Fig. 9. Concentrations of Nd, Eu, and Lu in solutions Žpmolrkg. as a function of R r W. Circles: REE concentrations measured in the interstitial fluids of the volcanic rocks at Mururoa. Solid lines: concentrations calculated assuming reversible ŽEq. Ž3.. or irreversible ŽEq. Ž5.. incorporation of REE in the secondary phases. The distribution coefficients between the secondary phases Žsaponites and zeolites. and the solutions, used in this simulation Nd Eu Lu are: Dsaponiterw s 4.2=10 6 ; Dsaponiterw s Dsaponiterw s 3=10 5 ; Nd Eu Lu Dzeoliterw s 2.1=10 5 ; Dzeoliterw s Dzeoliterw s1=10 4 .
C. Guy et al.r Chemical Geology 158 (1999) 21–35
Fig. 10. Distribution coefficients of the REE between solution and secondary products Ž Djirw ; log scale. corresponding to the best fit between data ŽREE concentrations in Mururoa interstitial fluids. and concentrations in solutions modeled assuming irreversible incorporation of REE in secondary products.
permanent equilibrium of the secondary phases with the solution, all the REE should be found in each alteration skin. 6.2.1. Distribution coefficients of REE along the series The REE distribution coefficients calculated assuming irreversible incorporation are reported in Fig. 10; Table 4. These distribution coefficients imply that more than 99% of all the REE are incorporated in the secondary products. This result is in agreement with the concentrations of REE in secondary phases and basalts being very close from one another. The apparent distribution coefficients of REE decrease slightly from light to heavy REE ŽCe and Eu excluded.. Cerium displays a higher affinity for the secondary products, while europium shows a lower tendency for partitioning. The general trend in Fig. 10 is in agreement with the preferential uptake of
31
LREE over HREE described in montmorillonites, calcite, aragonite, Fe-, Al- and Ti-oxides and Fe–Mn crusts and coatings ŽBruque et al., 1980; Byrne and Kim, 1990; Sholkovitz et al., 1994; Bau et al., 1996.. The preferential uptake of LREE is likely due to the fact that the proportion of LREE as free ions in solution is higher than the proportion of HREE as free ions ŽFig. 7.. This change in speciation in turn results in fractionation whereby the light REE are preferentially trapped in the secondary phases ŽKoeppenkastrop et al., 1991; Sholkovitz, 1992; Koeppenkastrop and De Carlo, 1993.. In the interstitial fluids of the Mururoa volcanic massif, the valency of europium is II. The reduction of Eu increases its mobility Žcompared to the other REE. and explains its lower distribution coefficients. The distribution coefficient of Ce is found to be the highest of all REE Žfour times higher than the coefficient of neighbour La.. The enrichment of Ce with respect to the other REE has already been observed in manganese nodules ŽGlasby, 1973., organic matter ŽByrne and Kim, 1990., and clays precipitated from seawater altered basalt ŽFrey et al., 1974; Juteau et al., 1979; Bonnot-Courtois, 1980; Desprairies and Bonnot-Courtois, 1980; Nesbitt and Wilson, 1992; Prudencio et al., 1993.. This behaviour is often explained by the oxidation of CeŽIII. to the insoluble CeŽIV.. However, the reduction potential of interstitial water sampled from the atoll is not supposed to promote the oxidation of cerium. Moffett Ž1990. has experimentally shown that microbially mediated oxidation of dissolved CeŽIII. to the insoluble form of CeŽIV. occurs in seawater. The organic oxidation of cerium may be responsible for Ce depletion in the interstitial fluids, resulting in Ce apparent enrichment in the secondary products.
Table 4 REE distribution coefficients in saponites and zeolites Žmean values. calculated assuming the irreversible incorporation of the REE in the secondary products i Dsaponiterw = 10y6 y6
SD = 10 i Dzeoliterw = 10y5 SD = 10y5
SD: standard deviation.
La
Ce
Pr
Nd
Sm
Eu
Tb
Dy
Ho
Er
Yb
Lu
3.3 1.4 1.6 0.7
13.5 5.9 6.8 2.9
2.6 0.8 1.3 0.4
4.2 2.0 2.1 1.0
1.4 0.5 0.7 0.2
0.3 0.1 0.14 0.06
1.4 0.5 0.7 0.3
2.9 1.2 1.4 0.6
1.3 0.5 0.7 0.2
1.7 0.6 0.8 0.3
1.1 0.4 0.6 0.2
0.3 0.1 0.14 0.05
32
C. Guy et al.r Chemical Geology 158 (1999) 21–35
˚ in Given their ionic radius Žfrom 0.86 to 1.17 A coordinance 6; Baes and Mesmer, 1976., REE can be located in the interlayer or in the octahedral sites ˚ ., replacing Mg or Fe. of clay Žradius clay site s 0.97 A ˚ ., they can replace In zeolites Žradius zeo site s 1.10 A Na or K. The irreversibility of the incorporation suggests that the octahedral sites are occupied. 6.2.2. Distribution coefficients of REE in relation with the nature of the secondary phases Given the high ion exchange capacity of zeolites, the low REE concentrations found in zeolites from the Nerite core were unexpected. Berger Ž1992. conducted dissolutions of basaltic and Na-glasses in a REE-spiked seawater solution. For each experiment, smectites, chlorites or zeolites Ža mixture of analcimes, Na-phillipsites and chabazites. were formed. The distribution coefficients of REE were ) 10 5 in the clays, and ) 10 6 in the zeolites. We obtain similar distribution coefficients for clays but significantly lower values for zeolites. This discrepancy may be partly due to the fact that clays and zeolites are not formed simultaneously in Berger’s experiments. In addition, the zeolites sampled at Mururoa are K-phillipsites formed in situ under physicochemical conditions different from Berger’s experimental conditions Že.g., a pH 10 during Na-glass dissolution.. The existence of a REE concentration gradient from the basalt interface, through the clays, to the bulk solution where zeolites precipitate, can also account for the low concentration of REE in the zeolites. The dissolution of basalt by seawater promotes the precipitation of clays near the basalt dissolution interface while the formation of zeolites occurs in the bulk solution ŽGuy et al., 1992.. It is clear that the dissolving REE coming from the interface of basaltrseawater interaction are first trapped in the clays while zeolites filling up the veins incorporate the remaining REE.
7. Summary–conclusion The analyses of REE concentrations in the interstitial fluids and alteration products of Mururoa atoll volcanic rocks have made possible quantifying the distribution of REE between the solution and the
secondary phases during the seawaterrbasalt interaction at low temperature. The REE exhibit a very low mobility during the alteration processes. There is a preferential uptake of light REE from the solutions. This is related to the greater stability of the HREE complexes in solution. EuŽII., the dominant Eu species in the fluids under reducing conditions, is the most mobile REE while Ce is the least mobile. Although the adsorption capacity of zeolites is very high, they exhibit a lower REE enrichment than clays. These secondary products precipitate near the interface of basaltrseawater interaction and first incorporate the dissolving REE while zeolites located in the center of the veins incorporate only the remaining REE. Our results indicate the incorporation of REE into secondary products to be irreversible. The 10 000– 100 000 years long residence time of the interstitial fluids in the volcanic rocks demonstrates that the REE are actually steadily trapped in secondary minerals over a long period of time. Rare earth elements and some actinides ŽAm and Cm. have many common features Žsame oxidation state, close ionic radii, similar hydrolysis constants; Brookins, 1984; Krauskopf, 1986; Byrne and Kim, 1990.. Due to these substantially similar characteristics, the chemical controls on REE during the alteration of basalts are expected to be quite similar to those which govern the behaviour of ions such as Am3q, Cm3q. From that point of view, this study suggest that secondary phases such as clays and zeolites can retain radionuclides at least over the most critical time of intense radioactive disintegration. [CA]
Appendix A Thermodynamic formation constants of inorganic REE complexes for the reactions expressed as REE 3qq x Liys REEŽL x . 3y i x and equilibrium constants of the hydrolysis reaction for REE solid phases Žs.; from Ža. Brookins Ž1989., Žb. Millero Ž1992. and Žc. Byrne and Kim Ž1993.. No thermodynamic constants available for Pr, Tb and Tm.
C. Guy et al.r Chemical Geology 158 (1999) 21–35
a a a a a a a a a a b b b a a c
REEŽOH. 2q REEŽOH.q 2 REEŽOH. 3 REEŽOH.y 4 REEF 2q REECl 2q REEClq 2 REEO4q REEŽSO4 .y 2 REEŽCO 3 .q REEŽCO 3 .y 2 REEŽHPO4 .q REEŽHPO4 .y 2 REEŽOH. 3 s REE 2 ŽCO 3 . 3 s REEŽPO4 .s
La 5.5 10.6 14.6 17.2 3.6 0.8 y0.29 3.64 5.29 6.16 11.31 4.87 8.17 21.72 28.7 25.44
Ce 5.7 10.9 15.6 18.4 4.0 0.8 1.19 3.59 5.27 6.78 11.5 4.98 8.34 22.16 29.6 24.26
Nd 6.0 11.1 15.5 18.9 3.99 0.8 y0.29 3.64 5.10 6.72 11.8 5.18 8.66 23.41 30.6 25.42
Sm 6.1 11.4 16.2 20.3 4.02 0.8 y0.29 3.67 5.2 6.86 12.11 5.35 8.96 25.54 29.6 25.31
Eu 6.2 11.4 16.4 20.7 4.09 0.8 0.99 3.59 5.41 6.83 12.24 5.42 9.10 26.05 29.6 24.24
References Baes, C.F., Mesmer, R.E., 1976. The Hydrolysis of Cations. Wiley-Interscience, New York. Bardintzeff, J.M., Demange, J., Gachon, A., 1986. Petrology of the volcanic bedrock of Mururoa atoll ŽTuamotu Archipelago French Polynesia.. J. Volcanol. Geotherm. Res. 28, 55–83. Bau, M., Koschinsky, A., Dulski, P., Hein, J.R., 1996. Comparison of the partitioning behaviours of yttrium, rare earth elements, and titanium between hydrogenic marine ferromanganese crusts and seawater. Geochim. Cosmochim. Acta 60, 1709–1725. Berger, G., 1992. Distribution of trace elements between clays and zeolites and aqueous solutions similar to seawater. Appl. Geochem. Special Issue No. 1—Geochemistry of Radioactive Waste Disposal: A French Contribution, pp. 193–204. Berger, G., Claparols, C., Guy, C., Daux, V., 1994. Dissolution rate of a basalt glass in silica-rich solutions: implications for long-term alteration. Geochim. Cosmochim. Acta 58, 4875– 4886. Bonnot-Courtois, C., 1980. Le comportement des terres rares au cours de l’alteration sous-marine et ses consequences. Chem. ´ ´ Geol. 30, 119–131. Bonnot-Courtois, C., Jaffrezic-Renault, N., 1982. Etude des echanges entre terres rares et cations interfoliaires de deux ´ argiles. Clay Miner. 17, 409–420. Bourcier, W.L., 1989. Improvements in the solid solution modelling capabilities of the EQ3r6 geochemical code. Lawrence Livermore National Laboratory, Livermore, CA, UCID- 20587, 1989, 145 pp. Brookins, D.G., 1984. Geochemical Aspects of Radioactive Waste Disposal. Springer, New York, 347 pp. Brookins, D.G., 1989. Aqueous geochemistry of REE elements. In: Lipin, B.R., McKay, G.A. ŽEds.., Geochemistry and Min-
Gd 6.0 11.6 16.8 21.6 4.3 0.8 y0.29 3.66 5.21 7.29 12.39 5.49 9.24 26.49 29.6 24.13
Dy 6.0 11.8 17.3 22.5 4.36 0.8 y0.29 3.62 4.8 7.17 12.65 5.60 9.49 25.98 30.4 24.41
33
Ho 6.0 11.9 17.4 22.6 4.42 0.8 y0.29 3.59 4.9 7.23 12.77 5.64 9.62 26.63 29.6 25.35
Er 6.1 12.1 17.8 23.4 4.44 0.8 y0.29 3.59 5.10 7.32 12.88 5.68 9.73 27 29.6 25.07
Yb 6.3 12.2 17.9 23.3 4.48 0.7 y0.29 3.58 5.2 7.6 13.08 5.73 9.95 27.29 30.8 25.53
Lu 6.4 12.3 18.3 24.2 4.51 0.5 y0.29 3.52 5.3 7.57 13.2 5.75 10.05 27.51 29.6 24.7
eralogy of REE Elements. Reviews in Mineralogy 21, Miner. Soc. Am., pp. 201–225. Bruque, S., Mozas, T., Rodriguez, A., 1980. Factors influencing retention of lanthanides ions by montmorillonite. Clay Miner. 15, 413–420. Buigues, D., Gachon, A., Guille, G., 1992. L’atoll de Mururoa ŽPolynesie Geologique. ´ franc¸aise.: I. Structure et evolution ´ ´ Bull. Soc. Geol. ´ France 163 Ž5., 645–657. Byrne, R.H., Kim, K.H., 1990. REE element scavenging in seawater. Geochim. Cosmochim. Acta 54, 2645–2656. Byrne, R.H., Kim, K.H., 1993. Rare earth precipitation and coprecipitation behaviour: the limiting role of PO43y on dissolved rare earth concentrations in seawater. Geochim. Cosmochim. Acta 57, 519–526. Campbell, A.C., Palmer, M.R., Klinkhammer, G.P., Bowers, T.S., Edmond, J.M. et al., 1988. Chemistry of hot springs on the Mid-Atlantic Ridge. Nature 335, 514–519. Cantrell, K.J., Byrne, R.H., 1987. REE element complexation by carbonate and oxalate ions. Geochim. Cosmochim. Acta 51, 597–605. Claparols, C., 1992. Comportement des elements en trace et ´´ variations des compositions isotopiques du Nd et du Sr au cours de l’alteration de depots de cas et ´ ´ ˆ volcaniques: etudes ´ applications aux series de black shales de l’Ocean ´ mesozoıques ´ ¨ ´ Austral. Unpublished Thesis, Univ. Orsay, France, 305 pp. Daux, V., Crovisier, J.L., Hemond, C., Petit, J.C., 1994. Geochemical evolution of a basaltic rock submitted to weathering: fate of the major elements, rare earth elements and thorium. Geochim. Cosmochim. Acta 58, 4941–4954. De Baar, H.J.L., Bacon, M.P., Brewer, P.G., 1985. Rare earth in the Pacific and Atlantic oceans. Geochim. Cosmochim. Acta 49, 1943–1959. De Baar, H.J.L., German, C.R., Elderfield, H., van Gaans, P., 1988. Rare earth element distributions in anoxic waters of the Cariaco Trench. Geochim. Cosmochim. Acta 52, 1203–1219.
34
C. Guy et al.r Chemical Geology 158 (1999) 21–35
Desprairies, A., Bonnot-Courtois, C., 1980. Relation entre la composition des smectites d’alteration sous-marine et leur ´ cortege ` de terres rares. Earth Planet. Sci. Lett. 48, 124–130. Destrigneville, C., 1991. Etude de l’alteration dans le massif ´ basaltique de Mururoa ŽPolynesie ´ Franc¸aise.: analyses des phases solides et fluides et modelisation thermodynamique. ´ Unpublished Thesis, Univ. Toulouse, France, 163 pp. Drake, M.J., 1975. The oxidation state of europium as an indicator of oxygen fugacity. Geochim. Cosmochim. Acta 39, 55–64. Dudoignon, P., 1988. Alterations hydrothermales des roches vol´ caniques de l’atoll de Mururoa ŽPolynesie ´ franc¸aise.. Unpublished Thesis, Univ. Poitiers, France, 276 pp. Dudoignon, P., Meunier, A., Beaufort, D., Gachon, A., Buigues, D., 1989. Hydrothermal alteration at Mururoa Atoll ŽFrench Polynesia.. Chem. Geol. 76, 385–401. Frey, F.A., Bryan, W.B., Thompson, G., 1974. Atlantic ocean floor: geochemistry and petrology of basalts from legs 2 and 3 of the Deep Sea Drilling Project. J. Geophys. Res. 79, 5507– 5527. Glasby, G.P., 1973. Mechanisms of enrichment of the rarer elements in marine manganese nodules. Mar. Chem. 1, 105–125. Gobelet, P., 1981. Modelisation des transferts de masse et ´ d’energie en aquifere. ´ ` Unpublished Thesis, Ecole Nat. Sup. Mines de Paris, Univ. Paris VI, 225 pp. Guille, G., Goutiere, G., Sornein, J.F., 1993. Les atolls de Mururoa et Fangataufa ŽPolynesie ´ Franc¸aise.: I. Geologie–Petrol´ ´ ogie–Hydrogeologie. In: Louis-Jean ŽEd.., Edification et ´ evolution des edifices. Gap, France, 168 pp. ´ ´ Guy, C., Schott, J., Destrigneville, C., Chiappini, R., 1992. Low temperature alteration of basalt by seawater, Mururoa, French Polynesia. Geochim. Cosmochim. Acta 56, 4169–4189. Hajash, A., Chandler, G.W., 1984. An experimental investigation of high temperature interactions between seawater and rhyolite, andesite, basalt and peridotite. Contrib. Mineral. Petrol. 78, 240–254. Helgeson, H.C., Kirkham, D.H., 1974a. Theoretical prediction of the thermodynamic behaviour of aqueous electrolytes at high pressures and temperatures: I. Summary of the thermodynamicrelectrostatic properties of the solvent. Am. J. Sci. 274, 1089–1198. Helgeson, H.C., Kirkham, D.H., 1974b. Theoretical prediction of the thermodynamic behaviour of aqueous electrolytes at high pressures and temperatures: II. Debye–Huckel parameters for ¨ activity coefficients and partial molal properties. Am. J. Sci. 274, 1199–1261. Helgeson, H.C., Delany, J.M., Nesbitt, H.W., Bird, D.K., 1978. Summary and critique of the thermodynamics properties of rock-forming minerals. Am. J. Sci. 278, 1–229. Helgeson, H.C., Kirkham, D.H., Flowers, G.C., 1981. Theoretical prediction of the thermodynamic behaviour of aqueous electrolytes at high pressures and temperatures: IV. Calculation of activity coefficients, osmotic coefficients, and apparent molal and standard and relative partial molal properties to 6008C and 5 kb. Am. J. Sci. 281, 1249–1516. Henry, P., Guy, C., Cattin, R., Dudoignon, N.P., Sornein, J.F., Caristan, Y., 1996. A convective model of water flow in Mururoa basalt. Geochim. Cosmochim. Acta 60, 2087–2109.
Jercinovic, M.J., Keil, K., Smith, M.R., Schmitt, R.A., 1990. Alteration of basaltic glasses from Northcentral British Columbia, Canada. Geochim. Cosmochim. Acta 54, 2679– 2696. Juteau, T., Noack, Y., Whitechurch, H., Courtois, C., 1979. Mineralogy and geochemistry of alteration products in holes 417A and 417D basement samples. Proc. Ocean Drill. Program 51, 1273–1297. Klinkhammer, G., Elderfield, H., Hudson, A., 1983. REE elements in seawater near hydrothermal vents. Nature 305, 185– 188. Klinkhammer, G.P., Chin, C.S., Wilson, C., German, C.R., 1995. X Venting from the Mid-Atlantic Ridge at 37817 N: the Lucky Strike hydrothermal site. In: Parson, L.M., Walker, C.L., Dixon, D.R. ŽEds.., Hydrothermal Vents and Processes. Geol. Soc. Spec. Publ., pp. 87–96. Koeppenkastrop, D., De Carlo, E.H., 1993. Uptake of rare earth elements from solution by metal oxides. Environ. Sci. Technol. 27, 1796–1802. Koeppenkastrop, D., De Carlo, E.H., Roth, M., 1991. A method to investigate the interaction of rare earth elements in aqueous solution with metal oxides. J. Radioanal. Nucl. Chem. 152, 337–346. Krauskopf, K.B., 1986. Thorium and rare-earth metals as analogues for actinide elements. Chem. Geol. 55, 323–335. Lee, J.H., Byrne, R.H., 1992. Examination of comparative rare earth element complexation behaviour using linear free-energy relationships. Geochim. Cosmochim. Acta 56, 1127–1138. Lee, J.H., Byrne, R.H., 1993. Complexation of trivalent rare earth elements ŽCe, Eu, Gd, Tb, Yb. by carbonate ions. Geochim. Cosmochim. Acta 57, 295–302. Ludden, J.N., Thompson, G., 1979. An evaluation of the behaviour of the REE elements during the weathering of seafloor basalt. Earth Planet. Sci. Lett. 43, 85–92. Maury, R.C., Caroff, M., Achard, S., Guille, G., Joron, J.L., Racaboy, A., Leterrier, J., 1992. L’atoll de Mururoa ŽPolynesie ´ franc¸aise. IILa serie ´ magmatique. Bull. Soc. Geol. ´ France 163 Ž5., 659–679. Menzies, M., Seyfried, W.E., 1979. Basaltrseawater interaction: trace element and strontium isotopic variations in experimentally altered basalt and peridotite. Earth Planet. Sci. Lett. 44, 463–473. Michard, A., 1989. REE element systematics in hydrothermal fluids. Geochim. Cosmochim. Acta 53, 745–750. Michard, A., Albarede, F., 1986. The REE content of some ` hydrothermal fluids. Chem. Geol. 55, 51–60. Michard, A., Albarede, ` F., Michard, G., Minster, J.F., Charlou, J.L., 1983. REE elements and uranium in high temperature solutions from East Pacific Rise hydrothermal vent field Ž138N.. Nature 303, 795–797. Millero, F.J., 1992. Stability constants for the formation of rare earth inorganic complexes as a function of ionic strength. Geochim. Cosmochim. Acta 56, 3123–3132. Moffett, J.W., 1990. Microbially mediated cerium oxidation in seawater. Nature 345, 421–423. Nesbitt, H.W., Wilson, R.E., 1992. Recent chemical weathering of basalts. Am. J. Sci. 292, 740–777.
C. Guy et al.r Chemical Geology 158 (1999) 21–35 Piepgras, D.J., Jacobsen, S.B., 1992. The behaviour of REE elements in seawater: precise determination of variations in the North Pacific water column. Geochim. Cosmochim. Acta 56, 1851–1862. Prudencio, M.I., Braga, M.A.S., Gouveia, M.A., 1993. REE mobilization, fractionation and precipitation during weathering of basalts. In: Kump, L.R. ŽEd.., Geochemistry of the Earth Surface. Chem. Geol. 107, pp. 251–254 Žspecial issue.. Ridley, W.I., Perfot, M.R., Jonasson, I.R., Smith, M.F., 1994. Hydrothermal alteration in oceanic ridge volcanics: a detailed study at the Galapagos Fossil Hydrothermal Field. Geochim. Cosmochim. Acta 58, 2477–2494. Rougerie, F., Fagerstom, J.A., Andrie, C., 1992. Geothermal endo-upwelling: a solution to the reef nutrient paradox. Cont. Shelf Res. 12, 785–798. Samaden, G., Dallot, P., Roche, R., 1985. Atoll d’Eniwetok. Systeme geothermique insulaire a` l’etat ` ´ ´ naturel. La Houille Blanche 2, 143–151. Schiano, P., Dupre, ´ B., Lewin, E., 1993. Application of element concentration variability to the study of basalt alteration ŽFangataufa atoll, French Polynesia.. Chem. Geol. 104, 99– 124. Sholkovitz, E.R., 1992. Chemical evolution of rare earth elements:
35
fractionation between colloidal and solution phases of filtered river water. Earth Planet. Sci. Lett. 114, 77–84. Sholkovitz, E.R., Landing, W.M., Lewis, B.L., 1994. Ocean particle chemistry: the fractionation of rare earth elements between suspended particles and seawater. Geochim. Cosmochim. Acta 58, 1567–1579. Sverjensky, D.A., 1984. Europium redox equilibria in aqueous solution. Earth. Planet. Sci. Lett. 67, 70–78. Swartz, J.H., 1958. Geothermal Measurements. Eniwetok and Bikini Atolls. Geol. Sur. Prof. 260U. Wolery, T.J., 1983. EQ3rNR, A Computer Program for Geochemical Aqueous Speciation–Solubility Calculations: User’s Guide, Documentation. Lawrence Livermore National Laboratory UCRL-53414, Livermore, CA. Wolery, T.J., 1986. EQ3r6—status and future development. Proc. Workshop Geochem. Modeling, September 14–17, 1986, Fallen Leaf Lake, CA. Lawrence Livermore CONF-8609134, pp. 10–19. Wood, S.A., 1990. The aqueous geochemistry of the rare-earth elements and yttrium: 1. Review of available low-temperature data for inorganic complexes and the inorganic REE speciation of natural waters. Chem. Geol. 82, 159–186.
Behaviour of rare earth elements during seawaterrbasalt interactions in the Mururoa Massif C. Guy a
a,)
, V. Daux b, J. Schott
c
Commissariat a` l’Energie Atomique, Departement Analyses SurÕeillance EnÕironnement, serÕice RCE, BP 12, 91680 Bruyeres-Le-Chatel, ´ ` ˆ France b Laboratoire de Geologie des bassins sedimentaires, UniÕersite´ Paris VI, 4 place Jussieu, Paris, France ´ ´ c Laboratoire de Geochimie, 38 rue des trente six ponts, 31400 Toulouse, France ´ Received 13 January 1998; received in revised form 14 December 1998; accepted 14 December 1998
Abstract The rare earth elements ŽREE. were analysed in the interstitial fluids, in the basalts and in the alteration products Žclays and zeolites. of Mururoa atoll volcanic rocks ŽFrench Polynesia. in order to investigate REE behaviour during seawaterrbasalt interactions at low temperature. The REE distribution coefficients between fluids and secondary products were calculated using the EQ3r6 geochemical code. The reversiblerirreversible character of the REE entrapment in secondary products was tested. The REE concentrations in the clays are close to that of basalt. The concentrations in zeolites are 10 times lower. Comparison of measured and modeled concentrations in solutions favours the irreversible incorporation assumption. The affinity of the REE for the secondary products is shown to be high. The calculated distribution coefficients range from 3 = 10 5 for Lu and Eu to 10 7 for Ce in clays and from 10 4 to 7 = 10 5 in zeolites. The distribution coefficients exhibit a progressive decrease from La to Lu. This behaviour is related to the greater stability of HREE complexes relative to LREE complexes in solution. Under reducing conditions in the interstitial fluids ŽEh down to y500 mV., Eu undergoes reduction to the mobile EuŽII.. This enhanced mobility is reflected by Eu distribution coefficient being lower than those of adjacent REE. The distribution coefficient of Ce is higher. As Ce is trivalent in the fluids, fractionation from the other trivalent REE was unexpected. The calculated distribution coefficients of REE are lower in zeolites than in clays and this can be related to the spatial distribution and paragenesis of the secondary phases: the dissolution of the basalt promotes the precipitation of clays at the basalt–fluid interface and of zeolites in the bulk solution. The REE released by dissolution are incorporated into the clays and further uptake occurs in zeolites for the REE left in solution. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Rare earth element; Seawaterrbasalt interactions; Mururoa Massif
1. Introduction The chemical coherence of the rare earth elements ŽREE. makes them useful tracers of a variety of
)
Corresponding author
geochemical processes. For instance, they can be used for understanding low temperature reactions between seawater and the oceanic crust and many studies have documented the distribution and mobility of REE during seawaterrbasalt interactions at high and low temperature. REE have been shown to be more concentrated in hydrothermal fluids than in
0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 0 1 9 - 4
22
C. Guy et al.r Chemical Geology 158 (1999) 21–35
seawater ŽKlinkhammer et al., 1983; Michard et al., 1983; Michard and Albarede, ` 1986; Campbell et al., 1988; Michard, 1989.. However, their concentrations in the alteration products were shown to be close to those of the fresh basalt, thus indicating the low mobility of these elements. The overall low abundance of REE in seawater is further evidence for their low solubility in both continent and oceanic crust weathering reactions ŽFrey et al., 1974; Ludden and Thompson, 1979; Menzies and Seyfried, 1979; Bonnot-Courtois, 1980; Desprairies and BonnotCourtois, 1980; Hajash and Chandler, 1984; Schiano et al., 1993; Daux et al., 1994; Ridley et al., 1994.. Experimentally, this poor mobility has been also observed ŽBonnot-Courtois and Jaffrezic-Renault, 1982; Berger, 1992; Berger et al., 1994. and, moreover, it has been demonstrated that the light REE are even less mobile than the heavy REE ŽBruque et al., 1980; Byrne and Kim, 1990.. It has furthermore been emphasised that the behaviour of REE depends heavily on the intensity of weathering Žwaterrrock ratio.. To model the behaviour of REE during basalt alteration, it is necessary to examine the distribution of REE in the secondary phases. Slight changes in the rock composition during alteration may induce dramatic variation in the associated fluid. Thus, very useful information can be obtained from the concomitant study of the fluid. The Mururoa atoll drilling program provides a unique opportunity to document the behaviour of REE during basalt weathering via sampling of altered basalts together with the associated interstitial fluids. This work examines the distribution of REE between solutions and secondary products sampled from two drill-holes ŽNerite in 1986, Exocet in 1989. in the Mururoa Atoll. Using the EQ3r6 geochemical code, this dataset is modelled to understand the behaviour of REE during the low temperature alteration of basalts by seawater. The Mururoa situation in terms of physical properties of the crust is not analogous to ocean-crust: the hydraulic regime Žresidence time and velocity of the fluid in the crust. is probably different, the temperature of the alteration fluid may be higher on average than the low temperature conditions on the seabed. However, from a chemical point of view, we assume that the model presented in this study is relevant to REE behaviour during seawaterrocean-crust interactions.
2. Geological and hydrogeological settings The Mururoa atoll is located in the Tuamotu Archipelago Ž21850X S and 138853X W.. Its formation, between 12 and 9.5 Ma ago, is attributed to hot spot intraplate volcanism that generated the Pitcairn– Gambier chain ŽBardintzeff et al., 1986.. Today, it is covered by a thick Ž180 to 400 m. carbonate layer deposited after subsidence ŽBuigues et al., 1992.. Volcanic rocks are of alkali magmatic series extending from oceanites and Mg-rich basalts to comenditic trachytes. Hawaiites and alkali basalts are the most common rocks among the volcanic series ŽBardintzeff et al., 1986; Dudoignon et al., 1989; Maury et al., 1992; Guille et al., 1993.. In 1986 and 1989, two drill holes were cored under the lagoon ŽFig. 1.: one to a depth of 400 m at the Nerite site and the other to a depth of 690 m at the Exocet site. The sequence found from the top to the bottom of the holes shown in Fig. 1 is: Ž1. limestones, Ž2. basalts erupted in aerial and subaerial conditions, and Ž3. submarine basaltic pillow lavas. Numerous drilling campaigns at Eniwetok, Bikini ŽSwartz, 1958. and Mururoa ŽGuille et al., 1993. and theoretical studies using numerical codes ŽSamaden et al., 1985; Henry et al., 1996., have provided information about the general hydrogeology of atolls. Models of fluid circulation through porous media by diffusion and convection ŽGobelet, 1981; Samaden et al., 1985. show that interstitial water circulates through the atolls due to a thermal gradient between the cold oceanic water and the geothermally heated atoll base. For Mururoa, the reliability of the simulation has been demonstrated by a close agreement between calculated and measured temperatures down to a depth of 1200 m ŽGuille et al., 1993.. The water circulation is described as a centripetal infiltration that starts on the external slope of the atoll and rises to the centre. Circulation modelling suggests that the residence time of the interstitial fluids in the volcanic rocks ranges between 10 4 and 10 5 years ŽHenry et al., 1996.. 3. Fluid sampling and analytical methods Solids and interstitial fluids were sampled during Exocet and Nerite drillings campaigns at Mururoa. Since surface seawater was used as drilling fluid, it
C. Guy et al.r Chemical Geology 158 (1999) 21–35
23
Fig. 1. The Mururoa atoll. Southwestrnortheast schematised section and lithology; and location of the Nerite and Exocet drill holes.
was spiked with LiCl Žabout 50 ppm of Li. in order to quantify its mixing with the interstitial fluid. The drilling was generally carried out using a coring tool which enabled the entire core to be withdrawn, but when drilling was destructive, core cuttings were taken. After the sampler tubing had reached the appropriate depth, it was raised several meters. A packer was then brought down, fixed to the sampler tubing and inflated in order to isolate the volume between the packer and the bottom of the hole. A rupture disc was then broken to establish communication between the casing and the core, and the interstitial fluid was pumped from the surrounding rock up to the surface. The Li content of the solution was monitored as a function of time until it reached
a low value, ensuring a minimal contribution of surface seawater to the sampled fluid. Fluid sampling was carried out by a cable-operated borehole sampler. The apparatus included a 1-l vacuum chamber. After sampling, in-situ pressure was maintained as the apparatus was brought to the surface. Temperature was monitored during the drillings. It was constant at about 258C Ž"38C. throughout the two drill cores. The secondary phases in the volcanic rocks were hand-picked under a binocular microscope and analysed for REE. Basalts and interstitial fluids were analysed for major elements and REE. Major elements in the basalts were determined by X-ray fluorescence ŽXRF.. The analytical precision
C. Guy et al.r Chemical Geology 158 (1999) 21–35
24
is 5%. The major element compositions of the interstitial fluids determined by gas chromatography, ion chromatography and spectrophotometry were presented in previous papers. The main results of these studies are summarized in Section 4. REE analyses were performed by inductively coupled plasma mass spectrometry ŽICP-MS.. The interstitial fluids were diluted and acidified with HNO 3 in order to improve the signal stability. For solid samples, 100 mg of powder were attacked using HFrHNO 3 . These solutions were then diluted 10 to 50 times. Indium was added to all solutions as an internal standard. The analytical precision is 5%. Gd analyses are not reported due to analytical interferences.
4. Previous studies The behaviour of the major elements and strontium isotopes during the alteration of Mururoa basalts by seawater has been documented by Destrigneville Ž1991., Guy et al. Ž1992. and Guille et al. Ž1993..
Fig. 2. Si and K concentrations Ž10y3 molrkg. in the interstitial fluids of Mururoa volcanic rocks as a function of the mass of basalt dissolved per volume of interstitial water Ž R r W . Žsee Table 1; data from Guy et al., 1992.. Samples Exocet 1 and Nerite 3 correspond to the lowest and highest R r W ratios, respectively.
The major element composition of the interstitial fluids sampled in the carbonate layer is very close to that of seawater. In contrast, the chemical composi-
Table 1 Major element and REE concentrations in the interstitial fluids of Mururoa volcanic rocks Fluid
Depth
RrW
pH
Si
8.35 8.15 7.52 8.10 9.00 9.00
7 122 132 160 362 331 348
Ce
Pr
Nd
5.7 360 390 330 340 380 290 348 37
3.8 60 71 120 110 160 110 105 36
16.2 265 350 370 540 585 375 414 122
a a a a a a a
Lagoon Exocet 1 Exocet 2 Nerite 1 Nerite 2 Nerite 3 Nerite 4
571–580 613–624 207–223 341–353 390–399 411–423
22 37 52 97 110 94
Fluid
La
b c c c c c c
Seawater Exocet 1 Exocet 2 Nerite 1 Nerite 2 Nerite 3 Nerite 4 Mean SD
25.7 420 520 580 490 550 480 507 56
Mg
Ca
Na
11.4 46.0 82.1 141.0 127.0 99.2 80.7
466 478 450 327 293 264 275
Sm
Eu
Tb
3.0 87 135 280 370 320 230 237 109
0.8 113 174 365 520 485 550 368 186
53.4 34.1 14.4 3.2 32.5 69.4 52.8
0.6 67 39 21 26 30 18 34 18
K 8.7 6.00 3.50 2.20 0.86 0.86 1.10
Sr 0.11 0.31 0.40 0.50 0.72 0.83 0.77
SO4
Cl
Alkalinity
28.2 28.1 19.8 14.1 15.5 13.1 13.4
545 601 609 572 554 579 507
2.38 0.48 0.15 -
Tm
Yb
Lu
4.7 18 29 57 68 52 41 44 19
0.9 8 12 28 43 51 39 30 17
Dy
Ho
Er
5.3 46 80 48 60 76 60 62 14
1.1 11 19 9.5 17 30 17 17 7
5.0 78 57 54 59 54 41 57 12
0.7 6 7 26 29 24 20 19 10
Si concentrations in micromole per kilogram; Mg, Ca, Na, K, Sr, SO4 , Cl in millimole per kilogram and alkalinity in milliequivalent per liter; - for inferior to detection limit; RrW in gram per liter; REE in picomole per kilogram; depth in meter. The major element concentrations in the Mururoa lagoon and the REE concentrations in the Pacific surface seawater are given for comparison. References: Ža. from Guy et al. Ž1992.; Žb. average surface seawater 0–1000 m derived from De Baar et al. Ž1985. and Piepgras and Jacobsen Ž1992.; Žc. this study.
C. Guy et al.r Chemical Geology 158 (1999) 21–35
25
Increasing RrW ratios are related to the following changes in fluid composition ŽTable 1; Fig. 2.: Ž1. Sr and Si are progressively enriched, while K, dissolved carbonates and ÝS are depleted; Ž2. Ca and the pH decrease at first, and then increase as alteration progresses to higher RrW, while Mg exhibits the opposite behaviour; Ž3. the solutions change from oxidising to reducing. The secondary phases formed at Mururoa have already been described in detail ŽDudoignon, 1988; Dudoignon et al., 1989.. Chlorite, aluminium saponite, and calcic zeolites such as chabazite are generally observed from the rim to the centre of veins in the subaerial basalt. Gypsum and apophyllite crystals are observed locally. In the pillow lavas, the secondary minerals assemblages comprised celadonite, iron-rich saponite, magnesium saponite and potassium zeolites Žphillipsite.. The behaviour of major elements during basaltr seawater interactions at Mururoa was modelled by Guy et al. Ž1992. using EQ3r6 code. The paragenesis of secondary minerals calculated as a function of RrW is given in Fig. 3. The results of the geochemical simulations agree with the observations made at Mururoa. Magnesium-rich clays Žsaponites. precipitate at low RrW, yielding a Mg depletion and Ca enrichment in the solution. The precipitation of calcite induces the total removal of dissolved carbonates and a decrease of pH from 8.35 to 7.5. After calcite precipitation, the dissolution process causes
Fig. 3. Sequence of the secondary phases calculated using EQ3r6 as a function of R r W, at 258C. Saponites and zeolites are the most common secondary products. Minors refer to: gibbsite, goethite, pyrite and calcite. After Guy et al. Ž1992..
tion of the interstitial fluids of the basalts exhibits a dramatic change which can be related to the progress of the alteration reaction. The strontium isotope ratios Ž87 Srr86 Sr. of the fluids were used as proxies for the rockrwater ratios Ž RrW: mass of basalt dissolved per volume of interstitial water.. RrW ratios were shown to increase with depth up to 50 grl and 110 grl in Exocet and Nerite cores, respectively. This trend is consistent with seawater percolating from the atoll rim to the centre ŽHenry et al., 1996..
Table 2 Major element Žin %. and REE Žin ppm. concentrations in Mururoa aerial basalts at Exocet and Nerite drilling sites Basalt
SiO 2
TiO 2
Al 2 O 3
3.97 3.35 3.66 0.31
12.16 15.71 13.94 1.78
Fe 2 O 3 13.62 12.50 13.06 0.56
MnO
MgO
0.18 0.18 0.18 0.00
9.86 4.67 7.27 2.60
Exocet Nerite Mean SD
43.9 47.5 45.7 1.8
Basalt
Depth
La
Ce
Pr
Nd
Sm
Exocet Nerite Nerite Nerite Nerite Nerite Mean SD
735 311 346 377 391 407
31.2 29 45 45 51.6 55 42.9 9.5
76.2 71 108 106 115 135 102 22
9.9 r r r 15.5 r 12.7 2.8
42.3 39 64 64 67.9 79 59.4 14.1
8.23 r r r 14.1 r 11.2 2.9
CaO
Na 2 O
K 2O
P2 O5
PF
H 2O
Total
11.29 7.49 9.39 1.90
2.06 3.40 2.73 0.67
0.62 1.90 1.26 0.64
0.50 1.30 0.90 0.40
0.87 1.70 1.29 0.42
0.34 0.35 0.35 0.01
99.37 100.05 99.71 0.34
Eu
Tb
Dy
Ho
Er
Tm
Yb
0.88 r r r 1.53 r 1.2 0.3
4.76 5.1 7.7 7.7 8.13 8.8 7.0 1.5
0.85 r r r 1.43 r 1.1 0.3
2.04 2.4 3.3 3.3 3.4 3.6 3.0 0.6
0.26 r r r 0.44 r 0.35 0.09
2.64 2.7 4.0 4.0 4.9 4.8 3.8 0.9
1.57 1.7 2.5 2.5 2.65 2.8 2.3 0.5
Lu 0.20 r r r 0.35 r 0.27 0.08
Basaltrseawater interactions Žsee Section 6. was modeled using the mean values of the concentrations in the basalts. SD: standard deviation.
C. Guy et al.r Chemical Geology 158 (1999) 21–35
26
Table 3 REE concentrations in the saponites and zeolites sampled at Exocet and Nerite drilling site in Mururoa atoll Žppm. Depth
La
Ce
Pr
Nd
Sm
Eu
Tb
Dy
Ho
Er
Tm
Yb
Lu
Saponites 337 386 403 Mean SD
16.3 26.2 20.5 21 4.1
39.1 52.8 65.0 52.3 10.6
4.6 5.7 6.2 5.5 0.7
19.9 23.5 28.0 23.8 3.3
4.2 4.4 5.9 4.8 0.8
1.4 1.3 1.8 1.5 0.2
0.77 0.75 0.77 0.76 0.01
3.0 2.9 3.9 3.3 0.4
0.49 0.45 0.66 0.53 0.09
1.7 1.6 1.8 1.7 0.1
0.17 0.15 0.23 0.18 0.03
1.2 1.0 1.5 1.2 0.2
0.19 0.15 0.20 0.18 0.02
1.9
4.1
0.58
2.5
0.52
1.9
0.06
0.32
0.06
0.14
Zeolites 326
.018
0.11
.015
Depth in meter SD: standard deviation.
proton consumption which results in an increase in pH up to 9. The oxidation of Fe 2q consumes the dissolved oxygen and consequently, Eh decreases from about q350 mV to y500 mV. While RrW increases, ÝS is gradually removed from the solution due to the precipitation of gypsum and pyrite. The increase in Ca concentration and pH in the solution promotes the precipitation of zeolites Ž RrW ) 25 grl.. At higher RrW ratios, the magnesium-rich saponites are destabilised, inducing the formation of Ca- and Na-rich saponites.
5. Results The REE concentrations measured in the interstitial fluids, in the basalts and in the saponites and the zeolites are listed in Tables 1–3.
Fig. 4. Seawater-normalised REE profiles for the mean interstitial fluid of Mururoa volcanic rocks. The error bars correspond to the standard deviation. The seawater composition used for the normalisation correspond to Pacific surface seawater Ždata in Table 1..
5.1. REE concentrations in the interstitial fluids It can be seen in Fig. 4 that the interstitial fluids are significantly enriched in REE with respect to seawater. Hydrothermal solutions also exhibit REE enrichments ŽRefs. in Section 1. which increase with decreasing pH ŽMichard, 1989. and increasing temperature ŽKlinkhammer et al., 1995.. This observation is consistent with Mururoa interstitial fluids being less concentrated Žapproximately tenfold. than the hotter and more acidic fluids of the East Pacific Rise ŽMichard et al., 1983.. The other features illustrated in Fig. 4 are: Ž1. a large Eu positive anomaly, and Ž2. a slight decrease of the enrichment across the REE series. Except for Ce, Tb and Er concentrations which are nearly constant, the REE concentrations in the interstitial fluids increase with increasing RrW ŽTable 1; Fig. 5..
Fig. 5. Nd and Ce concentrations Ž10y1 0 molrkg. in the interstitial fluids of Mururoa volcanic rocks as a function of R r W Ždata in Table 1..
C. Guy et al.r Chemical Geology 158 (1999) 21–35
5.2. REE concentrations in the secondary products Relative to the basalt, the saponites display a slight depletion ŽFig. 6a.. REE concentrations in zeolites are distinctly lower by an order of magnitude. The basalt-normalised REE patterns of the secondary products are almost flat. The main features of the REE normalisation of the secondary products to seawater are ŽFig. 6b.: Ž1. a systematic and nearly tenfold decrease of the normalised concentration across the REE series, and Ž2. a huge positive Ce anomaly. The REE enrichment of the interstitial fluids relative to seawater does not imply that these elements are mobile. The dissolution of 20 g of basalt per litre of solution ŽExocet 1., should result in REE concentrations in solution of ca. 10y7 molrl. The measured concentrations in Exocet 1 do not exceed a few picomoles per litre. These low concentrations result from the incorporation of REE into saponites and, to a lesser extent, into zeolites. The REE content of the clays is close to that of basalts. This low variability of the REE concentrations during basalt to clay transformation has been reported by several authors
Fig. 6. Ža. Basalt-normalised Žlog scale. and Žb. seawater-normalised Žlog scale. REE profiles for the secondary products sampled in Mururoa volcanic rocks. For basalt and saponites, the data used are the mean concentrations. The error bars correspond to the standard deviation Žsee Table 2Table 3..
27
ŽLudden and Thompson, 1979; Jercinovic et al., 1990; Claparols, 1992; Daux et al., 1994..
6. Discussion 6.1. REE in the interstitial fluids: thermodynamic approach The distribution of REE in seawater and in Exocet 1 and Nerite 3 interstitial fluids Žlowest and highest RrW ratios, respectively. and the saturation indexes of these solutions with respect to REE hydroxides, carbonates and phosphates have been calculated using EQ3r6 computer code. 6.1.1. Thermodynamic data EQ3r6 computer code was developed by Wolery Ž1983, 1986.. EQ3 computes the equilibrium distribution of species in the aqueous solution and generates concentrations and activities of ions and complexes. EQ6 calculates chemical equilibrium and mass transfer in aqueous solution–mineral systems using a step-wise procedure involving titration of increments of solid phases into solution. To perform these calculations, thermodynamic parameters for minerals were taken from Helgeson et al. Ž1978. and Bourcier Ž1989., the equation of state of water from Helgeson and Kirkham Ž1974a,b. and the data for aqueous species from Helgeson et al. Ž1981.. The standard molal Gibbs free energies Ž D f G8. and enthalpies Ž D f H8. of formation adopted in this study represent the change in those thermodynamic quantities when 1 mol of a substance in its standard state is formed isothermally from the elements at 1 bar pressure. The standard state for solid phases and H 2 O are unit activity for the pure phases at all temperatures and pressures. For aqueous species, the standard state corresponds to unit activity coefficient for a hypothetical ideal 1 m solution. Molal activity coefficients of neutral aqueous species were assumed to be unity. Activity coefficients of charged species were calculated using the extended Debye–Huckel equation with values of the electro¨ static parameters taken from Helgeson and Kirkham Ž1974a,b.. The thermodynamic parameters for the REE were taken from Drake Ž1975., Sverjensky
28
C. Guy et al.r Chemical Geology 158 (1999) 21–35
Ž1984., Brookins Ž1989., Wood Ž1990. and Millero Ž1992.. For saturation index calculations, REE hydroxides REEŽOH. 3 , carbonates REE 2 ŽCO 3 . 3 and phosphates REEŽPO4 . were considered. The thermodynamic constants used in the calculations are given in Appendix A. In the interstitial waters from the volcanic rocks, the concentration of phosphates is inferior to 10y8 molrl ŽDestrigneville, 1991.. We assumed that phosphates were thermodynamically controlled by hydroxyapatite, a basalt constitutive mineral, to calculate the saturation index of REEŽPO4 .. The hydroxyapatite Ca 5 ŽPO4 . 3 OH Žlog K s y11.52 at 258C. was used in the calculation. The calculated ÝPO4 are 3 = 10y1 0 and 1.5 = 10y1 1 molrl for Exocet 1 and Nerite 3 samples, respectively. For seawater, we used the concentration of REE reported in Table 1 and a mean phosphate concentration of 5 = 10y7 molrl measured at Tikehau atoll ŽRougerie et al., 1992..
6.1.2. Speciation and saturation The REE speciation in seawater is dominated by the carbonate complexes ŽFig. 7a.. The most common light REE species are REEŽCO 3 .q and REEqqq. The proportion of REEŽCO 3 .y 2 gradually increases from La to Lu and this complex becomes the most abundant species for the heavy REE. These results are in good agreement with previous studies ŽCantrell and Byrne, 1987; Byrne and Kim, 1990; Lee and Byrne, 1992, 1993; Millero, 1992.. Calculated saturation indexes Ž QrK . reported on Fig. 8a show that seawater is close to equilibrium with respect to light REE phosphates but undersaturated with respect to heavy REE phosphates. Seawater is strongly undersaturated with respect to the REE carbonates and hydroxides. Although dissolved carbonate concentrations in Exocet 1 solution are low Žsee alkalinity in Table 1., REE with the exception of La and Ce are mainly present as carbonate complexes ŽFig. 7b.. The proportion of carbonate complexes increases along the series. For La and Ce, the dominant species are La3q and Ce 3q. REE 3q, REEŽCl. 2q and REEŽCO 3 .q represent 90% of the aqueous species. Saturation indexes calculations ŽFig. 8b. show that these solutions are undersaturated with respect to REE phosphates,
Fig. 7. Trivalent REE speciation in Ža. seawater, Žb. low R r W interstitial fluids ŽExocet 1., Žc. high R r W interstitial fluids ŽNerite 3.. The thermodynamic constants used for the calculation can be found in Appendix A. The percentages of REE 3q and Žor REEŽOH.y . are represented by white areas at REEŽCO 3 .y 2 4 the top and at the bottom of the diagrams, respectively.
carbonates and hydroxides. The saturation indexes of the interstitial fluids with respect to REE 2 ŽCO 3 . 3 are lower than 10y3 0 and are not reported in Fig. 8. Nerite 3 is a carbonate-free solution Žsee alkalinity in Table 1. in which the hydrolysed species predominate ŽFig. 7c.. The proportion of these complexes increases from light to heavy REE Žat the expense of free ions. as does the degree of hydroxylation: La and Ce are present as REE 3q, Nd and Sm as REEŽOH. 2q, Dy, Ho, Er, Yb and Lu as REEŽOH.y 4 . In the same way as for low RrW solutions, the high RrW ones are undersaturated with respect to REE phosphates, carbonates and hydroxides.
C. Guy et al.r Chemical Geology 158 (1999) 21–35
29
phates suggests that the concentrations of REE in the solutions are not controlled by the solubility of REE-phases but, as it is shown in Section 6.2, by REE incorporation in the saponites and zeolites.
6.2. Incorporation of REE in secondary phases The incorporation of REE in the secondary products can be described using two different reactional schemes: Ø Reversible incorporation involving continuous equilibrium between the solution and the secondary phases; Ø Irreversible incorporation: the secondary phases being formed are in equilibrium with the reacting fluid but not with previously formed phases. If reversible incorporation is postulated, the mass conservation for each REE i can be expressed as: n
Fig. 8. Logarithm of the saturation index Ž Qr K . of seawater, and of low and high R r W interstitial fluids of the volcanic rocks ŽExocet 1 and Nerite 3, respectively. with respect to REE hydroxides, carbonates and phosphates. Q is the activity product and K the thermodynamic constant. The K constants used for the saturation calculation can be found in Appendix A. The fluids are undersaturated with respect to REE minerals Žexcept seawater with respect to LaPO4 .. The supersaturation of seawater with respect to CeŽIV.O 2 Žlog Qr K s 4.5. is not reported.
According to our calculation, the percentage of cerium in the form CeŽIV. is negligible in seawater and in the interstitial solutions. In agreement with the thermodynamic calculation reported by De Baar et al. Ž1988., the oxygenated seawater is found to be supersaturated with respect to CeŽIV.O 2 Žlog QrK s 4.5.. In contrast, the reduced interstitial fluids are undersaturated with respect to this oxide. Eu is also present as EuŽIII. in seawater but it is reduced to EuŽII. in the interstitial fluids. The dominant Eu Ž15%. for species are: Eu2q Ž77%. and EuŽCO 3 .y 2 2q Ž . low RrW solutions, Eu 89% and EuŽOH. 2q Ž4%. for high RrW solutions. The undersaturation of the interstitial fluids with respect to the REE hydroxides, carbonates and phos-
R Ž t . C bi s WC wi Ž t . q
Ý M j Ž t . C ji Ž t . ,
Ž 1.
js1
where t is the time, C bi , C wi and C ji refer to element i concentration in the basalt, the solution and the secondary phase j, respectively. R, W and M j stand for the mass of dissolved basalt, the mass of solution involved in the dissolution and the mass of secondary product j. n refers to the number of secondary phases. The distribution coefficient Djirw of the REE i between a secondary phase j and the solution is:
Djir w s
C ji Ž t . C wi Ž t .
.
Ž 2.
Combining Eqs. Ž1. and Ž2. leads to: R Ž t . C bi
C wi Ž t . s
n
Wq
Ý js1
Djir w M j Ž t .
.
Ž 3.
C. Guy et al.r Chemical Geology 158 (1999) 21–35
30
If the incorporation of REE in secondary products is assumed to be irreversible, the Eq. Ž1. applies only during the time frame D t. This can be expressed as: C wi Ž t . W q C bi D R s C wi Ž t q D t . W n
q
Ý C ji Ž t q D t . D M j .
Ž 4.
js1
Combining Eqs. Ž2. and Ž4. leads to: C wi Ž t q D t . s
C wi Ž t . W q D RC bi n
Wq
Ý
.
Ž 5.
Djir w D M j
js1
The incorporation of REE in saponites and zeolites, which are the main secondary phases, has been modelled using EQ3r6 according to the expressions Ž3. and Ž5. Žreversible and irreversible models, respectively.. The distribution coefficients Ž Djirw . were adjusted so that the calculated concentrations fitted the measured REE concentrations. This requires an iterative calculation, which converges after a few cycles. From Eqs. Ž3. and Ž5., the REE concentrations in solution depend on the following parameters. Ž1. The masses of dissolved basalt Ž R . and solution ŽW .. The calculation of REE concentrations in the interstitial fluids was performed as a function of RrW. Ž2. REE concentrations in the basalt Ž C b .. The REE concentrations used in the calculations are the mean values in Table 2. The volcanic rocks in Nerite and Exocet range from basalts to hawaiites; hawaiites are enriched with respect to basalt 1.6 to 1.9 times in REE Ždepending on the element—see Table 2.. This variability was taken into account in the calculation. Ž3. The mass of secondary products Ž M j . can be derived with a good precision from geochemical EQ3r6 simulations constrained by electron microscope observations, laboratory experiments and interstitial fluids sampled in Nerite and Exocet ŽGuy et al., 1992.. The evolution of the amount of secondary products as a function of RrW Žreported in Fig. 3. is used to model the incorporation of the REE in the secondary products. Through the comparison of measured and modeled concentrations in solutions, we can test the
irreversiblerreversible assumptions. Fig. 9 shows that Nd, Eu, and Lu concentrations can be modeled successfully by the irreversible model. The injection of distinct distribution coefficients in the reversible equation is figured by parallel lines in a concentration vs. RrW plot. As can be seen in Fig. 9, translating the reversible lines cannot lead to a satisfactory fit of the measured Nd, Eu and Lu concentrations. The same conclusions are drawn for the other REE. These results corroborate experimental studies from Claparols Ž1992. and Berger et al. Ž1994.. These authors performed dissolution experiments of REE-spiked spherules of basaltic glass. Each individual spherule was doped with a different REE and all the spherules altered together at 3008C in the same solution. It was shown that the altered skin formed on a spherule was enriched in the lanthanide originally present in the spherule. No REE contamination between spherules was observed. If there was a
Fig. 9. Concentrations of Nd, Eu, and Lu in solutions Žpmolrkg. as a function of R r W. Circles: REE concentrations measured in the interstitial fluids of the volcanic rocks at Mururoa. Solid lines: concentrations calculated assuming reversible ŽEq. Ž3.. or irreversible ŽEq. Ž5.. incorporation of REE in the secondary phases. The distribution coefficients between the secondary phases Žsaponites and zeolites. and the solutions, used in this simulation Nd Eu Lu are: Dsaponiterw s 4.2=10 6 ; Dsaponiterw s Dsaponiterw s 3=10 5 ; Nd Eu Lu Dzeoliterw s 2.1=10 5 ; Dzeoliterw s Dzeoliterw s1=10 4 .
C. Guy et al.r Chemical Geology 158 (1999) 21–35
Fig. 10. Distribution coefficients of the REE between solution and secondary products Ž Djirw ; log scale. corresponding to the best fit between data ŽREE concentrations in Mururoa interstitial fluids. and concentrations in solutions modeled assuming irreversible incorporation of REE in secondary products.
permanent equilibrium of the secondary phases with the solution, all the REE should be found in each alteration skin. 6.2.1. Distribution coefficients of REE along the series The REE distribution coefficients calculated assuming irreversible incorporation are reported in Fig. 10; Table 4. These distribution coefficients imply that more than 99% of all the REE are incorporated in the secondary products. This result is in agreement with the concentrations of REE in secondary phases and basalts being very close from one another. The apparent distribution coefficients of REE decrease slightly from light to heavy REE ŽCe and Eu excluded.. Cerium displays a higher affinity for the secondary products, while europium shows a lower tendency for partitioning. The general trend in Fig. 10 is in agreement with the preferential uptake of
31
LREE over HREE described in montmorillonites, calcite, aragonite, Fe-, Al- and Ti-oxides and Fe–Mn crusts and coatings ŽBruque et al., 1980; Byrne and Kim, 1990; Sholkovitz et al., 1994; Bau et al., 1996.. The preferential uptake of LREE is likely due to the fact that the proportion of LREE as free ions in solution is higher than the proportion of HREE as free ions ŽFig. 7.. This change in speciation in turn results in fractionation whereby the light REE are preferentially trapped in the secondary phases ŽKoeppenkastrop et al., 1991; Sholkovitz, 1992; Koeppenkastrop and De Carlo, 1993.. In the interstitial fluids of the Mururoa volcanic massif, the valency of europium is II. The reduction of Eu increases its mobility Žcompared to the other REE. and explains its lower distribution coefficients. The distribution coefficient of Ce is found to be the highest of all REE Žfour times higher than the coefficient of neighbour La.. The enrichment of Ce with respect to the other REE has already been observed in manganese nodules ŽGlasby, 1973., organic matter ŽByrne and Kim, 1990., and clays precipitated from seawater altered basalt ŽFrey et al., 1974; Juteau et al., 1979; Bonnot-Courtois, 1980; Desprairies and Bonnot-Courtois, 1980; Nesbitt and Wilson, 1992; Prudencio et al., 1993.. This behaviour is often explained by the oxidation of CeŽIII. to the insoluble CeŽIV.. However, the reduction potential of interstitial water sampled from the atoll is not supposed to promote the oxidation of cerium. Moffett Ž1990. has experimentally shown that microbially mediated oxidation of dissolved CeŽIII. to the insoluble form of CeŽIV. occurs in seawater. The organic oxidation of cerium may be responsible for Ce depletion in the interstitial fluids, resulting in Ce apparent enrichment in the secondary products.
Table 4 REE distribution coefficients in saponites and zeolites Žmean values. calculated assuming the irreversible incorporation of the REE in the secondary products i Dsaponiterw = 10y6 y6
SD = 10 i Dzeoliterw = 10y5 SD = 10y5
SD: standard deviation.
La
Ce
Pr
Nd
Sm
Eu
Tb
Dy
Ho
Er
Yb
Lu
3.3 1.4 1.6 0.7
13.5 5.9 6.8 2.9
2.6 0.8 1.3 0.4
4.2 2.0 2.1 1.0
1.4 0.5 0.7 0.2
0.3 0.1 0.14 0.06
1.4 0.5 0.7 0.3
2.9 1.2 1.4 0.6
1.3 0.5 0.7 0.2
1.7 0.6 0.8 0.3
1.1 0.4 0.6 0.2
0.3 0.1 0.14 0.05
32
C. Guy et al.r Chemical Geology 158 (1999) 21–35
˚ in Given their ionic radius Žfrom 0.86 to 1.17 A coordinance 6; Baes and Mesmer, 1976., REE can be located in the interlayer or in the octahedral sites ˚ ., replacing Mg or Fe. of clay Žradius clay site s 0.97 A ˚ ., they can replace In zeolites Žradius zeo site s 1.10 A Na or K. The irreversibility of the incorporation suggests that the octahedral sites are occupied. 6.2.2. Distribution coefficients of REE in relation with the nature of the secondary phases Given the high ion exchange capacity of zeolites, the low REE concentrations found in zeolites from the Nerite core were unexpected. Berger Ž1992. conducted dissolutions of basaltic and Na-glasses in a REE-spiked seawater solution. For each experiment, smectites, chlorites or zeolites Ža mixture of analcimes, Na-phillipsites and chabazites. were formed. The distribution coefficients of REE were ) 10 5 in the clays, and ) 10 6 in the zeolites. We obtain similar distribution coefficients for clays but significantly lower values for zeolites. This discrepancy may be partly due to the fact that clays and zeolites are not formed simultaneously in Berger’s experiments. In addition, the zeolites sampled at Mururoa are K-phillipsites formed in situ under physicochemical conditions different from Berger’s experimental conditions Že.g., a pH 10 during Na-glass dissolution.. The existence of a REE concentration gradient from the basalt interface, through the clays, to the bulk solution where zeolites precipitate, can also account for the low concentration of REE in the zeolites. The dissolution of basalt by seawater promotes the precipitation of clays near the basalt dissolution interface while the formation of zeolites occurs in the bulk solution ŽGuy et al., 1992.. It is clear that the dissolving REE coming from the interface of basaltrseawater interaction are first trapped in the clays while zeolites filling up the veins incorporate the remaining REE.
7. Summary–conclusion The analyses of REE concentrations in the interstitial fluids and alteration products of Mururoa atoll volcanic rocks have made possible quantifying the distribution of REE between the solution and the
secondary phases during the seawaterrbasalt interaction at low temperature. The REE exhibit a very low mobility during the alteration processes. There is a preferential uptake of light REE from the solutions. This is related to the greater stability of the HREE complexes in solution. EuŽII., the dominant Eu species in the fluids under reducing conditions, is the most mobile REE while Ce is the least mobile. Although the adsorption capacity of zeolites is very high, they exhibit a lower REE enrichment than clays. These secondary products precipitate near the interface of basaltrseawater interaction and first incorporate the dissolving REE while zeolites located in the center of the veins incorporate only the remaining REE. Our results indicate the incorporation of REE into secondary products to be irreversible. The 10 000– 100 000 years long residence time of the interstitial fluids in the volcanic rocks demonstrates that the REE are actually steadily trapped in secondary minerals over a long period of time. Rare earth elements and some actinides ŽAm and Cm. have many common features Žsame oxidation state, close ionic radii, similar hydrolysis constants; Brookins, 1984; Krauskopf, 1986; Byrne and Kim, 1990.. Due to these substantially similar characteristics, the chemical controls on REE during the alteration of basalts are expected to be quite similar to those which govern the behaviour of ions such as Am3q, Cm3q. From that point of view, this study suggest that secondary phases such as clays and zeolites can retain radionuclides at least over the most critical time of intense radioactive disintegration. [CA]
Appendix A Thermodynamic formation constants of inorganic REE complexes for the reactions expressed as REE 3qq x Liys REEŽL x . 3y i x and equilibrium constants of the hydrolysis reaction for REE solid phases Žs.; from Ža. Brookins Ž1989., Žb. Millero Ž1992. and Žc. Byrne and Kim Ž1993.. No thermodynamic constants available for Pr, Tb and Tm.
C. Guy et al.r Chemical Geology 158 (1999) 21–35
a a a a a a a a a a b b b a a c
REEŽOH. 2q REEŽOH.q 2 REEŽOH. 3 REEŽOH.y 4 REEF 2q REECl 2q REEClq 2 REEO4q REEŽSO4 .y 2 REEŽCO 3 .q REEŽCO 3 .y 2 REEŽHPO4 .q REEŽHPO4 .y 2 REEŽOH. 3 s REE 2 ŽCO 3 . 3 s REEŽPO4 .s
La 5.5 10.6 14.6 17.2 3.6 0.8 y0.29 3.64 5.29 6.16 11.31 4.87 8.17 21.72 28.7 25.44
Ce 5.7 10.9 15.6 18.4 4.0 0.8 1.19 3.59 5.27 6.78 11.5 4.98 8.34 22.16 29.6 24.26
Nd 6.0 11.1 15.5 18.9 3.99 0.8 y0.29 3.64 5.10 6.72 11.8 5.18 8.66 23.41 30.6 25.42
Sm 6.1 11.4 16.2 20.3 4.02 0.8 y0.29 3.67 5.2 6.86 12.11 5.35 8.96 25.54 29.6 25.31
Eu 6.2 11.4 16.4 20.7 4.09 0.8 0.99 3.59 5.41 6.83 12.24 5.42 9.10 26.05 29.6 24.24
References Baes, C.F., Mesmer, R.E., 1976. The Hydrolysis of Cations. Wiley-Interscience, New York. Bardintzeff, J.M., Demange, J., Gachon, A., 1986. Petrology of the volcanic bedrock of Mururoa atoll ŽTuamotu Archipelago French Polynesia.. J. Volcanol. Geotherm. Res. 28, 55–83. Bau, M., Koschinsky, A., Dulski, P., Hein, J.R., 1996. Comparison of the partitioning behaviours of yttrium, rare earth elements, and titanium between hydrogenic marine ferromanganese crusts and seawater. Geochim. Cosmochim. Acta 60, 1709–1725. Berger, G., 1992. Distribution of trace elements between clays and zeolites and aqueous solutions similar to seawater. Appl. Geochem. Special Issue No. 1—Geochemistry of Radioactive Waste Disposal: A French Contribution, pp. 193–204. Berger, G., Claparols, C., Guy, C., Daux, V., 1994. Dissolution rate of a basalt glass in silica-rich solutions: implications for long-term alteration. Geochim. Cosmochim. Acta 58, 4875– 4886. Bonnot-Courtois, C., 1980. Le comportement des terres rares au cours de l’alteration sous-marine et ses consequences. Chem. ´ ´ Geol. 30, 119–131. Bonnot-Courtois, C., Jaffrezic-Renault, N., 1982. Etude des echanges entre terres rares et cations interfoliaires de deux ´ argiles. Clay Miner. 17, 409–420. Bourcier, W.L., 1989. Improvements in the solid solution modelling capabilities of the EQ3r6 geochemical code. Lawrence Livermore National Laboratory, Livermore, CA, UCID- 20587, 1989, 145 pp. Brookins, D.G., 1984. Geochemical Aspects of Radioactive Waste Disposal. Springer, New York, 347 pp. Brookins, D.G., 1989. Aqueous geochemistry of REE elements. In: Lipin, B.R., McKay, G.A. ŽEds.., Geochemistry and Min-
Gd 6.0 11.6 16.8 21.6 4.3 0.8 y0.29 3.66 5.21 7.29 12.39 5.49 9.24 26.49 29.6 24.13
Dy 6.0 11.8 17.3 22.5 4.36 0.8 y0.29 3.62 4.8 7.17 12.65 5.60 9.49 25.98 30.4 24.41
33
Ho 6.0 11.9 17.4 22.6 4.42 0.8 y0.29 3.59 4.9 7.23 12.77 5.64 9.62 26.63 29.6 25.35
Er 6.1 12.1 17.8 23.4 4.44 0.8 y0.29 3.59 5.10 7.32 12.88 5.68 9.73 27 29.6 25.07
Yb 6.3 12.2 17.9 23.3 4.48 0.7 y0.29 3.58 5.2 7.6 13.08 5.73 9.95 27.29 30.8 25.53
Lu 6.4 12.3 18.3 24.2 4.51 0.5 y0.29 3.52 5.3 7.57 13.2 5.75 10.05 27.51 29.6 24.7
eralogy of REE Elements. Reviews in Mineralogy 21, Miner. Soc. Am., pp. 201–225. Bruque, S., Mozas, T., Rodriguez, A., 1980. Factors influencing retention of lanthanides ions by montmorillonite. Clay Miner. 15, 413–420. Buigues, D., Gachon, A., Guille, G., 1992. L’atoll de Mururoa ŽPolynesie Geologique. ´ franc¸aise.: I. Structure et evolution ´ ´ Bull. Soc. Geol. ´ France 163 Ž5., 645–657. Byrne, R.H., Kim, K.H., 1990. REE element scavenging in seawater. Geochim. Cosmochim. Acta 54, 2645–2656. Byrne, R.H., Kim, K.H., 1993. Rare earth precipitation and coprecipitation behaviour: the limiting role of PO43y on dissolved rare earth concentrations in seawater. Geochim. Cosmochim. Acta 57, 519–526. Campbell, A.C., Palmer, M.R., Klinkhammer, G.P., Bowers, T.S., Edmond, J.M. et al., 1988. Chemistry of hot springs on the Mid-Atlantic Ridge. Nature 335, 514–519. Cantrell, K.J., Byrne, R.H., 1987. REE element complexation by carbonate and oxalate ions. Geochim. Cosmochim. Acta 51, 597–605. Claparols, C., 1992. Comportement des elements en trace et ´´ variations des compositions isotopiques du Nd et du Sr au cours de l’alteration de depots de cas et ´ ´ ˆ volcaniques: etudes ´ applications aux series de black shales de l’Ocean ´ mesozoıques ´ ¨ ´ Austral. Unpublished Thesis, Univ. Orsay, France, 305 pp. Daux, V., Crovisier, J.L., Hemond, C., Petit, J.C., 1994. Geochemical evolution of a basaltic rock submitted to weathering: fate of the major elements, rare earth elements and thorium. Geochim. Cosmochim. Acta 58, 4941–4954. De Baar, H.J.L., Bacon, M.P., Brewer, P.G., 1985. Rare earth in the Pacific and Atlantic oceans. Geochim. Cosmochim. Acta 49, 1943–1959. De Baar, H.J.L., German, C.R., Elderfield, H., van Gaans, P., 1988. Rare earth element distributions in anoxic waters of the Cariaco Trench. Geochim. Cosmochim. Acta 52, 1203–1219.
34
C. Guy et al.r Chemical Geology 158 (1999) 21–35
Desprairies, A., Bonnot-Courtois, C., 1980. Relation entre la composition des smectites d’alteration sous-marine et leur ´ cortege ` de terres rares. Earth Planet. Sci. Lett. 48, 124–130. Destrigneville, C., 1991. Etude de l’alteration dans le massif ´ basaltique de Mururoa ŽPolynesie ´ Franc¸aise.: analyses des phases solides et fluides et modelisation thermodynamique. ´ Unpublished Thesis, Univ. Toulouse, France, 163 pp. Drake, M.J., 1975. The oxidation state of europium as an indicator of oxygen fugacity. Geochim. Cosmochim. Acta 39, 55–64. Dudoignon, P., 1988. Alterations hydrothermales des roches vol´ caniques de l’atoll de Mururoa ŽPolynesie ´ franc¸aise.. Unpublished Thesis, Univ. Poitiers, France, 276 pp. Dudoignon, P., Meunier, A., Beaufort, D., Gachon, A., Buigues, D., 1989. Hydrothermal alteration at Mururoa Atoll ŽFrench Polynesia.. Chem. Geol. 76, 385–401. Frey, F.A., Bryan, W.B., Thompson, G., 1974. Atlantic ocean floor: geochemistry and petrology of basalts from legs 2 and 3 of the Deep Sea Drilling Project. J. Geophys. Res. 79, 5507– 5527. Glasby, G.P., 1973. Mechanisms of enrichment of the rarer elements in marine manganese nodules. Mar. Chem. 1, 105–125. Gobelet, P., 1981. Modelisation des transferts de masse et ´ d’energie en aquifere. ´ ` Unpublished Thesis, Ecole Nat. Sup. Mines de Paris, Univ. Paris VI, 225 pp. Guille, G., Goutiere, G., Sornein, J.F., 1993. Les atolls de Mururoa et Fangataufa ŽPolynesie ´ Franc¸aise.: I. Geologie–Petrol´ ´ ogie–Hydrogeologie. In: Louis-Jean ŽEd.., Edification et ´ evolution des edifices. Gap, France, 168 pp. ´ ´ Guy, C., Schott, J., Destrigneville, C., Chiappini, R., 1992. Low temperature alteration of basalt by seawater, Mururoa, French Polynesia. Geochim. Cosmochim. Acta 56, 4169–4189. Hajash, A., Chandler, G.W., 1984. An experimental investigation of high temperature interactions between seawater and rhyolite, andesite, basalt and peridotite. Contrib. Mineral. Petrol. 78, 240–254. Helgeson, H.C., Kirkham, D.H., 1974a. Theoretical prediction of the thermodynamic behaviour of aqueous electrolytes at high pressures and temperatures: I. Summary of the thermodynamicrelectrostatic properties of the solvent. Am. J. Sci. 274, 1089–1198. Helgeson, H.C., Kirkham, D.H., 1974b. Theoretical prediction of the thermodynamic behaviour of aqueous electrolytes at high pressures and temperatures: II. Debye–Huckel parameters for ¨ activity coefficients and partial molal properties. Am. J. Sci. 274, 1199–1261. Helgeson, H.C., Delany, J.M., Nesbitt, H.W., Bird, D.K., 1978. Summary and critique of the thermodynamics properties of rock-forming minerals. Am. J. Sci. 278, 1–229. Helgeson, H.C., Kirkham, D.H., Flowers, G.C., 1981. Theoretical prediction of the thermodynamic behaviour of aqueous electrolytes at high pressures and temperatures: IV. Calculation of activity coefficients, osmotic coefficients, and apparent molal and standard and relative partial molal properties to 6008C and 5 kb. Am. J. Sci. 281, 1249–1516. Henry, P., Guy, C., Cattin, R., Dudoignon, N.P., Sornein, J.F., Caristan, Y., 1996. A convective model of water flow in Mururoa basalt. Geochim. Cosmochim. Acta 60, 2087–2109.
Jercinovic, M.J., Keil, K., Smith, M.R., Schmitt, R.A., 1990. Alteration of basaltic glasses from Northcentral British Columbia, Canada. Geochim. Cosmochim. Acta 54, 2679– 2696. Juteau, T., Noack, Y., Whitechurch, H., Courtois, C., 1979. Mineralogy and geochemistry of alteration products in holes 417A and 417D basement samples. Proc. Ocean Drill. Program 51, 1273–1297. Klinkhammer, G., Elderfield, H., Hudson, A., 1983. REE elements in seawater near hydrothermal vents. Nature 305, 185– 188. Klinkhammer, G.P., Chin, C.S., Wilson, C., German, C.R., 1995. X Venting from the Mid-Atlantic Ridge at 37817 N: the Lucky Strike hydrothermal site. In: Parson, L.M., Walker, C.L., Dixon, D.R. ŽEds.., Hydrothermal Vents and Processes. Geol. Soc. Spec. Publ., pp. 87–96. Koeppenkastrop, D., De Carlo, E.H., 1993. Uptake of rare earth elements from solution by metal oxides. Environ. Sci. Technol. 27, 1796–1802. Koeppenkastrop, D., De Carlo, E.H., Roth, M., 1991. A method to investigate the interaction of rare earth elements in aqueous solution with metal oxides. J. Radioanal. Nucl. Chem. 152, 337–346. Krauskopf, K.B., 1986. Thorium and rare-earth metals as analogues for actinide elements. Chem. Geol. 55, 323–335. Lee, J.H., Byrne, R.H., 1992. Examination of comparative rare earth element complexation behaviour using linear free-energy relationships. Geochim. Cosmochim. Acta 56, 1127–1138. Lee, J.H., Byrne, R.H., 1993. Complexation of trivalent rare earth elements ŽCe, Eu, Gd, Tb, Yb. by carbonate ions. Geochim. Cosmochim. Acta 57, 295–302. Ludden, J.N., Thompson, G., 1979. An evaluation of the behaviour of the REE elements during the weathering of seafloor basalt. Earth Planet. Sci. Lett. 43, 85–92. Maury, R.C., Caroff, M., Achard, S., Guille, G., Joron, J.L., Racaboy, A., Leterrier, J., 1992. L’atoll de Mururoa ŽPolynesie ´ franc¸aise. IILa serie ´ magmatique. Bull. Soc. Geol. ´ France 163 Ž5., 659–679. Menzies, M., Seyfried, W.E., 1979. Basaltrseawater interaction: trace element and strontium isotopic variations in experimentally altered basalt and peridotite. Earth Planet. Sci. Lett. 44, 463–473. Michard, A., 1989. REE element systematics in hydrothermal fluids. Geochim. Cosmochim. Acta 53, 745–750. Michard, A., Albarede, F., 1986. The REE content of some ` hydrothermal fluids. Chem. Geol. 55, 51–60. Michard, A., Albarede, ` F., Michard, G., Minster, J.F., Charlou, J.L., 1983. REE elements and uranium in high temperature solutions from East Pacific Rise hydrothermal vent field Ž138N.. Nature 303, 795–797. Millero, F.J., 1992. Stability constants for the formation of rare earth inorganic complexes as a function of ionic strength. Geochim. Cosmochim. Acta 56, 3123–3132. Moffett, J.W., 1990. Microbially mediated cerium oxidation in seawater. Nature 345, 421–423. Nesbitt, H.W., Wilson, R.E., 1992. Recent chemical weathering of basalts. Am. J. Sci. 292, 740–777.
C. Guy et al.r Chemical Geology 158 (1999) 21–35 Piepgras, D.J., Jacobsen, S.B., 1992. The behaviour of REE elements in seawater: precise determination of variations in the North Pacific water column. Geochim. Cosmochim. Acta 56, 1851–1862. Prudencio, M.I., Braga, M.A.S., Gouveia, M.A., 1993. REE mobilization, fractionation and precipitation during weathering of basalts. In: Kump, L.R. ŽEd.., Geochemistry of the Earth Surface. Chem. Geol. 107, pp. 251–254 Žspecial issue.. Ridley, W.I., Perfot, M.R., Jonasson, I.R., Smith, M.F., 1994. Hydrothermal alteration in oceanic ridge volcanics: a detailed study at the Galapagos Fossil Hydrothermal Field. Geochim. Cosmochim. Acta 58, 2477–2494. Rougerie, F., Fagerstom, J.A., Andrie, C., 1992. Geothermal endo-upwelling: a solution to the reef nutrient paradox. Cont. Shelf Res. 12, 785–798. Samaden, G., Dallot, P., Roche, R., 1985. Atoll d’Eniwetok. Systeme geothermique insulaire a` l’etat ` ´ ´ naturel. La Houille Blanche 2, 143–151. Schiano, P., Dupre, ´ B., Lewin, E., 1993. Application of element concentration variability to the study of basalt alteration ŽFangataufa atoll, French Polynesia.. Chem. Geol. 104, 99– 124. Sholkovitz, E.R., 1992. Chemical evolution of rare earth elements:
35
fractionation between colloidal and solution phases of filtered river water. Earth Planet. Sci. Lett. 114, 77–84. Sholkovitz, E.R., Landing, W.M., Lewis, B.L., 1994. Ocean particle chemistry: the fractionation of rare earth elements between suspended particles and seawater. Geochim. Cosmochim. Acta 58, 1567–1579. Sverjensky, D.A., 1984. Europium redox equilibria in aqueous solution. Earth. Planet. Sci. Lett. 67, 70–78. Swartz, J.H., 1958. Geothermal Measurements. Eniwetok and Bikini Atolls. Geol. Sur. Prof. 260U. Wolery, T.J., 1983. EQ3rNR, A Computer Program for Geochemical Aqueous Speciation–Solubility Calculations: User’s Guide, Documentation. Lawrence Livermore National Laboratory UCRL-53414, Livermore, CA. Wolery, T.J., 1986. EQ3r6—status and future development. Proc. Workshop Geochem. Modeling, September 14–17, 1986, Fallen Leaf Lake, CA. Lawrence Livermore CONF-8609134, pp. 10–19. Wood, S.A., 1990. The aqueous geochemistry of the rare-earth elements and yttrium: 1. Review of available low-temperature data for inorganic complexes and the inorganic REE speciation of natural waters. Chem. Geol. 82, 159–186.