Types Of Chondrites Research Articles

Determination of the petrologic type of CV3 chondrites by Raman spectroscopy of included organic matter

This paper reports the first reliable quantitative determination of the thermal metamorphism grade of a series of nine CV3 chondrites: Allende, Axtell, Bali, Mokoia, Grosnaja, Efremovka, Vigarano, Leoville, and Kaba. The maturity of the organic matter in matrix, determined by Raman spectroscopy, has been used as a powerful metamorphic tracer, independent of the mineralogical context and extent of aqueous alteration. This tracer has been used along with other metamorphic tracers such as Fe zoning in type-I chondrules of olivine phenocrysts, presolar grain abundance and noble gas abundance (bulk and P3 component). The study shows that the petrologic types determined earlier by Induced ThermoLuminescence were underestimated and suggests the following values: PT (Allende-Axtell) >3.6; PT (Bali-Mokoia-Grosnaja) ∼3.6; PT (Efremovka-Leoville-Vigarano) = 3.1–3.4; PT (Kaba) ∼3.1. The most commonly studied CV3, Allende, is also the most metamorphosed. Bali is a breccia containing clasts of different petrologic types. The attribution suggested by this study is that of clasts of the highest petrologic types, as pointed out by IOM maturity and noble gas bulk abundance. CV3 chondrites have complex asteroidal backgrounds, with various degrees of aqueous alteration and/or thermal metamorphism leading to complex mineralogical and petrologic patterns. (Fe,Mg) chemical zoning in olivine phenocrysts, on the borders of type I chondrules of porphyritic olivine- and pyroxene-rich textural types, has been found to correlate with the metamorphism grade. This suggests that chemical zoning in some chondrules, often interpreted as exchanges between chondrules and nebular gas, may well have an asteroidal origin. Furthermore, the compositional range of olivine matrix is controlled both by thermal metamorphism and aqueous alteration. This does not support evidence of a nebular origin and does not necessarily mirror the metamorphism grade through (Fe,Mg) equilibration. On the other hand, it may provide clues on the degree of aqueous alteration vs. thermal metamorphism and on the timing of both processes. In particular, Mokoia experienced significant aqueous alteration after the metamorphism peak, whereas Grosnaja, which has similar metamorphism grade, did not.

The nature of the KT impactor. A 54Cr reappraisal

According to the variations observed in the various meteorite classes, 54Cr represents an isotopic tool for planetary body discrimination. In the search for the nature of the Cretaceous-Tertiary (KT) impactor, Cr isotopic measurements were already performed at KT boundary. The current work is aimed to modernize Cr isotopic data with high-precision measurements to confirm a carbonaceous chondrite type infall, the contribution of which is also estimated. Isotopic signatures of two marine clays (Caravaca, Stevens Klint) exhibit an isotopic ratio which would represent a mixing of a carbonaceous chondrite of CM2 type with terrestrial material in a ratio 6% to 19%. A single impactor may account for both marine and continental Cr isotopic signatures.

Simulations of self-magnetization in dusty space plasmas comparing different charge polarities and charge numbers of the dust

In partially ionized dusty space plasmas collisional momentum transfer between neutral and charged components of different inertia yields significant self-induced magnetic fields on remarkable short time scales of several dust Alfven times. Considering the proto-solar accretion disk previous first self-consistent plasma–neutral gas–dust simulations have shown, that this process yields a self-magnetization of 10 - 5 – 10 - 3 T on time scales of about 100 days. Thus, this mechanism is able to explain the remanent magnetization of chondrite type meteorite matter. New simulations show the quantitative dependence of the self-magnetization on polarity and charge numbers of the dust grains.

Origin of chondritic forsterite grains

Iron-poor and refractory lithophile element (RLE) rich forsterite grains occur in all major types of unequilibrated chondrites. In our laser ablation inductively coupled mass spectrometry (LA-ICPMS) minor and trace element study we show that refractory forsterites (RF) from carbonaceous (CC), unequilibrated ordinary (UOC) and a Rumuruti chondrite (RC) have similar chemical compositions with high RLE concentrations and low concentrations of Mn, Fe, Co and Ni. Fractionation of RLEs and rare earth elements (REEs) is in agreement with formation by crystallization from a RLE rich silicate melt. Low concentrations and the fractionation of moderately siderophile elements (Fe, Co, Ni) in RFs suggests formation at low oxygen fugacity, possibly in equilibrium with primitive Fe,Ni metal condensates in a gas of solar composition. Anomalously high Ti in the parental melt can be explained by Ti 3+/Ti 4+ ∼1.5, supporting formation of RF in highly reducing conditions. Low Mn concentrations indicate formation at high temperatures (>∼1160 K). The model of formation of RFs and the accompanying physico-chemical conditions during their formation as well as their relation to non refractory olivine are discussed.

Evolution of Oxygen Isotopic Composition in the Inner Solar Nebula

Changes in the chemical and isotopic composition of the solar nebula with time are reflected in the properties of different constituents that are preserved in chondritic meteorites. CR-group carbonaceous chondrites are among the most primitive of all chondrite types and must have preserved solar nebula records largely unchanged. We have analyzed the oxygen and magnesium isotopes in a range of the CR constituents of different formation temperatures and ages, including refractory inclusions and chondrules of various types. The results provide new constraints on the time variation of the oxygen isotopic composition of the inner (<5 AU) solar nebula—the region where refractory inclusions and chondrules most likely formed. A chronology based on the decay of short-lived 26 Al (t1=2 � 0:73 Myr) indicates that the inner solar nebula gas was 16 O-rich when refractory inclusions formed, but less than 0.8 Myr later, gas in the inner solar nebula became 16 O-poor, and this state persisted at least until CR chondrules formed � 1‐2 Myr later. We suggest that the inner solar nebula became 16 O-poor because meter-sized icy bodies, which were enriched in 17 Oa nd 18 O as a result of isotopic self-shielding during the ultraviolet photodissociation of CO in the protosolar molecular cloud or protoplanetary disk, agglomerated outside the snow line, drifted rapidly toward the Sun, and evaporated at the snow line. This led to significant enrichment in 16 O-depleted water,whichthenspreadthroughtheinnersolarsystem.Astronomicalstudiesofthespatialandtemporalvariations of water abundance in protoplanetary disks may clarify these processes. Subject headingg meteors, meteoroids — nuclear reactions, nucleosynthesis, abundances — planetary systems: formation — solar system: formation

Self-magnetization of the early Solar System matter

The remnant magnetization of chondrite type meteorite matter indicates the existence of 10 −5–10 −3 T magnetic fields in the early Solar System accretion disk. Taking into account parameter regimes being typical for this evolutionary stage of Sun and planets we consider the protosolar disk matter as partially ionized dusty plasma consisting of massive charged dust grains, neutral gas, electrons and ions. Results of systematic multifluid neutral gas–plasma–dust simulations show that shear flow driven collisional interactions yield a self-magnetization of the early Solar System matter which is able to explain the measured remnant magnetization of meteorite material.

Infrared spectroscopic taxonomy for carbonaceous chondrites from speciation of hydrous components

Abstract— Mid‐infrared absorption spectra for all types of carbonaceous chondrites were obtained in this study to establish a versatile method for spectroscopic classification of carbonaceous chondrites. Infrared spectra were measured using a conventional KBr pellet method and diamond press method. Spectra of hydrous carbonaceous chondrites exhibit intense O‐H stretching vibrations. CI chondrites are identifiable by a characteristic sharp absorption band appearing at 3685 cm−1, which is mainly attributable to serpentine. X‐ray diffraction analysis showed the presence of serpentine. However, Yamato (Y‐) 82162 (C1) does not have the band at 3685 cm−1 because of its thermal metamorphism. CM and CR chondrites have an intense absorption band at approximately 3600 cm−1. This absorption tends to appear in CM chondrites more strongly than CR chondrites because the intensity ratios of an OH stretching mode at 3520 cm−1 compared to 3400 cm−1 for CM chondrites are in the range of 0.95–1.04, which is systematically higher than those of CR chondrites (0.86–0.88). Therefore, the two types of chondrites are distinguishable by their respective infrared spectra. The spectrum feature of the Tagish Lake meteorite is attributable to neither CI nor CM chondrites. CO chondrites are characterized by weak and broad absorption at 3400 cm−1. CV chondrites have weak or negligible absorption of water. CK chondrites also have no water‐induced absorption. CH and CB chondrites have a sharp absorption at 3692 cm−1 indicating the presence of chrysotile, which is also supported by observations of X‐ray diffraction and TEM. The combination of spectroscopic classification and the diamond press method allows classification of carbonaceous chondrites of very valuable samples with small quantities. As one example, carbonaceous chondrite clasts in brecciated meteorites were classified using our technique. Infrared spectra for a fragment of carbonaceous clasts (&lt;1 μg) separated from Willard (b) and Tsukuba were measured. The 3685 cm−1 band found in CI chondrites was clearly detected in the clasts, indicating that they are CI‐like clasts.

SIMS analyses of Mg, Cr, and Ni isotopes in primitive meteorites and short-lived radionuclides in the early solar system

SIMS analyses of 26 Al – 26 Mg , 60 Fe – 60 Ni , and 53 Mn – 53 Cr systems in unequilibrated enstatite chondrites provide evidence for the former existence of 26 Al , 60 Fe , and 53 Mn in this highly reduced meteorite group, suggesting the widespread of these short-lived radionuclides in the early solar system. Calcium–aluminum-rich inclusions and Al-rich chondrules in enstatite chondrites show 26 Al – 26 Mg characteristics similar to their counterparts in other types of chondrites. Assuming its homogenous distribution, fine-scale 26 Al relative chronology can be obtained for different components in meteorites. Well-defined isochrons yield initial 60 Fe / 56 Fe ratios of (0.3−1)×10 −6 in sulfides from enstatite chondrites. The former presence of 60 Fe suggests a stellar source and its possible high abundance points to a supernova origin. 26 Al and 60 Fe could be important heat sources for the early planetary processes. The large variations of initial 60 Fe / 56 Fe and 53 Mn / 55 Mn ratios and the discordance between 60 Fe – 60 Ni and 53 Mn – 53 Cr systems observed in sulfides indicate later disturbance of the two isotopic systems, and therefore, bear no strict chronological significance. SIMS isotopic analysis can be utilized to attain important information about the early evolution of the solar system.

FTIR and Raman analyses of the Tagish Lake meteorite: Relationship with the aliphatic hydrocarbons observed in the Diffuse Interstellar Medium

Using FTIR and Raman microspectroscopies we have analysed 6 fragments of the Tagish Lake meteorite. The data obtained show that all the fragments belong to the carbonate-rich lithology, where an organic material, including a highly disordered macromolecular carbonaceous component is found. The FTIR approach shows that part of the organic material present in Tagish Lake is aliphatic. The Raman approach shows that there is also highly disordered polyaromatic organic material, which is abundant. Furthermore, the comparison of Raman data of Tagish Lake to other carbonaceous chondrites (CI, CM2, CR2) shows that the carbon in Tagish Lake is different, supporting the assertion that this meteorite is a unique and new type of carbonaceous chondrite. The comparison of the aliphatic hydrocarbon FTIR data found in the Tagish Lake meteorite with the aliphatic hydrocarbon IR data of the carbonaceous chondrites Orgueil and Murchison and with the diffuse Interstellar Medium (ISM) shows that they are different, in that the Tagish Lake meteorite has longer aliphatic chains.

Petrographic and oxygen-isotopic study of refractory forsterites from R-chondrite Dar al Gani 013 (R3.5–6), unequilibrated ordinary and carbonaceous chondrites

We have conducted petrographic, chemical and in-situ oxygen isotopic studies of refractory forsterites from unequilibrated ordinary and carbonaceous chondrites as well as an unequilibrated R-chondrite. Refractory forsterites occur in all types of unequilibrated chondrites and all have very similar chemical composition with low FeO and high refractory lithophile element (RLE) contents. Refractory forsterites are typically enriched in 16O relative to ‘normal’ olivine independent of the bulk O-isotope ratios of the parent meteorites. Analyses of refractory forsterites spread along a Δ 17O mixing line with Δ 17O ranging from +2 to −10‰. Due to similarities in oxygen isotopes and chemical compositions, we conclude that refractory forsterites of various types of chondrites come from a single common reservoir. Implications of this hypothesis for the chemical and O-isotope evolution of silicates in the early solar nebular are discussed.

Formation of metal in Grosvenor Mountains 95551 and comparison to ordinary chondrites

Siderophile element abundances in individual metal grains in the ungrouped chondrite Grosvenor Mountains (GRO) 95551 and in the ordinary chondrites Tieschitz H3.6, Soko-Banja LL4, and Allegan H5 were measured with laser ablation–inductively coupled plasma mass spectrometry. Matrix metal in GRO 95551 falls into two distinct compositional groups, a high-Ni group with 7.2 ± 0.4 wt% Ni and a low-Ni group with 3.7 ± 0.1 wt% Ni, indicating that kamacite/taenite equilibration at ∼1020 K was followed by rapid cooling. The nonrefractory siderophile elements P, Co, Cu, Ga, Ge, As, Pd, and Au also partition between the high-Ni and low-Ni metal in a manner consistent with kamacite/taenite fractionation, but the refractory siderophiles Ru, Re, Os, Ir, and Pt show correlated variations that are unrelated to kamacite/taenite partitioning and indicate that variations in refractory components of the metal were not completely erased during equilibration at ∼1020 K. The Ni-normalized bulk metal composition of GRO 95551 is refractory depleted and volatile rich relative to Bencubbin and related metal-rich chondrites but bears strong similarities to equilibrated ordinary chondrite metal. GRO 95551 represents a new chondrite type with chemical affinity to the ordinary chondrites. Individual metal grains in unequlibrated ordinary chondrites also have correlated variations in refractory siderophile contents that cannot be produced by redox processes alone; these variations span three orders of magnitude and diminish with increasing metamorphic grade of the ordinary chondrites.

Carbonates in CM2 chondrites: constraints on alteration conditions from oxygen isotopic compositions and petrographic observations

High-precision measurements of the oxygen isotopic compositions of carbonates (calcite and dolomite) from five CM2 chondrites are presented and put into context of the previously determined mineralogic alteration index (MAI), which places these meteorites into an alteration sequence. The carbonate oxygen isotopic compositions range from +20.0 to +35.7‰ for δ 18O, +8.0 to +17.7‰ for δ 17O, and −0.7 to −2.7‰ for Δ 17O. Carbonate Δ 17O values are inversely correlated with MAI and track the evolution of fluid composition from higher to lower Δ 17O values with increasing alteration on the CM parent body. Similar Δ 17O values for calcite and dolomite fractions from the same splits of the same meteorites indicate that calcite and dolomite in each split precipitated from a single fluid reservoir. However, reversed calcite dolomite fractionations (δ 18O dol − δ 18O cc) indicate that the fluid was subject to processes, such as freeze–thaw or evaporation, that fractionated isotopes in a mass-dependent way. Consideration of the carbonate isotopic data in the context of previously proposed models for aqueous alteration of carbonaceous chondrites has provided important insights into both the evolving alteration conditions and the utility of the models themselves. The data as a whole indicate that the isotopic evolution of the fluid was similar to that predicted by the closed-system, two-reservoir models, but that a slightly larger matrix–water fractionation factor may apply. In the context of this model, more altered samples largely reflect greater reaction progress and thus probably indicate more extended times of fluid exposure. Petrographic observations of carbonates reveal a trend of variable carbonate morphology correlated with alteration that is also consistent with changes in the duration of fluid–rock interaction. The data can also be reconciled with fluid-flow models in a restricted region of the parent body, which is consistent with assertions that the different types of carbonaceous chondrites derive from different regions of their parent bodies. In this case, the model results for a 9-km-radius body, and our data place the location of the CM chondrite formation in a 100-m-thick zone 1 km from the surface. The size of this zone could be increased if the model parameters were adjusted.

Early Archean spherule beds: Chromium isotopes confirm origin through multiple impacts of projectiles of carbonaceous chondrite type

Three Early Archean spherule beds from Barberton, South Africa, have anomalous Cr isotope compositions in addition to large Ir anomalies, confirming the presence of meteoritic material with a composition similar to that in carbonaceous chondrites. The extra-terrestrial components in beds S2, S3, and S4 are estimated to be approx. l%, 50% - 60%, and 15% - 30%, respectively. These beds are probably the distal, and possibly global, ejecta from major large-body impacts. These impacts were probably much larger than the Cretaceous-Tertiary event, and all occurred over an interval of approx. 20 m.y., implying an impactor flux at 3.2 Ga that was more than an order of magnitude greater than the present flux.

Early Archean spherule beds: Chromium isotopes confirm origin through multiple impacts of projectiles of carbonaceous chondrite type: Comment and Reply

[Kyte et al. (2003)][1] and [Lowe et al. (2003)][2] are to be congratulated on their study of Early to Middle Archean impact fallout units in the Barberton greenstone belt, South Africa, representing a major advance in the understanding of the nature of early Precambrian Earth. Here I point to some

Early Archean spherule beds: Chromium isotopes confirm origin through multiple impacts of projectiles of carbonaceous chondrite type: Comment and Reply

This is a exchange in the form of a comment and a reply in regards to an earlier article. The authors of the original article, consider it likely that virtually all of the projectile will condense with the silicate fraction, resulting in very little platinum group element fractionation in the final ejecta deposit. Further, we find no evidence in the commentator's, (i.e., Glikson), comment to support vapor fractionation. We note that the Pd/Ir ratios of published data on 2.56 Ga Hamersley Basin spherules are all greater than in chondrites, contrary to the assertion by Glikson. This is consistent with relatively high Pd concentrations (and Pd/Ir ratios) in crustal rocks.

A multi-isotopic and trace element investigation of the Cretaceous–Tertiary boundary layer at Stevns Klint, Denmark – inferences for the origin and nature of siderophile and lithophile element geochemical anomalies

Os, Sr, Nd and Pb isotope data were collected from a profile across the Cretaceous–Tertiary (K–T) boundary layer at Stevns Klint, Denmark. ϵ Nd [ T=65 Ma] values from within the boundary layer (Fish Clay) are lower by ∼1 ϵ unit than those of the underlying Maastrichtian limestone and the overlying Danian chalk sequences. Systematic profile-upward changes of Pb, Sr and Os isotopic compositions and concentrations in the boundary layer cannot be accounted for by in situ growth of daughter products since the sedimentation of the Fish Clay. While Os, Nd and Pb isotopes indicate the admixing of less radiogenic components to the Fish Clay, Sr isotopes show elevated radiogenic values in the boundary layer, relative to the carbonate sequences beneath and above it. The sudden change in lithophile (e.g., Sr, Pb and Nd) isotope compositions at the base of the Fish Clay and profile-upward trends of 87Sr/ 86Sr and 206Pb/ 204Pb ratios towards those of the overlying Danian chalk are interpreted to reflect recovery from enhanced, acid rain-induced continental (local?) weathering input to the seawater. However, a continental crustal source is invalid for the siderophile element Os. In the light of evidence from chromium isotopes for a cosmic origin of the platinum group elements (PGEs) and certain moderately siderophile elements (Cr, Ni, Co, V) in K–T boundary sediments, including Stevns Klint [Shukolyukov and Lugmair, Science 282 (1998) 927–929], and supported by the finding of projectile debris [Bauluz et al., Earth Planet. Sci. Lett. 182 (2000) 127–136] and the occurrence of abundant Ni-rich spinel at many K–T sites [Robin et al., Nature 363 (1993) 615–617; Kyte, Nature 396 (1998) 237–239], we favor to explain the sudden drop of 187Os/ 188Os ratios from 0.210 to 0.160 at the K–T boundary to derive from global fall-out of extraterrestrial matter. The present 186Os/ 188Os ratio of 0.119836±0.000004 measured in the basal layer of the Fish Clay is within the uncertainty a chondritic value. We therefore exclude the possibility of a major contribution of PGEs to the sediment from iron meteorites. Chondrite-normalized (Ru/Ir) N ratios of ∼0.95±0.14 and (Os/Ir) N ratios of ∼0.93±0.14 in the Fish Clay cannot distinguish between abundance ratios of different types of chondrites, and strongly sub-chondritic (Pt/Ir) N ratios of ∼0.62±0.09 (2σ) suggest differential PGE remobilization through the sedimentary column (and consequently the alteration of inter-element ratios). PGEs and the moderately siderophile elements Cr, Ni, V, and Co form an elemental association with systematically upward-decreasing concentrations in the Fish Clay. Low Co/Ni ratios of ∼0.12 in the Fish Clay relative to values of ∼0.35 in the over- and underlying carbonate sequences support mixing of meteorite-derived (Co/Ni ∼0.05) and terrestrial upper mantle/crustal (Co/Ni >∼0.3) sources. While lithophile element isotope data indicate an increased continental crustal input to the Fish Clay at the K–T transition, the uncertainty with respect to possible post-depositional alteration of abundance patterns of siderophile and moderately siderophile elements – though not affecting the chondritic isotopic composition of Os – does not allow confirmation of indications from chromium isotopes for a carbonaceous (CV-type) meteorite as the preferred K–T impactor type by Shukolyukov and Lugmair [Science 282 (1998) 927–929].

443 Eros: Problems with the meteorite magnetism record in attempting an asteroid match

Abstract— The magnetometer experiment (MAG) onboard the Near‐Earth Asteroid Rendezvous (NEAR)‐Shoemaker spacecraft detected no global scale magnetization and established a maximum magnetization of 2.1 times 10−6 Am2 kg−1 for asteroid 433 Eros. This is in sharp contrast with the estimated magnetization of other S‐class asteroids (Gaspra, ˜2.4 times 10−2 Am2 kg−1; Braille, ˜2.8 times 10−2 Am2 kg−1) and is below published values for all types of ordinary chondrites. This includes the L/LL types considered to most closely match 433 Eros based on preliminary interpretations of NEAR remote geochemical experiments.The ordinary chondrite meteorite magnetization intensity data was reviewed in order to assess the reasonableness of an asteroid‐meteorite match based on magnetic property measurements. Natural remanent magnetization (NRM) intensities for the ordinary chondrite meteorites show at least a 2 order of magnitude range within each of the H, L, and LL groups, all well above the 2.1 times 10−6 Am2 kg−1 level for 433 Eros. The REM values (ratio of the NRM to the SIRM (saturation remanent magnetization)) range over 3 orders of magnitude for all chondrite groups indicating no clear relationship between NRM and the amount of magnetic material. Levels of magnetic noise in chondrite meteorites can be as much as 70% or more of the NRM. Consequently, published values of the NRM should be considered suspect unless careful evaluation of the noise sources is done. NASA Goddard SFC studies of per unit mass intensities in large (&gt;10 000 g) and small (down to &lt;1 g) samples from the same meteorite demonstrate magnetic intensity decreases as size increases. This would appear to be explained by demagnetization due to magnetic vector randomness at unknown scale sizes in the larger samples. This would then argue for some level of demagnetization of large objects such as an asteroid. The possibility that 433 Eros is an LL chondrite cannot be discounted.

Noble gases in enstatite chondrites II: The trapped component

Abstract— The trapped noble gas record of 57 enstatite chondrites (E chondrites) has been investigated. Basically, two different gas patterns have been identified dependent on the petrologic type. All E chondrites of type 4 to 6 show a mixture of trapped common chondritic rare gases (Q) and a subsolar component (range of elemental ratios for E4–6 chondrites: 36Ar/132Xe = 582 ± 270 and 36Ar/84Kr = 242 ± 88). E3 chondrites usually contain Q gases, but also a composition with lower 36Ar/132Xe and 36Ar/84Kr ratios, which we call sub‐Q (36Ar/132Xe = 37.0 ± 18.0 and 36Ar/84Kr = 41.7 ± 18.1). The presence of either the subsolar or the sub‐Q signature in particular petrologic types cannot be readily explained by parent body metamorphism as postulated for ordinary chondrites. We therefore present a different model that can explain the bimodal distribution and composition of trapped heavy noble gases in E chondrites.Trapped solar noble gases have been observed only in some E3 chondrites. About 30% of each group, EH3 and EL3 chondrites, amounting to 9% of all analyzed E chondrites show the solar signature. Notably, only one of those meteorites has been explicitly described as a regolith breccia.

Composition and stability of salts on the surface of Europa and their oceanic origin

We present theoretical models of the composition, the relative abundances, and the stability of hydrated salts on the surface and in the icy shell of Jupiter's satellite Europa and discuss whether those salts have an oceanic origin. The evaluations were done with thermodynamic calculations of (1) salt dehydration equilibria at the conditions of the surface of Europa and its icy shell, (2) chemical equilibria involving solids and water vapor in the Na‐K‐Mg‐Ca‐S‐Cl‐H2O system at surface temperatures and variable partial pressures of water vapor, and (3) changes in aquatic chemistry and sequences of salt precipitation from freezing oceanic water, using cosmochemical, mass balance, and physical‐chemical constraints on the elemental and ionic composition of the ocean. Mass balance calculations of total or partial extraction of elements into an ocean from a carbonaceous chondrite type mantle show that magnesium and sulfate rather than chloride and sodium could be the most abundant solutes in the ocean. Freezing oceanic water of this composition leads to brines that differ in composition from the original water and to deposition of ice and highly hydrated sulfates of Mg, Na, and Ca as well as alkali chlorides. After freezing is complete, highly hydrated salts remain stable in ice‐bearing surface materials and throughout the icy shell. For hypothetical surface salt lag deposits, formed through sublimation/sputtering of ice and dehydration of salts, we predict hydration stratification with depth, approaching the highest hydration states in ice‐bearing materials in the lowest parts of the deposits. We discuss the effects of fast disequilibrium freezing and variable dehydration rates of salts on the predicted mineral assemblages at the surface. All of our models, which are independent of observations, predict the predominance of Mg and Na sulfates in surface salts, in agreement with spectroscopic models for the nonicy surface material in the near infrared spectral region.

The Tagish Lake meteorite: a possible sample from a D-type asteroid.

A new type of carbonaceous chondrite, the Tagish Lake meteorite, exhibits a reflectance spectrum similar to spectra observed from the D-type asteroids, which are relatively abundant in the outer solar system beyond the main asteroid belt and have been inferred to be more primitive than any known meteorite. Until the Tagish Lake fall, these asteroids had no analog in the meteorite collections. The Tagish Lake meteorite is a carbon-rich (4 to 5 weight %), aqueously altered carbonaceous chondrite and contains high concentrations of presolar grains and carbonate minerals, which is consistent with the expectation that the D-type asteroids were originally made of primitive materials and did not experience any extensive heating.