Water in Earth's mantle: Hydrogen analysis of mantle olivine, pyroxenes and garnet using the SIMS

Hydrogen (or water) in the Earth's interior plays a key role in the evolution and dynamics of the planet. However, the abundance and the existence form of the hydrogen have scarcely been clear in practice. Hydrogen in the mantle was incorporated in the interior during the formation of the Earth. The incorporated hydrogen was hardly possible to concentrate locally inside the Earth considering its high mobility and high reactivity. The hydrogen, preferably, could be distributed homogeneously over the mantle and the core by the subsequent physical and chemical processes. Therefore, hydrogen in the mantle could be present in the form of trace hydrogen in nominally anhydrous mantle minerals. The hydrogen and the other trace elements in mantle olivines, orthopyroxenes, clinopyroxenes, and garnets were determined using secondary ion mass spectrometry (SIMS) for elucidating (1) the exact hydrogen contents, (2) the correlation between the hydrogen and the other trace elements, (3) the dependence of the hydrogen contents on the depth, and (4) the dependence of the whole rock water contents on the depth

Possible sub-arc origin of podiform chromitites

金沢大学理工研究域自然システム学系The sub-arc mantle condition possibly favors the formation of podiform chromitites. The Cr/(Cr + Al) atomic ratio (= Cr no.) of their chromian spinel frequently is higher than 0.7. This almost excludes the possibility of their sub-oceanic origin, because both oceanic peridotites and MORB have chromian spinel with the Cr no. < 0.6. Precipitation of chromitite and associated dunite enhances a relative depletion of HFSE to LILE, one of chemical characteristics of arc magmas, for the involved magma. This cannot alter completely, however, the MORB to the arc-type magma, especially for Ti and Zr. The presence of chromitite xenoliths, similar both in texture and in chemistry to podiform chromitites of some ophiolitic complexes, in some Cenozoic alkali basalts from the southwest Japan arc indicates directly that the upper mantle beneath the Japan arcs has chromitites. -from Author

Ion microprobe mesearments of Mg isotopes in Type B1 CAI of Allende meteorite

Magnesium isotopes in individual mineral grains of a Ca-Al rich inclusion from the Allende meteorite have been measured by secondary ion mass spectrometry. An electrostatic peak switching system was used to make a precise isotopic measurement in high mass resolution mode (M/&lrtri;M=&acd;4000). The inclusion shows excess of ^Mg correlated with the ^Al/^Mg ratio. The results suggest that live ^Al decayed in the inclusion which formed simultaneously in the solar nebula. The relative abundance of ^Al(^Al/^Al=3.12×10^) is close to the "canonical" value (^Al/^Al=&acd;5×10^) for coarse-grained CAIs

Search for 60Ni excesses in MET-78008 ureilite: An ion microprobe study

We have developed a technique for in-situ Ni isotopic analysis using the ion microprobe, in order to detect ^Ni excess from the decay of the short lived nuclide ^Fe (half life=1.5Ma) in ureilite samples. The silicate minerals from MET-78008 ureilite with an old U-Pb age of 4.563±0.006 Ga were analyzed. The ^Fe/^Ni ratios of olivine and orthopyroxene are between 2700 and 5400. In spite of the high Fe/Ni ratios, we could not observe any detectable ^Ni excess. From the mean value of olivine core data, we obtain an upper limit of the ^Fe/^Fe ratio at the time of ureilite formation of 1.8×10^. The time difference between CAI formation and ureilite formation was estimated to be more than 4 million years, which is consistent with the UPb data from the same meteorite. We concluded that the impact event for the disruption of the ureilite parent body happened more than 4 million years after CAI formation. However, a large uncertainty in the initial ^Fe/^Fe ratio is introduced by the possibility that the ^Ni excess observed in CAIs is of nucleosynthetic origin. Our conclusion may change if the initial ^Fe/^Fe ratio of the solar system using CAI data is too high

ロシア・南バイカルKhenteyドーム、Burkal川流域に産するマントル捕獲岩中の単斜輝石の微量元素研究

Trace element chemistry of clinopyroxene in the mantle xenoliths from melanephelinites of the Burkal volcanic group has been studied. The Burkal group is composed of several local outcrops of 5-8 Ma melanephelinites within the Khentey domal uplift near the Russia/Mongolia boundary. Cr-diopside group xenoliths include garnet and spinel lherzolite, spinel harzburgite and dunite, and garnet and spinel pyroxenites. Hydrous minerals were not detected, however shallow mantle feldspatic metasomatism is present. Clinopyroxene from garnet lherzolites has high TiO_2, Al_2O_3, and Na_2O relative to clinopyroxene from spinel lherzolites. Olivine has composition of Fo_. Spinel has Mg#=60-80 and contains 10-46 wt.% Cr_2O_3. Clinopyroxene from garnet lherzolites has REE patterns typical for fertile peridotites. Trace element patterns of clinopyroxene from depleted spinel peridotites show progressive depletion in HREE and HFSE and enrichment in LREE toward more depleted varieties of harzburgites and dunites. REE patterns of clinopyroxene in harzburgites are strongly U-shaped and have (La/Sm)n=5-36 and (Sm/Yb)n=0.4-2.1. Clinopyroxene in harzburgites has also extremely low Zr content (0.4-3.4ppm) and high Ti/Zr ratio ranged in 190-240. The patterns of clinopyroxene in depleted peridotites are indicative of significant partial melting (up to 15-20%) of the primary substrate followed by cryptic metasomatic enrichment by silicate or carbonatitic melt. Estimation of T-P parameters for garnet lherzolites reveals equilibration at 17-23 kbar (60-90km depths) and 1050-1150℃. T-estimations for harzburgites and dunites indicate, that they may form veins at 50-70km depth, whereas shallow mantle (low-T) depleted peridotites were not detected. The uppermost mantle may be composed of fertile spinel lherzolites.論文Articl

Silicon self-diffusion in wadsleyite: Implications for rheology of the mantle transition zone and subducting plates

/s] exp (À248 [kJ/mol]/RT), respectively. Si diffusion rates in wadsleyite are about 5 orders of magnitude slower than Mg-Fe interdiffusion rates at 1400°C. Assuming that Si is the slowest diffusing species in wadsleyite, the geophysical model of the viscosity in the mantle transition zone can be explained by diffusion creep in wadsleyite for a grain size of about 0.5-5 mm. Some portions in cold subducting slabs, where the grain size reduces to less than 1 mm after the olivinespinel transformation, become weaker than the surrounding mantle

Mineralogical and oxygen isotopic study of a new ultrarefractory inclusion in the Northwest Africa 3118 CV3 chondrite

Calcium‐aluminum‐rich inclusions (CAIs) are the first solid materials formed in the solar nebula. Among them, ultrarefractory inclusions are very rare. In this study, we report on the mineralogical features and oxygen isotopic compositions of minerals in a new ultrarefractory inclusion CAI 007 from the CV3 chondrite Northwest Africa (NWA) 3118. The CAI 007 inclusion is porous and has a layered (core–mantle–rim) texture. The core is dominant in area and mainly consists of Y‐rich perovskite and Zr‐rich davisite, with minor refractory metal nuggets, Zr,Sc‐rich oxide minerals (calzirtite and tazheranite), and Fe‐rich spinel. The calzirtite and tazheranite are closely intergrown, probably derived from a precursor phase due to thermal metamorphism on the parent body. The refractory metal nuggets either exhibit thin exsolution lamellae of Fe,Ni‐dominant alloy in Os,Ir‐dominant alloy or are composed of Os,Ir,Ru,Fe‐alloy and Fe,Ni,Ir‐alloy with troilite, scheelite, gypsum, and molybdenite. The later four phases are apparently secondary minerals. The Zr,Sc,Y‐rich core is surrounded by a discontinuous layer of closely intergrown hibonite and spinel. The CAIs are rimmed by Fe‐rich spinel and Al‐rich diopside. Perovskite has high concentrations of the most refractory rare earth elements (REEs) but is relatively depleted in the moderately refractory and volatile REEs, consistent with the ultrarefractory REE pattern. Based on this unusual Zr,Sc,Y‐rich mineral assemblage, the layered distribution in CAI 007, and the REE concentrations in perovskite, we suggest that CAI 007 is an ultrarefractory inclusion of condensation origin. In CAI 007, hibonite, spinel, and probably Al‐rich diopside are ¹⁶O‐rich (Δ¹⁷O ~–22‰) whereas perovskite and davisite are ¹⁶O‐poor (Δ¹⁷O ~–3‰). Such oxygen isotope heterogeneity suggests that the UR inclusion formed in the various degrees of ¹⁶O‐rich nebular setting or was originally ¹⁶O‐rich and then experienced oxygen isotope exchange with ¹⁶O‐poor fluid on the CV3 chondrite parent body