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Constraints on rare gas partition coefficients from analysis of olivine-glass from a picritic mid-ocean ridge basalt
ChemicalGeology, 106 (1993) 1-7 Elsevier Science Publishers B.V., Amsterdam
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Letter Section Constraints on rare gas partition coefficients from analysis of olivine-glass from a picritic mid-ocean ridge basalt Bernard Marty ~ and Patricia Lussiez Laboratoire MAGIE/URA CNRS 736, Universit~ Pierre et Marie Curie, 4 Place Jussieu, F- 7.5252 Paris Cedex 05. France (Received February 2, 1993; revised and accepted February 10, 1993 )
ABSTRACT Marty, B. and Lussiez, P., 1993. Constraints on rare gas partition coefficients from analysis of olivine-glass from a picritic mid-ocean ridge basalt. Chem. Geol., 106: 1-7. Knowledge of the rare gas distribution between crystal and silicate melt is a key requisite when modelling the evolution of the Earth-atmosphere system. The concentrations and isotope compositions of He and Ar in glass and coexisting olivine in chemical equilibrium of a picritic MORB basalt from the Mid-Atlantic Ridge at 38°N have been measured. In an attempt to resolve components trapped in fluid inclusions from components dissolved in the glass or the olivine phases, gases were extracted from both phases by vacuum crushing and fusion. Since ( 1 ) olivine also contains glass inclusions and (2) the glass may be partially degassed, only upper limits of the partition coefficients between olivine and melt can he proposed. These upper limits (0.008 for He and 0.003 for Ar) indicate that rare gases behave as incompatible elements during magmatic processes. This validates assumptions about the efficiency of rare gas degassing from the solid Earth.
1. Introduction It is generally assumed that rare gases behave as incompatible elements during partial melting because they are chemically inert and their atomic radii are large (Ozima and Podosek, 1983; Sarda et al., 1985; All6gre et al., 1986/87; Azbel and Tolstikhin, 1988; Zhang andZindler, 1989), from 1.22 /~ for He to 2.18 /~ for Xe (Lux, 1986 ). Experimental measurements of crystal-basalt melt distribution have been attempted by Hiyagon and Ozima ( 1986 ) and Broadhurst et al. (1992). Both groups sug~Present address: Centre de Recherches P6trographiques et G6ochimiques, Rue Notre-Dame des Pauvres, B.P. 20, F-54501 Vand~euvre Cedex, France.
0009-2541/93/$06.00
gested from their experimental data that rare gases are only moderately incompatible, with partition coefficients increasing from He to Xe, in contrast with the increase of the atomic radius from He to Xe. Batiza et al. (1979) reported rare gas contents in primitive and evolved rocks reputedly from the same magma. From estimates of the degree of crystal fractionation, they derived partition coefficients between l 0 - t and l 0. From these unlikely high values, they concluded that the system was not closed and that the calculated coefficients were not real. Oceanic basalt glasses chilled under low temperature and hydrostatic pressure at the bottom of oceans are known to contain mantie-derived volatiles. Moreover, the rapid cooling of magma prohibits secondary diffusion el'-
© 1993 Elsevier Science Publishers B.V. All rights reserved.
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B. MARTY AND P. LUSSIEZ
fects from or to crystal phases. Kurz et al. (1983) estimated the partition coefficient of He between plagioclase and mid-ocean ridge basalt (MORB) glass and suggested an upper limit of 0.01. In this work we have measured rare gases in glass and coexisting olivine in a picritic basalt from the Mid-Atlantic Ridge at 38°N (FAMOUSzone). The comparison between the He and Ar isotope contents of the olivine and the glass phases allows to put constraints on the partition coefficients of these rare gases, 2. Sample and analytical procedure The sample used in this study labelled
CH31DRI2 was dredged from the inner floor of the Atlantic Rift Valley at 38 °N by the R.V. "Jean Charcot" in 1972 (Bougault and H6kinian, 1974). Some of the rocks recovered during this dredge were picritic basalts, characterized by abundant large olivine crystals ( > 25%, some of them reaching 1 cm) set in a groundmass of a dark mesostasis, tiny plagioclase laths, olivine and glass. The dredge portion that we have studied (CH31DR12-734) contains abundant fresh glass coexisting with 3-10-mm olivine megacrysts, allowing a clear phase separation for rare gas analysis. The olivine phenocrysts contain a few 15-25-/~m fluidand glass inclusions and some of the olivine phenocrysts show resorbed interfaces. The glassy matrix contains thin lattices of skeletal plagioclases and olivine microcrystals. The sample composition (Table 1 ) shows that the rock is a primitive ( M g # = 70) picritic tholeiite with low K20 content characteristic of N-MORB. Analyses of several glass inclusions within olivine phenocrysts yielded compositions similar to that of the glass, indicating that the same liquid was trapped during olivine growth, Analyses of various olivine ( F o = 9 0 % ) phenocrysts (Table 1) and different locations within single olivine phenocrysts do not show a significant compositional variation, even between olivine phenocrysts and olivine micro-
crysts. The equilibrium of olivine with the basaltic liquid was tested using the Fe and Mg contents of the two phases. Roeder and Emslie's (1970) partition coefficient, KD, is 0.23 for the present sample (runs G 1 and O 1, Table 1 ), comparable to the experimentally determined value of 0.30 for equilibrium. Using the Ca-Mg exchange equilibration geothermometer of Watson (1979), a plausible olivine-melt equilibrium temperature of 1260 °C is derived. In the Cameh VS. Caolivine diagram from the same author, the plot corresponding to the present sample falls close to the experimental equilibrium line. All these considerations indicate that olivine is likely to be in chemical equilibrium withthe melt. Most of the analytical procedures will be described elsewhere (B. Malty and M. Lenoble, in prep.). Glassy chips ( ~ 5 m m long) were separated from the groundmass and washed in 0.1 N HNO3, distilled water and high-grade acetone. An aliquot of the sample was gently crushed and millimeter-.sized olivine grains were selected for their absence of alteration and attached glassy matrix under a binocular microscope. Both olivine and MORB glass are known to contain fluid inclusions and, due to the low solubility of rare gases in silicate melts (Jambon et al., 1986; Lux, 1986), rare gases tend to concentrate in the gas phase as soon as bubbles start to grow. In the case of olivine inclusions, they were probably trapped during olivine growth in a magma where micro-bubbles were already exsolved. In order to take into account the occurrence of such traps, both olivine and glass were analysed by fusion and by crushing under vacuum. Vacuum crushing was performed on-line in a stainless-steel tube. The olivine sample was baked overnight at 200°C while the glass fraction was baked at only 100°C in order to prevent He diffusion. After purification, gaseswere separated on a He cold trap and analysed with a new sector-type mass spectrometer (VG 5400, Fizons Instruments ®). Analytical blanks were 2.4.10-15
ANALYSISOF OLIVINE-GLASSFROM A PICRITIC MID-OCEAN R1DGE BASALT
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TABLE 1
Composition of glass and coexisting olivine of sample CH31DRI2-734 analysed by electron microprobe at the analytical centre Camparis (Universil6 Pierre et Marie Curie, Paris) Glass
Olivine
G1
G2
G3
O1
SiO2 AI203 FeOT M nO MgO CaO Na20 K20 TiO2 Cr203 NiO
49.6 16. l 7.9 0.17 9.2 13.5 1.83 0.04 0.7 0.09 -
51.4 17.5 6.5 0.24 5.5 14.9 2.00 0.03 0.91 0.03 -
50.0 15.3 8.1 0.17 8.2 14.2 1.72 0.08 0.66 0.06
41.0 0.03 9.5 0.24 48.1 0.35
Tolal
99.0
99.0
98.5
99.5
I00.1
100.2
100.4
99.7
Mga: Fo (%) Fa (%)
70.5 -
63.5 -
67.6
91.3 90.1 9.9
92.1 91.0 9.0
91.2 90.0 10.0
90.8 89.6 10.4
90.9 89.7 10.3
0.09 0.21
02 4 i .0 0.03 8.8 0.22 49.6 0.25 0.01 0.07 0.23
03 40.8 0.06 9.7 0.25 48.8 0.25 0.01 0.01 0.05 0.06 0.22
04 4 1.0 0.10 10.1 0.13 48.5 0.30 0.03 0.08 0.23
05 40.7 0.07 9.9 0.11 48.1 0.35 0.09 0.36
The M g a [ = M g / ( Mg + Fe ) ] is computed assuming that FeOT = 1.15 × FeO. G 1 = glassy matrix; G2 = glass inclusions in a massive olivine phenocryst; G3 =glass inclusion in a partially resorbed olivine phenocryst; O1 = m a s s i v e olivine, centre; O 2 = massive olivine, centre; O 3 = t h e same as 0 2 , border; O4= partially resorbed olivine, border: O5 = skeletal olivine in the matrix.
mol 4He and 6.7- I0-14 mol 4°Ar for the crushing experiments with atmospheric ratios for Ar (He isotope ratios in blanks could not be precisely determined). For the fusion experiments, the blanks were 1.7.10-13 mol 4He and 1.0.10-13 mol 4°Ar. The relatively high He blank is due to the use of glass for the housing of the induction heater, 3. Results Results of rare gas analyses are given in Table 2. The 3He/4He ratios measured in the glass (8.3Ra) and in the olivine (8.4R,) are indistinguishable within the uncertainties, and are in the range of N-MORB. The 4°Ar/36Ar ratios up to 2589 are also within the range of MORB values, albeit at the lower end (e.g., Sarda et al., 1985 ). These rather low values may relate to the occurrence of microcrysts in the glassy matrix. Indeed, the holocrystalline portion of deep-sea basalts have been shown to be richer
in atmospheric gases than pure glasses (e.g., Sarda et al., 1985 ), probably as a result of enhanced water-rock interaction during slower cooling. In detail, the 4°Ar/36Ar isotope ratios vary from 847 to 2589 in the glass and from 406 to 614 in the olivine. In the case of olivine runs, lower Ar isotope ratios may be related to the problem of correcting for blank when the amount of Ar is very low. These variations represent variable additions of air-like Ar during magma extraction, storage, eruption and, eventually, sample handling. This assumption is tested in Fig. 1 which is a ( 1/36Ar) vs. (4°Ar/ 36Ar) diagram. In such a format, atmospheric contamination of a sample having a 4°Ar/36Ar ratio higher than air and a low 36Ar content leads to a (approximate) straight line whose origin is (0, 295.5; the 4°Ar/36Ar ratio of air), and whose slope represents the amount of 4°Ar in excess of the atmospheric composition (4°Ar*). This diagram allows one to check if: ( 1 ) the atmospheric contamination hypothe-
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B. MARTYAND P. LUSSIEZ
Ta~BLE 2 Rare gas data of olivine and glass phases from sample CH31DRI2-734 Mass (g)
4He (10 ~2mol g 1)
3He/4He (×Ra)
4°Ar ( 10-~2 mol
4°Ar/36Ar
0.1151
2.06-+0.34 [30]
160i) (
Total
4°Ar* ( 10-L2mol g 1)
-
1.08-+0.23 [34]
468-+85
55.0+3.4 [3.11
-
29.1_+2,5 [1.3]
3,112_+17
23.0_+2.4
-
57.1 +_3.5
-
30.3_+2.6
2,589_+ 19
23.4+_2.2
2.44+0.27
g 1)
4He/4°Ar*
(liars: Fusion 600C
0.4o+o.lo
-
Crushing-/
0.2019 70.4+_4.2 [0.016]
8.4+0.1
38.9_+2.5 [1.8]
982+_2
27.5+2.5
2.68+0.28
Crushing-2
0.5810 109.0_+8.7 [0.012]
8.2+-0.1
67.3_+4.0 [0.071
847_+3
43.8+2.7
2.49+0.25
Fusion 1600 C
0.0828
1.26_+0.51 [62]
-
0.48_+0.27 [71]
6t4_+133
Crushing-/
0.2242
0.47_+0.04 [2.2]
8.5+0.3
.
Crushing-2
0.2287
0.70_+0.12 [4.81
8.3_+0.3
1.25+_0.12 [11.11
Ohvim':
.
.
406_+32
0.25:)-0.16
5.1_+3.7
0.34-+0.07
2.06-+0.55
.
Ra is the atmospheric 3He/nile ratio of 1.39.10 -6. Values between brackets are the respective percentages of the blanks over the total signal during analyses. For computations of uncertainties on abundances, see the text. For R / R . the uncertainties (2c0 include standard and sample analytical errors.
sis is valid; and (2) if the sample is homogeneous with respect to its 4°Ar* content (Takaoka and Nagao, 1980; Fisher, 1981; Marty and Ozima, 1986 ). Its appears from Fig. 1 that both hypotheses are valid. There are in fact two correlations, one corresponding to the glass analyses and the other to the olivine analyses. The equation of the glass correlation gives an intercept at the origin of 454.2 and a slope of 25.4.10-12 mol g-1 4OAr,"When forced to pass through the air value of 295.5, the slope of the correlation becomes 27.2- 10-12 mol g - 1. Since this correlation is based on fusion and crush-
ing extractions and because the respective data cannot be resolved (correlation coefficient of 0.991 for all data), this correlation also shows that vesicles contain most of radiogenic argon and are representative of the bulk Ar content of the glass. This is exactly what is expected from rare gas solubility coefficients in basaltic melt and typical vesicularities of MORB (e.g., Marty and Ozima, 1986). Hence the rare gas content variations recorded from duplicate analyses of the glass (Table 2 ) probably reflect the heterogeneous distribution of vesicles in the sample.
.ANALYSIS OF OLIVINE-GLASS FROM A PICRITIC MID-OCEAN RIDGE BASALT
.•
3000
G,
<
200° ~, A,, ~ - - - - - - - - - ~ o ~ 0 1 2 1/36Ar' 1015g/mot
....
Fig. 1. 1/36Ar vs. 4°Ar/36Ar diagram (Gl=glass; O/=olivine). Solid dots correspond to fusion extractions and open circles are the crushing data. The two correlation lines have been forced to pass trough the air dot (0,295.5). The slopes give the amounts of radiogenic Ar (4°Ar*) in excess ofthe air composition.
This observation raises an important issue for this study. Is the rare gas content of the glass representative of the melt or is it thoroughly degassed of its magmatic volatiles, including rare gases? Although typical of N-MORB the He content of the sample ( 1.10- l O mol g- l ) is low since He contents of 10-9 mol g-1 a r e often recorded in MORB samples (e.g., Kurz et al., 1983 ). This peculiarity may be the result of previous degassing events but could be also related to the primitive nature of the picritic magma. In this hypothesis the higher concentrations recorded in some of the MORB may be due to subsequent fractional crystallisation, Clearly an independent test is needed. The 4He/a°Ar* ratio is a good index of degassing because both isotopes are non-atmospheric and because the solubilities of He and Ar differ by one order of magnitude (e.g., Jambon et al., 1986 ). Since He is more soluble, any degassing of a magma should result in enrichment of He vs. Ar, the degree of which depends on the extent of degassing and on the nature of the degassing process (e.g., bulk or Rayleigh; e.g., Bottinga and Javoy, 1990). The production/ accumulation range, computed from the abundances of the parent nuclides [U, Th and K, with K / U = 12,700 (Jochum et al., 1983) and T h / U = 3 . 3 ] is 1.8-4.1, depending on the assumption concerning mantle degassing and evolution (Ozima and Podosek, 1983; Allrgre
5
et al., 1986/87). MORB show 4He/4°Ar* ratios varying between this radiogenic end-member and much higher values up t o 102, as a probable effect of fractional degassing. The 4He/4°Ar* ratios recorded during the various runs of this study are surprisingly constant and vary little around 2.5 (Fig. 2 ). These values are within the field of production/accumulation, which implies that the sample does not show evidenceofmagmadegassing. We nevertheless consider here that the rare gas content of the glass represents a lower limit of the original rare gas c o n t e n t of the magma. The similarity of the 4 H e / 4 ° A r * ratios between olivine and glass
yields interesting information about the nature of the phases which were analysed. For the glass, the value of the 4He/4°Ar* ratio between crushing (vesicle fraction) and fusion (bulk fraction) is further evidence for the preferential partitioning of rare gases into the vesicles. Indeed, the vesicle ratio is representative of the bulk ratio and also of the magmatic ratio if we assume limited degassing of the melt. The fraction extracted by olivine crushing has a 4He/ 4°Ar* ratio of 2.06_+ 0.55, marginally lower than the values recorded in the glass, but still compatible if one takes into account the respective uncertainties. Since crushing is likely to open olivine microvesicles, such a low value may represent the first degassed fraction which,
~ ,o ~ ~ 8 8 , ~i~~ ....... ~, 2
o,
.,...._
o.
.,2
-,,
-,o
.,
log 4He (mot/g) Fig. 2.4He/4°Ar*vs. 4He content. Same symbols as in Fig. 1. The shaded area is the radiogenic production/accu-
mulation domain.
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B. MARTY AND P. LUSSIEZ
following the solubility law, is enriched in the least soluble gas, that is, Ar. In the case of the fusion extraction, the rare gas component is a mixture of the microvesicle fraction, the melt inclusion fraction and the olivine fraction. The low Ar content recovered during this run prevents precise computation of the 4He/a°Ar* ratio, which appears higher, but still compatible with the glass ratio. Given the complexity of the phase components recovered during this run, we cannot decipher if this marginally higher ratio is or not an effect of phase partition (e.g., solubility in olivine), 4. Discussion Partition coefficients between olivine and melt are estimated from the ratios between the rare gas content in olivine and that in the glass. For He, atmospheric contamination is negligible and the bulk coefficient is: D He __ C oHe l , tot ~ +0.007 bulk--p--~e =0"01u-0"007 "~ GIAot where subscripts Ol,tot and Gl,tot refer to the total content of He in olivine and in glass, respectively, extracted by fusion (for the glass content, a mean between all runs has been taken). From the crushing experiments of olivine, it is clear that part of He in olivine does not reside within the matrix but most likely in fluid inclusions. Hence the fraction of He recovered following crushing needs to be removed from the total He content in olivine: He He COI'Mx 0 0 0 8 +0.007 DMx-- He = • -0006 CGI.tot
where Mx is assumed to represent the olivine matrix. Olivine also contains glass inclusions and therefore the real He content in olivine must be much lower than the content cornputed above. As expounded in the preceding section, the He content of the glass represents a lower limit of the He content in the magma, For these two reasons, we conclude that:
D ~ e ~<0.008 For Ar, the partition coefficient is computed for 4°Ar*, that is, the slopes of the correlations in Fig. 1: Ar Dbo~kAr--~=Col't°t 0.009 +°°°5-o.oo4 '~GI,tot The Ar fraction of olivine needs also to be corrected for the fluid inclusion fraction. In the case of Ar however, such a correction is not possible because both crushing and fusion runs lead to Ar contents undistinguishable each other, given the low Ar contents and the resulting high uncertainties. Taking into account these uncertainties, an upper limit for the partition coefficient is given by: Ar ~<0.003 OMx These estimations show that the rare gas fraction in the olivine matrix is likely to be very low and can hardly be distinguished from inherited fractions such as fluid or glass inclusions trapped in the crystal. Even taking the bulk rare gas contents that we know to be overestimated, we get partition coefficients lower than those claimed from experimental works. Hiyagon and Ozima (1986) synthesized olivine in rare-gas saturated silicate melt under various pressure conditions and f o u n d t h a t the He partition coefficient should be lower than 0.07, but that of Ar was between 0.05 and 0.15, in contrast with the present study. Broadhurst et al. (1992) attempted to equilibrate natural crystal samples with rare gases at 1 bar and 1300°C. They derived Ar partition coefficients between 0.15 and 0.84. Both groups claimed an increase of the partition coefficients with atomic mass number, Kr and Xe being slightly incompatible or not at all incompatible. These unexpected results were explained by differential polarizability. It must be noted that such a trend is qualitatively consistent with adsorption of atmospheric rare gases, particularly when crystal grain sizes are small ( ~< 10/tm in Broadhurst et al., 1992 ).
ANALYSISOF OLIVINE-GLASSFROM A PICRITIC MID-OCEAN RIDGE BASALT
Keeping in mind the limitations inherent t o the procedure adopted in this study, the He and Ar partition coefficients appear comparable t o those of large-ion lithophile elements (LILE) such as K, U and Th (e.g., McKenzie and O'Nions, 1991 ). Hence a similar behaviour during magmatic processes is expected, complicated in the case of rare gases by degassing fractionation. If rare gases are strongly incompatible as the present results suggest, then rare gas evolution models based on partial melting partition (e.g., Azbel and Tolstikhin, 1988) will need to be reconsidered, Acknowledgements We are grateful to N. Vassard and M. Lenoble for help during rare gas analysis and to two anonymous reviewers for suggestions. The English was improved by Mike Topliss. This paper is dedicated to the late I.Ya. Azbel. References Allegre, C.J., Staudacher, T. and Sarda, P., 1986/87. Rare gas systematics: formation of the atmosphere, evolution and structure of the Earth's mantle. Earth Planet. Sci. Lett., 81: 127-150. Azbel, l.Ya. and Tolstikhin, I.N., 1988. Geodynamics, magmatism and degassing of the Earth. Geochim. Cosmochim. Acta, 54:139-154. Batiza, R., Bernatowicz, T.J., Honhenberg, C.M. and Podosek, F.A., 1979. Relation of noble gas abundances to petrogenesis and magmatic evolution of some oceanic basalts and related differentiated rocks. Contrib. Mineral. Petrol., 69: 301-314. Bottinga, Y. and Javoy, M., 1990. MORB degassing: Bubble growth and ascent. Chem. Geol., 81: 255-270. Bougault, H. and H6kinian, R., 1974. Rift valley in the Atlantic Ocean near 36 ° 50'N: Petrology and chemistry of basaltic rocks. Earth Planet. Sci. Lett., 4: 249261. Broadhurst, C.L., Drake, M.J., Hagee, B.E. and Bernatowicz, T.J., 1992. Solubilities and partitioning of no-
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ble gases in mineral and synthetic basaltic melts. Geo-
chim. Cosmochim.Acta, 56: 709-724.
Fisher, D.E., 1981. Quantitative retention of magmatic argon in a glassy basalt. Nature (London), 290: 4243. Hiyagon, H. and Ozima, M., 1986. Partition of rare gases
between olivine and basalt melt. Geochim. Cosmochim. Acta, 50: 2045-2057. Jambon, A., Weber, H. and Braun, 0.. 1986. Solubility of He, Ne, Ar, Kr and Xe in a basalt melt in the range 1250-1600 cc. Geochim. Cosmochim. Acta, 50:40 l -
408. Jochum, K.P., Hofmann, A.W., lto, E., Seufert, H.M. and White, W.H., 1983. K, U and Th in Mid-Ocean Ridge Basalt glasses and heat production, K / U and K / R b in the mantle. Nature (London), 306: 431-436. Kurz, M.D., Jenkins, W.J., Schilling, J,G. and Hart, S.R., 1983. Helium isotopic variations in the mantle beneath the Central North Atlantic Ocean. Earth Planet. Sci. Lett., 58: 1-14. Lux, G., 1986. The behaviour of noble gases in silicate liquids: Solution, diffusion, bubbles and surface effects, with applications to natural samples. Geochim. Cosmochim. Acta, 51:1549-1560. Marty, B. and Ozima, M., 1986. Noble gas distribution in oceanic basalt glasses. Geochim. Cosmochim. Acta, 50: 1093-1097. McKenzie, D. and O'Nions, R.K., 1991. Partial melt coefficients from inversion of rare earth element concentrations. J. Petrol., 23:1021-109 I. Ozima, M. and Podosek, F.A., 1983. Noble Gas Geochemistry. Cambridge University Press, Cambridge, 367 pp. Roeder, P.L. and Emslie, R.F., 1970. Olivine-liquid equilibrium. Contrib. Mineral. Petrol., 29: 275-289. Sarda, P., Staudacher, T. and Allbgre, C.J., 1985. 4 ° A t / 36Ar in MORB glasses: constraints on the atmosphere and mantle evolution. Earth Planet. Sci. Lett., 72: 357375. Takaoka, N. and Nagao, K., 1980. Rare-gas studies of Cretaceous deep-sea basalts. Init. Rep. Deep Sea Drill. Proj., Legs 1-3, 35:1121-1126. Watson, E.B., 1979. Calcium content of forsterite coexisting with silicate liquid in the system Na20-CaO-MgOA1203-SIO 2. Am. Mineral., 64: 824-829. Zhang, Y. and Zindler, A., 1989. Noble gas constrainst on the evolution of the Earth's atmosphere. J. Geophys. Res., 94B 10:13,7129-13,7137.
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Letter Section Constraints on rare gas partition coefficients from analysis of olivine-glass from a picritic mid-ocean ridge basalt Bernard Marty ~ and Patricia Lussiez Laboratoire MAGIE/URA CNRS 736, Universit~ Pierre et Marie Curie, 4 Place Jussieu, F- 7.5252 Paris Cedex 05. France (Received February 2, 1993; revised and accepted February 10, 1993 )
ABSTRACT Marty, B. and Lussiez, P., 1993. Constraints on rare gas partition coefficients from analysis of olivine-glass from a picritic mid-ocean ridge basalt. Chem. Geol., 106: 1-7. Knowledge of the rare gas distribution between crystal and silicate melt is a key requisite when modelling the evolution of the Earth-atmosphere system. The concentrations and isotope compositions of He and Ar in glass and coexisting olivine in chemical equilibrium of a picritic MORB basalt from the Mid-Atlantic Ridge at 38°N have been measured. In an attempt to resolve components trapped in fluid inclusions from components dissolved in the glass or the olivine phases, gases were extracted from both phases by vacuum crushing and fusion. Since ( 1 ) olivine also contains glass inclusions and (2) the glass may be partially degassed, only upper limits of the partition coefficients between olivine and melt can he proposed. These upper limits (0.008 for He and 0.003 for Ar) indicate that rare gases behave as incompatible elements during magmatic processes. This validates assumptions about the efficiency of rare gas degassing from the solid Earth.
1. Introduction It is generally assumed that rare gases behave as incompatible elements during partial melting because they are chemically inert and their atomic radii are large (Ozima and Podosek, 1983; Sarda et al., 1985; All6gre et al., 1986/87; Azbel and Tolstikhin, 1988; Zhang andZindler, 1989), from 1.22 /~ for He to 2.18 /~ for Xe (Lux, 1986 ). Experimental measurements of crystal-basalt melt distribution have been attempted by Hiyagon and Ozima ( 1986 ) and Broadhurst et al. (1992). Both groups sug~Present address: Centre de Recherches P6trographiques et G6ochimiques, Rue Notre-Dame des Pauvres, B.P. 20, F-54501 Vand~euvre Cedex, France.
0009-2541/93/$06.00
gested from their experimental data that rare gases are only moderately incompatible, with partition coefficients increasing from He to Xe, in contrast with the increase of the atomic radius from He to Xe. Batiza et al. (1979) reported rare gas contents in primitive and evolved rocks reputedly from the same magma. From estimates of the degree of crystal fractionation, they derived partition coefficients between l 0 - t and l 0. From these unlikely high values, they concluded that the system was not closed and that the calculated coefficients were not real. Oceanic basalt glasses chilled under low temperature and hydrostatic pressure at the bottom of oceans are known to contain mantie-derived volatiles. Moreover, the rapid cooling of magma prohibits secondary diffusion el'-
© 1993 Elsevier Science Publishers B.V. All rights reserved.
2
B. MARTY AND P. LUSSIEZ
fects from or to crystal phases. Kurz et al. (1983) estimated the partition coefficient of He between plagioclase and mid-ocean ridge basalt (MORB) glass and suggested an upper limit of 0.01. In this work we have measured rare gases in glass and coexisting olivine in a picritic basalt from the Mid-Atlantic Ridge at 38°N (FAMOUSzone). The comparison between the He and Ar isotope contents of the olivine and the glass phases allows to put constraints on the partition coefficients of these rare gases, 2. Sample and analytical procedure The sample used in this study labelled
CH31DRI2 was dredged from the inner floor of the Atlantic Rift Valley at 38 °N by the R.V. "Jean Charcot" in 1972 (Bougault and H6kinian, 1974). Some of the rocks recovered during this dredge were picritic basalts, characterized by abundant large olivine crystals ( > 25%, some of them reaching 1 cm) set in a groundmass of a dark mesostasis, tiny plagioclase laths, olivine and glass. The dredge portion that we have studied (CH31DR12-734) contains abundant fresh glass coexisting with 3-10-mm olivine megacrysts, allowing a clear phase separation for rare gas analysis. The olivine phenocrysts contain a few 15-25-/~m fluidand glass inclusions and some of the olivine phenocrysts show resorbed interfaces. The glassy matrix contains thin lattices of skeletal plagioclases and olivine microcrystals. The sample composition (Table 1 ) shows that the rock is a primitive ( M g # = 70) picritic tholeiite with low K20 content characteristic of N-MORB. Analyses of several glass inclusions within olivine phenocrysts yielded compositions similar to that of the glass, indicating that the same liquid was trapped during olivine growth, Analyses of various olivine ( F o = 9 0 % ) phenocrysts (Table 1) and different locations within single olivine phenocrysts do not show a significant compositional variation, even between olivine phenocrysts and olivine micro-
crysts. The equilibrium of olivine with the basaltic liquid was tested using the Fe and Mg contents of the two phases. Roeder and Emslie's (1970) partition coefficient, KD, is 0.23 for the present sample (runs G 1 and O 1, Table 1 ), comparable to the experimentally determined value of 0.30 for equilibrium. Using the Ca-Mg exchange equilibration geothermometer of Watson (1979), a plausible olivine-melt equilibrium temperature of 1260 °C is derived. In the Cameh VS. Caolivine diagram from the same author, the plot corresponding to the present sample falls close to the experimental equilibrium line. All these considerations indicate that olivine is likely to be in chemical equilibrium withthe melt. Most of the analytical procedures will be described elsewhere (B. Malty and M. Lenoble, in prep.). Glassy chips ( ~ 5 m m long) were separated from the groundmass and washed in 0.1 N HNO3, distilled water and high-grade acetone. An aliquot of the sample was gently crushed and millimeter-.sized olivine grains were selected for their absence of alteration and attached glassy matrix under a binocular microscope. Both olivine and MORB glass are known to contain fluid inclusions and, due to the low solubility of rare gases in silicate melts (Jambon et al., 1986; Lux, 1986), rare gases tend to concentrate in the gas phase as soon as bubbles start to grow. In the case of olivine inclusions, they were probably trapped during olivine growth in a magma where micro-bubbles were already exsolved. In order to take into account the occurrence of such traps, both olivine and glass were analysed by fusion and by crushing under vacuum. Vacuum crushing was performed on-line in a stainless-steel tube. The olivine sample was baked overnight at 200°C while the glass fraction was baked at only 100°C in order to prevent He diffusion. After purification, gaseswere separated on a He cold trap and analysed with a new sector-type mass spectrometer (VG 5400, Fizons Instruments ®). Analytical blanks were 2.4.10-15
ANALYSISOF OLIVINE-GLASSFROM A PICRITIC MID-OCEAN R1DGE BASALT
3
TABLE 1
Composition of glass and coexisting olivine of sample CH31DRI2-734 analysed by electron microprobe at the analytical centre Camparis (Universil6 Pierre et Marie Curie, Paris) Glass
Olivine
G1
G2
G3
O1
SiO2 AI203 FeOT M nO MgO CaO Na20 K20 TiO2 Cr203 NiO
49.6 16. l 7.9 0.17 9.2 13.5 1.83 0.04 0.7 0.09 -
51.4 17.5 6.5 0.24 5.5 14.9 2.00 0.03 0.91 0.03 -
50.0 15.3 8.1 0.17 8.2 14.2 1.72 0.08 0.66 0.06
41.0 0.03 9.5 0.24 48.1 0.35
Tolal
99.0
99.0
98.5
99.5
I00.1
100.2
100.4
99.7
Mga: Fo (%) Fa (%)
70.5 -
63.5 -
67.6
91.3 90.1 9.9
92.1 91.0 9.0
91.2 90.0 10.0
90.8 89.6 10.4
90.9 89.7 10.3
0.09 0.21
02 4 i .0 0.03 8.8 0.22 49.6 0.25 0.01 0.07 0.23
03 40.8 0.06 9.7 0.25 48.8 0.25 0.01 0.01 0.05 0.06 0.22
04 4 1.0 0.10 10.1 0.13 48.5 0.30 0.03 0.08 0.23
05 40.7 0.07 9.9 0.11 48.1 0.35 0.09 0.36
The M g a [ = M g / ( Mg + Fe ) ] is computed assuming that FeOT = 1.15 × FeO. G 1 = glassy matrix; G2 = glass inclusions in a massive olivine phenocryst; G3 =glass inclusion in a partially resorbed olivine phenocryst; O1 = m a s s i v e olivine, centre; O 2 = massive olivine, centre; O 3 = t h e same as 0 2 , border; O4= partially resorbed olivine, border: O5 = skeletal olivine in the matrix.
mol 4He and 6.7- I0-14 mol 4°Ar for the crushing experiments with atmospheric ratios for Ar (He isotope ratios in blanks could not be precisely determined). For the fusion experiments, the blanks were 1.7.10-13 mol 4He and 1.0.10-13 mol 4°Ar. The relatively high He blank is due to the use of glass for the housing of the induction heater, 3. Results Results of rare gas analyses are given in Table 2. The 3He/4He ratios measured in the glass (8.3Ra) and in the olivine (8.4R,) are indistinguishable within the uncertainties, and are in the range of N-MORB. The 4°Ar/36Ar ratios up to 2589 are also within the range of MORB values, albeit at the lower end (e.g., Sarda et al., 1985 ). These rather low values may relate to the occurrence of microcrysts in the glassy matrix. Indeed, the holocrystalline portion of deep-sea basalts have been shown to be richer
in atmospheric gases than pure glasses (e.g., Sarda et al., 1985 ), probably as a result of enhanced water-rock interaction during slower cooling. In detail, the 4°Ar/36Ar isotope ratios vary from 847 to 2589 in the glass and from 406 to 614 in the olivine. In the case of olivine runs, lower Ar isotope ratios may be related to the problem of correcting for blank when the amount of Ar is very low. These variations represent variable additions of air-like Ar during magma extraction, storage, eruption and, eventually, sample handling. This assumption is tested in Fig. 1 which is a ( 1/36Ar) vs. (4°Ar/ 36Ar) diagram. In such a format, atmospheric contamination of a sample having a 4°Ar/36Ar ratio higher than air and a low 36Ar content leads to a (approximate) straight line whose origin is (0, 295.5; the 4°Ar/36Ar ratio of air), and whose slope represents the amount of 4°Ar in excess of the atmospheric composition (4°Ar*). This diagram allows one to check if: ( 1 ) the atmospheric contamination hypothe-
4
B. MARTYAND P. LUSSIEZ
Ta~BLE 2 Rare gas data of olivine and glass phases from sample CH31DRI2-734 Mass (g)
4He (10 ~2mol g 1)
3He/4He (×Ra)
4°Ar ( 10-~2 mol
4°Ar/36Ar
0.1151
2.06-+0.34 [30]
160i) (
Total
4°Ar* ( 10-L2mol g 1)
-
1.08-+0.23 [34]
468-+85
55.0+3.4 [3.11
-
29.1_+2,5 [1.3]
3,112_+17
23.0_+2.4
-
57.1 +_3.5
-
30.3_+2.6
2,589_+ 19
23.4+_2.2
2.44+0.27
g 1)
4He/4°Ar*
(liars: Fusion 600C
0.4o+o.lo
-
Crushing-/
0.2019 70.4+_4.2 [0.016]
8.4+0.1
38.9_+2.5 [1.8]
982+_2
27.5+2.5
2.68+0.28
Crushing-2
0.5810 109.0_+8.7 [0.012]
8.2+-0.1
67.3_+4.0 [0.071
847_+3
43.8+2.7
2.49+0.25
Fusion 1600 C
0.0828
1.26_+0.51 [62]
-
0.48_+0.27 [71]
6t4_+133
Crushing-/
0.2242
0.47_+0.04 [2.2]
8.5+0.3
.
Crushing-2
0.2287
0.70_+0.12 [4.81
8.3_+0.3
1.25+_0.12 [11.11
Ohvim':
.
.
406_+32
0.25:)-0.16
5.1_+3.7
0.34-+0.07
2.06-+0.55
.
Ra is the atmospheric 3He/nile ratio of 1.39.10 -6. Values between brackets are the respective percentages of the blanks over the total signal during analyses. For computations of uncertainties on abundances, see the text. For R / R . the uncertainties (2c0 include standard and sample analytical errors.
sis is valid; and (2) if the sample is homogeneous with respect to its 4°Ar* content (Takaoka and Nagao, 1980; Fisher, 1981; Marty and Ozima, 1986 ). Its appears from Fig. 1 that both hypotheses are valid. There are in fact two correlations, one corresponding to the glass analyses and the other to the olivine analyses. The equation of the glass correlation gives an intercept at the origin of 454.2 and a slope of 25.4.10-12 mol g-1 4OAr,"When forced to pass through the air value of 295.5, the slope of the correlation becomes 27.2- 10-12 mol g - 1. Since this correlation is based on fusion and crush-
ing extractions and because the respective data cannot be resolved (correlation coefficient of 0.991 for all data), this correlation also shows that vesicles contain most of radiogenic argon and are representative of the bulk Ar content of the glass. This is exactly what is expected from rare gas solubility coefficients in basaltic melt and typical vesicularities of MORB (e.g., Marty and Ozima, 1986). Hence the rare gas content variations recorded from duplicate analyses of the glass (Table 2 ) probably reflect the heterogeneous distribution of vesicles in the sample.
.ANALYSIS OF OLIVINE-GLASS FROM A PICRITIC MID-OCEAN RIDGE BASALT
.•
3000
G,
<
200° ~, A,, ~ - - - - - - - - - ~ o ~ 0 1 2 1/36Ar' 1015g/mot
....
Fig. 1. 1/36Ar vs. 4°Ar/36Ar diagram (Gl=glass; O/=olivine). Solid dots correspond to fusion extractions and open circles are the crushing data. The two correlation lines have been forced to pass trough the air dot (0,295.5). The slopes give the amounts of radiogenic Ar (4°Ar*) in excess ofthe air composition.
This observation raises an important issue for this study. Is the rare gas content of the glass representative of the melt or is it thoroughly degassed of its magmatic volatiles, including rare gases? Although typical of N-MORB the He content of the sample ( 1.10- l O mol g- l ) is low since He contents of 10-9 mol g-1 a r e often recorded in MORB samples (e.g., Kurz et al., 1983 ). This peculiarity may be the result of previous degassing events but could be also related to the primitive nature of the picritic magma. In this hypothesis the higher concentrations recorded in some of the MORB may be due to subsequent fractional crystallisation, Clearly an independent test is needed. The 4He/a°Ar* ratio is a good index of degassing because both isotopes are non-atmospheric and because the solubilities of He and Ar differ by one order of magnitude (e.g., Jambon et al., 1986 ). Since He is more soluble, any degassing of a magma should result in enrichment of He vs. Ar, the degree of which depends on the extent of degassing and on the nature of the degassing process (e.g., bulk or Rayleigh; e.g., Bottinga and Javoy, 1990). The production/ accumulation range, computed from the abundances of the parent nuclides [U, Th and K, with K / U = 12,700 (Jochum et al., 1983) and T h / U = 3 . 3 ] is 1.8-4.1, depending on the assumption concerning mantle degassing and evolution (Ozima and Podosek, 1983; Allrgre
5
et al., 1986/87). MORB show 4He/4°Ar* ratios varying between this radiogenic end-member and much higher values up t o 102, as a probable effect of fractional degassing. The 4He/4°Ar* ratios recorded during the various runs of this study are surprisingly constant and vary little around 2.5 (Fig. 2 ). These values are within the field of production/accumulation, which implies that the sample does not show evidenceofmagmadegassing. We nevertheless consider here that the rare gas content of the glass represents a lower limit of the original rare gas c o n t e n t of the magma. The similarity of the 4 H e / 4 ° A r * ratios between olivine and glass
yields interesting information about the nature of the phases which were analysed. For the glass, the value of the 4He/4°Ar* ratio between crushing (vesicle fraction) and fusion (bulk fraction) is further evidence for the preferential partitioning of rare gases into the vesicles. Indeed, the vesicle ratio is representative of the bulk ratio and also of the magmatic ratio if we assume limited degassing of the melt. The fraction extracted by olivine crushing has a 4He/ 4°Ar* ratio of 2.06_+ 0.55, marginally lower than the values recorded in the glass, but still compatible if one takes into account the respective uncertainties. Since crushing is likely to open olivine microvesicles, such a low value may represent the first degassed fraction which,
~ ,o ~ ~ 8 8 , ~i~~ ....... ~, 2
o,
.,...._
o.
.,2
-,,
-,o
.,
log 4He (mot/g) Fig. 2.4He/4°Ar*vs. 4He content. Same symbols as in Fig. 1. The shaded area is the radiogenic production/accu-
mulation domain.
6
B. MARTY AND P. LUSSIEZ
following the solubility law, is enriched in the least soluble gas, that is, Ar. In the case of the fusion extraction, the rare gas component is a mixture of the microvesicle fraction, the melt inclusion fraction and the olivine fraction. The low Ar content recovered during this run prevents precise computation of the 4He/a°Ar* ratio, which appears higher, but still compatible with the glass ratio. Given the complexity of the phase components recovered during this run, we cannot decipher if this marginally higher ratio is or not an effect of phase partition (e.g., solubility in olivine), 4. Discussion Partition coefficients between olivine and melt are estimated from the ratios between the rare gas content in olivine and that in the glass. For He, atmospheric contamination is negligible and the bulk coefficient is: D He __ C oHe l , tot ~ +0.007 bulk--p--~e =0"01u-0"007 "~ GIAot where subscripts Ol,tot and Gl,tot refer to the total content of He in olivine and in glass, respectively, extracted by fusion (for the glass content, a mean between all runs has been taken). From the crushing experiments of olivine, it is clear that part of He in olivine does not reside within the matrix but most likely in fluid inclusions. Hence the fraction of He recovered following crushing needs to be removed from the total He content in olivine: He He COI'Mx 0 0 0 8 +0.007 DMx-- He = • -0006 CGI.tot
where Mx is assumed to represent the olivine matrix. Olivine also contains glass inclusions and therefore the real He content in olivine must be much lower than the content cornputed above. As expounded in the preceding section, the He content of the glass represents a lower limit of the He content in the magma, For these two reasons, we conclude that:
D ~ e ~<0.008 For Ar, the partition coefficient is computed for 4°Ar*, that is, the slopes of the correlations in Fig. 1: Ar Dbo~kAr--~=Col't°t 0.009 +°°°5-o.oo4 '~GI,tot The Ar fraction of olivine needs also to be corrected for the fluid inclusion fraction. In the case of Ar however, such a correction is not possible because both crushing and fusion runs lead to Ar contents undistinguishable each other, given the low Ar contents and the resulting high uncertainties. Taking into account these uncertainties, an upper limit for the partition coefficient is given by: Ar ~<0.003 OMx These estimations show that the rare gas fraction in the olivine matrix is likely to be very low and can hardly be distinguished from inherited fractions such as fluid or glass inclusions trapped in the crystal. Even taking the bulk rare gas contents that we know to be overestimated, we get partition coefficients lower than those claimed from experimental works. Hiyagon and Ozima (1986) synthesized olivine in rare-gas saturated silicate melt under various pressure conditions and f o u n d t h a t the He partition coefficient should be lower than 0.07, but that of Ar was between 0.05 and 0.15, in contrast with the present study. Broadhurst et al. (1992) attempted to equilibrate natural crystal samples with rare gases at 1 bar and 1300°C. They derived Ar partition coefficients between 0.15 and 0.84. Both groups claimed an increase of the partition coefficients with atomic mass number, Kr and Xe being slightly incompatible or not at all incompatible. These unexpected results were explained by differential polarizability. It must be noted that such a trend is qualitatively consistent with adsorption of atmospheric rare gases, particularly when crystal grain sizes are small ( ~< 10/tm in Broadhurst et al., 1992 ).
ANALYSISOF OLIVINE-GLASSFROM A PICRITIC MID-OCEAN RIDGE BASALT
Keeping in mind the limitations inherent t o the procedure adopted in this study, the He and Ar partition coefficients appear comparable t o those of large-ion lithophile elements (LILE) such as K, U and Th (e.g., McKenzie and O'Nions, 1991 ). Hence a similar behaviour during magmatic processes is expected, complicated in the case of rare gases by degassing fractionation. If rare gases are strongly incompatible as the present results suggest, then rare gas evolution models based on partial melting partition (e.g., Azbel and Tolstikhin, 1988) will need to be reconsidered, Acknowledgements We are grateful to N. Vassard and M. Lenoble for help during rare gas analysis and to two anonymous reviewers for suggestions. The English was improved by Mike Topliss. This paper is dedicated to the late I.Ya. Azbel. References Allegre, C.J., Staudacher, T. and Sarda, P., 1986/87. Rare gas systematics: formation of the atmosphere, evolution and structure of the Earth's mantle. Earth Planet. Sci. Lett., 81: 127-150. Azbel, l.Ya. and Tolstikhin, I.N., 1988. Geodynamics, magmatism and degassing of the Earth. Geochim. Cosmochim. Acta, 54:139-154. Batiza, R., Bernatowicz, T.J., Honhenberg, C.M. and Podosek, F.A., 1979. Relation of noble gas abundances to petrogenesis and magmatic evolution of some oceanic basalts and related differentiated rocks. Contrib. Mineral. Petrol., 69: 301-314. Bottinga, Y. and Javoy, M., 1990. MORB degassing: Bubble growth and ascent. Chem. Geol., 81: 255-270. Bougault, H. and H6kinian, R., 1974. Rift valley in the Atlantic Ocean near 36 ° 50'N: Petrology and chemistry of basaltic rocks. Earth Planet. Sci. Lett., 4: 249261. Broadhurst, C.L., Drake, M.J., Hagee, B.E. and Bernatowicz, T.J., 1992. Solubilities and partitioning of no-
7
ble gases in mineral and synthetic basaltic melts. Geo-
chim. Cosmochim.Acta, 56: 709-724.
Fisher, D.E., 1981. Quantitative retention of magmatic argon in a glassy basalt. Nature (London), 290: 4243. Hiyagon, H. and Ozima, M., 1986. Partition of rare gases
between olivine and basalt melt. Geochim. Cosmochim. Acta, 50: 2045-2057. Jambon, A., Weber, H. and Braun, 0.. 1986. Solubility of He, Ne, Ar, Kr and Xe in a basalt melt in the range 1250-1600 cc. Geochim. Cosmochim. Acta, 50:40 l -
408. Jochum, K.P., Hofmann, A.W., lto, E., Seufert, H.M. and White, W.H., 1983. K, U and Th in Mid-Ocean Ridge Basalt glasses and heat production, K / U and K / R b in the mantle. Nature (London), 306: 431-436. Kurz, M.D., Jenkins, W.J., Schilling, J,G. and Hart, S.R., 1983. Helium isotopic variations in the mantle beneath the Central North Atlantic Ocean. Earth Planet. Sci. Lett., 58: 1-14. Lux, G., 1986. The behaviour of noble gases in silicate liquids: Solution, diffusion, bubbles and surface effects, with applications to natural samples. Geochim. Cosmochim. Acta, 51:1549-1560. Marty, B. and Ozima, M., 1986. Noble gas distribution in oceanic basalt glasses. Geochim. Cosmochim. Acta, 50: 1093-1097. McKenzie, D. and O'Nions, R.K., 1991. Partial melt coefficients from inversion of rare earth element concentrations. J. Petrol., 23:1021-109 I. Ozima, M. and Podosek, F.A., 1983. Noble Gas Geochemistry. Cambridge University Press, Cambridge, 367 pp. Roeder, P.L. and Emslie, R.F., 1970. Olivine-liquid equilibrium. Contrib. Mineral. Petrol., 29: 275-289. Sarda, P., Staudacher, T. and Allbgre, C.J., 1985. 4 ° A t / 36Ar in MORB glasses: constraints on the atmosphere and mantle evolution. Earth Planet. Sci. Lett., 72: 357375. Takaoka, N. and Nagao, K., 1980. Rare-gas studies of Cretaceous deep-sea basalts. Init. Rep. Deep Sea Drill. Proj., Legs 1-3, 35:1121-1126. Watson, E.B., 1979. Calcium content of forsterite coexisting with silicate liquid in the system Na20-CaO-MgOA1203-SIO 2. Am. Mineral., 64: 824-829. Zhang, Y. and Zindler, A., 1989. Noble gas constrainst on the evolution of the Earth's atmosphere. J. Geophys. Res., 94B 10:13,7129-13,7137.