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Effects of heating on the geochemistry of biogenic carbonates
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ELSEVIER
ISOTOPE
GEOSCIENCE
Chemical Geology 129 (1996) 173-183
Effects of heating on the geochemistry of biogenic carbonates Christophe LCcuyer Laboratoire des Sciences de la Terre, CNRS URA 726, hole
Nonnale
SupCrieure, 46, Alike d’ltalie, F-69364 Lyon Cedex 07, France
Received 15 March 1995; accepted 5 December 1995
Abstract Heating and leaching experiments were performed on biogenic carbonates to investigate the behavior of trace elements during diagenetic and metamorphic processes. Samples were treated with H,O, to remove organic compounds and heated in vacuum in the range lOO-500°C. The effects of heating on the geochemistry of these shells were studied by measuring: (1) Cl-, SO:-, Naf, Ca*‘, big’+ and Sr2+ concentrations in leachates; and (2) the amount of CO, released by the carbonate during heating experiments. No significant increase in Mg2+ and Sr*+ was observed in the leachates for heated samples relative to the blank reference which corresponds to the leaching of unheated samples. An enrichment in Ca2+ from 100 to 700 ppm is only detectable at high temperatures (> 400°C) as a consequence of a decarbonation process. The effects of heating on the chemistry are especially sensitive for the aragonite shells. Leachates were obtained on aragonites heated above 200°C and enrichment factors up to 100, 200 and 500 were measured for Cl-, SOi- and Nat, respectively. Na+ contents display a positive correlation with temperature whereas Cl- ion contents are rather constant. The source of Cl- is probably the fluid inclusions which represent - 1 wt% of the shell. Na contents in samples treated with H,O, are lower than those untreated, this result suggests that a fraction of Na + is linked to the organic matrix of the shells. On the basis of increasing Na+ and SO:- contents with peak temperatures, another fraction of Nat and SOi- ions is considered associated with defects in the crystal lattice. These ions are thus expelled during the heating of samples which is responsible for the annealing of defects and the observed progressive aragonite-calcite inversion. Mass-balance calculations using the chemistry of shell fragments reveal that heating and leaching of aragonites above 200°C are responsible for reductions by up to 70%, 55% and 25% of initial Cl-, SO:- and Na+ contents, respectively. Sr’+ and Mg ” ions are more stable in the crystal structure of the biogenic carbonates and are probably more reliable indicators of paleoenvironment than SO-, Cl- and Na+ ions. In skeletal carbonates, especially aragonites, the abundance of the latter elements can be strongly modified when they are exposed to temperatures of only 200°C and leached by low-salinity fluids.
1. Introduction The chemical composition of seawater is a fundamental topic in environmental research. Variations in salt concentrations through time should reflect changes in chemical fluxes, most particularly the competition between marine volcanism and continental erosion. The perturbations of the seawater chem0009-2541/96/$15.00 Copyright PII SOOOS-2541(96)00005-8
8 1996 Published
istry should be reflected in the composition of the carbonate shells secreted by marine organisms (Hallam and Price, 1968; Railsback and Anderson, 1987). Many attempts have been made to use the skeletal chemistry as a potential indicator of past seawater compositions (e.g., Veizer et al., 1977; White, 1977, 1978; Ishikawa and Ichikuni, 1984; Bates and Brand, 1991; McCulloch et al., 1994; Mii and Grossman,
by Elsevier Science B.V.
174
C. Le’cuyer / Chemical Geology 129 f 1996) 173-l 83
1994; Puechmaille, 1994). A great part of the work was mainly focused on Sr2+ and Mgzf, and less on the Na+ and SO:- concentrations in skeletal structures of both calcite and aragonite. However, several factors complicate the application of these data to paleoenvironmental reconstructions: (1) kinetic factors which control the distribution of trace elements in carbonates (Busenberg and Plummer, 1985); (2) the presence of an organic matrix intimately associated with carbonate crystals; (3) ions and complexes adsorbed or trapped within crystal defects; (4) the abundance of solid and fluid inclusions; and (5) the alteration of original chemistry during diagenetic or metamorphic processes. This last point has commonly been suspected of modifying the chemistry of numerous fossil carbonates via diffusion and dissolution-precipitation processes induced by diagenetic fluids (Turekian and Armstrong, 1960; Curtis and Krinsley, 1965; Schroeder, 1969; Land and Hoops, 1973; Pingitore, 1976; Veizer and Wendt, 1976; Buchardt and Weiner, 1981). Generally, the location of trace elements such as Naf and SO:- in carbonate shells is not well known and the matter is of great debate in the literature. Solid-solution substitutions proposed by Amiel et al. (1973) and Kitano et al. (1975) do not provide satisfactory mechanisms to explain trace-element abundances in skeletal carbonates. Blake and Peacor (1981) and Mackenzie et al. (1983) suggested that Na+ and SOi- are included in the organic matrix rather than be substituted into the crystals. White (1977, 1978) also suggested that the presence of organics may be responsible for the high concentrations of Naf in biogenic calcites, an interpretation in conflict with the results of Ishikawa and Ichikuni (1984) who reported concentrations of Na+ in synthetic calcites entirely comparable to those of biogenie calcites. The chemical changes which may affect biogenic carbonates during heating and leaching by very lowsalinity fluids were quantified to evaluate how these shells may be used to interpret paleoenvironments. The goal of this study is also to examine the possible modification of diagenetic or metamorphic fluids by the release of chemical components from altered carbonates. Indeed, biogenic carbonates represent a substantial reservoir of trace elements that could be
transferred to fluids circulating through sediments. Cloud (1962) determined that recognisable skeletal debris constitute 11 wt% of the total sediment. Bathurst (1975) proposed 20-25 wt%, a fraction probably excessive because the amount of redissolved carbonates in deep seas, especially aragonite, had been underestimated (Betzer et al., 1984; Byrne et al., 1984). The contribution of fluid inclusions was examined, as well as the contribution of organic matrix to the total chemical budget of skeletal matter. The general approach entailed measurement of variations of concentrations in Na+, SO:-, Cl-, Sr2+ M g ‘+ and Ca2+ in fluids leaching biogenic carbonates before and after heating in the range lOO-500°C using a thermal decrepitation vacuum line.
2. Samples and analytical techniques Four marine biogenic carbonates were selected for this study. Pododesmus macrochismu is a calcite shell that was sampled along the south California coast. Arca zebra and Anadara notabilis are arago-
Table 1 X-ray diffractometry analysis of unheated and heated aragonite species with a semi-quantitative estimate of the remaining aragonite fraction Genus and species
Anadara Anadara Anadara Annaiwa Anadara Arca Arca Arca Arca Arca
notabilis notabilis notabilis notabilis notabilis
zebra zebra zebra zebra zebra
Strombus Strombus Strombus Strombus Strombus Strombus
gigas gigas gigas gigas gigas gigas
Aragonite fraction estimate 25 170 193 310 415
1.00 0.25 0.25 0.10 0.00
25 199 215 287 461
1.00 0.50 0.45 0.33 0.00
25 118 158 215 329 450
1.00 1.00 1.00 0.25 0.20 0.00
C. Lkuyer/Chemical
Geology 129 (1996) 173-183
nite shells that were collected in the open seas of Saint Maarten Island in the Can&an Sea. Strombus gigas, which has aragonite shell, was sampled in the Florida Keys. At the time of collection, the organisms were living so that the shells were in an excellent state of preservation. All these organisms live in the shallow marine environment and are subtidal or intertidal. They are bivalves except Strombus gigas which is a gastropod. The high porosity of the shell structure was responsible for the incorporation of significant amounts of trapped fluids (Gaffey, 1988). LCcuyer and O’Neil(1994) measured the amounts of water (wt%) included in the shells before and after treatment with H,O,. No meaningful differences were detected. Amounts of water vary among the species: the lowest value was found for Pododesmus macrochisma (0.7%), Arca zebra and Anadara notabilis have 1% of water, and Strombus gigas provides the maximum amount found with 1.2%. The X-ray diffractometry (XRD) determination of carbonate polymorphs was performed at the University of Michigan on powders of both unheated and heated biogenic aragonites. For each experiment, a
Table 2 Cl-, SOi-,
Na+, Ca2+, Mg2*
175
semi-quantitative estimate of the aragonite/calcite ratio is presented in Table 1. 2.1. Leachutes Fifty mg of millimeter-size grains of carbonate were treated with H,O, for 48 hr to remove organic compounds. After this treatment, no detectable concentration of organic carbon was found using coulometry. However, it must be noted that the size of shell fragments may be large enough to contain protected micro environments where small amounts of organic compounds may have survived to the H,O, treatment. Samples were then washed with double deionized water and the ion contents of this water was analyzed to determine the blanks used as reference levels for unheated shells (Table 2). Other aliquots of carbonate extracted from the same shell were heated at various temperatures in the range lOO-500°C and maintained for 20 min to break the fluid inclusions and extract quantitatively all the water (peak temperatures are quoted in Table 2). The amounts of CO, liberated by the shells dur-
and Sr2+ contents (pg g-’ ) of leachates obtained for the four studied skeletal carbonates
Sample (leachates)
Peak T
cl-
so;-
Na+
Ca”
Mg2+
Sr2+
CO,/CaCO,
(“0
(p,g g-‘)
(kg g-‘1
(pgg-‘)
(pg g-‘)
@g g-‘)
(pg g-‘)
(km01 mol-‘1
Pododesmus macrochisma Pododesmus macrochisma
n.h. 317
6 9
11 ,457
269
244 331
2.5 5.0
0.5 0.7
3598
Anadara Anadara Anadara Anadara Anadara Anaaiwa Anadara
n.h. 187 201 222 310 414 519
7 106 64 100 102 132 73
5 578 521 697 615 1025 932
12 1074 309 394 430 1725 1098
84 105 51 23 61 344 670
0.9 0.2 1.8 0.0 0.9 0.3 0.2
0.4 0.5 0.2 0.0 0.3 1.8 1.1
2762 1618 2750 4011 5187 11869
n.h. 199 215 256 354
4 44 42 30 32
5 490 613 432 357
1 234 860 334 559
80 163 107 448 201
0.2 1.7 2.5 0.8 0.0
0.0 0.8 0.6 4.1 0.5
3303 4479 5798 10303
n.h. 118 221 329 440
2 10 37 36 37
5 9 14.5 90 112
15 20 460 747 892
47 93 29 19 59
0.1 0.0 0.0 0.2 0.0
0.1 0.3 0.1 0.1 0.1
192 9798 11410 13531
Arca Arca Arca Arca Arca
notabilis notabilis notabilis notabilis notabilis notabilis notabilis
zebra zebra zebra zebra zebra
Strombus Strombus Strombus Strombus Strombus
gigas gigas gigas gigas gigas
The peak temperature
of decrepitation
4
is reported for heated samples. n.h. = no heating.
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176
Chemical Geology 129 (1996) 173-183
ing the heating experiments were manometrically measured and reported in Table 2. The products of each experiment were split in two parts. One aliquot was analyzed by XRD to determine the advancement of the polymorphic inversion from aragonite to calcite. The other aliquot was leached with double deionized water for 10 min. Analysis of Mg*+, Ca’+, Sr*+ and Na’ was then carried out using a Leeman’ Labs Plasma-Spec III inductively coupled plasma spectrometer. Cl- and SOi- were analyzed by a Dionex” 4000i ion chromatograph, using a Dionex@ AS2 column and suppressed conductivity detection. Instrument precision in both methods was k 2.5% relative standard deviation for those analyses performed at the Laboratory of Hydrothermal Geochemistry, University of Michigan.
Samples weighing from 10 to 14 mg were dissolved in dilute 0.1 N HCl before chemical analysis by inductively coupled plasma spectrometry and ion chromatography. Cl- and SOiconcentrations were analyzed at the “Centre de Recherches Petrographiques et GCochimiques” of Nancy (Table 3). Cl- was analyzed by calorimetry following the method of Vernet et al. (1987). Sulfur was analyzed coulometrically after total decomposition of carbonates at 1700°C. Concentrations in sulfur were attributed entirely to sulfate and are reported as SOiconcentrations in Table 3.
3. Chemical data 3.1. Biogenic carbonates
2.2. Biogenic carbonates Millimeter-size fragments of shells, both untreated and treated with H 202, were also analyzed at the University of Michigan for the same batch of elements except for Cl- and SOiions (Table 3).
Table 3 Cl-, SO:-,
Na+, Ca2’,
Mg”
and Sr2* contents (p,g g-’ ) of skeletal carbonates
Pododesmus macrochisma n.t.
Genus Species Comment
The chemical analyses obtained for untreated and treated samples (H 202) of Pododesmus macrochisma are indistinguishable. For aragonite species treated with H,O,, small differences can be noted for Anadara notabizis: treated samples have
Anadara notabilis ns.
Arca zebra n.t.
Strombus gigas n.t.
Pododesmus macrochisma
Anadara notabilis
Arca zebra
Strombus gigas
Hz02
Hz02
Hz02
H202
Anions:
so;- (pg g-l) cl-
(tLg g-‘)
n.d. n.d.
n.d. n.d.
n.d. n.d.
n.d. nd.
12.3
11.2
11.8
3300 44
1830 171
1830 124
510 129
Cations: 14
Aliquot (mg) Na+ (kg gSt. dv.
’1
Ca’+ (ug g-‘) St. dv. ;g;;,
(ILg g-‘)
;:,*;“(Pg
g-
’)
13.3
10.9
13.7
11.4
7990 157
7750 131
7760 90
9120 180
7770 135
7230 161
7320 117
396000 2340
394000 645
400000 1410
395000 978
399000 2710
397000 1420
401000 957
2500 21
214 3
244 2
124 2
2560 21
260 0
131 1
80 2
1050 11
1530 6
2810 17
1290 3
1040 5
1970 8
1860 5
1210 7
Except for the anions, other elemental concentrations are given for samples untreated and treated with hydrogen deviation; n.t. = not treated with H,O,; n.d. = not determined.
9010 194 4OOOtIO 2080
peroxide. St. dv. = standard
C. L&uyer/Chemical
lower Na+ and higher Mg2+ and Sr2+ concentrations. More pronounced differences can be observed for Arca zebra where treatment resulted in systematically lower concentrations in Na+, Mg2+ and Sr2+ (Table 3). Cl- and SOianalyses were only performed on samples treated with H,O,. The SO:- concentration is much higher in calcite shells (3300 ppm) than in aragonite shells, especially for Strombus gigas which only contains 510 ppm. Cl- concentrations are very low, 44 ppm in the calcite shell compared with 124-171 ppm for the aragonite species. Mg2+ contents
Geology 129 (1996) 173-183
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3.2. Leachates Chemical analyses of leachates are given in Table 2. Leachates from unheated samples were used as blanks and constitute reference’ data during the discussion of the concentrations obtained when the same samples were heated at different temperatures. Concentrations in leachates of unheated samples of the four species are generally very low, < 10 ppm for Cl- and SO:-, 15 ppm for Na+, 100 ppm for Ca2+ in aragonite s,pecies, and < 3 ppm for Mg2+ and Sr’+. Contrasting with these data, leachates of heated shells generally have significantly higher values. In detail, each element displays a specific distribution of values which varies as a function of the peak temperature. Cl - concentrations may vary from one species to another: they are lower than 140 pg gg ’ (Table 2) and do not correlate with the peak temperature (Fig. la). SOi- concentrations also are species-dependent and only Anadara notabilis shows
177
1500 7 M
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+Ld
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500
0
100
200
300
400
peak T”C Fig. 1. Cl- (a), SOi- (b) and Na+ (c) concentrations in leachates vs. peak temperature of decrepitation for the four selected biogenie carbonates. Open square = Pododesmus macrochisma; open circle = Anadara notabilis; open diamand = Arca zebra: filled triangle = Strombus gigas.
a strong increase of SOi- content at peak temperatures of > 400°C (Fig. lb). In the case of Naf, a positive correlation between concentration and tem-
178
C. Lkuyer/
Chemical Geology 129 (1996) 173-183
perature is observed for Anadara notabilis and Strombus gigas. In contrast, no such trend is apparent in Arca zebra data (Fig. 1~). Ca” concentrations are notably higher above 300°C in the case of Anaa’ara notabilis and Arca zebra (Table 2). This can be mainly attributed to the decarbonation process which becomes dramatic above 400°C (Spry, 1969), as is confirmed by the higher amounts of CO, collected during these experiments (Table 2). Mg2+ and Sr2+ concentrations are very low, generally close to the analytical detection limit and do not vary with temperature (Table 2). Although only one analysis of leachate is availPododesmus able for the calcite species macrochisma; Na+ and SOi- concentrations are obviously much higher in the leachate obtained on the sample heated up to 3 17°C than in the leachate of the unheated sample. It is noteworthy that the Naf content of 269 ppm is significantly lower than those found for the three aragonites heated at similar temperatures (430, 559 and 747 ppm for Anadara notabilk, Arca zebra and Strombus gigas, respectively). The SOi- content in this leachate is of the same magnitude as those obtained from the aragonite species, despite a much higher concentration of SOiin the Pododesmus shell: 3300 ppm vs. 510-1830 ppm in the aragonites.
4. Discussion 4.1. Heating and polymorphic inversion Spry (1969) showed that a temperature of N 400°C is required for rapid inversion of aragonite to calcite under dry, solid state conditions. Below 400°C the inversion is very slow, of the order of the geological time scale. The estimates of aragonite/calcite ratios for the three heated skeletal aragonites reveal that the polymorphic inversion starts at - 200°C and is complete around 400°C (Table 1). Water has a known catalytic effect upon this transformation and could explain, by its presence in the shells, the growth of calcite in a few minutes at temperatures of > 200°C. Other factors such as the small grain size and poor crystallinity can also favor the polymorphic inversion which does not require, in this case, the presence of
-1 _I
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7
12000
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10000 -
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6000 -
1
4000 -
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*
I c
8000 0 0 0 o"o 3
2000 -
0
0
t=+-l--r--rT-’ 0
100
200
I
300
400
-r-r 500
600
peak T”C
Fig. 2. bmoles of CO, per mole of CaCO, biogenic carbonates during heating experiments. in Fig. 1.
liberated by the Same symbols as
external fluids. The behavior of trace elements during the polymorphic inversion must be examined to test whether or not they remain stable within the crystal lattice. 4.2. Variations of trace-element concentrations in leachates with temperature
Fig. 1 reveals that trace elements behave in two different ways when shells are heated and then leached: (1) No significant increase in Mg2+ and Sr2+ concentrations can be detected for all the samples relative to the blank reference. Higher amounts of Ca2+ are released only at high temperatures, generally associated to the highest amounts of CO, loss (except for Strombus gigas) during heating experiments, and reflecting the beginning of the decarbonation process (Fig. 2; Table 2). (2) For samples heated above 2OO”C,SO:-, Na+ and Cl- concentrations are higher than for unheated samples, except for Pododesmus macrochisma for which Cl- content in the leachate were similar to the blanks. In the case of Naf concentrations, there is a positive correlation between concentration in the leachate and temperature for Anadara norubilis. An increase of SOi- contents was only observed for Anadaru notabilis at temperatures of > 400°C. By contrast, Cl- concentrations do not show any correlation with peak temperatures.
C. LRcuyer / Chemical Geology 129 (1996) 173-183
Double-charged cations are generally supposed to substitute for Ca*+ in solid solutions. The absence of these cations in the leaching fluid confirms their stability within the carbonate lattice. The location of SOi- and Naf is less certain and is still debated in the literature. Some authors (White, 1977, 1978; Blake and Peacor, 1981; Mackenzie et al., 1983) have proposed that organic matter contains a great part of the total budget of these elements. Because the most important fraction of organic matter was removed from the studied samples by treatment with H,O, before they were heated and leached, the presence of these ions in the leaching fluid is more likely related to their expulsion from the crystal lattice. Such a process could be the consequence of the advancing aragonite-calcite inversion but SOiand Na+ losses also take place during the leaching of the calcite shell and cannot be attributed only to the polymorphic inversion. Busenberg and Plummer (1985) suggested on the basis of their experiments that Na+ concentrations in solid-solutions depend on the density of defects in the crystal structure. It is possible that, as sug,gested by Gaffey et al. (19911, the heating experiments carried out during this study could have resulted in the annealing of defects. This would have reduced the number of defects and the ions would be expelled and easily leached with an external fluid. Cl- concentrations in the leachates are high in comparison with the total carbonate content, up to 72% in the case of Anadara notabilis and generally close to 30-50% (Ta.ble 2). The absence of correlation of Cl- concentra.tions with temperature suggests that the source of Cl - may be essentially the fluid inclusions. The contribution of fluid inclusions is very difficult to quantify because of the poorly known chemical partitioning between ambient seawater and the extrapallial fluid (Simkiss, 1965; Weber, 1973; Lorens and Bender, 1980; Wilbur and Saleuddin, 1983). However, this contribution cannot be neglected because the significant amount of water (1 wt%) trapped in the aragonites is quantitatively liberated at temperatures of > 200°C (Gaffey et al., 1991; LCcuyer and O’Neil, 1994). The Cl- concentrations in the carbonate differ between species, and reflect the biological fractionation of ions between the extrapallial fluid and seawater (Land and Hoops, 1973; Rosenthal and Katz, 1989). Assuming that all
179
the Cl- ions came from the fluid inclusions, the Clconcentration appears to be about half (10,000 ppm) its concentration in seawater (19,350 ppm; Bearman, 1991). These estimates confirm that it is not necessary to invoke another source of Cl- to explain its concentration in the leachates. Following the hypothesis that Cl- concentration in the carbonate is inherited from fluid inclusions, it means that a fraction of the Na+ and SO:- is also derived from the extrapallial fluids. This contribution is difficult to assess in the absence of an accurate knowledge of the biological partitioning effects. Assuming an unmodified seawater composition, it is possible, for example, to estimate on the basis of charge balance that the Na+ contribution of fluid inclusions to the leachate is no more than lo-15% for Anaakra notabilis heated in the range 20022O’C. The contribution of Na+ from fluid inclusions to the net budget of Na+ in the shell is thus very limited, but cannot be totally neglected despite the conclusions of Rosenthal and Katz (1989) who studied freshwater samples and considered that the contribution of fluid inclusions to the chemical composition of the leachates is insignificant. 4.3. Chemical changes of biogenic carbonates The chemical compositions of carbonates untreated and treated with H,O, are presented in Table 3. Only Na+ concentrations provide a coherent pattern of chemical modifications: in all species the concentration of Na+ was lowered by the H,O, treatment. These results suggest that a small fraction of Na+ (- 7%) is complexed with the organic matrix. This value must be considered as a minimum value since some organic matter may be still present in the carbonate after the H,O, treatment. The contribution of Na+ from the crystal lattice to the net budget of Na+ in the shell may be thus estimated from 75% to 85%. Elemental losses (in wt% for Na+, Cl- and SO:-) of skeletal aragonites after heating and leaching were calculated and presented in Fig. 3. For the aragonite species, the most pronounced effect is a loss of Cl(values range from 20% to 72%). The SO:- loss is also substantial, up to 56%, whereas the Naf loss varies from 5% up to 24%. Rosenthal and Katz (1989) emphasized the loss of Na+ from the shell
180
C. L.&uyer/Chemical
during early diagenetic processes. They found during their dissolution experiments that 10% of the Na+ is leached, a value well bracketed by the results obtained in this study.
Geology 129 (1996) 173-183
(a) 2o
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PeakTC Fig. 3. Cl- (a), SOi(b) and Na+ (c) losses of biogenic aragonites against peak temperature of decrepitation. Percentages of ions released by the shells were calculated using ion concentrations in leachates corrected from blank values. Crosses = Clions; open circle = SO:- ions; filled square = Na+ ions.
The calcite shell Pododesmus macrochisma displays low elemental losses compared to aragonite species. In this species, only the SOiloss was significant with a value of 14%, cf. 3% and 6% for Na+ and Cl- ions, respectively (Tables 2 and 3). The differential elemental losses recorded in biogenie carbonates are responsible for the various ion ratios found in leachates (Fig. 4). The SO:-/Clratio of the leachate preserves the gross chemical characteristics of the carbonate. This is not the case for the Na+/Clratio which displays a positive correlation with the temperature (Fig. 4b). The Na+ losses increase with temperature but do not exceed 24% at 400°C for Anadara notabilis (Fig. 3a). The Nat/Clratio of the leachates are consequently systematically lower (in the range 3-25) than the carbonate ratio (in the range 40-70). The chemical composition of fluids which leach heated biogenic
C. L&uyer/Chemical
aragonites is thus controlled by the differential loss of trace elements by the skeletal carbonate. The SO:-/Cland Na+/Cll ratios for Pododesmus macrochisma are not presented in Fig. 4 because their values of 150 and 90, respectively, are much higher than those of aragonites. These high ratios also reflect the high ratios for these elements in the shells, 75 and 176, respectively, which also indicate that SO:-- is more mobile than Na+. The modification of concentrations in trace elements like Na+, SOi- and Cl- (and their ratios) have important consequences for paleo-environmental reconstructions. The trace elements SO:- and Na+ are constituents of all marine carbonates and have considered as potential indicators of past ocean salinities (White, 1977, 1978; Veizer et al., 1977; Ishikawa and Ich:ikuni, 1984). However, this study shows that Naf, SOi- and Cl- contents of skeletal aragonites are particularly sensitive to heating and become mobile in the presence of an external fluid. Mg*+ and Sr*+ could appear thus as more reliable tracers for studying aqueous paleoenvironments. The preservation of trace elements in skeletal carbonates are, for example, well illustrated by the varying chemistry of foraminifera through time. Graham et al. (1982) made a compilation of Sr2+/Ca2+ and Na+/Ca*+ ratios of fossil planktonic foraminifera during the entire Cenozoic Era. No major changes in the Sr2+/Ca2+ ra.tio of foraminifera have been observed during the past 80 Ma. At the opposite, the Na+/Ca*+ ratios are significantly lowered for samples more than 10 Ma old: the average ratio of 6 for modem samples decreases down to 3. These observations suggest that the use of Na+ contents in fossil biogenic carbonatles is certainly very limited. Decomposition of the organic matrix, burying, and circulation of pore waters can have strongly lowered the initial Na+ content of shells and biased the record of paleo-salinities through time.
5. Concluding remarks Leaching experiments were performed on skeletal carbonates heated in the range lOO-500°C to cast some light on their diagenetic evolution. The main results of this study are:
Geology 129 (1996) 173-183
181
(1) After heating above 200°C leachates become enriched in Na+, SO:- and Cl- whereas Sr*+ and Mg*+ ions remain stable within the crystal lattice of the skeletal carbonate. (2) Naf and SOi- contents increase in the leaching fluid with increasing peak temperature. The annealing of crystal defects and the aragonite-calcite inversion are possible mechanisms responsible for the “cleaning” of the crystal lattice and the expulsion of these ions into the leaching fluid. (3) Cl- contents of leachates are species-dependent and do not correlate with temperature. The source of Cl- in these biogenic carbonates is likely the fluid inclusions which probably provide only a small fraction of Na+ and SOi- (I 10%) to the net budget of these ions in the shells. (4) The Na+/Cl- ra h‘o of the shell and leachates differs because of differential elemental loss during the heating experiments. (5) The Cl-, Na+ and SOi- contents of aragonites can be strongly modified. Aragonites can loose up to 70% of the initial Cl- content and up to 55% and 25% of their SOi- and Na+ contents, respectively. A fraction estimated to N 7% of the initial trapped Naf is linked to the organic matrix of the skeletal carbonates. If external very low-salinity fluids (e.g., meteoric fluids) are incorporated into large marine biogenic carbonate deposits during burial metamorphism, the rapid and easy release of SO:-, Cl- and Na+ may strongly influence the trace-element contents of these fluids. The use of SO:-, Cl- and Na+ contents in biogenic carbonates to reconstruct paleosalinities of oceans appears thus to be limited to the best preserved samples.
Acknowledgements
P. Grandjean performed the X-ray diffractometry analyses of biogenic carbonates at the University of Michigan. The author thanks L. Walther and T. Huston for the analytical data obtained at the Laboratory of Hydrothermal Geochemistry, University of Michigan. N.T. Arndt, A. Dia and B. Reynard ‘are also gratefully acknowledged for helpful comments. (SB)
182
C. L.&uyer/
Chemical Geology I29 (1996) 173-183
References Amiel, A., Friedman, GM. and Miller, D.S., 1973. Distribution and nature of incorporation of trace elements in aragonite corals. Sedimentology, 20: 47-64. Bates, N.R. and Brand, U., 1991. Environmental and physiological influences on isotopic and elemental compositions of brachiopod shell calcite: Implications for the isotopic evolution of Paleozoic oceans. Chem. Geol., 94: 67-78. Bathurst, R.C.G., 1975. Carbonate Sediments and Their Diagenesis. Developments in Sedimentology, Vol. 12. Elsevier, Amsterdam, 2nd enlarged ed., 658 pp. Bearman, G., 1991. Seawater: Its Composition, Properties and Behaviour. Oceanography Series, Vol. 2. The Open University and Pergamon Press, Oxford, 165 pp. Betzer, P.R., Byrne, R.H., Acker, J.G., Lewis, C.S., Jolley, R.R. and Feely, R.A., 1984. The oceanic carbonate system: A reassessment of biogenic controls. Science, 226: 1074-1077. Blake, D.F. and Peacor, D.R., 1981. Biomineralization in crinoid echinoderms: characterization of crinoid skeletals elements using TEM and STEM microanalysis. Scann. Electron Microsc., 3: 321-328. Buchardt, B. and Weiner, S., 1981. Diagenesis of aragonite from Upper Cretaceous ammonites: a geochemical case study. Sedimentology, 28: 423-438. Busenberg, E. and Plummer, L.N., 1985. Kinetic and thermodynamic factors controlling the distribution of SO:- and NaC in calcites and selected aragonites. Geochim. Cosmochim. Acta, 49: 713-725. Byrne, R.H., Acker, J.G., Betzer, P.R., Feely, R.A. and Cates, M.H., 1984. Water column dissolution of aragonite in the Pacific ocean. Nature (London), 312: 321-326. Cloud, Jr., P.E., 1962. Environment of calcium carbonate deposition, west of Andros Island, Bahamas. U.S. Geol. Surv., Prof. Pap., 350: l-138. Curtis, C.D. and Krinsley, D., 1965. The detection of minor diagenetic alteration in shell material. Geochim. Cosmochim. Acta, 29: 71-84. Gaffey, S.J., 1988. Water in skeletal carbonates. J. Sediment. Petrol., 58: 397-414. Gaffey, S.J., Kolak, J.J. and Bronnimann, C.E., 1991. Effects of drying, heating, annealing, and roasting on carbonate skeletal material, with geochemical and diagenetic implications. Geochim. Cosmochim. Acta, 55: 1627-1640. Graham, D.W., Bender, M.L., Williams, D.F. and Keigwin, Jr., L.D., 1982. Strontium-calcium ratios in Cenozoic planktonic foraminifera. Geochim. Cosmochim. Acta, 46: 1281-1292. Haham, A. and Price, N.B., 1968. Further notes on the strontium contents of unaltered fossil cephalopod shells. Geol. Mag., 105: 52-55. Ishikawa, M. and Ichikuni, M., 1984. Uptake of sodium and potassium by calcite. Chem. Geol., 42: 137-146. Kitano, Y., Okumura, M. and Idogaki, M., 1975. Incorporation of sodium, chloride and sulfate. with calcium carbonate. Geochem. J., 9: 75-84. Land, L.S. and Hoops, G., 1973. Sodium in carbonate sediments
and rocks, a possible index to the salinity of diagenetic solutions. J. Sediment. Petrol., 43: 614-617. Lecuyer, C. and O’Neil, J.R., 1994. Stable isotope compositions of fluid inclusions in biogenic carbonates. Geochim. Cosmochim. Acta, 58: 353-363. Lorens, R.B. and Bender, M.L., 1980. The impact of solution chemistry on Mytilus edulis calcite and aragonite. Geochim. Cosmochim. Acta, 44: 1265-1278. Mackenzie, F.T., Bischoff, W.D., Bishop, F.C., Loijens, M., Schoonmaker, J. and Wollast, R., 1983. Magnesian calcites: Low temperature occurrence, solubility and solid solution behavior. In: R.J. Reeder (Carbonates): Mineralogy and Chemistry. Mineral. Sot. Am., Rev. Mineral., 11: 97-144. McCulloch, M.T., Gagan, M.K., Mortimer, G.E., Chivas, A.R. and Isdale, P.J., 1994. A high-resolution Sr/Ca and 8180 coral record from the Great Barrier Reef, Australia, and the 1982-1983 El Nino. Geochim. Cosmochim. Acta, 58: 27472754. Mii, H.S. and Grossman, E.L., 1994. Late Pennsylvanian seasonality reflected in the IsO and elemental composition of a brachiopod shell. Geology, 22: 661-664. Pingitore, N.E., 1976. Vadose and phreatic diagenesis: processes, products and their recognition in corals. J. Sediment. Petrol., 46: 985-1006. Puechmaille, C., 1994. Mg, Sr, and Na fluctuations in the test of modem and recent Globigerina bulloides. Chem. Geol., 116: 147-154. Railsback, L.B. and Anderson, T.F., 1987. Control of Triassic seawater chemistry and temperature on the evolution of postPaleozoic aragonite-secreting faunas. Geology, 15: 1002-1005. Rosenthal, Y. and Katz, A., 1989. The applicability of trace elements in freshwater shells for paleogeochemical studies. Chem. Geol., 78: 65-76. Schroeder, J.H., 1969. Experimental dissolution of calcium, magnesium and strontium from recent biogenic carbonates: a mode1 of diagenesis. J. Sediment. Petrol., 39: 1057-1073. Simkiss, K., 1965. The organic matrix of the oyster shell. Comp. B&hem. Physiol., 16: 427-435. Speer, J.A., 1990. Crystal chemistry and phase relations of orthorhombic carbonates. In: R.J. Reeder (Editor), Carbonates: Mineralogy and Chemistry. Mineral. Sot. Am., Rev. Mineral., 11: 145-190. Spry, A., 1969. Metamorphic Textures. Pergamon, Oxford, 350 PP. Turekian, K.K. and Armstrong, L., 1960. Magnesium, strontium and barium concentrations and calcite aragonite ratios of some recent molluscan shells. J. Mar. Res., 18: 133-151. Veizer, J. and Wendt, J., 1976. Mineralogy and chemical composition of recent and fossil skeletons of calcareous sponges. N. Jahrh. Geol. Pa1aontol. Monatsh., pp. 558-573. Veizer, J., Lemieux, J., Jones, B., Gibling, M.R. and Savelle, J., 1977. Sodium: Paleosalinity indicator in ancient carbonate rocks. Geology, 5: 177-179. Vemet, M., Marin, L., Boulmier, S., Lhomme, J. and Demange, J.C., 1987. Dosage du Fer et du Chlorure dam les materiaux gtologiques y compris les Cchantillons hyperalumineux. Analusis, 15: 490-498.
C. L.&yer/Chemical Weber, J.N., 1973. Incorporation of strontium into reef coral skeletal carbonate. Geochim. Cosmochim. Acta, 37: 21732190. White, A.F., 1977. Sodium and potassium coprecipitation in aragonite. Geochim. Cosmochim. Acta, 41: 613-625.
Geology 129 (1996) 173-183
183
White, A.F., 1978. Sodium coprecipitation in calcite and dolomite. Chem. Geol., 23: 65-72. Wilbur, K.M. and Saleuddin, A.S.M., 1983. Shell formation. In: A.S.M. Saleuddin and K.M. Wilbur (Editors), The MolluscaPhysiology. Academic Press, New York, N.Y., pp. 235-287.
ELSEVIER
ISOTOPE
GEOSCIENCE
Chemical Geology 129 (1996) 173-183
Effects of heating on the geochemistry of biogenic carbonates Christophe LCcuyer Laboratoire des Sciences de la Terre, CNRS URA 726, hole
Nonnale
SupCrieure, 46, Alike d’ltalie, F-69364 Lyon Cedex 07, France
Received 15 March 1995; accepted 5 December 1995
Abstract Heating and leaching experiments were performed on biogenic carbonates to investigate the behavior of trace elements during diagenetic and metamorphic processes. Samples were treated with H,O, to remove organic compounds and heated in vacuum in the range lOO-500°C. The effects of heating on the geochemistry of these shells were studied by measuring: (1) Cl-, SO:-, Naf, Ca*‘, big’+ and Sr2+ concentrations in leachates; and (2) the amount of CO, released by the carbonate during heating experiments. No significant increase in Mg2+ and Sr*+ was observed in the leachates for heated samples relative to the blank reference which corresponds to the leaching of unheated samples. An enrichment in Ca2+ from 100 to 700 ppm is only detectable at high temperatures (> 400°C) as a consequence of a decarbonation process. The effects of heating on the chemistry are especially sensitive for the aragonite shells. Leachates were obtained on aragonites heated above 200°C and enrichment factors up to 100, 200 and 500 were measured for Cl-, SOi- and Nat, respectively. Na+ contents display a positive correlation with temperature whereas Cl- ion contents are rather constant. The source of Cl- is probably the fluid inclusions which represent - 1 wt% of the shell. Na contents in samples treated with H,O, are lower than those untreated, this result suggests that a fraction of Na + is linked to the organic matrix of the shells. On the basis of increasing Na+ and SO:- contents with peak temperatures, another fraction of Nat and SOi- ions is considered associated with defects in the crystal lattice. These ions are thus expelled during the heating of samples which is responsible for the annealing of defects and the observed progressive aragonite-calcite inversion. Mass-balance calculations using the chemistry of shell fragments reveal that heating and leaching of aragonites above 200°C are responsible for reductions by up to 70%, 55% and 25% of initial Cl-, SO:- and Na+ contents, respectively. Sr’+ and Mg ” ions are more stable in the crystal structure of the biogenic carbonates and are probably more reliable indicators of paleoenvironment than SO-, Cl- and Na+ ions. In skeletal carbonates, especially aragonites, the abundance of the latter elements can be strongly modified when they are exposed to temperatures of only 200°C and leached by low-salinity fluids.
1. Introduction The chemical composition of seawater is a fundamental topic in environmental research. Variations in salt concentrations through time should reflect changes in chemical fluxes, most particularly the competition between marine volcanism and continental erosion. The perturbations of the seawater chem0009-2541/96/$15.00 Copyright PII SOOOS-2541(96)00005-8
8 1996 Published
istry should be reflected in the composition of the carbonate shells secreted by marine organisms (Hallam and Price, 1968; Railsback and Anderson, 1987). Many attempts have been made to use the skeletal chemistry as a potential indicator of past seawater compositions (e.g., Veizer et al., 1977; White, 1977, 1978; Ishikawa and Ichikuni, 1984; Bates and Brand, 1991; McCulloch et al., 1994; Mii and Grossman,
by Elsevier Science B.V.
174
C. Le’cuyer / Chemical Geology 129 f 1996) 173-l 83
1994; Puechmaille, 1994). A great part of the work was mainly focused on Sr2+ and Mgzf, and less on the Na+ and SO:- concentrations in skeletal structures of both calcite and aragonite. However, several factors complicate the application of these data to paleoenvironmental reconstructions: (1) kinetic factors which control the distribution of trace elements in carbonates (Busenberg and Plummer, 1985); (2) the presence of an organic matrix intimately associated with carbonate crystals; (3) ions and complexes adsorbed or trapped within crystal defects; (4) the abundance of solid and fluid inclusions; and (5) the alteration of original chemistry during diagenetic or metamorphic processes. This last point has commonly been suspected of modifying the chemistry of numerous fossil carbonates via diffusion and dissolution-precipitation processes induced by diagenetic fluids (Turekian and Armstrong, 1960; Curtis and Krinsley, 1965; Schroeder, 1969; Land and Hoops, 1973; Pingitore, 1976; Veizer and Wendt, 1976; Buchardt and Weiner, 1981). Generally, the location of trace elements such as Naf and SO:- in carbonate shells is not well known and the matter is of great debate in the literature. Solid-solution substitutions proposed by Amiel et al. (1973) and Kitano et al. (1975) do not provide satisfactory mechanisms to explain trace-element abundances in skeletal carbonates. Blake and Peacor (1981) and Mackenzie et al. (1983) suggested that Na+ and SOi- are included in the organic matrix rather than be substituted into the crystals. White (1977, 1978) also suggested that the presence of organics may be responsible for the high concentrations of Naf in biogenic calcites, an interpretation in conflict with the results of Ishikawa and Ichikuni (1984) who reported concentrations of Na+ in synthetic calcites entirely comparable to those of biogenie calcites. The chemical changes which may affect biogenic carbonates during heating and leaching by very lowsalinity fluids were quantified to evaluate how these shells may be used to interpret paleoenvironments. The goal of this study is also to examine the possible modification of diagenetic or metamorphic fluids by the release of chemical components from altered carbonates. Indeed, biogenic carbonates represent a substantial reservoir of trace elements that could be
transferred to fluids circulating through sediments. Cloud (1962) determined that recognisable skeletal debris constitute 11 wt% of the total sediment. Bathurst (1975) proposed 20-25 wt%, a fraction probably excessive because the amount of redissolved carbonates in deep seas, especially aragonite, had been underestimated (Betzer et al., 1984; Byrne et al., 1984). The contribution of fluid inclusions was examined, as well as the contribution of organic matrix to the total chemical budget of skeletal matter. The general approach entailed measurement of variations of concentrations in Na+, SO:-, Cl-, Sr2+ M g ‘+ and Ca2+ in fluids leaching biogenic carbonates before and after heating in the range lOO-500°C using a thermal decrepitation vacuum line.
2. Samples and analytical techniques Four marine biogenic carbonates were selected for this study. Pododesmus macrochismu is a calcite shell that was sampled along the south California coast. Arca zebra and Anadara notabilis are arago-
Table 1 X-ray diffractometry analysis of unheated and heated aragonite species with a semi-quantitative estimate of the remaining aragonite fraction Genus and species
Anadara Anadara Anadara Annaiwa Anadara Arca Arca Arca Arca Arca
notabilis notabilis notabilis notabilis notabilis
zebra zebra zebra zebra zebra
Strombus Strombus Strombus Strombus Strombus Strombus
gigas gigas gigas gigas gigas gigas
Aragonite fraction estimate 25 170 193 310 415
1.00 0.25 0.25 0.10 0.00
25 199 215 287 461
1.00 0.50 0.45 0.33 0.00
25 118 158 215 329 450
1.00 1.00 1.00 0.25 0.20 0.00
C. Lkuyer/Chemical
Geology 129 (1996) 173-183
nite shells that were collected in the open seas of Saint Maarten Island in the Can&an Sea. Strombus gigas, which has aragonite shell, was sampled in the Florida Keys. At the time of collection, the organisms were living so that the shells were in an excellent state of preservation. All these organisms live in the shallow marine environment and are subtidal or intertidal. They are bivalves except Strombus gigas which is a gastropod. The high porosity of the shell structure was responsible for the incorporation of significant amounts of trapped fluids (Gaffey, 1988). LCcuyer and O’Neil(1994) measured the amounts of water (wt%) included in the shells before and after treatment with H,O,. No meaningful differences were detected. Amounts of water vary among the species: the lowest value was found for Pododesmus macrochisma (0.7%), Arca zebra and Anadara notabilis have 1% of water, and Strombus gigas provides the maximum amount found with 1.2%. The X-ray diffractometry (XRD) determination of carbonate polymorphs was performed at the University of Michigan on powders of both unheated and heated biogenic aragonites. For each experiment, a
Table 2 Cl-, SOi-,
Na+, Ca2+, Mg2*
175
semi-quantitative estimate of the aragonite/calcite ratio is presented in Table 1. 2.1. Leachutes Fifty mg of millimeter-size grains of carbonate were treated with H,O, for 48 hr to remove organic compounds. After this treatment, no detectable concentration of organic carbon was found using coulometry. However, it must be noted that the size of shell fragments may be large enough to contain protected micro environments where small amounts of organic compounds may have survived to the H,O, treatment. Samples were then washed with double deionized water and the ion contents of this water was analyzed to determine the blanks used as reference levels for unheated shells (Table 2). Other aliquots of carbonate extracted from the same shell were heated at various temperatures in the range lOO-500°C and maintained for 20 min to break the fluid inclusions and extract quantitatively all the water (peak temperatures are quoted in Table 2). The amounts of CO, liberated by the shells dur-
and Sr2+ contents (pg g-’ ) of leachates obtained for the four studied skeletal carbonates
Sample (leachates)
Peak T
cl-
so;-
Na+
Ca”
Mg2+
Sr2+
CO,/CaCO,
(“0
(p,g g-‘)
(kg g-‘1
(pgg-‘)
(pg g-‘)
@g g-‘)
(pg g-‘)
(km01 mol-‘1
Pododesmus macrochisma Pododesmus macrochisma
n.h. 317
6 9
11 ,457
269
244 331
2.5 5.0
0.5 0.7
3598
Anadara Anadara Anadara Anadara Anadara Anaaiwa Anadara
n.h. 187 201 222 310 414 519
7 106 64 100 102 132 73
5 578 521 697 615 1025 932
12 1074 309 394 430 1725 1098
84 105 51 23 61 344 670
0.9 0.2 1.8 0.0 0.9 0.3 0.2
0.4 0.5 0.2 0.0 0.3 1.8 1.1
2762 1618 2750 4011 5187 11869
n.h. 199 215 256 354
4 44 42 30 32
5 490 613 432 357
1 234 860 334 559
80 163 107 448 201
0.2 1.7 2.5 0.8 0.0
0.0 0.8 0.6 4.1 0.5
3303 4479 5798 10303
n.h. 118 221 329 440
2 10 37 36 37
5 9 14.5 90 112
15 20 460 747 892
47 93 29 19 59
0.1 0.0 0.0 0.2 0.0
0.1 0.3 0.1 0.1 0.1
192 9798 11410 13531
Arca Arca Arca Arca Arca
notabilis notabilis notabilis notabilis notabilis notabilis notabilis
zebra zebra zebra zebra zebra
Strombus Strombus Strombus Strombus Strombus
gigas gigas gigas gigas gigas
The peak temperature
of decrepitation
4
is reported for heated samples. n.h. = no heating.
_
-
C. L&uyer/
176
Chemical Geology 129 (1996) 173-183
ing the heating experiments were manometrically measured and reported in Table 2. The products of each experiment were split in two parts. One aliquot was analyzed by XRD to determine the advancement of the polymorphic inversion from aragonite to calcite. The other aliquot was leached with double deionized water for 10 min. Analysis of Mg*+, Ca’+, Sr*+ and Na’ was then carried out using a Leeman’ Labs Plasma-Spec III inductively coupled plasma spectrometer. Cl- and SOi- were analyzed by a Dionex” 4000i ion chromatograph, using a Dionex@ AS2 column and suppressed conductivity detection. Instrument precision in both methods was k 2.5% relative standard deviation for those analyses performed at the Laboratory of Hydrothermal Geochemistry, University of Michigan.
Samples weighing from 10 to 14 mg were dissolved in dilute 0.1 N HCl before chemical analysis by inductively coupled plasma spectrometry and ion chromatography. Cl- and SOiconcentrations were analyzed at the “Centre de Recherches Petrographiques et GCochimiques” of Nancy (Table 3). Cl- was analyzed by calorimetry following the method of Vernet et al. (1987). Sulfur was analyzed coulometrically after total decomposition of carbonates at 1700°C. Concentrations in sulfur were attributed entirely to sulfate and are reported as SOiconcentrations in Table 3.
3. Chemical data 3.1. Biogenic carbonates
2.2. Biogenic carbonates Millimeter-size fragments of shells, both untreated and treated with H 202, were also analyzed at the University of Michigan for the same batch of elements except for Cl- and SOiions (Table 3).
Table 3 Cl-, SO:-,
Na+, Ca2’,
Mg”
and Sr2* contents (p,g g-’ ) of skeletal carbonates
Pododesmus macrochisma n.t.
Genus Species Comment
The chemical analyses obtained for untreated and treated samples (H 202) of Pododesmus macrochisma are indistinguishable. For aragonite species treated with H,O,, small differences can be noted for Anadara notabizis: treated samples have
Anadara notabilis ns.
Arca zebra n.t.
Strombus gigas n.t.
Pododesmus macrochisma
Anadara notabilis
Arca zebra
Strombus gigas
Hz02
Hz02
Hz02
H202
Anions:
so;- (pg g-l) cl-
(tLg g-‘)
n.d. n.d.
n.d. n.d.
n.d. n.d.
n.d. nd.
12.3
11.2
11.8
3300 44
1830 171
1830 124
510 129
Cations: 14
Aliquot (mg) Na+ (kg gSt. dv.
’1
Ca’+ (ug g-‘) St. dv. ;g;;,
(ILg g-‘)
;:,*;“(Pg
g-
’)
13.3
10.9
13.7
11.4
7990 157
7750 131
7760 90
9120 180
7770 135
7230 161
7320 117
396000 2340
394000 645
400000 1410
395000 978
399000 2710
397000 1420
401000 957
2500 21
214 3
244 2
124 2
2560 21
260 0
131 1
80 2
1050 11
1530 6
2810 17
1290 3
1040 5
1970 8
1860 5
1210 7
Except for the anions, other elemental concentrations are given for samples untreated and treated with hydrogen deviation; n.t. = not treated with H,O,; n.d. = not determined.
9010 194 4OOOtIO 2080
peroxide. St. dv. = standard
C. L&uyer/Chemical
lower Na+ and higher Mg2+ and Sr2+ concentrations. More pronounced differences can be observed for Arca zebra where treatment resulted in systematically lower concentrations in Na+, Mg2+ and Sr2+ (Table 3). Cl- and SOianalyses were only performed on samples treated with H,O,. The SO:- concentration is much higher in calcite shells (3300 ppm) than in aragonite shells, especially for Strombus gigas which only contains 510 ppm. Cl- concentrations are very low, 44 ppm in the calcite shell compared with 124-171 ppm for the aragonite species. Mg2+ contents
Geology 129 (1996) 173-183
-;
80
$
60
b
40 20 0 0
100
200
300
400
500
600
peak T”C
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200 0
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200
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400
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500
600
peak T”C
3.2. Leachates Chemical analyses of leachates are given in Table 2. Leachates from unheated samples were used as blanks and constitute reference’ data during the discussion of the concentrations obtained when the same samples were heated at different temperatures. Concentrations in leachates of unheated samples of the four species are generally very low, < 10 ppm for Cl- and SO:-, 15 ppm for Na+, 100 ppm for Ca2+ in aragonite s,pecies, and < 3 ppm for Mg2+ and Sr’+. Contrasting with these data, leachates of heated shells generally have significantly higher values. In detail, each element displays a specific distribution of values which varies as a function of the peak temperature. Cl - concentrations may vary from one species to another: they are lower than 140 pg gg ’ (Table 2) and do not correlate with the peak temperature (Fig. la). SOi- concentrations also are species-dependent and only Anadara notabilis shows
177
1500 7 M
pb
1000
s
+Ld
z
500
0
100
200
300
400
peak T”C Fig. 1. Cl- (a), SOi- (b) and Na+ (c) concentrations in leachates vs. peak temperature of decrepitation for the four selected biogenie carbonates. Open square = Pododesmus macrochisma; open circle = Anadara notabilis; open diamand = Arca zebra: filled triangle = Strombus gigas.
a strong increase of SOi- content at peak temperatures of > 400°C (Fig. lb). In the case of Naf, a positive correlation between concentration and tem-
178
C. Lkuyer/
Chemical Geology 129 (1996) 173-183
perature is observed for Anadara notabilis and Strombus gigas. In contrast, no such trend is apparent in Arca zebra data (Fig. 1~). Ca” concentrations are notably higher above 300°C in the case of Anaa’ara notabilis and Arca zebra (Table 2). This can be mainly attributed to the decarbonation process which becomes dramatic above 400°C (Spry, 1969), as is confirmed by the higher amounts of CO, collected during these experiments (Table 2). Mg2+ and Sr2+ concentrations are very low, generally close to the analytical detection limit and do not vary with temperature (Table 2). Although only one analysis of leachate is availPododesmus able for the calcite species macrochisma; Na+ and SOi- concentrations are obviously much higher in the leachate obtained on the sample heated up to 3 17°C than in the leachate of the unheated sample. It is noteworthy that the Naf content of 269 ppm is significantly lower than those found for the three aragonites heated at similar temperatures (430, 559 and 747 ppm for Anadara notabilk, Arca zebra and Strombus gigas, respectively). The SOi- content in this leachate is of the same magnitude as those obtained from the aragonite species, despite a much higher concentration of SOiin the Pododesmus shell: 3300 ppm vs. 510-1830 ppm in the aragonites.
4. Discussion 4.1. Heating and polymorphic inversion Spry (1969) showed that a temperature of N 400°C is required for rapid inversion of aragonite to calcite under dry, solid state conditions. Below 400°C the inversion is very slow, of the order of the geological time scale. The estimates of aragonite/calcite ratios for the three heated skeletal aragonites reveal that the polymorphic inversion starts at - 200°C and is complete around 400°C (Table 1). Water has a known catalytic effect upon this transformation and could explain, by its presence in the shells, the growth of calcite in a few minutes at temperatures of > 200°C. Other factors such as the small grain size and poor crystallinity can also favor the polymorphic inversion which does not require, in this case, the presence of
-1 _I
14000
!.I
I
j_' .
-I-
7
12000
7 d Y u B
10000 -
t dv
6000 -
1
4000 -
0
* 0
*
I c
8000 0 0 0 o"o 3
2000 -
0
0
t=+-l--r--rT-’ 0
100
200
I
300
400
-r-r 500
600
peak T”C
Fig. 2. bmoles of CO, per mole of CaCO, biogenic carbonates during heating experiments. in Fig. 1.
liberated by the Same symbols as
external fluids. The behavior of trace elements during the polymorphic inversion must be examined to test whether or not they remain stable within the crystal lattice. 4.2. Variations of trace-element concentrations in leachates with temperature
Fig. 1 reveals that trace elements behave in two different ways when shells are heated and then leached: (1) No significant increase in Mg2+ and Sr2+ concentrations can be detected for all the samples relative to the blank reference. Higher amounts of Ca2+ are released only at high temperatures, generally associated to the highest amounts of CO, loss (except for Strombus gigas) during heating experiments, and reflecting the beginning of the decarbonation process (Fig. 2; Table 2). (2) For samples heated above 2OO”C,SO:-, Na+ and Cl- concentrations are higher than for unheated samples, except for Pododesmus macrochisma for which Cl- content in the leachate were similar to the blanks. In the case of Naf concentrations, there is a positive correlation between concentration in the leachate and temperature for Anadara norubilis. An increase of SOi- contents was only observed for Anadaru notabilis at temperatures of > 400°C. By contrast, Cl- concentrations do not show any correlation with peak temperatures.
C. LRcuyer / Chemical Geology 129 (1996) 173-183
Double-charged cations are generally supposed to substitute for Ca*+ in solid solutions. The absence of these cations in the leaching fluid confirms their stability within the carbonate lattice. The location of SOi- and Naf is less certain and is still debated in the literature. Some authors (White, 1977, 1978; Blake and Peacor, 1981; Mackenzie et al., 1983) have proposed that organic matter contains a great part of the total budget of these elements. Because the most important fraction of organic matter was removed from the studied samples by treatment with H,O, before they were heated and leached, the presence of these ions in the leaching fluid is more likely related to their expulsion from the crystal lattice. Such a process could be the consequence of the advancing aragonite-calcite inversion but SOiand Na+ losses also take place during the leaching of the calcite shell and cannot be attributed only to the polymorphic inversion. Busenberg and Plummer (1985) suggested on the basis of their experiments that Na+ concentrations in solid-solutions depend on the density of defects in the crystal structure. It is possible that, as sug,gested by Gaffey et al. (19911, the heating experiments carried out during this study could have resulted in the annealing of defects. This would have reduced the number of defects and the ions would be expelled and easily leached with an external fluid. Cl- concentrations in the leachates are high in comparison with the total carbonate content, up to 72% in the case of Anadara notabilis and generally close to 30-50% (Ta.ble 2). The absence of correlation of Cl- concentra.tions with temperature suggests that the source of Cl - may be essentially the fluid inclusions. The contribution of fluid inclusions is very difficult to quantify because of the poorly known chemical partitioning between ambient seawater and the extrapallial fluid (Simkiss, 1965; Weber, 1973; Lorens and Bender, 1980; Wilbur and Saleuddin, 1983). However, this contribution cannot be neglected because the significant amount of water (1 wt%) trapped in the aragonites is quantitatively liberated at temperatures of > 200°C (Gaffey et al., 1991; LCcuyer and O’Neil, 1994). The Cl- concentrations in the carbonate differ between species, and reflect the biological fractionation of ions between the extrapallial fluid and seawater (Land and Hoops, 1973; Rosenthal and Katz, 1989). Assuming that all
179
the Cl- ions came from the fluid inclusions, the Clconcentration appears to be about half (10,000 ppm) its concentration in seawater (19,350 ppm; Bearman, 1991). These estimates confirm that it is not necessary to invoke another source of Cl- to explain its concentration in the leachates. Following the hypothesis that Cl- concentration in the carbonate is inherited from fluid inclusions, it means that a fraction of the Na+ and SO:- is also derived from the extrapallial fluids. This contribution is difficult to assess in the absence of an accurate knowledge of the biological partitioning effects. Assuming an unmodified seawater composition, it is possible, for example, to estimate on the basis of charge balance that the Na+ contribution of fluid inclusions to the leachate is no more than lo-15% for Anaakra notabilis heated in the range 20022O’C. The contribution of Na+ from fluid inclusions to the net budget of Na+ in the shell is thus very limited, but cannot be totally neglected despite the conclusions of Rosenthal and Katz (1989) who studied freshwater samples and considered that the contribution of fluid inclusions to the chemical composition of the leachates is insignificant. 4.3. Chemical changes of biogenic carbonates The chemical compositions of carbonates untreated and treated with H,O, are presented in Table 3. Only Na+ concentrations provide a coherent pattern of chemical modifications: in all species the concentration of Na+ was lowered by the H,O, treatment. These results suggest that a small fraction of Na+ (- 7%) is complexed with the organic matrix. This value must be considered as a minimum value since some organic matter may be still present in the carbonate after the H,O, treatment. The contribution of Na+ from the crystal lattice to the net budget of Na+ in the shell may be thus estimated from 75% to 85%. Elemental losses (in wt% for Na+, Cl- and SO:-) of skeletal aragonites after heating and leaching were calculated and presented in Fig. 3. For the aragonite species, the most pronounced effect is a loss of Cl(values range from 20% to 72%). The SO:- loss is also substantial, up to 56%, whereas the Naf loss varies from 5% up to 24%. Rosenthal and Katz (1989) emphasized the loss of Na+ from the shell
180
C. L.&uyer/Chemical
during early diagenetic processes. They found during their dissolution experiments that 10% of the Na+ is leached, a value well bracketed by the results obtained in this study.
Geology 129 (1996) 173-183
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Fig. 4. SOi-/Cl(a) and Na+/Cl(b) ratios in leachates against peak temperature of decrepitation for the three selected skeletal aragonites. Same symbols as in Fig. 1.
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PeakTC Fig. 3. Cl- (a), SOi(b) and Na+ (c) losses of biogenic aragonites against peak temperature of decrepitation. Percentages of ions released by the shells were calculated using ion concentrations in leachates corrected from blank values. Crosses = Clions; open circle = SO:- ions; filled square = Na+ ions.
The calcite shell Pododesmus macrochisma displays low elemental losses compared to aragonite species. In this species, only the SOiloss was significant with a value of 14%, cf. 3% and 6% for Na+ and Cl- ions, respectively (Tables 2 and 3). The differential elemental losses recorded in biogenie carbonates are responsible for the various ion ratios found in leachates (Fig. 4). The SO:-/Clratio of the leachate preserves the gross chemical characteristics of the carbonate. This is not the case for the Na+/Clratio which displays a positive correlation with the temperature (Fig. 4b). The Na+ losses increase with temperature but do not exceed 24% at 400°C for Anadara notabilis (Fig. 3a). The Nat/Clratio of the leachates are consequently systematically lower (in the range 3-25) than the carbonate ratio (in the range 40-70). The chemical composition of fluids which leach heated biogenic
C. L&uyer/Chemical
aragonites is thus controlled by the differential loss of trace elements by the skeletal carbonate. The SO:-/Cland Na+/Cll ratios for Pododesmus macrochisma are not presented in Fig. 4 because their values of 150 and 90, respectively, are much higher than those of aragonites. These high ratios also reflect the high ratios for these elements in the shells, 75 and 176, respectively, which also indicate that SO:-- is more mobile than Na+. The modification of concentrations in trace elements like Na+, SOi- and Cl- (and their ratios) have important consequences for paleo-environmental reconstructions. The trace elements SO:- and Na+ are constituents of all marine carbonates and have considered as potential indicators of past ocean salinities (White, 1977, 1978; Veizer et al., 1977; Ishikawa and Ich:ikuni, 1984). However, this study shows that Naf, SOi- and Cl- contents of skeletal aragonites are particularly sensitive to heating and become mobile in the presence of an external fluid. Mg*+ and Sr*+ could appear thus as more reliable tracers for studying aqueous paleoenvironments. The preservation of trace elements in skeletal carbonates are, for example, well illustrated by the varying chemistry of foraminifera through time. Graham et al. (1982) made a compilation of Sr2+/Ca2+ and Na+/Ca*+ ratios of fossil planktonic foraminifera during the entire Cenozoic Era. No major changes in the Sr2+/Ca2+ ra.tio of foraminifera have been observed during the past 80 Ma. At the opposite, the Na+/Ca*+ ratios are significantly lowered for samples more than 10 Ma old: the average ratio of 6 for modem samples decreases down to 3. These observations suggest that the use of Na+ contents in fossil biogenic carbonatles is certainly very limited. Decomposition of the organic matrix, burying, and circulation of pore waters can have strongly lowered the initial Na+ content of shells and biased the record of paleo-salinities through time.
5. Concluding remarks Leaching experiments were performed on skeletal carbonates heated in the range lOO-500°C to cast some light on their diagenetic evolution. The main results of this study are:
Geology 129 (1996) 173-183
181
(1) After heating above 200°C leachates become enriched in Na+, SO:- and Cl- whereas Sr*+ and Mg*+ ions remain stable within the crystal lattice of the skeletal carbonate. (2) Naf and SOi- contents increase in the leaching fluid with increasing peak temperature. The annealing of crystal defects and the aragonite-calcite inversion are possible mechanisms responsible for the “cleaning” of the crystal lattice and the expulsion of these ions into the leaching fluid. (3) Cl- contents of leachates are species-dependent and do not correlate with temperature. The source of Cl- in these biogenic carbonates is likely the fluid inclusions which probably provide only a small fraction of Na+ and SOi- (I 10%) to the net budget of these ions in the shells. (4) The Na+/Cl- ra h‘o of the shell and leachates differs because of differential elemental loss during the heating experiments. (5) The Cl-, Na+ and SOi- contents of aragonites can be strongly modified. Aragonites can loose up to 70% of the initial Cl- content and up to 55% and 25% of their SOi- and Na+ contents, respectively. A fraction estimated to N 7% of the initial trapped Naf is linked to the organic matrix of the skeletal carbonates. If external very low-salinity fluids (e.g., meteoric fluids) are incorporated into large marine biogenic carbonate deposits during burial metamorphism, the rapid and easy release of SO:-, Cl- and Na+ may strongly influence the trace-element contents of these fluids. The use of SO:-, Cl- and Na+ contents in biogenic carbonates to reconstruct paleosalinities of oceans appears thus to be limited to the best preserved samples.
Acknowledgements
P. Grandjean performed the X-ray diffractometry analyses of biogenic carbonates at the University of Michigan. The author thanks L. Walther and T. Huston for the analytical data obtained at the Laboratory of Hydrothermal Geochemistry, University of Michigan. N.T. Arndt, A. Dia and B. Reynard ‘are also gratefully acknowledged for helpful comments. (SB)
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Chemical Geology I29 (1996) 173-183
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