Petrology and geochemistry of the fine‐grained,unbrecciated diogenite Northwest Africa 4215

Abstract— We report on the petrology and geochemistry of Northwest Africa (NWA) 4215, an unbrecciated diogenite recovered in the Sahara. This single stone, weighing 46.4 g, displays a wellpreserved cumulative texture. It consists of zoned xenomorphic orthopyroxene grains on the order of 500 μm in size, along with a few large chromite crystals (<5 vol%, up to 3 mm). Accessory olivine and scarce diopside grains occur within the groundmass, usually around the chromite crystals. Minor phases are cristobalite, troilite, and metal. Unlike other diogenites, orthopyroxenes (En_76.2Wo_1.1Fs_22.7 to En_68.6Wo_5.5Fs_25.9), olivines (Fo₇₆ to Fo₇₁), and chromites (Mg# = 14.3 44.0, Cr# = 42.2–86.5) are chemically zoned. The minor element behavior in orthopyroxenes and the intricate chemical profiles obtained in chromites indicate that the zonings do not mirror the evolution of the parental melt. We suggest that they resulted from reaction of the crystals with intercumulus melt. In order to preserve the observed zoning profiles, NWA 4215 clearly cooled significantly faster than other diogenites. Indeed, the cooling rate determined from the diffusion of Cr in olivine abutting chromite is in the order of 10–50 °C/a, suggesting that NWA 4215 formed within a small, shallow intrusion. The bulk composition of NWA 4215 has been determined for major and trace elements. This meteorite is weathered and its fractures are filled with calcite, limonite, and gypsum, typical of hot desert alteration. In particular, the FeO, CaO abundances and most of the trace element concentrations (Sr, Ba, Pb, and REE among others) are high and indicate a significant contribution from the secondary minerals. To remove the terrestrial contribution, we have leached with HCl a subsample of the meteorite. The residue, made essentially of orthopyroxene and chromite, has similar major and trace element abundances to diogenites as shown by the shape of its REE pattern or by its high Al/Ga ratio. The connection of NWA 4215 with diogenites is confirmed by its O‐isotopic composition (δ¹⁷O = 1.431 ± 0.102‰, δ¹⁸O = 3.203 ± 0.205‰, Δ¹⁷O = ?0.248 ± 0.005‰).     

The Northwest Africa 1500 meteorite: Not a ureilite,maybe a brachinite

Abstract– The Northwest Africa (NWA) 1500 meteorite is an olivine‐rich achondrite containing approximately 2–3 vol% augite, 1–2 vol% plagioclase, 1 vol% chromite, and minor orthopyroxene, Cl‐apatite, metal and sulfide. It was originally classified as a ureilite, but is currently ungrouped. We re‐examined the oxygen three‐isotope composition of NWA 1500. Results of ultra‐high precision (～0.03‰ for Δ¹⁷O) laser fluorination analyses of two bulk chips, and high precision (～0.3‰) secondary ion mass spectrometry (SIMS) analyses of olivine and plagioclase in a thin section, show that the oxygen isotope composition of NWA 1500 (Δ¹⁷O = ?0.22‰ from bulk samples and ?0.18 ± 0.06‰ from 16 mineral analyses) is within the range of brachinites. We compare petrologic and geochemical characteristics of NWA 1500 with those of brachinites and other olivine‐rich primitive achondrites, including new petrographic, mineral compositional and bulk compositional data for brachinites Hughes 026, Reid 013, NWA 5191, NWA 595, and Brachina. Modal mineral abundances, texture, olivine and pyroxene major and minor element compositions, plagioclase major element compositions, rare earth element abundances, and siderophile element abundances of NWA 1500 are within the range of those in brachinites and, in most cases, well distinguished from those of winonaites/IAB silicates, acapulcoites/lodranites, ureilites, and Divnoe. NWA 1500 shows evidence of internal reduction, in the form of reversely zoned olivine (Fo ～65–73 core to rim) and fine‐grained intergrowths of orthopyroxene + metal along olivine grain margins. The latter also occur in Reid 013, Hughes 026, NWA 5191, and NWA 595. We argue that reduction (olivine→enstatite + Fe⁰ + O₂) is the best hypothesis for their origin in these samples as well. We suggest that NWA 1500 should be classified as a brachinite, which has implications for the petrogenesis of brachinites. Fe‐Mn‐Mg compositions of brachinite olivine provide evidence of redox processes among bulk samples. NWA 1500 provides evidence for redox processes on a smaller scale as well, which supports the interpretation that these processes occurred in a parent body setting. SIMS data for ²⁶Al‐²⁶Mg isotopes in plagioclase in NWA 1500 show no ²⁶Mg excesses beyond analytical uncertainties (1–2‰). The calculated upper limit for the initial ²⁶Al/²⁷Al ratio of the plagioclase corresponds to an age younger than 7 Ma after CAI. Compared to ⁵³Mn‐⁵³Cr data for Brachina ( Wadhwa et al. 1998b ), this implies either a much younger formation age or a more protracted cooling history. However, Brachina is atypical and this comparison may not extend to other brachinites.     

Petrology of Martian meteorite Northwest Africa 998

Abstract— Nakhlite Northwest Africa (NWA) 998 is an augite-rich cumulate igneous rock with mineral compositions and oxygen isotopic composition consistent with an origin on Mars. This 456-gram, partially fusion-crusted meteorite consists of (by volume) ∼75% augite (core composition Wo₃₉En₃₉Fs₂₂), ∼9% olivine (Fo₃₅), ∼7% plagioclase (Ab₆₁An₃₅) as anhedra among augite and olivine, ∼3.5% low-calcium pyroxenes (pigeonite and orthopyroxene) replacing or forming overgrowths on olivine and augite, ∼1% titanomagnetite, and other phases including potassium feldspar, apatite, pyrrhotite, chalcopyrite, ilmenite, and fine-grained mesostasis material. Minor secondary alteration materials include “iddingsite” associated with olivine (probably Martian), calcite crack fillings, and iron oxide/hydroxide staining (both probably terrestrial). Shock effects are limited to minor cataclasis and twinning in augite. In comparison to other nakhlites, NWA 998 contains more low-calcium pyroxenes and its plagioclase crystals are blockier. The large size of the intercumulus feldspars and the chemical homogeneity of the olivine imply relatively slow cooling and chemical equilibration in the late- and post-igneous history of this specimen, and mineral thermometers give subsolidus temperatures near 730 °C. Oxidation state was near that of the QFM buffer, from about QFM-2 in earliest crystallization to near QFM in late crystallization, and to about QFM + 1.5 in some magmatic inclusions. The replacement or overgrowth of olivine by pigeonite and orthopyroxene (with or without titanomagnetite), and the marginal replacement of augite by pigeonite, are interpreted to result from late-stage reactions with residual melts (consistent with experimental phase equilibrium relationships). Apatite is concentrated in planar zones separating apatite-free domains, which suggests that residual magma (rich in P and REE) was concentrated in planar (fracture?) zones and possibly migrated through them. Loss of late magma through these zones is consistent with the low bulk REE content of NWA 998 compared with the calculated REE content of its parent magma.     

Trace elements in olivine and the petrogenesis of the intermediate,olivine‐phyric shergottite NWA 10170

Olivine‐phyric shergottites represent primitive basaltic to picritic rocks, spanning a large range of Mg# and olivine abundances. As primitive olivine‐bearing magmas are commonly representative of their mantle source on Earth, understanding the petrology and evolution of olivine‐phyric shergottites is critical in our understanding of Martian mantle compositions. We present data for the olivine‐phyric shergottite Northwest Africa (NWA) 10170 to constrain the petrology with specific implications for magma plumbing‐system dynamics. The calculated oxygen fugacity and bulk‐rock REE concentrations (based on modal abundance) are consistent with a geochemically intermediate classification for NWA 10170, and overall similarity with NWA 6234. In addition, we present trace element data using laser ablation ICP‐MS for coarse‐grained olivine cores, and compare these data with terrestrial and Martian data sets. The olivines in NWA 10170 contain cores with compositions of Fo₇₇ that evolve to rims with composition of Fo₅₈, and are characterized by cores with low Ni contents (400–600 ppm). Nickel is compatible in olivine and such low Ni content for olivine cores in NWA 10170 suggests either early‐stage fractionation and loss of olivine from the magma in a staging chamber at depth, or that Martian magmas have lower Ni than terrestrial magmas. We suggest that both are true in this case. Therefore, the magma does not represent a primary mantle melt, but rather has undergone 10–15% fractionation in a staging chamber prior to extrusion/intrusion at the surface of Mars. This further implies that careful evaluation of not only the Mg# but also the trace element concentrations of olivine needs to be conducted to evaluate pristine mantle melts versus those that have fractionated olivine (±pyroxene and oxide minerals) in staging chambers.     

MINERALOGY AND CHEMISTRY OF THE KYLE,TEXAS, CHONDRITE

The Kyle, Texas, U.S.A., chondrite was identified in 1965. Electron microprobe analyses and microscopic examination show the following mineralogy: olivine (Fa 26.2 mole %), orthopyroxene (Fs 21.0 mole %), clinopyroxene, plagioclase (An 10.3 mole %), chlorapatite, whitlockite, kamacite, taenite, troilite, chromite, and an iron-bearing terrestrial weathering product. Eutectic intergrowths of metaltroilite and a brecciated matrix indicate that the Kyle chondrite was shocked. Recrystallization and shock have obliterated chondrule-matrix boundaries. A chemical analysis of the meteorite shows the following results (in weight %): Fe 0.38, Ni 1.22, Co 0.05, FeS 5.98, SiO₂ 38.41, TiO₂ 0.11, Al₂O₃ 2.13, Cr₂O₃ 0.55, Fe₂O₃ 8.02, FeO 14.83, MnO 0.31, MgO 23.10, CaO 1.60, Na₂O 0.74, K₂O 0.08, P₂O₅ 0.19, H₂O⁺ 1.73, H₂O^? 0.37, C 0.03, Sum 99.83. On the basis of bulk chemistry, composition of olivine and orthopyroxene, and the recrystallized matrix, the Kyle meteorite is classified as an L6 chondrite.     

Igneous and shock processes affecting chassignite amphibole evaluated using chlorine/water partitioning and hydrogen isotopes

Amphibole in chassignite melt inclusions provides valuable information about the volatile content of the original interstitial magma, but also shock and postshock processes. We have analyzed amphibole and other phases from NWA 2737 melt inclusions, and we evaluate these data along with published values to constrain the crystallization Cl and H₂O content of phases in chassignite melt inclusions and the effects of shock on these amphibole grains. Using a model for the Cl/OH exchange between amphibole and melt, we estimate primary crystallization OH contents of chassignite amphiboles. SIMS analysis shows that amphibole from NWA 2737 currently has 0.15 wt% H₂O. It has lost ~0.6 wt% H₂O from an initial 0.7–0.8 wt% H₂O due to intense shock. Chassigny amphibole had on average 0.3–0.4 wt% H₂O and suffered little net loss of H₂O due to shock. NWA 2737 amphibole has δD ≈ +3700‰; it absorbed Martian atmosphere‐derived heavy H in the aftermath of shock. Chassigny amphibole, with δD ≤ +1900‰, incorporated less heavy H. Low H₂O/Cl ratios are inferred for the primitive chassignite magma, which had significant effects on melting and crystallization. Volatiles released by the degassing of Martian magma were more Cl‐rich than on Earth, resulting in the high Cl content of Martian surface materials.     

Oxygen isotope systematics of chondrule olivine,pyroxene, and plagioclase in one of the most pristine CV3Red chondrites (Northwest Africa 8613)

We performed in situ oxygen three‐isotope measurements of chondrule olivine, pyroxenes, and plagioclase from the newly described CV_Red chondrite NWA 8613. Additionally, oxygen isotope ratios of plagioclase in chondrules from the Kaba CV3_OxB chondrite were determined to enable comparisons of isotope ratios and degree of alteration of chondrules in both CV lithologies. NWA 8613 was affected by only mild thermal metamorphism. The majority of oxygen isotope ratios of olivine and pyroxenes plot along a slope‐1 line in the oxygen three‐isotope diagram, except for a type II and a remolten barred olivine chondrule. When isotopic relict olivine is excluded, olivine, and low‐ and high‐Ca pyroxenes are indistinguishable regarding Δ¹⁷O values. Conversely, plagioclase in chondrules from NWA 8613 and Kaba plot along mass‐dependent fractionation lines. Oxygen isotopic disequilibrium between phenocrysts and plagioclase was caused probably by exchange of plagioclase with ¹⁶O‐poor fluids on the CV parent body. Based on an existing oxygen isotope mass balance model, possible dust enrichment and ice enhancement factors were estimated. Type I chondrules from NWA 8613 possibly formed at moderately high dust enrichment factors (50× to 150× CI dust relative to solar abundances); estimates for water ice in the chondrule precursors range from 0.2× to 0.6× the nominal amount of ice in dust of CI composition. Findings agree with results from an earlier study on oxygen isotopes in chondrules of the Kaba CV chondrite, providing further evidence for a relatively dry and only moderately high dust‐enriched disk in the CV chondrule‐forming region.     

H and Cl isotope systematics of apatite in brecciated lunar meteorites Northwest Africa 4472, Northwest Africa 773, Sayh al Uhaymir 169, and Kalahari 009

We have investigated the H and Cl systematics in apatite from four brecciated lunar meteorites. In Northwest Africa (NWA) 4472, most of the apatites contain ～2000–6000 ppm H₂O with δD between ?200 and 0‰, except for one grain isolated in the matrix, which contains ～6000 ppm H₂O with δD of ～500–900‰. This low‐δD apatite contains ～2500–7500 ppm Cl associated with δ³⁷Cl of ～15–20‰, while the high‐δD grain contains ～2500 ppm Cl with δ³⁷Cl of ～7–15‰. In NWA 773, apatites in a first group contain ～700–2500 ppm H₂O with δD values averaging around ～0 ± 100‰, while apatites in a second group contain ～5500–16500 ppm H₂O with δD ～250 ± 50‰. In Sayh al Uhaymir (SaU) 169 and Kalahari (Kal) 009, apatites are similar in terms of their H₂O contents (～600–3000 ppm) and δD values (?100 to 200‰). In SaU 169, apatites contain ～6000–10,000 ppm Cl, characterized by δ³⁷Cl of ～5–12‰. Overall, most of the analyzed apatite grains have δD within the range reported for carbonaceous chondrites, similar to apatite analyzed in ancient (>3.9 Ga) lunar magmatic. One grain in NWA 4472 has H and Cl isotope compositions similar to apatite from mare basalts. With an age of 4.35 Ga, this grain could be a representative of the oldest known lunar volcanic activity. Finally, since numerous evolved clasts in NWA 773 formed through silicate liquid immiscibility, the apatite grains with extremely high H₂O contents, reaching pure hydroxylapatite composition, could provide insights into the effects of such process on the evolution of volatiles in lunar magmas.     

Hydrogen isotopic composition of the Martian mantle inferred from the newest Martian meteorite fall,Tissint

The hydrogen isotopic composition of planetary reservoirs can provide key constraints on the origin and history of water on planets. The sources of water and the hydrological evolution of Mars may be inferred from the hydrogen isotopic compositions of mineral phases in Martian meteorites, which are currently the only samples of Mars available for Earth‐based laboratory investigations. Previous studies have shown that δD values in minerals in the Martian meteorites span a large range of ?250 to +6000‰. The highest hydrogen isotope ratios likely represent a Martian atmospheric component: either interaction with a reservoir in equilibrium with the Martian atmosphere (such as crustal water), or direct incorporation of the Martian atmosphere due to shock processes. The lowest δD values may represent those of the Martian mantle, but it has also been suggested that these values may represent terrestrial contamination in Martian meteorites. Here we report the hydrogen isotopic compositions and water contents of a variety of phases (merrillites, maskelynites, olivines, and an olivine‐hosted melt inclusion) in Tissint, the latest Martian meteorite fall that was minimally exposed to the terrestrial environment. We compared traditional sample preparation techniques with anhydrous sample preparation methods, to evaluate their effects on hydrogen isotopes, and find that for severely shocked meteorites like Tissint, the traditional sample preparation techniques increase water content and alter the D/H ratios toward more terrestrial‐like values. In the anhydrously prepared Tissint sample, we see a large range of δD values, most likely resulting from a combination of processes including magmatic degassing, secondary alteration by crustal fluids, shock‐related fractionation, and implantation of Martian atmosphere. Based on these data, our best estimate of the δD value for the Martian depleted mantle is ?116 ± 94‰, which is the lowest value measured in a phase in the anhydrously prepared section of Tissint. This value is similar to that of the terrestrial upper mantle, suggesting that water on Mars and Earth was derived from similar sources. The water contents of phases in Tissint are highly variable, and have been affected by secondary processes. Considering the H₂O abundances reported here in the driest phases (most likely representing primary igneous compositions) and appropriate partition coefficients, we estimate the H₂O content of the Tissint parent magma to be ≤0.2 wt%.     

Mineralogical study of brown olivine in Northwest Africa 1950 shergottite and implications for the formation mechanism of iron nanoparticles

Martian meteorites, in particular shergottites, contain darkened olivine (so‐called “brown olivine”) whose color is induced by iron nanoparticles formed in olivine during a shock event. The formation process and conditions of brown olivine have been discussed in the Northwest Africa 2737 (NWA 2737) chassignite. However, formation conditions of brown olivine in NWA 2737 cannot be applied to shergottites because NWA 2737 has a different shock history from that of shergottites. Therefore, this study observed brown olivine in the NWA 1950 shergottite and discusses the general formation process and conditions of brown olivine in shergottites. Our observation of NWA 1950 revealed that olivine is heterogeneously darkened between and within grains different from brown olivine in NWA 2737. XANES analysis showed that brown olivine contains small amounts of Fe³⁺ and TEM/STEM observation revealed that there is no SiO‐rich phase around iron metal nanoparticles. These observations indicate that iron nanoparticles were formed by a disproportionation reaction of olivine (3Fe²⁺_olivine → Fe⁰_metal + 2Fe³⁺_olivine + V_olivine, where V_olivine means a vacancy in olivine). Some parts of brown olivine show lamellar textures in SEM observation and Raman peaks in addition to those expected for olivine, implying that brown olivine experienced a phase transition (to e.g., ringwoodite). In order to induce heterogeneous darkening, heterogeneous high temperature of about 1500–1700 K and shock duration of at least ~90 ms are required. This heterogeneous high temperature resulted in high postshock temperature (>900 K) inducing back‐transformation of most high‐pressure phases. Therefore, in spite of lack of high‐pressure phases, NWA 1950 (= Martian meteorites with brown olivine) experienced higher pressure and temperature compared to other highly shocked meteorite groups.     

Determination of volatile concentrations in fluorapatite of Martian shergottite NWA 2975 by combining synchrotron FTIR,Raman spectroscopy,EMPA, and TEM,and inferences on the volatile budget of the apatite host‐magma

We combined the focused ion beam sample preparation technique with polarized synchrotron‐based FTIR (Fourier transform infrared) spectroscopy, laser‐Raman spectroscopy, electron microprobe analysis (EMPA), and transmission electron microscope (TEM) analysis to identify and quantify structurally bound OH, F, Cl, and CO₃ groups in fluorapatite from the Northwest Africa 2975 (NWA 2975) shergottite. In this study, the first FTIR spectra of the OH‐stretching region from a Martian apatite are presented that show characteristic OH‐bands of a F‐rich, hydroxyl‐bearing apatite. Depending on the method of apatite‐formula calculation and whether charge balance is assumed or not, the FTIR‐based quantification of the incorporated OH, expressed as wt% H₂O, is in variably good agreement with the H₂O concentration calculated from electron microprobe data. EMP analyses yielded between 0.35 and 0.54 wt% H₂O, and IR data yielded an average H₂O content of 0.31 ± 0.03 wt%, consistent with the lower range determined from EMP analyses. The TEM observations implied that the volatiles budget of fluorapatite is magmatic. The water content and the relative volatile ratios calculated for the NWA 2975 magma are similar to those established for other enriched or intermediate shergottites. It is difficult to define the source of enrichment: either Martian wet mantle or crustal assimilation. Comparing the environment of parental magma generation for NWA 2975 with the terrestrial mantle in terms of water content, it displays a composition intermediate between enriched and depleted MORB.     

Serpentinization and the inorganic synthesis of H2 in planetary surfaces

The near-surface inorganic synthesis of molecular hydrogen (H₂) is a fundamental process relevant to the origins and to the sustenance of early life on Earth and potentially other planets. Hydrogen production through the decomposition of water is thought to be a principal reaction that occurs during hydrothermal alteration of olivine, an iron-magnesium silicate abundant near planetary surfaces. We demonstrate that copious amounts of H₂ are produced only when the olivine undergoing alteration (serpentinization) contains 1 to 50 mol% iron over a variety of planetary surface P-T conditions. This suggests that extrasolar Earth-like planets that are hosted by a star with iron contents up to two times the solar value could support life provided they are hydrothermally active and fall within the habitable zone around the star.     

The water content and parental magma of the second chassignite NWA 2737: Clues from trapped melt inclusions in olivine

NWA 2737, the second known chassignite, mainly consists of cumulate olivine crystals of homogeneous composition (Fo = 78.7 ± 0.9). These brown colored olivine grains exhibit two sets of perpendicular planar defects due to shock. Two forms of trapped liquids, interstitial melts and magmatic inclusions, have been examined. Mineral assemblages within the olivine‐hosted magmatic inclusions include low‐Ca pyroxene, augite, kaersutite, fluorapatite, biotite, chromite, sulfide, and feldspathic glass. The reconstructed parental magma composition (A^#) of the NWA 2737 is basaltic and resembles both the experimentally constrained parental melt composition of chassiginites and the Gusev basalt Humphrey, albeit with lower Al contents. A^# also broadly resembles the average of shergottite parent magmas or LAR 06319. However, we suggest that the mantle source for the chassignite parental magmas was distinct from that of the shergottite meteorites, particularly in CaO/Al₂O₃ ratio. In addition, based on the analysis of the volatile contents of kaersutite, we derived a water content of 0.48–0.67 wt% for the parental melt. Finally, our MELTS calculations suggest that moderate pressure (approximately 6.8 kb) came closest to reproducing the crystallized melt‐inclusion assemblages.     

Northwest Africa 428: Impact‐induced annealing of an L6 chondrite breccia

Abstract— Northwest Africa (NWA) 428 is an L chondrite that was successively thermally metamorphosed to petrologic type‐6, shocked to stage S4–S5, brecciated, and annealed to approximately petrologic type‐4. Its thermal and shock history resembles that of the previously studied LL6 chondrite, Miller Range (MIL) 99301, which formed on a different asteroid. The petrologic type‐6 classification of NWA 428 is based on its highly recrystallized texture, coarse metal (150 ± 150 μm), troilite (100 ± 170 μm), and plagioclase (20–60 μm) grains, and relatively homogeneous olivine (Fa_{24.4 ± 0.6}), low‐Ca pyroxene (Fs_{20.5 ± 0.4}), and plagioclase (Ab_{84.2 ± 0.4}) compositions. The petrographic criteria that indicate shock stage S4–S5 include the presence of chromite veinlets, chromite‐plagioclase assemblages, numerous occurrences of metallic Cu, irregular troilite grains within metallic Fe‐Ni, polycrystalline troilite, duplex plessite, metal and troilite veins, large troilite nodules, and low‐Ca clinopyroxene with polysynthetic twins. If the rock had been shocked before thermal metamorphism, low‐Ca clinopyroxene produced by the shock event would have transformed into orthopyroxene. Post‐shock brecciation is indicated by the presence of recrystallized clasts and highly shocked clasts that form sharp boundaries with the host. Post‐shock annealing is indicated by the sharp optical extinction of the olivine grains; during annealing, the damaged olivine crystal lattices healed. If temperatures exceeded those approximating petrologic type‐4 (?600–700°C) during annealing, the low‐Ca clinopyroxene would have transformed into orthopyroxene. The other shock indicators, likewise, survived the mild annealing. An impact event is the most plausible source of post‐metamorphic, post‐shock annealing because any ²⁶Al that may have been present when the asteroid accreted would have decayed away by the time NWA 428 was annealed. The similar inferred histories of NWA 428 (L6) and MIL 99301 (LL6) indicate that impact heating affected more than 1 ordinary chondrite parent body.     

The abundance and isotopic composition of water in eucrites

Volatile elements play a key role in the dynamics of planetary evolution. Extensive work has been carried out to determine the abundance, distribution, and source(s) of volatiles in planetary bodies such as the Earth, Moon, and Mars. A recent study showed that the water in apatite from eucrites has similar hydrogen isotopic compositions compared to water in terrestrial rocks and carbonaceous chondrites, suggesting that water accreted very early in the inner solar system given the ancient crystallization ages (~4.5 Ga) of eucrites. Here, the measurements of water (reported as equivalent H₂O abundances) and the hydrogen isotopic composition (δD) of apatite from five basaltic eucrites and one cumulate eucrite are reported. Apatite H₂O abundances range from ~30 to ~3500 ppm and are associated with a weighted average δD value of ?34 ± 67‰. No systematic variations or correlations are observed in H₂O abundance or δD value with eucrite geochemical trend or metamorphic grade. These results extend the range of previously published hydrogen isotope data for eucrites and confirm the striking homogeneity in the H‐isotopic composition of water in eucrites, which is consistent with a common source for water in the inner solar system.     

Simulating OH/H2O formation by solar wind at the lunar surface

We simulate the OH/H₂O production from the action of keV protons on the lunar regolith using a vacuum chamber and a mass analyzer to examine the molecular products released from olivine and SiO₂ powders during their irradiation by deuterium ions. The measured mass spectra, showing the OD/D₂O signature, confirm the possibility of OH/H₂O formation on the lunar surface by solar-wind hydrogen.     

Olivine in Martian meteorite Allan Hills 84001: Evidence for a high-temperature origin and implications for signs of life

Abstract— Olivine from Martian meteorite Allan Hills (ALH) 84001 occurs as clusters within orthopyroxene adjacent to fractures containing disrupted carbonate globules and feldspathic shock glass. The inclusions are irregular in shape and range in size from ～40 μm to submicrometer. Some of the inclusions are elongate and boudinage-like. The olivine grains are in sharp contact with the enclosing orthopyroxene and often contain small inclusions of chromite. The olivine exhibits a very limited range of composition from Fo₆₅ to Fo₆₆ (n = 25). The δ¹⁸O values of the olivine and orthopyroxene analyzed by ion microprobe range from +4.3 to +5.3‰ and are indistinguishable from each other within analytical uncertainty. The mineral chemistries, O-isotopic data, and textural relationships indicate that the olivine inclusions were produced at a temperature >800 °C. It is unlikely that the olivines formed during the same event that gave rise to the carbonates in ALH 84001, which have more elevated and variable δ¹⁸O values, and were probably formed from fluids that were not in isotopic equilibrium with the orthopyroxene or olivine. The reactions most likely instrumental in the formation of olivine could be either the dehydration of hydrous silicates that formed during carbonate precipitation or the reduction of orthopyroxene and spinel. If the olivine was formed by either reaction during a postcarbonate heating event, the implications are profound with regards to the interpretations of McKay et al. (1996). Due to the low diffusion rates in carbonates, this rapid, high-temperature event would have resulted in the preservation of the fine-scale carbonate zoning, while partially devolatilizing select carbonate compositions on a submicrometer scale (Brearley, 1998a). This may have resulted in the formation of the minute magnetite grains that McKay et al. (1996) attributed to biogenic activity.     

Loss of water from Venus. I. Hydrodynamic escape of hydrogen

A one-dimensional photochemical-dynamic model is used to study hydrodynamic loss of hydrogen from a primitive, water-rich atmosphere on Venus. The escape flux is calculated as a function of the H₂O mixing ratio at the atmospheric cold trap. The cold-trap mixing ratio is then related in an approximate fashion to the H₂O concentration in the lower atmosphere. Hydrodynamic escape should have been the dominant loss process for hydrogen when the H₂O mass mixing ratio in the lower atmosphere exceeded ～0.1. The escape rate would have depended upon the magnitude of the solar ultraviolet flux and the atmospheric euv heating efficiency and, to a lesser extent, on the O₂ content of the atmosphere. The time required for Venus to have lost the bulk of a terrestrial ocean of water is on the order of a billion years. Deutrium would have been swept away along with hydrogen if the escape rate was high enough, but some D/H enrichment should have occurred as the escape rate slowed down.     

THE KRAMER CREEK,COLORADO METEORITE: A NEW L4 CHONDRITE

The Kramer Creek, Colorado, chondrite was found in 1966 and identified as a meteorite in 1972. Bulk chemical analysis, particularly the total iron content (20.36%) and the ratio of Fe_total/SiO₂ (0.52), as well as the compositions of olivine (Fa_21.7) and orthopyroxene (Fs_18.3) place the meteorite into the L-group of chondrites. The well-defined chondritic texture of the meteorite, the presence of igneous glass in the chondrules and of low-Ca clinopyroxene, as well as the slight variations in FeO contents of olivine (2.4% MD) and orthopyroxene (5.6% MD) indicate that the chondrite belongs to the type 4 petrologic class.     

Stability of hydrous minerals on the martian surface

The presence of water-bearing minerals on Mars has long been discussed, but little or no data exist showing that minerals such as smectites and zeolites may be present on the surface in a hydrated state (i.e., that they could contain H₂O molecules in their interlayer or extra-framework sites, respectively). We have analyzed experimental thermodynamic and X-ray powder diffraction data for smectite and the most common terrestrial zeolite, clinoptilolite, to evaluate the state of hydration of these minerals under martian surface conditions. Thermodynamic data for clinoptilolite show that water molecules in its extra-framework sites are held very strongly, with enthalpies of dehydration for Ca-clinoptilolite up to three times greater than that for liquid water. Using these data, we calculated the Gibbs free energy of hydration of clinoptilolite and smectite as a function of temperature and pressure. The calculations demonstrate that these minerals would indeed be hydrated under the very low-P (H₂O) conditions existing on Mars, a reflection of their high affinities for H₂O. These calculations assuming the partial pressure of H₂O and the temperature range expected on Mars suggest that, if present on the surface, zeolites and Ca-smectites could also play a role in affecting the diurnal variations in martian atmospheric H₂O because their calculated water contents vary considerably over daily martian temperature ranges. The open crystal structure of clinoptilolite and existing hydration and kinetic data suggest that hydration/dehydration are not kinetically limited. Based on these calculations, it is possible that hydrated zeolites and clay minerals may explain some of the recent observations of significant amounts of hydrogen not attributable to water ice at martian mid-latitudes.