臼井 寛裕

The hydrogen isotopic (D/H) ratio reflects the global cycling and evolution of water on Earth as it fractionates through planetary processes. We model the water cycle taking seafloor hydrothermal alteration, chemical alteration of continental crust, slab subduction, hydrogen escape from the early Earth, and degassing at mid-ocean ridges, hot spots, and arcs into account. The differences in D/H ratios between present-day oceans, oceanic and continental crust, and mantle are thought to reflect isotopic fractionation through seafloor alteration, chemical alteration, and slab dehydration. However, if the speed of plate tectonics has been nearly constant throughout Earth's history, the degassing and regassing rates are too small to reach the present-day D/H ratios. We show that (a) hydrogen escape from reduced early atmosphere, (b) secular net regassing, or (c) faster plate tectonics on early Earth is needed to reproduce the present-day D/H ratios of the water reservoirs. The low D/H ratio of Archean seawater at 3.8 Ga has previously been interpreted as a signature of (a) hydrogen escape, but we find it can also be explained either by (b) secular net degassing or by (c) faster plate tectonics on early Earth. The rates of hydrogen escape from early Earth and secular regassing on present-day Earth are constrained to be lower than 2.1 x 10(11) kg/yr and 3.9 x 10(11) kg/yr. Consequently, the volume of water in the present-day mantle could result entirely from the regassing through Earth's history. In that case, the volume of initial oceans could be 2 to 3 times larger than that of current Earth. We suggest that, in addition to the D/H ratio of Archean seawater, identifying the D/H ratios of both seawater and mantle throughout Earth's history would allow to distinguish these evolutionary scenarios. (C) 2018 Elsevier B.V. All rights reserved.

We review the geochemical observations of water, and volatile element abundances of the inner Solar System bodies, Mercury, Venus, the Moon, and Mars. We focus primarily on the inventories of water in these bodies, but also consider other volatiles when they can inform us about water. For Mercury, we have no data for internal water, but the reducing nature of the surface of Mercury would suggest that some hydrogen may be retained in its core. We evaluate the current knowledge and understanding of venusian water and volatiles and conclude that the venusian mantle was likely endowed with as much water as Earth of which it retains a small but non-negligible fraction. Estimates of the abundance of the Moon's internal water vary from Earth-like to one to two orders of magnitude more depleted. Cl, K, and Zn isotope anomalies for lunar samples argue that the giant impact left a unique geochemical fingerprint on the Moon, but not the Earth. For Mars, an early magma ocean likely generated a thick crust; this combined with a lack of crustal recycling mechanisms would have led to early isolation of the Martian mantle from later delivery of water and volatiles from surface reservoirs or late accretion. The abundance estimates of Martian mantle water are similar to those of the terrestrial mantle, suggesting some similarities in the water and volatile inventories for the terrestrial planets and the Moon.

We report Os isotope compositions of metal grains in two CBa chondrites (Bencubbin and Gujba) determined using a micromilling sampling coupled with thermal ionization mass spectrometry, together with the abundances of major and trace siderophile elements obtained by electron probe microanalysis and femtosecond laser ablation inductively coupled plasma-mass spectrometry. The CBa metal grains presented Os-187/Os-188 ratios akin to carbonaceous chondrites with limited variations (0.1257-0.1270). Most of the CBa metal grains were scattered along a Re-187-Os-187 reference isochron of IIIAB iron meteorites, indicating that the CBa metals experienced limited Re-Os fractionation at the time of their formation. The Re/Os ratios of sampling spots for the CBa metals, recast from the observed Os-187/Os-188 ratios, had a positive correlation with their Os/Ir ratios. In addition, the metal grains showed a positive correlation in a Pd/Fe versus Ni/Fe diagram. These correlations suggest that the CBa metal grains have formed via equilibrium condensation or evaporation from a gaseous reservoir at similar to 10(-4) bar with enhanced metal abundances. Compared to the Bencubbin metals, the Gujba metals are characterized by having systematically lower Pd/Fe and Ni/Fe ratios that span subchondritic values. Such a difference was most likely induced by the compositionally heterogeneous impact plume from which the metals were condensed.

Martian Moons eXploration (MMX) is a mission under study in ISAS/JAXA to be launched in 2024. This paper introduces the mission design of MMX mission. “How was water delivered to rocky planets and enabled the habitability of the solar system?” This is the key question to which MMX is going to answer. Solar system formation theories suggest that rocky planets must have been born dry. Delivery of water, volatiles, organic compounds etc. from outside the snow line entitles the rocky planet region to be habitable. Small bodies as comets and asteroids play the role of delivery capsules. Then, dynamics of small bodies around the snow line in the early solar system is the issue that needs to be understood. Mars was at the gateway position to witness the process, which naturally leads us to explore two Martian moons, Phobos and Deimos, to answer to the key question. The goal of MMX is to reveal the origin of the Martian moons, and then to make a progress in our understanding of planetary system formation and of primordial material transport around the border between the inner- and the outer-part of the early solar system. The mission is to survey two Martian moons, and return samples from one of them. Following the mission concepts study results presented in the previous conference, the following items will be reported in this paper. First, based on the mission goals and objectives defined, the requirements to the systems and operations are derived and their feasibility is evaluated. Second, as to the key technologies issues identified, partial models are built and their performance is evaluated. And third, collaborations with overseas space agency are discussed and the programmatic framework is defined.

<p>火星衛星Phobosからのサンプルリターンに挑む火星衛星探査計画 (Martian Moons eXploration: MMX) は，現在，宇宙航空研究開発機構 (JAXA) プリプロジェクトとして，2024年の打ち上げと5年の往還期間を設定し，精力的な検討・初期開発が進められている．MMXは，サンプル分析，Deimosを加えた火星衛星の近接観測，そして火星大気および火星圏のモニタリング観測を組み合わせることにより，惑星に寄りそう衛星という切り口と視座から，太陽系における大気と水を湛えたハビタブル惑星の形成と進化の解明に迫ろうとしている．</p>

We examine the history of the loss and replenishment of the Martian atmosphere using elemental and isotopic compositions of nitrogen and noble gases. The evolution of the atmosphere is calculated by taking into consideration various processes: impact erosion and replenishment by asteroids and comets, atmospheric escape induced by solar radiation and wind, volcanic degassing, and gas deposition by interplanetary dust particles. Our model reproduces the elemental and isotopic compositions of N and noble gases (except for Xe) in the Martian atmosphere, as inferred from exploration missions and analyses of Martian meteorites. Other processes such as ionization-induced fractionation, which are not included in our model, are likely to make a large contribution in producing the current Xe isotope composition. Since intense impacts during the heavy bombardment period greatly affect the atmospheric mass, the atmospheric pressure evolves stochastically. Whereas a dense atmosphere preserves primitive isotopic compositions, a thin atmosphere on early Mars is severely influenced by stochastic impact events and following escape-induced fractionation. The onset of fractionation following the decrease in atmospheric pressure is explained by shorter timescales of isotopic fractionation under a lower atmospheric pressure. The comparison of our numerical results with the less fractionated N (N-15/N-14) and Ar (Ar-38/Ar-36) isotope compositions of the ancient atmosphere recorded in the Martian meteorite Allan Hills 84001 provides a lower limit of the atmospheric pressure in 4 Ga to preserve the primitive isotopic compositions. We conclude that the atmospheric pressure was higher than approximately 0.5 bar at 4 Ga. (C) 2017 Elsevier Inc. All rights reserved.

The application of Martian meteorite U-Th-Pb isotope systematics to track the geochemical evolution of the Martian mantle has had limited success because of the difficulty in discriminating an indigenous magmatic Pb component from secondary near-surface components that have additionally been overprinted by terrestrial contamination. To mitigate this challenge, a successive acid-leaching experiment was conducted on the Tissint meteorite, the freshest, witnessed fall of a primitive, olivine-bearing Martian basalt. Trace element concentration analyses of acid leachates and residues indicate that secondary terrestrial contaminants were effectively removed by the early steps in the leaching experiments and that the acid residues contain pristine Pb from Tissint. The acid residue, which shows the most depleted REE signature, also has the least radiogenic Pb isotopic composition (Pb-206/Pb-204 = 10.948, Pb-207/Pb-204 = 11.187, Pb-208/Pb-204 = 30.228). A two-stage mantle evolution model based on this composition indicates that the Tissint mantle has the lowest mu-value (U-238/Pb-204 = 1.62 +/- 0.09) among the shergottite sources. (C) 2017 Elsevier B.V. All rights reserved.

It has long been suggested that mineral surfaces played an important role in peptide bond formation on the primitive Earth. However, it remains unclear which mineral species was key to the prebiotic processes. This is because great discrepancies exist among the reported catalytic efficiencies of minerals for amino acid polymerizations, owing to mutually different experimental conditions. This study examined polymerization of glycine (Gly) on nine oxide minerals (amorphous silica, quartz, alpha-alumina and gamma-alumina, anatase, rutile, hematite, magnetite, and forsterite) using identical preparation, heating, and analytical procedures. Results showed that a rutile surface is the most effective site for Gly polymerization in terms of both amounts and lengths of Gly polymers synthesized. The catalytic efficiency decreased as rutile > anatase > gamma-alumina > forsterite > alpha- alumina > magnetite > hematite > quartz > amorphous silica. Based on reported molecular-level information for adsorption of Gly on these minerals, polymerization activation was inferred to have arisen from deprotonation of the NH3 (+) group of adsorbed Gly to the nucleophilic NH2 group, and from withdrawal of electron density from the carboxyl carbon to the surface metal ions. The orientation of adsorbed Gly on minerals is also a factor influencing the Gly reactivity. The examination of Gly-mineral interactions under identical experimental conditions has enabled the direct comparison of various minerals' catalytic efficiencies and has made discussion of polymerization mechanisms and their relative influences possible Further systematic investigations using the approach reported herein (which are expected to be fruitful) combined with future microscopic surface analyses will elucidate the role of minerals in the process of abiotic peptide bond formation.

We present a new analytical technique for precise isotope measurement of sub-nanograms of Pb by total evaporation thermal ionization mass spectrometry (TE-TIMS) coupled with a Pb-204-Pb-207 double spike. The precise determination of Pb isotopic composition with a small (sub-nanograms) quantity of Pb is hampered due to the low signal intensity of Pb-204(+), which has the lowest isotope abundance in natural samples (1.4%). The advantage of TE-TIMS is that the measured signal intensities are larger and the measurement time is shorter than those by the measurement with constant filament current. In this study, we investigated optimization of the analytical protocol for Pb isotope measurement by TE-TIMS coupled with a Pb-204-Pb-207 double spike, including the sample loading technique, filament temperature control, and the reduction method of isotope data acquired. The Pb isotope analysis was performed with the heating rate of 18, 90, and 450 mA min(-1), with a short measurement time of 60, 30, and 20 minutes. The resulting reproducibilities of Pb isotope ratios were comparable to those of previous techniques including the Pb-202-Pb-205 double spike method and the Pb-204-Pb-207 double spike method using an amplifier with a state-of-the-art 10(13) Omega resistor for collecting the Pb-204 signal. The absolute Pb isotope ratios for NIST 981 measured by our method were biased due to anomalous Pb-207 behavior occurring at the filament temperature of > 1250 degrees C as well as the deterioration of Faraday cups. However, we confirmed that the accurate absolute Pb isotope ratios for an unknown sample can be obtained by normalizing the observed Pb isotope ratios to those of NIST 981 that are determined in the same analytical period and then multiplying the reference Pb isotope ratios of NIST 981. We measured the Pb isotope ratios of a standard rock material JB-3, of which the results were consistent with those reported by previous studies. We conclude that our method is suitable especially for the isotope analysis of subnanograms of Pb, and using a Pb-204-Pb-207 double spike instead of a Pb-202-Pb-205 double spike is beneficial for a number of laboratories due to the availability of the spikes. Furthermore, reduced measurement time compared to previous studies is helpful for measuring a large quantity of samples.

<p>最近の太陽系探査によって、地球以外の天体に液体の水が存在する(していた)証拠が続々と見つかっている。本発表は、これら天体上で水が駆動する化学反応や物質循環を解明することで、水が惑星の形成・進化に果たした役割を総合的に理解し、生命存在可能性の議論にまで至る「水惑星学」の創成を提案する。そのために、地球科学と惑星科学が有機的に融合し、はやぶさ２探査の機会を利用することで、太陽系天体の水・物質循環を記述する理論とその実試料による実証を両輪とする研究体系を構築する。これによる達成目標は、１)微惑星内の水・物質循環の解明と地球の水量の決定要因の理解、２)火星、氷衛星における水環境進化とエネルギー論に基づく生命圏の推定である。</p>

<p>火星由来の玄武岩であるシャーゴッタイト隕石は火星マントルの地球化学的特徴を保持しており、火星の内部進化・初期分化プロセスを知る上で非常に重要な研究対象である。本研究では、シャーゴッタイト隕石のPb同位体分析を行うことにより、U-Th-Pbシステマティクスに基づいて火星の初期分化プロセスに制約を与えることを目的とする。本研究の分析により、火星マントルのPb同位体組成が放射性起源のPbに著しく枯渇した特徴を保持していることが明らかとなった。この結果と、先行研究で報告されている火星マントルのSr, Nd, Hf同位体組成は、火星の初期分化時にザクロ石の結晶分別によって鉱物的に不均質な火星マントルが形成されたことを示唆している。</p>

<p>初期太陽系におけるユークライト母天体の火成活動に関する年代を制約するため、変成の影響を受けにくいSm–Nd放射壊変系に着目した。本研究で用いる玄武岩質ユークライトNWA 7188は天体衝突による角礫化の程度が低く、母天体形成初期の地殻情報を保持する重要な隕石であると考えられる。NWA 7188の酸素同位体測定の結果、Δ¹⁷ Oの値はユークライトフラクショネーションラインと一致した。また、ICP-MSを用いた元素分析から、NWA 7188は稀なStannern trendグループに分類されることが明らかになった。さらに、得られたSm–Nd年代(4553 +17/–20 Ma)は、先行研究による玄武岩質ユークライトのSm–Nd年代の中で非常に古く、玄武岩質ユークライトに含まれるジルコンのPb–Pb年代と誤差の範囲で一致している。従って、NWA 7188のSm–Nd年代は、母天体での火成活動の年代または地殻形成初期の広域にわたる熱変成の年代に対応すると考えられる。</p>

<p>TIMSを用いたジルコンの高精度Pb同位体分析は、その年代学的重要性から多くの研究が行われてきた。TIMSを用いた高精度同位体分析では、分析中に生じる質量依存同位体分別の補正が欠かせない。本研究では、分析中の同位体分別の影響が比較的小さいトータルエバポレーションTIMS(TE-TIMS)に着目し、ジルコンの高精度Pb同位体分析手法を開発した。TE-TIMSによる²⁰⁸Pb/²⁰⁶Pb比の同位体分析を行ったところ、その値は真値からわずかに変動することが分かった。変動の程度は測定試料の種類及びPb量によらず一定であり、この同位体分別係数を用いることでTE-TIMSによるジルコンのPb同位体分析が可能になる。本研究で開発した分析法を用いて測定したNIST 983の繰り返し再現性は、先行研究よりも約10倍程度良かった。</p>

Geochemical studies of shergottites (Martian basalts) based on Rb-Sr, Sm-Nd, and Lu-Hf isotopic systematics have provided clues to understanding the geochemical evolution of the Martian mantle and identification of the source reservoirs. However, U-Pb isotopic systematics has been used to a limited extent for shergottite petrogenesis, because it is generally difficult to discriminate indigenous magmatic Pb components from secondary Martian near-surface components and terrestrial contamination. This study compiles and reassesses all the available Pb isotopic data of shergottites, as well as their Rb-Sr, Sm-Nd, and Lu-Hf isotope systematics.The Sr-Nd-Hf isotopic systematics suggests that the geochemical variability of the shergottite suite (i.e., enriched, intermediate, and depleted shergottites) reflects a mixture of two distinct source reservoirs. In contrast, the Pb isotopic systematics does not support the two-component mixing model for shergottites, because the geochemically enriched, intermediate, and depleted shergottites do not participate in a two-component mixing array in Pb isotopic space. To reconcile the isotopic signatures of the Sr-Nd-Hf and Pb systems, we propose a new mixing model in which the geochemically enriched, intermediate, and depleted shergottites were derived from compositionally distinct mantle sources that had different mu (U-238/Pb-204) values. Moreover, a linear mixing trend defined by the enriched shergottites in Pb isotopic space is interpreted as the incorporation of a high-m Martian crustal component into a parental magma derived from a fertilized Martian mantle source. Our model implies that the geochemical diversity of shergottites reflects heterogeneous mantle sources and an assimilated high-m crustal component on Mars.

Ancient Mars is now considered to have had an environment somewhat similar to that of Earth in terms of the existence of large bodies of water, a wide range of surface oxidation states, an active dynamo and associated magnetic field, magmatism and tectonism which includes mountain building and basin formation, and appearances of variety of chemical components potentially building blocks of life. Similar to habitable Earth, ancient Mars included hydrological cycling among the atmosphere, ocean, and landmass (southern cratered highlands), and plate tectonism cannot be ruled out. Endogenic activities have continued until even very recently, and recent water-related geological features indicate prolonged existence of aquifer systems, where habitable environments may exist for a significant period of time. Occasional releases of volatiles from such aquifer systems may ultimately account for the detection of methane by the Curiosity rover in the Gale crater and the inconclusive result (i.e., not unambiguous denial) of metabolism-detection instrument onboard Viking landers. Unequivocal evidence of the existence of subsurface aquifers or extant endogenic activity is, however, still lacking possibly due to the existence of homogeneous regolith materials covering the surface of Mars. Also, even if a habitable environment exists at depth, accessing the environment by a spacecraft(either a lander or a rover) has been considered to be challenging especially because such an environment has been generally thought to exist more than several kilometers below the Martian surface. Recent findings of a recurring slope lineae (RSL) point to traces of possible seasonal liquid water flows along slopes, findings of which will likely change the above prevailing view; some of these features might result from the partial discharges from an aquifer. In other words, RSLs might provide a natural bridge between a subsurface aquifer and the surface accessible by a rover. Thus, subsurface structures near such features become prime targets to be explored through future missions. Once the presence of ground water is confirmed, especially an aquifer, mapping and characterizing the distribution of subsurface water would significantly help address the ever-important question of whether life exists on Mars.Given this view, we have selected possible landing sites for a future landing mission to detect life on Mars. Our selection is based on the possibility of the existence of near-surface water and recent geological and hydrological activities; specifically areas with (1) a higher possibility of releases of volatiles,(2) a relatively high water activity(Aw>0.6),(3) a relatively higher maximum environmental temperature (T>250 K), and (4) an existence of gradients of free energy. We propose Melas Chasma in Valles Marineris as a prime candidate because of its long-term water enrichment and energy conditions as evidenced through it:(1) comprising confirmed recurring slope lineae(RSL);(2) being the widest and deepest part of the Valles Marineris and thus a major catchment basin of Mars since its formation; (3) being connected to the outflow channels; (4) possible fog for at least part of a Martian day; (5) containing Interior Layered Deposits (ILDs) which comprise various sulfates deposits, as well as phyllosilicates among the canyon units, both of which are suggestive of abundant past water; (6) comprising a volcanic field in its southeast part; and (7) being cut by deep-seated basement structures that served as conduits for the migration of both groundwater and heat. We also propose Tharsis/Elysium Corridor region as among the best candidates, which shows evidence of long-lived water enrichment and recent geologic activity. This includes possible magma, mud, and water interactions and related venting, in which case materials have been transferred to the surface environment from the subterranean, readily accessible for sampling and analyses such as in central Elysium Planitia.

Remote sensing data from orbiter missions have proposed that ground ice may currently exist on Mars, although the volume is still uncertain. Recent analyses of Martian meteorites have suggested that the water reservoirs have at least three distinct hydrogen isotope compositions (D/H ratios): primordial and high D/H ratios, which are approximately the same and six times that of ocean water on Earth, respectively, and a newly identified intermediate D/H ratio, which is approximately two to three times higher than that in ocean water on Earth. We calculate the evolution of the D/H ratios and the volumes of the water reservoirs on Mars by modeling the exchange of hydrogen isotopes between multiple water reservoirs and the atmospheric escape. The D/H ratio is slightly higher in the topmost thin surface-ice layer than that in the atmosphere because of isotopic fractionation by sublimation, whereas the water-ice reservoir just below the exchangeable topmost surface layer retains the intermediate D/H signature found in Martian meteorites. We propose two possible models for constraining the volume of the ground ice considering the observed D/H ratios and geomorphological estimates of Paleo-oceans. The first assumes that the atmospheric loss is dominated by the Jeans escape. In this case, the volume of ground ice should be larger than the total volume of the observable surface ice that mainly occurs as polar layered ice deposits. The other model assumes diffusion-limited atmospheric loss in which the interactive evolution of the multiple water reservoirs naturally accounts for the observed D/H ratios. In this scenario, a large volume of ground ice does not necessarily exist currently on Mars as opposed to the perspective view proposed on the basis of recent orbiter missions.

The origin and petrogenesis of the Cameroon Volcanic Line (CVL), composed of volcanoes that form on both the ocean floor and the continental crust, are difficult to understand because of the diversity, heterogeneity, and nature of available data. Major and trace elements, and Sr-Nd-Pb isotope data of volcanic rocks of the CVL spanning four decades have been compiled to reinterpret their origin and petrogenesis. Volcanic rocks range from nephelinite, basanite and alkali basalts to phonolite, trachyte and rhyolite with the presence of a compositional gap between SiO2 58-64 wt.%. Similarities in geochemical characteristics, modeled results for two component mixing, and the existence of mantle xenoliths in most mafic rocks argue against significant crustal contamination. Major and trace element evidences indicate that the melting of mantle rocks to generate the CVL magma occurred dominantly in the garnet lherzolite stability field. Melting models suggest small degree (<3%) partial melting of mantle bearing (6-10%) garnet for Mt. Etinde, the Ngaoundere Plateau and the Biu Plateau, and <5% of garnet for the oceanic sector of the CVL, Mt. Cameroon, Mt. Bambouto, Mt. Manengouba and the Oku Volcanic Group. The Sr-Nd-Pb isotope systematics suggest that mixing in various proportions of Depleted MORB Mantle (DMM) with enriched mantle 1 and 2 (EMI and EM2) could account for the complex isotopic characteristics of the CVL lavas. Low Mg number (Mg-# = 100 x MgO/(MgO + FeO)) and Ni, Cr and Co contents of the CVL mafic lavas reveal their crystallization from fractionated melts. The absence of systematic variation in Nb/Ta and Zr/Hf ratios, and Sr-Nd isotope compositions between the mafic and felsic lavas indicates progressive evolution of magmas by fractional crystallization. Trace element ratios and their plots corroborate mantle heterogeneity and reveal distinct geochemical signatures for individual the CVL volcanoes. (C) 2015, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

Lake Nyos is located at the summit of a stratovolcano in the Oku Volcanic Group (OVG) along the Cameroon Volcanic Line. The sudden release of magmatic CO2 trapped at the bottom of Lake Nyos in August 1986 caused historical casualties of 1750 people and over 3000 cattle. New geochemical data of volcanic rocks from the Nyos volcano and the first available data for volcanic rocks from other maar-bearing volcanoes (Lakes Elum, Wum and Oku) in the OVG are presented and compared. Lavas from the Nyos, Elum and Wum volcanoes show similarities in major and trace elements and Sr-Nd-Pb isotopes, suggestive of a similar mantle source. However, this source is slightly different from that of the Oku volcano. The samples from Lake Oku have lower alkali, higher TiO2 and more depletion and enrichment in most incompatible trace elements than those from the Nyos, Elum and Wum volcanoes. These differences and those observed in the Sr-Nd-Pb results are consistent with a heterogeneous source for lavas in the OVG. Trace element compositions suggested the presence of garnet in the source (<6% garnet) and modelled melting results indicate <2% partial melting of the source material. Isotope data plot within the focal zone, extending towards enriched mantle 1 (EM1; e.g. Lakes Oku and Nyos samples). This indicates the involvement of at least three mantle components: depleted mid-ocean ridge basalt mantle, high-mu and EM1 components in the magmatism of the lavas studied. The contributions of these components in different proportions, originating from asthenospheric and subcontinental lithospheric mantle sources, can account for the observed variations in geochemical characteristics. The geochemical characteristics of the studied lavas indicate that the magma source need not necessarily have an abnormal CO2 concentration to pose a potential threat. Degassing of an ordinary magma chamber and the migration of gas to the bottom of the lakes through cracks and faults can lead to the accumulation of CO2 in lake bottoms. This is controlled by tectonic parameters (fractures and faults) that enhance degassing from the magma chamber to the lake bottom and physical parameters of the lake (e.g. size, depth, temperature and solubility) that control CO2 stability. (C) 2015 Elsevier B.V. All rights reserved.

Melting experiments of a synthesized, alkali-bearing, H-chondrite composition were conducted at ambient pressure with three distinct oxygen fugacity conditions (IW-1, IW, and IW+2). Oxygen fugacity conditions significantly influence the compositions of partial melts. Partial melts at IW-1 are distinctly enriched in SiO2 relative to those of IW and IW+2 melts. The silica-enriched, reduced (IW-1) melts are characterized by high alkali contents and have silica-oversaturated compositions. In contrast, the silica-depleted, oxidized (IW) melts, which are also enriched in alkali contents, have distinctly silica-undersaturated compositions. These experimental results suggest that alkali-rich, felsic, asteroidal crusts as represented by paired achondrites Graves Nunataks 06128 and 06129 should originate from a low-degree, relatively reduced partial melt from a parent body having near-chondritic compositions. Based on recent chronological constraints and numerical considerations as well as our experimental results, we propose that such felsic magmatism should have occurred in a parent body that is smaller in size and commenced accreting later than those highly differentiated asteroids having basaltic crusts and metallic cores.

Fluvial landforms on Mars suggest that it was once warm enough to maintain persistent liquid water on its surface. The transition to the present cold and dry Mars is closely linked to the history of surface water, yet the evolution of surficial water is poorly constrained. Based on in situ hydrogen isotope (D/H) analyses of quenched and impact glasses in Martian meteorites, we provide evidence for the existence of a distinct but ubiquitous water/ice reservoir (D/H = similar to 2-3 times Earth's ocean water) that lasted from at least the time when the meteorites crystallized (173-472 million years ago) to the time they were ejected by impacts (0.7-3.3 million years ago), but possibly much longer. The origin of this reservoir appears to predate the current Martian atmospheric water (D/H = similar to 5-6 times Earth's ocean water) and is unlikely to be a simple mixture of atmospheric and primordial water retained in the Martian mantle (D/H approximate to Earth's ocean water). This reservoir could represent hydrated crust and/or ground ice interbedded within sediments. Our results corroborate the hypothesis that a buried cryosphere accounts for a large part of the initial water budget of Mars. (C) 2014 Elsevier B.V. All rights reserved.

火星マントルの化学進化の解明には、火星マントルの溶融液を起源とするシャーゴッタイト隕石を用いた地球化学的研究が最も有効である。本研究では、火星隕石Tissintに5段階の酸処理を行うことによって、5つの抽出液（L1-5）と残留物を得た。それらの初生Pb同位体組成に関し、火成作用後期に晶出するリン酸塩鉱物を多く含む抽出液（L3）が、初期に晶出するカンラン石・輝石からなる残留物と比較して多くの放射性鉛成分を含むことが明らかとなった。このことは、火成作用の後期に地球化学的に富む特徴を持つ火星地殻成分が混入したことを示す。以上から、枯渇した地球化学的特徴を持つTissint隕石でさえ、地殻同化作用を経験していることが明らかとなり、シャーゴッタイトを用いた火星マントルの議論を行う際には、本研究で用いたような段階的酸処理法により、純粋な火星マントル成分を抽出することが必要不可欠であると言える。

Dawn has recently revealed that the surface of Vesta is heterogeneously covered by polymictic regoliths represented by mixtures of howardite, eucrite, and diogenite (HED) meteorites. Mixing relations of the HED suite are examined here using a new computational statistical approach of independent component analysis (ICA). We performed eight-component ICA (Si, Ti, Al, Cr, Fe, Mn, Mg, and Ca) for 209 HED bulk-rock compositions. The ICA results indicate that the HED bulk-rock compositions can be reduced into three independent components (IC) and these IC vectors can reasonably explain compositional variation, petrographic observations, and the mixing relations of the HED suite. The IC-1 vector represents a eucrite variation that extends from cumulate eucrite toward main-group (MG) and incompatible-element enriched eucrites. The IC-2 vector represents a compositional variation of howardites that extends from diogenites to MG-eucrites, indicating the well-known two-component mixing trend of diogenite and eucrite. The IC-3 vector represents a compositional variation defined by diogenites and olivine-bearing diogenites, suggesting mixing of olivine and orthopyroxene. Among the three ICs, the diogenite-eucrite mixing trend IC-2 is most statistically robust and dominates the compositional variations of the HED suite. Our ICA study further indicates that the combination of only three elements (Mg, Si, and Fe) approximates the eight-component ICA model, and that the limited number of resolvable -ray spectra obtained by the Dawn mission possibly discriminates olivine lithologies from the olivine-free regolith breccias on the surface of Vesta.

Shergottites sampled two distinct geochemical reservoirs on Mars. Basaltic and olivine-phyric shergottites individually sampled both geochemically enriched and depleted reservoirs, whereas lherzolitic shergottites are previously known only to exhibit a relatively limited intermediate geochemical signature that may have resulted from the mixing of the two geochemical end-member reservoirs. Here we show that recently discovered shergottites Robert Massif (RBT) 04261 and RBT 04262 are the first examples of lherzolitic shergottites originating from the enriched reservoir.RBT 04261 and RBT 04262, initially identified as olivine-phyric shergottites, are actually lherzolitic shergottites. Both meteorites exhibit nearly identical textures and mineral compositions, suggesting that they should be paired. Each consists of two distinct textures: poikilitic and non-poikilitic. The poikilitic areas are composed of pyroxene oikocrysts enclosing olivine grains; all pyroxene oikocrysts have pigeonite cores mantled by augite. The non-poikilitic areas are composed of olivine, pyroxene, maskelynite and minor amounts of merrillite, chromite and ilmenite. Olivine and pyroxene show the lowest Mg-number, and maskelynite has the lowest anorthite component among the lherzolitic shergottites. Moreover, the modal abundances of maskelynite in these two meteorites are distinctly higher than the other lherzolitic shergottites.The rare earth element (REE) budgets of RBT 04261 and RBT 04262 are dominated by merrillite. The slightly light rare earth element (LREE)-enriched pattern of this mineral is similar to that of merrillite in the geochemically enriched basaltic shergottites Shergotty and Zagami, and unlike the LREE-depleted pattern of merrillite in the other lherzolitic shergottites. The REE patterns of both high- and low-Ca pyroxenes are also similar to those in Shergotty and Zagami. The REE pattern of a melt calculated to be in equilibrium with the core of a pyroxene oikocryst is parallel to that of the RBT 04262 whole-rock as well as whole-rock compositions of other geochemically enriched basaltic shergottites. These observations imply that RBT 04262 sampled an enriched and oxidized reservoir similar to that sampled by some of the basaltic shergottites and are consistent with an oxidizing condition for the formation of RBT 04262 (logfO(2) = QFM-1.6).The petrographic and geochemical observations presented here suggest that RBT 04261 and RBT 04262 represent the most evolved magma among the lherzolitic shergottites and that this magma originated from a geochemically enriched reservoir on Mars. Based on an evaluation of the relationship between petrographic, geochemical and chronological signatures for shergottites including RBT 04261 and RBT 04262, we propose that both geochemically enriched and depleted shergottites were ejected from the same launch site on Mars. (C) 2010 Elsevier Ltd. All rights reserved.

The Dawn spacecraft carries a gamma-ray and neutron detector (GRaND), which will measure and map the abundances of selected elements on the surface of asteroid 4 Vesta. We compare the variability of moderately volatile/refractory incompatible element ratios (K/Th and K/Ti) in howardite, eucrite, and diogenite (HED) meteorites with those in other achondrite suites that represent asteroidal crusts, because these ratios may be accurately measured by GRaND and likely reflect initial chemical compositions of the HED parent body. The K/Th and K/Ti variations can differentiate HED meteorites from angrites and some unique eucrite-like lithologies. The results suggest that K, Th, and Ti abundances determined from GRaND data could not only confirm that Vesta is the parent body of HED meteorites but might also allow recognition of as-yet unsampled compositional terranes on Vesta. Besides the K-Th-Ti systematics study, we propose a new three-component mixing model for interpretation of GRaND spectra, required because the spatial resolution of GRaND is coarser than the spectral (compositional) heterogeneity of Vesta's surface. The mixing model uses abundances of K, Ti, Fe, and Mg that will be analyzed more accurately than other prospective GRaND-analyzed elements. We examine propagated errors due to GRaND analytical uncertainties and intrinsic errors that stem from an assumption introduced into the mixing model. The error investigation suggests that the mixing model can adequately estimate not only the diogenite/eucrite mixing ratio but also the abundances of most major and minor elements within the GRaND propagated errors.

The Spirit rover has analyzed a wide variety of alkaline volcanic rocks in Gusev Crater. Among them are Wishstone Class tephrites with unusually high P2O5 (>5 wt %) abundances. To obtain the chemical compositions of rock interiors, we extrapolated a trend defined by brushed and abraded surface compositions. The extrapolation analysis suggests the high-P2O5 tephrite signature is not attributable to secondary aqueous alteration but represents an igneous rock composition. Assessment of the compositional trend also suggests that merrillite is the phosphate mineral in Wishstone Class. Modified CIPW norm calculations using merrillite instead of apatite show that Wishstone Class rocks are silica saturated (hypersthene-olivine normative) and alkaline. The high-P2O5 whole rock compositions plot above solubility curves of merrillite in a diagram of P2O5 versus aluminosity, suggesting that mechanical admixture of merrillite is required. A source supplying merrillite cannot be a common silicate magma; instead, it could be a carbonatitic melt or a highly fractionated residual melt (jotunite) after anorthosite crystallization. Comparison with Earth analogs suggests that the Wishstone Class is akin to alkaline basalts in the eastern Snake River Plain, with regard to its chemistry, texture, and geologic context. These alkaline basalts are interpreted to originate from mantle metasomatized by CO2-fluids and highly alkaline melts in the wake of the Yellowstone hot spot. Thus, we propose that the Wishstone Class represents a silica-saturated alkaline suite that has mechanically mixed xenocrystic merrillites, probably during explosive volcanic eruption; the merrillites crystallized from carbonate-rich magma produced by melting of a CO2-bearing Martian mantle.

Primitive magmas provide critical information on mantle sources, but most Martian meteorites crystallized from fractionated melts. An olivine-phyric shergottite, Yamato 980459 (Y-980459), has been interpreted to represent a primary melt, because its olivine megacrysts have magnesian cores (Fo84-86) that appear to be in equilibrium with the Y-980459 whole-rock composition based on Fe-Mg partitioning. However, crystal size distribution (CSD) plots for Y-980459 olivines show a size gap, suggesting a cumulus origin for some megacrysts. Because melting experiments using the Y-980459 whole-rock composition have been used to infer the thermal structure and volatile contents of the Martian mantle, the interpretation that this rock is primitive should be scrutinized.We report major, minor and trace element compositions of Y-980459 olivines and compare them with results from melting experiments (both hydrous and anhydrous) and thermodynamic calculations. Cores of the olivine megacrysts have major and minor element contents identical to those of the most magnesian olivines from the experiments, but they differ slightly from those of thermodynamic calculations. This is probably because the Y-980459 whole-rock composition lies near the limit of the range of liquids used to calibrate these models. The megacryst cores (Fo80-85) exhibit minor and trace element (Mn-Ni-Co-Cr-V) characteristics distinct from other olivines (megacryst rims and groundmass olivines, Fo < 80), implying that the megacryst cores crystallized under more reduced conditions (similar to IW + 1).Y-980459 contains pyroxenes with orthopyroxene cores mantled by pigeonite and augite. We also found some reversely zoned pyroxenes that have augite cores (low-Mg#) mantled by orthopyroxenes (high-Mg#), although they are uncommon. These reversely zoned pyroxenes are interpreted to have grown initially as atoll-like crystals with later crystallization filling in the hollow centers, implying disequilibrium crystallization at a moderate cooling rate (3-7 degrees C/h). The calculated REE pattern of a melt in equilibrium with normally zoned pyroxene is parallel to those of glass and the Y-980459 whole-rock as well as other depleted olivine-phyric shergottites, suggesting that Y-980459 was derived from a depleted mantle reservoir.Considering the CSD patterns of Y-980459 olivines, we propose that the olivine megacrysts are cumulus crystals which probably formed in a feeder conduit by continuous melt replenishment, and the parent melt composition was indistinguishable from the Y-980459 whole-rock with 0-2 wt% of H2O and 0-5 Wt% Of CO2. The final magma pulse entrained these cumulus olivines and then crystallized groundmass olivines and pyroxenes. Although Y-980459 contains small amounts of cumulus olivine (<similar to 6 vol%), we conclude that the Y-980459 whole-rock composition closely approximates a Martian primary melt composition. (C) 2008 Elsevier Ltd. All rights reserved.

A coesite-bearing, lawsonite-eclogite xenolith from the Colorado Plateau, interpreted as a fragment of the subducted Farallon plate, is used to characterize trace element behavior in subducted oceanic crust. The xenolith consists of almandine-rich garnet, omphacite, lawsonite, phengite, rutile, pyrite and zircon as the primary mineral assemblage. Garnet crystals are extremely zoned with respect to their Mn contents, with core to rim variation from similar to 1.4 to similar to 0.2 wt.%. The euhedral zoning feature of garnet crystals and its included mineral assemblages suggest that the garnet continued to grow in the coesite stability field during prograde lawsonite eclogite facies metamorphism. In the lawsonite-eclogite xenoliths, garnet dominates the heavy rare earth elements (HREE), and lawsonite dominates both light rare earth elements (LREE) and Sr inventories. Combining the mineralogical and petrographic observations with precise spatial resolution ion microprobe analyses (< 15 mu m) of zoned garnet as well as lawsonite inclusions in garnet, we investigated trace element fractionation in coesite stability field during lawsonite eclogite facies metamorphism. Garnet shows progressive HREE depletion from core to rim, suggesting that HREE, which once partitioned into garnet crystal, would not be involved in postdated metamorphic reactions due to the high partition coefficients of HREE into garnet. Lawsonite inclusions in garnet, which represent lower metamorphic condition relative to lawsonite in the matrix, have LREE concentrations similar to 10 times lower than those of matrix lawsonite. On the contrary, the concentration of Sr in the included lawsonite is (< 20 relative %) lower than that of the matrix lawsonite. Based on constraints from metamorphic history recorded in the prograde-zoned garnet and mass balance among all constituent minerals in the lawsonite-eclogite xenolith, this contrasting feature for Sr and LREE of lawsonite is most plausibly explained by the hypothesis that allanite coexisting with included lawsonite might have decomposed during prograde metamorphism. The LREE released from the decomposing allanite would have been incorporated into lawsonite crystals. Consequently, REE and Sr could be retained in subducting oceanic crust even in the coesite stability field, if the slab is sufficiently cold enough to pass though the lawsonite eclogite facies. (c) 2006 Elsevier B.V. All rights reserved.

A steroid 4 Vesta, believed to be the parent body of the howardite, eucrite, and diogenite (HED) meteorites, will be investigated by the Dawn orbiting spacecraft. Dawn carries a gamma ray and neutron detector (GRaND) that will measure and map some major- and trace-elernent abundances. Drawing on HED geochemistry, we propose a mixing model that uses element ratios appropriate for the interpretation of GRaND data.Because the spatial resolution of GRaND is relatively coarse, the analyzed chemical compositions on the surface of Vesta will likely reflect mixing of three endmember components: diogenite, cumulate eucrite, and basaltic eucrite. Reliability of the mixing model is statistically investigated based on published whole-rock data for HED meteorites. We demonstrate that the mixing model can accurately estimate the abundances of all the GRaND-analyzed major elements, as well as of minor elements (Na, Cr, and Mn) not analyzed by this instrument. We also show how a similar mixing model can determine the modal abundance of olivine, and we compare estimated and non-native olivine data for olivine-bearing diogenites. By linking the compositions of well-analyzed HED meteorites with elemental mapping data from GRaND, this study may help constrain the geological context for HED meteorites and provide new insight into the magmatic evolution of Vesta.

Eclogite xenoliths from the Colorado Plateau, interpreted as fragments of the subducted Farallon plate, are used to constrain the trace element and Sr-Nd-Pb isotopic compositions of oceanic crust subducted into the upper mantle. The xenoliths consist of almandine-rich garnet, Na-clinopyroxene, lawsonite and zoisite with minor amounts of phengite, rutile, pyrite and zircon. They have essentially basaltic bulk-rock major element compositions their Na2O contents are significantly elevated, but K2O contents are similar to those of unaltered mid-ocean ridge basalt (MORB). These alkali element characteristics are explained by spilitization or albitization processes on the sea floor and during subduction-zone metasomatism in the fore-arc region. The whole-rock trace element abundances of the xenoliths are variable relative to sea-floor-altered MORB, except for the restricted Zr/Hf ratios (36.9-37.6). Whole-rock mass balances for two Colorado Plateau eclogite xenoliths are examined for 22 trace elements, Rb, Cs, Sr, Ba, Y, rare earth elements, Pb, Th and U. Mass balance considerations and mineralogical observations indicate that the whole-rock chemistries of the xenoliths were modified by near-surface processes after emplacement and limited interaction with their host rock, a serpentinized ultramafic microbreccia. To avoid these secondary effects, the Sr, Nd and Pb isotopic compositions of minerals separated from the xenoliths were measured, yielding 0.70453-0.70590 for 87Sr/86Sr, -3.1 to 0.5 for εNd and 18.928-19.063 for 206Pb/204Pb. These isotopic compositions are distinctly more radiogenic for Sr and Pb and less radiogenic for Nd than those of altered MORB. Our results suggest that the MORB-like protolith of the xenoliths was metasomatized by a fluid equilibrated with sediment in the fore-arc region of a subduction zone and that this metasomatic fluid produced continental crust-like isotopic compositions of the xenoliths. © 2006 Oxford University Press.

A new method has been developed for the simultaneous determination of Pb abundance and Pb isotopic composition with high precision and accuracy for small test portion masses by thermal ionisation mass spectrometry. In this method, a Pb-205-Pb-204 double spike is added to samples prior to the chemical separation of Pb, and the isotopic composition of the spike-sample mixture is determined rigorously by the double spike technique using a Pb-207-Pb-204 spike. The isotopic composition and concentration of Pb in the sample are then obtained by utilising the principle of isotope dilution. Using this technique, replicate determinations of Pb from NIST SRM 981 and GSJ JP-1 (peridotite; 0.07 mu g g(-1) Pb) were performed. The measured concentration and isotopic data were identical, within uncertainty, to published data or to data that were determined independently in this study. The application of this method to U-Pb dating and the determination of the "initial" Pb isotopic composition was also tested. Lead isotopic compositions and the concentrations of Pb, Th and U were determined for a single batch of samples, through the addition of Pb-205-Pb-204, Th-230 and U-235 spikes to samples prior to chemical separation. Also in these experiments, we confirmed that this routine gives accurate data for Pb, Th and U concentrations and Pb isotopic compositions.

We present the first finding of the high-pressure mineral coesite in lawsonite-bearing eclogite xenoliths from the Colorado Plateau, United States. The presence of coesite in these xenoliths supports the hypothesis that the eclogite formed in a low-temperature-high-pressure environment such as envisaged inside subducted oceanic lithosphere. Ion-microprobe U-Pb dating of micrometer-scale zircons in the eclogites yields ages ranging from 81 Ma to 33 Ma, the two extremes in age likely indicating the age of crystallization during subduction-related metamorphism and the age of recrystallization by the host magmatic event, respectively. These observations conclusively demonstrate that certain eclogite xenoliths from the Colorado Plateau originated as fragments of the subducted Farallon plate, which had been residing in the upper mantle since the Late Cretaceous. This is the first conclusive evidence that any eclogite xenoliths can be directly linked to a known subducted plate.

We improved the U-Pb zircon dating method employing HR-SIMS by applying i) preparation of precise and homogeneous zircon standards by ID-TIMS, ii) simultaneous analysis of Pb isotopes by multi-collection system, and iii) use of a highly focused primary ion beam as small as 5 μm in diameter. Using the improved HR-SIMS technique, U-Pb ages of zircons in an eclogite xenolith from the Colorado Plateau were determined. Zircon occurs as small inclusions (&lt 20 μm) in most constituent minerals. Ten zircons were measured, and the concordant analyses of these zircon data yielded a weighted mean value of 65.2 ± 0.7 Ma for the age of crystallization. Petrographic and geochemical observations suggest that this zircon age represents the age of initial subduction of basaltic oceanic crust. We conclude that the eclogite xenolith was a fragment of basaltic oceanic crust subducted to &gt 90 km depth along a cold geotherm with a subduction rate of &gt 2 cm/y in Tertiary.