Mineralogical characterization of a unique material having heavy oxygen isotope anomaly in matrix of the primitive carbonaceous chondrite Acfer 094

We report the mineral compositions and micro-texture of the isotopically anomalous (δ^17,18O_SMOW ∼ +180‰) Fe-S-Ni-O material recently discovered in matrix of the primitive carbonaceous chondrite Acfer 094 [Sakamoto N., Seto Y., Itoh S., Kuramoto K., Fujino K., Nagashima K., Krot A. N. and Yurimoto H. (2007) Oxygen isotope evidence for remnants of the early solar system primordial water. Science317, 231-233]. Synchrotron radiation X-ray diffraction and transmission electron microscopy studies indicate that this material consists of the symplectitically intergrown magnetite (Fe₃O₄) and pentlandite (Fe_5.7Ni_3.3S₈) with magnetite/pentlandite volume ratio of ∼2.3. Magnetite forms column-shaped grains (10-30 nm in diameter and 100-200 nm in length); pentlandite occurs as worm-shaped grains or aggregates of grains 100-300 nm in size between magnetite crystals. Although both the X-ray diffraction and electron energy loss spectra support identification of iron oxide as magnetite, the electron diffraction patterns show that magnetite has a weak 3-fold superstructure, possibly due to ordering of vacancies. We infer that the isotopically anomalous symplectite formed by sulfurization and oxidization of metal grains either in the solar nebula or on an icy planetesimal. The intersite cation distribution of pentlandite suggests that timescale of oxidation was no longer than 1000 years.     

A comparison of methods for calculating the motion of a semi-submersible

The 17th ITTC Ocean Engineering Committee undertook the comparison of methods for calculating semi-submersible wave motion, and 34 programs from 28 different organizations participated in the project. The summary of the results are reported in the Technical Report of the 17th ITTC (ITTC, 1984).In this paper, the details of the project are described. Namely, almost all the calculation results of the 34 programs are shown and examined from the viewpoints of the validity of the program itself and of the correlation between the differences in the various calculation methods and the differences in the calculation results. The calculation results are also compared with the experiments, the details of which are also illustrated.     

Unexpectedly high-PGE chromitite from the deeper mantle section of the northern Oman ophiolite and its tectonic implications

Unusually high, platinum-group element (PGE) enrichments are reported for the first time in a podiform chromitite of the northern Oman ophiolite. The chromitite contains Б.5 ppm of total PGE, being highly enriched in the IPGE subgroup (Ir, Os and Ru) and strongly depleted in the PPGE subgroup (Rh, Pt and Pd). Its platinum-group minerals (PGMs) are classified into three types arranged in order of abundance: (1) sulphides (Os-rich laurite, laurite-erlishmanite solid solution and an unnamed Ir sulphide), (2) alloys (Os-Ir alloy and Ir-Rh alloy), and (3) sulpharsenides (irarsite and hollingworthite). The high PGE concentrations are observed only in a discordant chromitite deep in the mantle section, which has high-Cr# (>0.7) spinel with an olivine matrix. All the other types of chromitite (in the Moho transition zone (MTZ) and concordant pods in the deeper mantle section) are poor in PGEs and tend to have spinels with lower Cr# (up to 0.6). This diversity of chromitite types suggests two stages of magmatic activity were responsible for the chromitite genesis, in response to a switch of tectonic setting. The first is residual from lower degree, partial melting of peridotite, which produced low-Cr#, PGE-poor chromitites at the Moho transition zone and, to a lesser extent, within the mantle, possibly beneath a fast-spreading mid-ocean ridge. The second chromitite-forming event involves higher degree partial melting, which produced high-Cr#, PGE-rich discordant chromitite in the upper mantle, possibly in a supra-subduction zone setting.     

Petrological feature of spinel lherzolite xenolith from Oki-Dogo Island: An implication for variety of the upper mantle peridotite beneath southwestern Japan

Abstract   Spinel lherzolite is a minor component of the deep-seated xenolith suite in the Oki-Dogo alkaline basalts, whereas other types of ultramafic (e.g. pyroxenite and dunite) and mafic (e.g. granulite and gabbro) xenoliths are abundant. All spinel lherzolite xenoliths have spinel with a low Cr number (Cr#; < 0.26). They are anhydrous and are free of modal metasomatism. Their mineral assemblages and microtextures, combined with the high NiO content in olivine, suggest that they are of residual origin. But the Mg numbers of silicate minerals are lower (e.g. down to Fo₈₆) in some spinel lherzolites than in typical upper mantle residual peridotites. The clinopyroxene in the spinel lherzolite shows U-shaped chondrite-normalized rare-earth element (REE) patterns. The abundance of Fe-rich ultramafic and mafic cumulate xenoliths in Oki-Dogo alkali basalts suggests that the later formation of those Fe-rich cumulates from alkaline magma was the cause of Fe- and light REE (LREE)-enrichment in residual peridotite. The similar REE patterns are observed in spinel peridotite xenoliths from Kurose and also in those from the South-west Japan arc, which are non-metasomatized in terms of major-element chemistry (e.g. Fo > 89), and are rarely associated with Fe-rich cumulus mafic and ultramafic xenoliths. This indicates that the LREE-enrichment in mantle rocks has been more prominent and prevalent than Fe and other major-element enrichment during the metasomatism.     

Origin of low-Ca pyroxene in amoeboid olivine aggregates: Evidence from oxygen isotopic compositions

Amoeboid olivine aggregates (AOAs) in primitive carbonaceous chondrites consist of forsterite (Fa_<2), Fe,Ni-metal, spinel, Al-diopside, anorthite, and rare gehlenitic melilite (Åk_<15). ∼10% of AOAs contain low-Ca pyroxene (Fs_1-3Wo_1-5) that is in corrosion relationship with forsterite and is found in three major textural occurrences: (i) thin (<15 μm) discontinuous layers around forsterite grains or along forsterite grain boundaries in AOA peripheries; (ii) 5-10-μm-thick haloes and subhedral grains around Fe,Ni-metal nodules in AOA peripheries, and (iii) shells of variable thickness (up to 70 μm), commonly with abundant tiny (3-5 μm) inclusions of Fe,Ni-metal grains, around AOAs. AOAs with the low-Ca pyroxene shells are compact and contain euhedral grains of Al-diopside surrounded by anorthite, suggesting small (10%-20%) degree of melting. AOAs with other textural occurrences of low-Ca pyroxene are rather porous. Forsterite grains in AOAs with low-Ca pyroxene have generally ¹⁶O-rich isotopic compositions (Δ¹⁷O < −20‰). Low-Ca pyroxenes of the textural occurrences (i) and (ii) are ¹⁶O-enriched (Δ¹⁷O < −20‰), whereas those of (iii) are ¹⁶O-depleted (Δ¹⁷O = −6‰ to −4‰). One of the extensively melted (>50%) objects is texturally and mineralogically intermediate between AOAs and Al-rich chondrules. It consists of euhedral forsterite grains, pigeonite, augite, anorthitic mesostasis, abundant anhedral spinel grains, and minor Fe,Ni-metal; it is surrounded by a coarse-grained igneous rim largely composed of low-Ca pyroxene with abundant Fe,Ni-metal-sulfide nodules. The mineralogical observations suggest that only spinel grains in this igneous object were not melted. The spinel is ¹⁶O-rich (Δ¹⁷O ∼ −22‰), whereas the neighboring plagioclase mesostasis is ¹⁶O-depleted (Δ¹⁷O ∼ −11‰).We conclude that AOAs are aggregates of solar nebular condensates (forsterite, Fe,Ni-metal, and CAIs composed of Al-diopside, anorthite, spinel, and ±melilite) formed in an ¹⁶O-rich gaseous reservoir, probably CAI-forming region(s). Solid or incipiently melted forsterite in some AOAs reacted with gaseous SiO in the same nebular region to form low-Ca pyroxene. Some other AOAs appear to have accreted ¹⁶O-poor pyroxene-normative dust and experienced varying degrees of melting, most likely in chondrule-forming region(s). The most extensively melted AOAs experienced oxygen isotope exchange with ¹⁶O-poor nebular gas and may have been transformed into chondrules. The original ¹⁶O-rich signature of the precursor materials of such chondrules is preserved only in incompletely melted grains.     

The Nature of Mantle Rocks in Ophiolites of the Polar Urals

This work presents the results of geochemical (LA-ICP-MS) study of minerals of peridotites from ophiolite complexes of the Polar Urals to clarify the nature of these formations. The distribution of trace and rare earth elements in clinopyroxenes testifies that there were three types of the mantle substratum, which formed in different geodynamic settings. Two types of primary peridotites were formed upon partial melting of the mantle at different-depth levels in the subduction zone. The first type is represented by lherzolites and diopside harzburgites, formed at partial melting under the spinel facies conditions; the second type is represented by diopside harzburgites, formed under polybaric partial melting under the garnet and spinel facies conditions. In the suprasubduction zone, peridotites experienced fluid-induced partial melting that resulted in crystallization of harzburgites. All types of harzburgites were transformed by ascending melts and fluids (refertilization) and high-temperature hydration with the formation of amphibole. These processes are recorded in variations in the REE spectra of minerals.     

Pressure-temperature dependent compositional variation of phlogopitic micas in upper mantle peridotites

Compositional variation of phlogopitic micas in upper mantle peridotites is reviewed. Phlogopitic micas in garnet peridotites are systematically lower in Al (or eastonite component) than those in spinel peridotites. The core of phlogopite megacryst and phenocryst of kimberlite is always lower in Al than the rim. It is apparent that Al/(Al + Si) ratio or eastonite component in phlogopitic micas in ultramafic rocks is controlled by the equilibrium pressure and temperature. In the upper mantle peridotites containing garnet or spinel, the Al/(Al + Si) ratio of phlogopitic mica decreases with increasing pressure at constant temperature. Phlogopitic mica is a potential thermo-barometer in both garnet- and spinel-peridotite facies.     

Field survey of the East Java earthquake and tsunami of June 3, 1994

A field survey of the June 3, 1994 East Java earthquake tsunami was conducted within three weeks, and the distributions of the seismic intensities, tsunami heights, and human and house damages were surveyed. The seismic intensities on the south coasts of Java and Bali Islands were small for an earthquake with magnitudeM 7.6. The earthquake caused no land damage. About 40 minutes after the main shock, a huge tsunami attacked the coasts, several villages in East Java Province were damaged severely, and 223 persons perished. At Pancer Village about 70 percent of the houses were swept away and 121 persons were killed by the tsunami. The relationship between tsunami heights and distances from the source shows that the Hatori's tsunami magnitude wasm=3, which seems to be larger for the earthquake magnitude. But we should not consider this an extraordinary event because it was pointed out byHatori (1994) that the magnitudes of tsunamis in the Indonesia-Philippine region generally exceed 1–2 grade larger than those of other regions.