Eiichi Takazawa

Whole rock compositional variations in an upper mantle peridotite (Horoman, Hokkaido, Japan): Are they consistent with a partial melting process?

Abstract Whole rock major and trace element abundances of the Horoman peridotites were used to understand processes forming lithological and compositional variations in the upper mantle. Similar to other orogenic peridotites, Horoman peridotites range from fertile lherzolites (3 to 4% Al2O3 and CaO) to depleted harzburgites (∼0.5% Al2O3 and CaO). Abundances of major oxides and compatible to moderately incompatible elements vary systematically with variations in MgO content. Such trends are commonly interpreted as indicating that the peridotites formed as residues from varying degrees of partial melting. The fertile end of these trends coincides with estimates of primitive mantle composition. Because of a mismatch between experimental melting trends for spinel peridotite, especially the Na2O-MgO trend, the compositional variations of Horoman peridotites are not consistent with formation as residues from partial melting of spinel peridotite. Non-linear trends in minor and trace element versus major element abundance diagrams also preclude a two-component mixing model. Recent melting experiments on garnet peridotite demonstrate that at 3 GPa the near-solidus peridotite has a large amount of subcalcic clinopyroxene (ca. 27%) coexisting with small amount of garnet (ca. 2%). Residues from polybaric melting of such garnet peridotite are consistent with the abundance variations of major and moderately incompatible elements, such as Na and heavy rare-earth elements, in the Horoman peridotites. A similar conclusion is applicable to other orogenic peridotites such as the Ronda peridotite because their major element compositional variations are similar to the Horoman peridotite.

Non-chondritic platinum-group element ratios in oceanic mantle lithosphere: petrogenetic signature of melt percolation?

Abstract The concentrations of the platinum-group elements (PGE) Ir, Ru, Pt and Pd were determined in 11 abyssal peridotites from ODP Sites 895 and 920, as well in six ultramafic rocks from the Horoman peridotite body, Japan, which is generally thought to represent former asthenospheric mantle. Individual oceanic peridotites from ODP drill cores are characterized by variable absolute and relative PGE abundances, but the average PGE concentrations of both ODP suites are very similar. This indicates that the distribution of the noble metals in the mantle is characterized by small-scale heterogeneity and large-scale homogeneity. The mean Ru/Ir and Pt/Ir ratios of all ODP peridotites are within 15% and 3%, respectively, of CI-chondritic values. These results are consistent with models that advocate that a late veneer of chondritic material provided the present PGE budget of the silicate Earth. The data are not reconcilable with the addition of a significant amount of differentiated outer core material to the upper mantle. Furthermore, the results of petrogenetic model calculations indicate that the addition of sulfides derived from percolating magmas may be responsible for the variable and generally suprachondritic Pd/Ir ratios observed in abyssal peridotites. Ultramafic rocks from the Horoman peridotite have PGE signatures distinct from abyssal peridotites: Pt/Ir and Pd/Ir are correlated with lithophile element concentrations such that the most fertile lherzolites are characterized by non-primitive PGE ratios. This indicates that processes more complex than simple in-situ melt extraction are required to produce the geochemical systematics, if the Horoman peridotite formed from asthenospheric mantle with chondritic relative PGE abundances. In this case, the PGE results can be explained by melt depletion accompanied or followed by mixing of depleted residues with sulfides, with or without the addition of basaltic melt.

EVOLUTION OF THE HOROMAN PERIDOTITE (HOKKAIDO, JAPAN) : IMPLICATIONS FROM PYROXENE COMPOSITIONS

Abstract Processes occurring in the Earths upper mantle are important in controlling evolution of the crust-mantle system. The effects of multiple igneous and metamorphic processes are recorded in upper-mantle peridotites, such as the Horoman Peridotite in Hokkaido, Japan. Geochemical studies of these peridotites and their minerals, combined with determination of the spatial geochemical variations, can be used to understand the sequence of processes that affected the perioditite. In this study we show that compositional zoning patterns of major and trace elements in clinopyroxene porphyroclasts reflect a sub-solidus, closed-system transition from garnet periodotite, equilibrated at 20–24 kbar and 1040–1160°C, to plagioclase periodotite equilibrated at ∼ 7 kbar and 850–950°C. The preservation of compositionally zoned pyroxenes indicates that this transition was a relatively recent process that probably occurred as the Horoman Peridotite was emplaced into the Hidaka metamorphic belt. The clinopyroxene compositions also show that: (1) harzburgites and lherzolites reacted with a fluid/melt that resulted in relative enrichment of highly incompatible elements, such as the light rare-earth elements; and (2) this enrichment process preceded the sub-solidus breakdown of garnet.

COLLAPSING CARBON NANOTUBES AND DIAMOND FORMATION UNDER SHOCK WAVES

Abstract In order to investigate the mechanical properties of carbon nanotubes, dynamic shock wave pressures (⩽50 GPa) were applied on arc-discharge-generated carbon nanotubes containing polyhedral nanoparticles. High-resolution transmission electron microscopy (HRTEM) studies of the shock-recovered samples reveal that: (i) layers of the outer shells of the nanotubes break and transform into curled graphitic structures and (ii) the inner tube walls and bulk material display structural defects. Further X-ray powder diffraction and HRTEM analyses exhibit the presence of diamond nanocrystals which are produced after the shock-wave compression of polyhedral particles (present in the starting material).

Model of layering formation in a mantle peridotite (Horoman, Hokkaido, Japan)

Abstract The Horoman peridotite complex exhibits a conspicuous layered structure. It is found, from the geological and petrological survey, that the pattern of layering has three characteristics: symmetry, asymmetry (subtly collapsed symmetry) and scale invariance. Especially, symmetric and asymmetric patterns clearly recognized in the sequence of mafic layers at the Northern ridge of Apoi-dake peak, and at the Western ridge of Bozu-yama peak are noticeable. We present a simple mathematical model describing stretching (thinning) and folding during deformation that accounts for the three characteristics. The model quantitatively reproduces the slope in cumulative frequency distribution of the width of mafic layers and indicates that the frequency distribution is strongly influenced by the spatial strain contrast. Applying the model result to the observational data for mafic layers, it is found that the strain contrast approximately ranges several to 10 times between regions with the highest and the lowest strain rates.

Shock-induced phase transitions among SiC polytypes

A series of recovery experiments was conducted using a propellant single-stage gun on starting materials of both α-SiC and β-SiC. X-ray examination on the recovered samples indicated that obvious polytype transformations among 3C, 6H, and 15R took place. To the α-SiC starting material, 15R tends to increase and 6H tends to decrease, while a small amount of α-SiC form transforms to 3C type, along with increasing the shock temperature and pressure. X-ray diffraction analysis showed that the β-SiC polytype is transformed into rhombohedral forms. From results of both types of SiC samples, rhombohedral polytypes seem to be the favored shock modification. The effects of shock pressure and shock temperature and their heterogeneous distribution on these polytype transitions are discussed in detail. Analysis showed that these polytype transitions resulted from the stacking sequence changes of SiC atom layers.

A kilometre-scale highly refractory harzburgite zone in the mantle section of the northern Oman Ophiolite (Fizh Block): implications for flux melting of oceanic lithospheric mantle

Abstract We report the major element compositions of constituent minerals in 278 harzburgites and of 101 whole rocks from the northern Fizh mantle section in the northern Oman Ophiolite to investigate the formation and evolution of oceanic lithospheric mantle. Olivine Fo varies from 90 to 92 whereas spinel Cr# (= Cr/(Cr+Al) atomic ratio) varies from 0.15 to 0.78. The Cr# of spinels in a large number of harzburgites exceeds 0.6, which is the upper bound for abyssal peridotites. In the northern Fizh mantle section, highly refractory harzburgites with spinel Cr# greater than 0.7 are distributed in a 3-km-wide band along a NW–SE-striking shear zone. We infer a two-stage depletion process in the northern Fizh mantle section. In the first stage, asthenospheric mantle was partially melted beneath a mid-ocean ridge, producing a harzburgitic residual column. In the second stage during detachment of oceanic lithosphere an H2O-rich fluid, released from the metamorphic sole due to thermal metamorphism of altered oceanic crust, extensively infiltrated the northern Fizh mantle section where the ridge segment boundary region was previously located. The residual harzburgites were subjected to flux melting, resulting in a highly refractory harzburgite zone with spinel Cr# greater than 0.7. Supplementary material: Locality map, names of the wadis and UTM coordinates and further analytical data are available at http://www.geolsoc.org.uk/SUP18679.

Hugoniot equation of state and high‐pressure transformation of jadeite

The Hugoniot equation of state of jadeite (NaAlSi206) was investigated with a two-stage light gas gun extending the range of shock pressure up to 150 GPa. From the present results coupled with previous data a high-pressure phase transformation was observed in jadeite over 60 GPa. Moreover, the Hugoniot elastic limit was found to be ---7 GPa. Fitting the theoretical Hugoniot equation of state to the high-pressure phase results in a bulk modulus at ambient condition of 280 _+ 10 GPa for a zero-pressure density of 4.26 _+ 0.05 g cm -3. These physical properties for high-pressure NaAlSi20 6 are consistent with the perovskite-type structure at pressure above 120 GPa. Because its density is close to MgSiO 3 and CaSiO 3 perovskites, NaA1Si20 6 perovskite is a candidate to be a host for sodium and aluminum near the core-mantle boundary.

Radiation temperatures of soda-lime glass in its shock-compressed liquid state

A radiation pyrometer in conjunction with a two-stage light gas gun has been used to measure shock temperatures of soda-lime glass in the pressure range 54–109 GPa. This pyrometer consists of two parts, i.e., an optical multichannel analyzer which measures the radiation spectrum over the visible range (∼450 nm window) and a four-channel photomultiplier tube system which records the time-varying behavior of shock temperatures. The measured radiation spectra are compared with the Planck function to estimate the shock temperatures and emissivities. Obtained spectra are well fit by the Planck function with moderate emissivities, indicating that relatively homogeneous thermal radiation is the main component of radiation. Obtained shock temperatures range from 2800 (100) to 5700 (300) K and they seem to represent shock temperatures of liquefied soda-lime glass (melt). The Hugoniot is well described by a linear relation, us=0.14(21)+1.92(5)up km/s. It is deduced that the radiation from liquids under shock compre...

Distribution and hosts of arsenic in a sediment core from the Chianan Plain in SW Taiwan: Implications on arsenic primary source and release mechanisms

High arsenic abundance of 50-700μg/L in the groundwater from the Chianan Plain in southwestern Taiwan is a well-known environmental hazard. The groundwater-associated sediments, however, have not been geochemically characterized, thus hindering a comprehensive understanding of arsenic cycling in this region. In this study, samples collected from a 250m sediment core at the centre of the Chianan Plain were analyzed for arsenic and TOC concentrations (N=158), constituent minerals (N=25), major element abundances (N=105), and sequential arsenic extraction (N=23). The arsenic data show a prevalence of >10mg/kg with higher concentrations of 20-50mg/kg concentrated at 60-80 and 195-210m. Arsenic was extracted mainly as an adsorbate on clay minerals, as a co-precipitate in amorphous iron oxyhydroxide, and as a structural component in clay minerals. Since the sediments consist mainly of quartz, chlorite, and illite, the correlations between arsenic concentration and abundances of K2O and MgO pinpoint illite and chlorite as the major arsenic hosts. The arsenic-total iron correlation reflects the role of chlorite along with the contribution from amorphous iron oxyhydroxide as indicated by arsenic extraction data. Organic matter is not the dominant arsenic host for low TOC content, low arsenic abundance extracted from it, and a relatively low R(2) of the arsenic-TOC correlation. The major constituent minerals in the sediments are the same as those of the upriver metapelites, establishing a sink-source relationship. Composition data from two deep groundwater samples near the sediment core show Eh values and As(V)/As(III) ratios of reducing environments and high arsenic, K, Mg, and Fe contents necessary for deriving arsenic from sediments by desorption from clay and dissolution of iron oxyhydroxide. Therefore, groundwater arsenic was mainly derived from groundwater-associated sediments with limited contributions from other sources, such as mud volcanoes.