Earth and Planetary Science Letters 298 (2010) 443–449
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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l
Composition and evolution of the early oceans: Evidence from the Tagish Lake meteorite M.R.M. Izawa a,⁎, H.W. Nesbitt a, N.D. MacRae a, E.L. Hoffman b a b
Department of Earth Sciences, University of Western Ontario, 1151 Richmond St., London, Ontario, Canada N6A 5B7 Activation Laboratories Ltd., 1336 Sandhill Drive, Ancaster, Ontario, Canada L9G 4V5
a r t i c l e
i n f o
Article history: Received 17 February 2010 Received in revised form 16 August 2010 Accepted 20 August 2010 Available online 16 September 2010 Editor: R.W. Carlson Keywords: carbonaceous chondrites aqueous alteration early oceans icy satellites
a b s t r a c t Laboratory leaching studies of the Tagish Lake meteorite demonstrate that the most readily leached cations of carbonaceous chondrite meteorites are Mg, Ca, Na and K; with the most readily leached anions being SO4, Cl and PO4 (in decreasing order of abundances). Soluble organics were not analyzed due to limited sample sizes. Magnesium and sulfate – the most abundant solutes leached from Tagish Lake meteorite – are also the most abundant ions of salts lining fractures of aqueously altered meteorites. These same ions probably were the most abundant solutes of the ﬁrst permanent oceans, which probably formed during or soon after the “late heavy bombardment” stage (~ 4.2 to 3.7 Ga). Considering the residence time of Cl in modern seawater, and the composition of the exposed continental crust to at least 3.3 Ga, evolution of seawater from Mg–SO4dominated to NaCl-dominated would have required about 3 × 105 yr and could not have been complete until about 3.0 to 3.3 Ga. The calculations also suggest that the ﬁrst permanent oceans were more saline than modern seawater and, as argued by others, these aspects have signiﬁcant implications for evolution of life. Many solar system bodies besides Earth probably derived a signiﬁcant proportion of their volatile and soluble constituents from carbonaceous chondrite material, therefore, primordial solutions similar to those presented here were likely important in the early evolution of aqueous systems throughout the solar system, including those present on carbonaceous chondrite parent asteroids and icy outer solar system satellites. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The Tagish Lake meteorite is unusual in a number of respects: its orbit is known; it is perhaps the meteorite most representative of bulk solar system composition; and some samples were collected prior to contamination by or reaction with either the lithosphere or the hydrosphere (i.e., in a highly pristine state) (Brown et al., 2000). These materials provide a unique opportunity to determine which constituents of a volatile-rich carbonaceous chondrite are readily soluble. Such information is relevant to the origin of solutes in primitive seawater, to compositional evolution of the early oceans and, consequently, for evolution of life on the planet. Identiﬁcation of these highly leachable constituents and discussion of implications are the focus of this communication. Recent arguments and indirect analytical evidence suggest the early oceans were 1.5 to 2 times more saline than modern seawater (de Ronde et al., 1997). Morse and Mackenzie (1998) suggest, and Knauth (1998) implies, that the major contribution to the high salinity of the early oceans was NaCl. One of the most-commonly cited arguments supporting an early NaCldominated ocean is based on uniformitarianism: acids of the
⁎ Corresponding author. E-mail address: [email protected]
(M.R.M. Izawa). 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.08.026
atmosphere leach the crust, with rates of supply and residence times of elements determining seawater compositions (e.g., Garrels and Mackenzie, 1971; Holland, 1984). Application of the argument to the accretionary and late heavy bombardment stages of the Earth is, however, problematic unless it can be demonstrated that the sources of solutes were the same then as now. By the uniformitarian principle, the crust is the ultimate source of solutes. It is now of granodioritic composition and covered by a veneer of continental shelf-type sedimentary rocks (e.g., Garrels and Mackenzie, 1971; Holland, 1984). The available evidence indicates that the crust was different before about 3.3 Ga, with komatiitic lavas produced on the continents and a virtual absence of mineralogically mature continental shelf-type sedimentary rocks (Fedo et al., 1996; Arndt et al., 2008). These considerations, and experimental results presented here, indicate that NaCl was not the major salt of the ﬁrst permanent oceans, that primitive seawater was more saline than modern seawater, and that uniformitarian arguments do not apply to the early oceans, or to primitive ﬂuid reservoirs on other solar system bodies. The case against the uniformitarian explanation for early seawater composition is made stronger by considering ﬁndings concerning the sources of H2O and organic carbon of the early hydrosphere and atmosphere. Volatiles were captured during and following the accretionary and “late heavy bombardment” stages of Earth evolution, although some were lost due to heavy impact during the late phases of
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the above stages. Even considering such losses, numerous independent calculations agree that capture of exogenous material including carbonaceous chondrites, hydrous interplanetary dust particles, and comets contributed a major portion of the volatiles to the Earth's atmosphere and oceans (Anders and Owen, 1977; Anders, 1989; Chyba, 1990; Zahnle and Grinspoon, 1990; Chyba and Sagan, 1992; Delsemme, 2006; Óro et al., 2006). Óro et al. (2006) emphasized that if just 10% of the bodies captured were comets and carbonaceous chondrites, they would have contributed the bulk of oceanic waters and more than ten times the amount of carbon in the biosphere (Anders and Owen, 1977; Chyba, 1987; Anders, 1989; Delsemme, 2006). From these arguments it is apparent that captured carbonaceous chondrites and comets can account for effectively all solvent (H2O) and carbon of the Earth's hydrosphere and biosphere. The deliberations may be extended to include dissolved salts of the early oceans. Just as carbonaceous chondritic dust and cometary materials could have supplied the bulk of solvent (H2O) to the oceans, these same sources likely supplied solutes to the early oceans, the amount supplied being dependent upon the abundance of soluble solutes in these sources. We have examined this possibility by leaching Tagish Lake meteorite to determine the nature and amount of inorganic solutes released to solution. 1.1. Ephemeral and permanent oceans Formation of the Earth's core between about 100 and 200 Ma probably led to the formation of a primitive crust which continued to evolve through accretion and by planetary processes (Ahrens et al., 1989; Greenberg, 1989; Weissman, 1989; Greenberg et al., 1991; Yang et al., 2007). Accordingly, the compositions of the early oceans must similarly have evolved. The earliest oceans, although “ephemeral”, likely formed while the Earth was still accreting (4.5 to 4.2 Ga) (Ahrens et al., 1989; Weissman, 1989). Near the end of the accretionary period, when planetesimals were largest, portions of any existing hydrosphere and atmosphere may have been driven off by energetic impacts (Greenberg, 1989; Yang et al., 2007). Chemical and dynamical arguments suggest that during the late accretionary stage, and after differentiation of Earth, Moon formed as a result of an exceptionally large Earth impact (Greenberg, 1989; Cohen et al., 2000). The hydrosphere would have been lost during this event, with regeneration proceeding by subsequent capture of volatiles and solutes. Ephemeral oceans produced during the accretionary phase may have displayed compositions different from oceans formed later, primarily because early accreted materials should have contained a high proportion of volatile-poor constituents, possibly similar to enstatite chondrites (e.g., Javoy, 1995; Righter et al., 2004); whereas materials captured later (~4.2 to 3.7 Ga) contained more volatile-rich carbonaceous chondritic (and possibly cometary) materials derived from beyond the orbit of Mars (e.g., Righter et al., 2004; Strom et al., 2005). These arguments suggest that the ﬁrst permanent oceans were created during, and soon after, the late heavy bombardment stage (4.2 to 3.7 Ga) (Ahrens et al., 1989; Weissman, 1989). Degassing of the Earth's interior likely contributed to establishment of the ﬁrst permanent oceans, but Chyba (1987) argues forcefully that during this period, a large proportion (40%) of H2O of the hydrosphere was derived from exogenous sources. The argument is supported by Weissman (1989). Carbonaceous chondrites are volatile-rich and are widely thought to be representative of small bodies in the outer asteroid belt and beyond (Burbine et al., 2002; Scott, 2007). The CI chondrites are widely considered to be most representative of solar system bulk composition because they are compositionally most similar to the solar photosphere (e.g., Palme and Jones, 2003; Scott, 2007). Tagish Lake is a C2 chondrite that is chemically mineralogically, and isotopically most similar to CI and CM chondrites (e.g., Brown et al., 2000; Mittlefehldt, 2002; Zolensky et al., 2002; Izawa et al., 2010; Russell et al, 2010. Tagish Lake is, therefore, potentially representative
of the hydrous carbonaceous chondrite material captured during planetary accretion. Chondritic micrometeorites are another important potential source of volatiles and solutes during planetary accretion. Multiple studies support a relationship between hydrous carbonaceous chondrites (especially CM and CI types) and chondritic micrometeorites, as well as hydrous interplanetary dust particles, comets, and carbonaceous chondrite microclasts in howardites (e.g., Engrand et al., 1996; Gounelle et al., 2003; Gounelle et al., 2005; Zolensky et al., 2008; Aléon et al., 2009; Dobrica et al., 2009). Orbital (Brown et al., 2000; Hildebrand et al., 2006) and spectroscopic (Hiroi et al., 2001) data suggest that Tagish Lake may be related to the D-type asteroids, a class of objects which are likely to be very common in the outer main belt, and may be related to both comets and carbonaceous asteroids. The recent discovery of very rare aqueous alteration phases in cometary samples (Zolensky, 2007; Flynn, 2008; Nakamura et al., 2008) supports the existence of a continuum of objects between carbonaceous asteroids and comet nuclei, of which Tagish Lake may be the ﬁrst known sample. For these reasons (recovery in highly pristine state, kinship with common hydrous meteorites and related materials, linkage to a very common asteroid type) Tagish Lake is plausibly representative of objects that delivered abundant solutes during the late stages of planetary accretion. The relatively pristine nature of the Tagish Lake material is particularly relevant to the present study, as this greatly increases the likelihood that the leachable constituents are of pre-terrestrial origin and therefore that the leachate compositions are minimally inﬂuenced by the terrestrial atmosphere and hydrosphere. 2. Samples and experimental methods: leaching of Tagish Lake meteorite Three Tagish Lake samples, each with different post-entry histories, were investigated. Material which fell on snow without having melted it and that was collected within one week of the fall is referred to as “pristine”; it contains a full complement of leachable constituents. The second sample (PM-05) was exposed to melt water derived from snow (into which it fell); this sample is referred to as “partially degraded”. The third sample (MG-02) was immersed in melt water derived both from snow and ice of the frozen Tagish Lake (though it did not contact lake water); it is referred to as “degraded”. The previously published observation that the degraded Tagish Lake sample was depleted in H, Na and Cl (Brown et al., 2000) was the primary stimulus for this investigation. Table 1 provides analyses of the three Tagish Lake samples, as well as one sample of Allende (CV3) for comparison. Ten milligram aliquots of each sample were leached in three sequential experiments using 4 ml of 18 MΩ distilled, deionized water at each step. The ﬁrst experiment used 20 °C water for 10 min, the second repeated this procedure on the solid residue, and the third used 60 °C water for 10 min on the residue of the ﬁrst two. The results and implications of each experiment are discussed in turn. The leachate of the ﬁrst experiment was analyzed by ion chromatography for PO4, SO4, Cl and Br, then diluted and analyzed by ICP-MS for a wide range of elements; Mg and SO4 are the major cation and anion in all leachates. Mg, Ca and K increase systematically in leachate from pristine to partially degraded and degraded samples, the reason for which is uncertain. The two degraded samples, having been wetted (by melt waters) and subsequently dried, may have additional (precipitated) soluble salts lining pore walls; alternatively, freezing and thawing of the samples may have produced fractures and channels (and access to additional solutes) absent from the pristine sample. Petrographic evidence of some mineral alteration (e.g., saponite–serpentine, secondary magnetite, and carbonates) from water on the parent body has been identiﬁed in the Tagish Lake meteorite (Brown et al., 2000; Zolensky et al., 2002; Greshake et al., 2005). Aqueous alteration of primitive meteorites, generally, has probably promoted release of soluble salts. Gounelle and Zolensky (2001) concluded that the soluble phases reported in chondritic
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Table 1 Partial bulk analyses of pristine, partially degraded (PM-05) and degraded (MG-02) Tagish Lake meteorite, and Allende meteorite. Abbreviations for methods: Prt-Gm, prompt gamma ray; LL/SL-INAA, long-lived/short-lived INAA; TD-ICPMS, total digestion (HNO3/HCLO4/HF,HCl, 260 °C) ICP-MS; WRA–ICP, whole rock analysis (lithium metaborate/ tetraborate fusion) ICP-OES; F–ICPMS, lithium metaborate/tetraborate fusion ICP-MS. See Brown et al. (2000) for details of analytical techniques. Z
1 11 12 15 17 19 20 20 26 35 38
H Na Mg P Cl K Ca Ca Fe Br Sr
wt.% ppm wt.% ppm ppm ppm wt.% wt.% wt.% ppm ppm
Prt-Gm LL-INAA WRA/ICP TD-ICP SL-INAA TD-ICP WRA/ICP TD-ICPMS LL-INAA LL-INAA F/ICPMS
Tagish Lake samples
1.5 ± 0.3 4450 ± 60 10.8 ± 0.5 927 ± 50 560 ± 90 650 ± 50 0.99 ± 0.09 1.21 ± 0.04 19.3 ± 0.9 2.8 ± 0.2 9.4 ± 0.5
N/A 1800 ± 10 12.0 ± 0.2 790 ± 50 220 ± 120 580 ± 50 1.77 ± 0.05 1.98 ± 0.03 20.2 ± 0.4 0.7 ± 0.3 6.9 ± 0.4
0.7 ± 0.3 2230 ± 80 11.8 ± 0.2 1040 ± 70 b60 520 ± 50 1.21 ± 0.02 1.40 ± 0.03 19.5 ± 0.5 0.7 ± 0.3 11.0 ± 0.9
0.2 ± 0.1 3310 ± 20 15.5 ± 0.2 1120 ± 50 260 ± 100 340 ± 40 1.70 ± 0.02 1.87 ± 0.03 24.1 ± 1.2 1.6 ± 0.2 14.1 ± 0.9
N/A = not analyzed for this sample.
meteorites may be entirely the result of terrestrial alteration. Our observations of greater quantities of soluble phases, notably Na, Cl and Br, in the pristine Tagish Lake material, which was never exposed to terrestrial melt water, argue strongly for an extraterrestrial origin for at least some proportion of the soluble phases observed. Phosphate, Fe and Ni were detected in all leachates, but at low levels. For purposes of the present study, the question of the ultimate origin of the soluble phases is largely irrelevant. For whether the leached solutes derive from extraterrestrial sulfates, halides, etc., or were formed as a result of alteration during very brief exposure to terrestrial melt water; our observations of the soluble phases evolved during the aqueous leaching of carbonaceous chondrite material, and our inferences on the subsequent evolution of such a solution, remain the same. Moreover, the implications of the initial composition and subsequent evolution of Tagish Lake leachate are not affected if the solutes are exogenous, or formed very soon after the fall event upon contact with the terrestrial hydrosphere. The partially degraded and degraded samples yield lower quantities of Na, Cl and Br than did the pristine sample (Table 2), probably because these elements were leached from the ﬁrst two samples by melt waters. Compared with the pristine sample, Na in the degraded/partially degraded samples is almost halved and Cl is effectively absent (Table 1). The effective absence of Cl indicates that it and about half of Na are readily leached, thus probably reside on pore walls and linings of meteorites. The low Na and Cl of the bulk samples (Table 1) provide partial conﬁrmation of Na and Cl leaching results. Much of these may be derived from soluble salts; this is consistent with the known presence in meteorites of halite (Zolensky and McSween, 1988; Gooding et al., 1991; Rubin et al., 2002) and brine ﬂuid inclusions (Zolensky et al., 1999). Conﬁrmation of abundant Mg and SO4 leaching is derived from petrographic studies of chondritic meteorites. Epsomite (MgSO4·7H2O) and hexahydrite (MgSO4·6H2O) are among the most abundant fractureﬁlling salts of aqueously altered chondritic meteorites (Zolensky and McSween, 1988; Brearley and Jones, 1998; Gounelle and Zolensky, 2001). It has since become clear that most or all of the hydrous Mg-sulfate veins observed in meteorites such as Orgueil that have suffered long exposure to the terrestrial environment formed during their residence on Earth (Gounelle and Zolensky, 2001). The possibility that some of the sulfate was present in carbonaceous chondrites prior to their falls was recognized by Gounelle and Zolensky (2001), and is supported by our observation of abundant sulfate in leachates of the Tagish Lake samples, most notably the pristine material. We suggest that extraterrestrial Mgsulfates are present in aqueously altered carbonaceous chondrites, and that the terrestrial sulfate veins in Orgueil and similar samples reﬂect both remobilization (and probably hydration) of existing sulfates as well as the formation of purely “terrestrial” sulfate via the hydrolysis of sulﬁdes.
Sulfate of extraterrestrial origin was almost certainly formed during aqueous processing on the parent asteroid, not in the nebula (though we cannot deﬁnitively rule out such an origin for some sulfate). Additional sulfate is very likely to have been formed in partially degraded samples during (very brief) exposure to terrestrial conditions. This mechanism is also likely responsible for the formation of hydrated Mg-sulfate veins in carbonaceous chondrites as has been pointed out by Gounelle and Zolensky (2001). We emphasize, however, the implausibility of a terrestrial origin for all sulfate leached from Tagish Lake, especially from the pristine sample which was recovered frozen and was kept frozen until analysis. Studies of the aqueous alteration of carbonaceous chondrites indicate that Mg-rich ﬂuids are produced during late-stage aqueous processing (e.g., Hanowski and Brearley, 2000; Hanowski and Brearley, 2001; Airieau et al., 2005; Jones and Brearley, 2006; Rubin et al., 2007; Chizmadia and Brearley, 2008). Sulfate was likely produced by the hydrolysis of primary sulﬁdes, also during the later stages of alteration (Airieau et al., 2005; Jones and Brearley, 2006). The existence of oxidized secondary phases, notably magnetite, that pseudomorphically replace sulﬁdes in Tagish Lake and other aqueously altered C chondrites indicates that at least some oxidization of sulﬁdes took place on the parent body (Zolensky et al., 2002; Takayama and Tomeoka, 2008; Izawa et al., 2010). Subsequent second and third leachates were analyzed by ICP-MS only. The third leach at 60 °C yielded (generally) an increased abundance of solutes (Table 2). The Allende leachates consistently yield much lower solute contents than Tagish Lake samples, possibly because of a different starting composition or because of a different mineral distribution (i.e., fewer interstitial salts). The CI carbonaceous chondrites, Orgueil and Murchison, have been leached and their compositions plotted on Fig. 1b (Fanale et al., 1977; Fanale et al., 2001). The high Mg/(Na +Mg) values of both leachates demonstrate their afﬁnity with Tagish Lake leachate. The chloride content of Orgueil leachate was not analyzed and for plotting was assumed to be negligible. These data conﬁrm the Tagish Lake experimental leaching results, demonstrating that the results are not unique and that MgSO4-dominated solutions are produced by leaching carbonaceous chondrites. The conclusion is corroborated by the numerous observations of Mg and Na sulfates ﬁlling fractures of aqueously altered carbonaceous chondrites (Zolensky and McSween, 1988; Airieau et al., 2005) where natural leaching from these materials occurred during the early evolution of the solar system (Zolensky and McSween, 1988; Airieau et al., 2005) and/or in the terrestrial environment (Gounelle and Zolensky, 2001). Allende was also leached (Table 2); it yielded similar proportions of solutes, but much lower total dissolved solids. Perhaps thermally altered carbonaceous chondrites have had their soluble constituents either removed or altered to less reactive phases (Zolensky and McSween, 1988). The total solute contents derived from each Tagish Lake sample (Table 2, last column) indicate that
M.R.M. Izawa et al. / Earth and Planetary Science Letters 298 (2010) 443–449
about 0.2 wt.% Mg, 0.1 wt.% Ca and 0.01 wt.% Na and K were extracted by leaching. From perusal of Table 2 it is evident that continued leaching will yield still greater amounts of solutes. Because a signiﬁcant amount of the H present in Tagish Lake is incorporated into organic compounds, and because it is unlikely that all OH and H2O structurally bound in minerals would be released, these concentrations are lower bounds only. With these minimum values, and if all H of the pristine Tagish Lake sample were present as H2O (about 11 wt.% of sample), then complete removal of all water from the sample would yield a solution of at least 20,000 mg/L Mg, 10,000 mg/L Ca and about 1000 mg/L of Na and K; Fe would be present at about 50 to 100 mg/L. These cation concentrations are balanced primarily by SO4 and Cl, with SO4 favored by at least a 7:1 wt. ratio (Table 2). The maximum Na+ content of modern seawater is 10,000 to 11,000 mg/L. If the solvent and solutes of primordial
Table 2 Analyses of sequential leaching experiments, all values are ppm (of Tagish Lake meteorite), that is, ppm lost from meteorite. Anions are reported from the ﬁrst leach only. Z
Pristine 11 Na 12 Mg 19 K 20 Ca 26 Fe 38 Sr Anions PO4 SO4 Cl Br
20 °C 1st leach 105 467 49 121 7 0.3
60 °C 3rd leach
14 418 35 244 2 0.2
13 723 50 784 1 0.8
132 1608 134 1149 10 1.3
6 576 33 263 0 0.1
13 930 55 836 2.6 0.8
67 2125 158 1331 2.8 1.2
6 417 38 245 1.6 0.2
5 629 44 635 1.7 0.7
64 1902 167 1289 5.5 1.4
24 2980 430 3.6
Partially degraded PM-05 11 Na 48 12 Mg 619 19 K 69 20 Ca 232 26 Fe 0.2 38 Sr 0.3 Anions PO4 24 SO4 1740 Cl 40 Br b0.8 Degraded MG-02 11 Na 53 12 Mg 856 19 K 86 20 Ca 409 26 Fe 2.1 38 Sr 0.5 Anions PO4 12 SO4 4400 Cl 40 Br b0.8 Allende 11 Na 12 Mg 19 K 20 Ca 26 Fe 38 Sr Anions PO4 SO4 Cl Br
20 °C 2nd leach
4 64 3 0 0 0.15
2 51 3 0 0.6 0.1
5 46 2 0 0 0.1
11 161 7 0 0.6 0.4
b3 350 40 b0.8
The lack of charge balance is due mostly to experimental errors, and the difference in sensitivity between ion chromatographic analysis used for anions and ICP-MS analysis used for cations. We are conﬁdent that the difference in charge is not due to a failure to analyze a species relevant to the conclusions of our study.
− Fig. 1. The system Na+–Mg2+–SO2− 4 –Cl may be considered a four component system with the restriction that all solids and solutions are charge-neutral. With this restriction, the phase rule can be applied to the system. The graphical equivalent of this four component system (with restriction) and the plane of neutrality are shown in panel a. Panel b is after Hardie and Eugster (1970) and illustrates phase relations in the reciprocal salt system NaSO4–MgSO4–Na2Cl2–MgCl2 (the plane of neutrality in panel a) at 25 °C and atmospheric pressure. A solution is everywhere present and the compositions of some leachates are plotted on the diagram. The Cl− content of Orgueil leachate is uncertain and is plotted as an ellipse to indicate such. TL; Tagish Lake (pristine) leachate, AL; Allende leachate, OL; Orgueil leachate (Fanale et al., 2001), ML Murchison leachate (Fanale et al., 2001). Ep; epsomite (MgSO4·7H2O), Hx; hexahydrite (MgSO4·6H2O), Bl; bloedite (NaMg[SO4]2·4H2O), Mi; mirabilite (NaSO4·10H2O), Th; thenardite (NaSO4) Ha; halite (NaCl), Bi; bischoﬁte (MgCl2·6H2O). Kieserite (MgSO4·H2O) has been omitted for clarity.
seawater were derived primarily from dehydration and leaching of chondritic meteorites of average solar system composition, exempliﬁed by the Tagish Lake meteorite, primordial seawater would be appreciably more saline, and of much different composition, than its modern counterpart. Fanale et al. (2001) leached several wt.% of materials from Orgueil, and as they noted, much of this likely was derived from salts lining pores and in veins of the meteorite. Only 1 to 2 wt.% of pristine Tagish Lake sample was leached (Table 2, total leached + SO4 + Cl). More than double this amount was released by leaching Orgueil in nearboiling water for an hour. This likely reﬂects the formation of additional sulfates during the residence of the Orgueil meteorite in terrestrial conditions as envisioned by Gounelle and Zolensky (2001). If carbonaceous chondrites provided both solvent and solute to the early oceans, then the data of Table 2 and results from the Orgueil experiment indicate a primitive ocean both with salinity greater than modern oceans and with a composition dominated by MgSO4 rather than NaCl.
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3. Leachate evolution and seawater composition The evolutionary path followed by Tagish Lake leachate can be approximated considering the main constituents: Mg, Na, SO4 and Cl (Fig. 1a), constrained to the plane of electrical neutrality. The saturation ﬁelds of salts stable at 25 °C and atmospheric pressure are illustrated in Fig. 1b. Modern seawater composition plots in the halite ﬁeld (Fig. 1b, large dot), thus NaCl would be the ﬁrst evaporite mineral to precipitate. The Tagish Lake leachate plots within the epsomite ﬁeld (Fig. 1b, cross). Evaporative concentration of the Tagish Lake leachate, thus the mineral sequence, is epsomite, epsomite + bloedite and epsomite + bloedite + halite. For closed systems, the ﬁnal solution composition remains at the peritectic composition (PI) and bloedite is consumed to produce halite + epsomite. Where previously precipitated salts are not available to react with evolved solutions (open system), the precipitation sequence is epsomite, epsomite + bloedite, epsomite + halite, hexahydrite + halite, and ﬁnally bischoﬁte + halite. Bloedite stability acts as a chemical divide (Hardie and Eugster, 1970), preventing the Tagish Lake leachates from evolving toward a NaCl-rich water. Instead, it must evolve toward an MgCl2-rich solution, distinctly different from modern seawater. The above analysis is simpliﬁed in that effects of carbonate and gypsum (or anhydrite) precipitation were not considered. Calcite precipitation cannot affect the evolutionary path of the leachate. Gypsum or anhydrite precipitation is likely to occur before precipitation of salts indicated in Fig. 1b (Hardie and Eugster, 1970). Their precipitation depletes the leachate in SO4, but does not affect the ordinate, thus the leachate composition follows a path of Cl enrichment (a horizontal path in Fig. 1b). Even with CaSO4 precipitation, evaporative concentration still yields an MgCl2-rich solution rather than a NaCl-rich solution: modern seawater composition cannot be realized from Tagish Lake leachate. Analysis of leachate evolution demonstrates that processes other than leaching of carbonaceous chondrite-like materials are required to produce modern seawater composition. Numerous competing processes contributed to the salinity and composition of the ﬁrst permanent oceans, one being partial dissolution of impactors including carbonaceous chondrites prior to 3.7 Ga. Weathering of the crust, marine sedimentation and hydrothermal alteration also made contributions prior to 3.7 Ga but they assumed dominance as rates of input from captured bolides, dust and comets subsided (Garrels and Mackenzie, 1971; Holland, 1984; Berner and Berner, 1996). Seawater compositional evolution may therefore be divided into two periods: a “preuniformitarian” period prior to about 3.7 Ga, where captured materials dominated solute content, that transitioned into a conventional “uniformitarian” period where weathering of crust dominated over a timescale dictated by the residence times of solutes, reaching a modern seawater composition by ~ 3.3–3.0 Ga. 4. Toward modern seawater composition Crustal composition was different from modern crust and evolved between 3.7 and about 3.2 Ga. For example, ultramaﬁc lavas (komatiites) were produced on the Rhodesian craton during 3.5 and 2.7 Ga (Arndt et al., 2008). The oldest recorded continental shelf-type sedimentary rocks are only about 3.3 to 3.0 Ga (Fedo et al., 1996). Because weathering of crustal rocks is a major contributor of solutes to the oceans, seawater composition has evolved in conjunction with the composition of exposed crust. This implies that the present “quasi” steady state composition of seawater was not realized until 3.3 to 3.0 Ga when, in the absence of major contributions from meteorites and comets, there was signiﬁcant compositional change to seawater. This argument is supported by the study of de Ronde et al. (1997) who found that Br/Cl and I/Cl ratios of hydrothermal ﬂuids trapped in 3.2 Ga old rocks were appreciably greater than for modern seawater. The Br/Cl value of leachate from pristine Tagish Lake meteorite is about 10−2, whereas modern seawater is near 1 × 10−6. de Ronde et al. (1997)
measured a Br/Cl ratio of 2 × 10−3 for 3.2 Ga “seawater”, a value intermediate between modern seawater and our leachates. Apparently Br/Cl had not yet achieved its modern ratio in seawater or its derivatives at 3.2 Ga. They also found that a mixing component of these hydrothermal solutions was somewhat more saline than modern seawater but was NaCl-rich. If this component is representative of 3.2 Ga seawater or its hydrothermally altered equivalent, then the major element content of seawater had approached modern seawater composition by about 3.2 Ga. It had not, however, achieved steady state in all constituents. If these ﬁndings and those of de Ronde et al. (1997) are accepted, seawater composition evolved to near-modern composition between about 3.7 and 3.0 Ga. Magnesium, Ca, K and SO4, derived from dissolution of meteorites, were likely more abundant in 3.7 Ga seawater than in modern seawater, and during evolution, these constituents were preferentially removed from the oceans while Na and Cl were preferentially enriched. Magnesium, Ca and K were likely incorporated into sedimentary and hydrothermal phases (e.g., phyllosilicates) at a rate greater than their supply from weathering or hydrothermal processes, thus their concentrations decreased in evolving seawater. Weathering of the crust, especially feldspar weathering, is likely responsible for Na enrichment. Continual and prolonged degassing of the Earth probably accounts for much of the Cl enrichment (over the amount originally supplied by meteorites). The fate of SO4 is more ambiguous but at least some probably was precipitated in sedimentary and hydrothermal systems as gypsum, anhydrite and barite, or was reduced in such systems to form sulﬁde minerals. The leachates of the Tagish Lake meteorite indicate neither an extremely reduced nor oxidized primitive ocean. Sulfate is the dominant anion of the leachates, thus a marine sulfate–sulﬁde buffer would inhibit oxidation potentials from decreasing to exceptionally low values (e.g., Garrels and Mackenzie, 1971). Similarly, Fe2+ should inhibit strongly oxidizing conditions by consuming oxidants as it is oxidized to Fe3+ (and probably precipitated). Neither is there indication of strong acid or base in these leachates, indicating that the ﬁrst permanent oceans were not made either strongly acidic or basic by input from meteoritic materials. 5. Implications for evolution of life Prebiotic organic compounds, including amino acids, exist widely on bodies through the solar system, but occur in particular abundance in carbonaceous chondrites including Tagish Lake (e.g., Pizzarello et al., 2001; Nakamura-Messenger et al., 2006; Garvie and Buseck, 2007). The signiﬁcance of the presence of such biological “building blocks” in primitive meteorites has long been recognized (e.g., Chyba, 1990). The importance of the inorganic soluble components delivered to early Earth (and Mars) by carbonaceous chondrites has received less attention. Schopf (1992) suggests the high inorganic synthesis rates likely in the primitive oceans of Earth would have led to a “relatively concentrated prebiotic soup” (Miller, 1992). Although life may have emerged and subsequently been destroyed at various times during the late heavy bombardment period, the oldest accepted evidence of life appears as fossil ﬁlaments in 3.5 Ga rocks of the Apex Chert, Western Australia; these are so remarkably similar to the single celled cyanobacteria of modern time that they are commonly so identiﬁed (Schopf, 1992); although this interpretation has been questioned (Brasier et al., 2002). Cyanobacteria are credited with being the ﬁrst organisms capable of oxygenic photosynthesis. Thus, with a remarkably short window of ~200–300 Ma, reasonably complex life forms developed in the ancient oceans. However, from that period to approximately 2.0–1.8 Ga, when an oxygen-rich atmosphere developed, further evolutionary advances were slow. On the basis of research which showed a strong correlation between biological diversity and salinity (Hughes Clarke and Keji, 1973), Knauth (1998) suggested that high salinity of primitive oceans could have been an impediment to the development of advanced life forms. A further
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hindrance was very likely the slow development of an oxygen-rich environment. During the time between the emergence of oxygenic photosynthesis and the rise of atmospheric oxygen, the strongest oxygen sink is thought to be reaction with dissolved ferrous iron to form banded iron formations. However, high salinity water is also an impediment to dissolved oxygen. If life had emerged by at least 3.5 Ga, and if the composition of seawater had not evolved to a modern NaCl dominant state, the waters in which life emerged and developed on Earth (and possibly Mars) were strongly inﬂuenced by Mg–SO4 rich carbonaceous chondrite leachates similar to those described in this study.
6. Implications for other planetary bodies The preceding discussion of the implications of the compositions and evolution of Tagish Lake leachates for terrestrial oceans is equally relevant (if not more so) to the evolution of primitive solutions or brines on other solar system bodies. In the context of late-veneer accretionary models of planet formation, carbonaceous chondrites are expected to contribute a signiﬁcant proportion of the volatiles and soluble components to all bodies in the solar system. Therefore, leachates of carbonaceous chondrite material would have contributed signiﬁcantly to the volatile and soluble component contents of the other terrestrial planets, carbonaceous asteroids, and icy satellites. Mg-sulfates have been directly observed on Mars (e.g., Wang et al., 2006) and their presence on Mars has been predicted based upon geochemical considerations (e.g., King et al., 2004). Reﬂectance spectroscopy of Europa has indicated the possible presence of Mg and Na sulfate salts and/or ices (Fanale et al., 2001). On bodies where the contribution to total volatile contents from outgassing, and the effects of active hydrologic and tectonic cycles are less than on Earth, the inﬂuence of primordial leachate-derived solutions should be proportionately larger. The results presented here apply to the initial composition and subsequent evolution of ﬂuid reservoirs on any solar system body that derived a signiﬁcant proportion of its initial solvent and solute inventory from carbonaceous chondrites. This includes the other terrestrial planets (notably Mars); the icy satellites of the outer solar system (Europa, Ganymede, Enceladus, etc); and carbonaceous chondrite parent asteroids (including the C, D, and related asteroids).
Acknowledgements This manuscript was much improved by insightful comments of two anonymous reviewers. We extend our thanks to R. W. Carlson for editorial handling. We also thank R.L. Flemming, P.J.A. McCausland, P.L. King, P.G. Brown, A.R. Hildebrand, S.D.J. Russell, C.D.K. Herd, R. Herd, and A. Blinova for the numerous valuable discussions.
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