Katsura Kobayashi

Fate of the subducted Farallon plate inferred from eclogite xenoliths in the Colorado Plateau

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.

Mantle metasomatism and rapid ascent of slab components beneath island arcs: Evidence from 238U‐230Th‐226Ra disequilibria of Miyakejima volcano, Izu arc, Japan

[1] 238 U- 230 Th- 226 Ra systematics in lavas from Miyakejima volcano, Japan, are presented to estimate the timescale of magmatic processes beneath an island arc. Miyakejima volcano has four recent eruptive stages (Stages 1-4) starting >7000 BP. 238 U- 230 Th- 226 Ra disequilibria observed in lavas with large 238 U and 226 Ra excesses imply metasomatism of depleted mantle by fluid-related processes. This metasomatism is also suggested by trace element and Sr-Nd-Pb isotopic systematics in the same lavas. In the equiline diagram, the trends for two magmatic stages (Stages 1 and 2) are regarded as two different isochrons with a common initial ( 230 Th/ 232 Th) ratio, although the trend for Stages 3 and 4 is a magma mixing line. Our model calculations show that slab-derived fluids can deliver some Th and a very rapid ascent time of the slab components in the mantle wedge (< 7 kyr) is inferred. This rapid ascent can be explained by nearly instantaneous material transport in the mantle wedge by a hydrofracture model for fluid and a channel flow model for melt. Such a timescale estimate is not increased even if melting processes that enhance 226 Ra are taken into account. The age difference in the equiline diagram corresponds to the interval of individual fluid-release events (13 kyr between Stages 1 and 2, and 5 kyr between Stages 2 and 3). Thus fluid release from the slab and subsequent magma generation occur as episodic events on a several-kiloyear timescale.

Space environment of an asteroid preserved on micrograins returned by the Hayabusa spacecraft

Records of micrometeorite collisions at down to submicron scales were discovered on dust grains recovered from near-Earth asteroid 25143 (Itokawa). Because the grains were sampled from very near the surface of the asteroid, by the Hayabusa spacecraft, their surfaces reflect the low-gravity space environment influencing the physical nature of the asteroid exterior. The space environment was examined by description of grain surfaces and asteroidal scenes were reconstructed. Chemical and O isotope compositions of five lithic grains, with diameters near 50 μm, indicate that the uppermost layer of the rubble-pile-textured Itokawa is largely composed of equilibrated LL-ordinary-chondrite-like material with superimposed effects of collisions. The surfaces of the grains are dominated by fractures, and the fracture planes contain not only sub-μm-sized craters but also a large number of sub-μm- to several-μm-sized adhered particles, some of the latter composed of glass. The size distribution and chemical compositions of the adhered particles, together with the occurrences of the sub-μm-sized craters, suggest formation by hypervelocity collisions of micrometeorites at down to nm scales, a process expected in the physically hostile environment at an asteroid’s surface. We describe impact-related phenomena, ranging in scale from 10-9 to 104 meters, demonstrating the central role played by impact processes in the long-term evolution of planetary bodies. Impact appears to be an important process shaping the exteriors of not only large planetary bodies, such as the moon, but also low-gravity bodies such as asteroids.

Origin of atoll garnets in eclogites and implications for the redistribution of trace elements during slab exhumation in a continental subduction zone

Abstract Detailed electron- and ion-microprobe analyses were carried out on atoll-shaped and normal garnets in ultrahigh-pressure (UHP) metamorphic eclogite from Dabie, east-central China. Compositional profiles of both normal garnets and rings of atoll garnets show well-preserved growth zoning with a decrease in Mn, Ca, and heavy rare earth elements (HREEs), and an increase in Mg toward rims. Manganese and middle rare earth element (MREE) enrichments are observed near garnet rims. Island- and peninsula-shaped garnet inside atolls are homogeneous in major elements and show the same composition as garnet rims, whereas the HREE concentrations are similar to those of the normal garnet cores. Electron back-scatter diffraction (EBSD) analyses show that the island- and peninsula-shaped garnet fractions inside atolls have crystallographic orientations identical to that of the atoll rings. These observations suggest that the atoll garnets were formed by the consumption of earlier-formed cores by fluid released from both hydroxyl exsolution from the nominally anhydrous minerals (NAMs) and lawsonite decomposition at the onset of exhumation, (i.e., garnet breakdown was from the inside and re-growth from outside to inside). Though somewhat restricted, this study reveals that because garnet and zircon act as sinks for HREEs and Zr, respectively, the majority of released HREEs and Zr were likely re-incorporated into newly grown garnet and zircon during the Dabie UHP metamorphic slab exhumation.

Transformation of Subcontinental Lithospheric Mantle through Peridotite-Melt Reaction: Evidence from a Highly Fertile Mantle Xenolith from the North China Craton

A highly fertile mantle xenolith entrained in the Upper Cretaceous Daxizhuang basalt, Shandong Province, China provides petrological evidence for reaction between basaltic melt and refractory peridotite in Mesozoic lithospheric mantle beneath the eastern North China craton. The xenolith is zoned with a lherzolite core, a sheared wehrlite mantle, and a reactant rim. The lherzolite has a medium-grained granular texture and contains olivine, orthopyroxene, clinopyroxene, and spinel with low Mg# values (≤87). This is the lowest value observed globally in mantle xenoliths entrained in both kimberlites and basalts from old cratons, but is close to low-Mg# spinel lherzolites entrained in adjacent region of the eastern North China craton. The wehrlite mantle shows clear orientation and foliation of constituent olivine and clinopyroxene. The reactant rim displays a symplectite texture of fine-grained olivine, clinopyroxene, and spinel, and is characterized by LREE-enriched patterns. Orthopyroxene is lacking from the sheared mantle and reactant rim. Clear zoning and systematic core-rim variations in major and trace elements of the constituent minerals demonstrate the occurrence of the refertilization through the melt-peridotite reaction in the xenolith. This reaction may have produced the low-Mg# peridotite from a high-Mg# protolith and could be an important mechanism for the change of the subcontinental lithospheric mantle from a Paleozoic refractory mantle to a Late Mesozoic fertile mantle.

Eclogite–high-pressure granulite metamorphism records early collision in West Gondwana: new data from the Southern Brasília Belt, Brazil

Abstract: Nappes in the southern sector of the Southern Brasília Belt record suturing of the Paranapanema Block and Socorro–Guaxupé Arc with a subducted passive margin on the western side of the São Francisco Craton. We report secondary ion mass spectrometry U–Pb zircon ages that for the first time constrain the age of: (1) retrograded eclogite from a block along the tectonic contact beneath the uppermost nappe in a stack of passive margin-derived nappes; (2) high-pressure granulite-facies metamorphism in the uppermost passive margin-derived nappe; (3) high-pressure granulite-facies metamorphism in the overlying arc-derived nappe. Rare zircons from a retrograded eclogite yield a 206Pb/238U age of 678 ± 29 Ma, which we interpret as most likely to date close-to-peak-P metamorphism and to provide a minimum age for detachment of the overlying passive margin-derived nappe from the subducting plate. Zircon associated with ilmenite in samples from two structural levels in the passive margin-derived high-pressure granulite nappe yields 206Pb/238U ages of 648 ± 12 and 647 ± 11 Ma, and Ti-in-zircon crystallization temperatures from c. 860 °C down to c. 785 °C, but skewed toward the lower part of the range. These data indicate zircon formation during cooling from around peak T to the solidus, consistent with the high-T retrograde P–T path deduced from microstructures linked to phase assemblage fields in isochemical phase diagrams. Rb–Sr multi-mineral–whole-rock isochrons from two samples from close to the bottom of this nappe date formation of a retrograde sillimanite-bearing penetrative fabric to c. 590 Ma at temperatures of c. 750 °C (based on Ti-in-quartz thermometry). Rare zircons from leucosome in high-pressure granulite from the overlying arc-derived nappe yield a 206Pb/238U age of 622 ± 28 Ma and Ti-in-zircon crystallization temperatures from c. 970 °C down to c. 820 °C, which we interpret to record formation of zircon during cooling from peak high-pressure granulite-facies conditions. These ages indicate that the first stage of craton amalgamation in West Gondwana may have occurred earlier than previously inferred. Supplementary material: Geochemical and geochronological data are available at http://www.geolsoc.org.uk/SUP18377.

U-Pb Zircon Dating of Regional Deformation in the Lower Crust of the Kohistan Arc

Felsic veins several cm thick to bodies several hundreds of m wide are sporadically distributed in the Kamila amphibolites of the Kohistan arc sequence. Some small dikes (less than one m thick) were deformed together with ductile deformation of wall-rock amphibolites; other small undeformed dikes crosscut foliation in the amphibolites. Large granitic intrusive bodies include gneissose and massive varieties. Ion-microprobe U-Pb dating of zircon grains reveal ages as follows: three deformed dikes, 107.7 ± 1.8 Ma, 94.0 ± 1.9 Ma, 81.0 ± 1.6 Ma; a single gneissose granitic body, 89.5 ± 4.2 Ma; three undeformed dikes, 82.0 ± 2.0 Ma, 80.6 ± 4.5 Ma, 75.8 ± 1.7 Ma; a massive granitic body, 75.7 ± 2.2 Ma; and a foliated amphibolite xenolith in the massive granitic body, 110.7 ± 4.9 Ma. Ages of the deformed dikes, the gneissose granitic body, and the amphibolite xenolith indicate that ductile deformation in the lower crust of the Kohistan arc took place episodically or successively during the period from 107 to 81 Ma. Ages of the undeformed dikes and the massive granitic body indicate that ductile deformation ceased prior to 80 Ma. Collision between Asia and the Kohistan arc elsewhere was inferred to be before 80 Ma. We conclude that the ductile deformation events did not extend beyond the time of Asia-Kohistan collision.

Discovery of Archean continental and mantle fragments inferred from xenocrysts in komatiites, the Belingwe greenstone belt, Zimbabwe

Controversy exists as to whether the 2.7 Ga Belingwe greenstone belt, Zimbabwe, is autochthonous or allochthonous. In this study we report direct evidence for an autochthonous continental setting for the Belingwe greenstone belt. Garnet and clinopyroxene xenocrysts were discovered in the fresh ultramafic komatiites. Major and trace element compositions of these xenocrysts suggest that they originated from mafic lower crust, presumably garnet granulite in composition, at a low temperature of ∼600 or ∼700 °C. Furthermore, in a komatiitic basalt sample, we have discovered orthopyroxene and clinopyroxene xenocrysts that may have originated from lithospheric mantle beneath the continental margin. Discoveries of these xenocrysts indicate that the komatiite magma was transported to the surface and quenched rapidly enough to prevent the complete melting of continental fragments.

5d electronic states in Pt alloys probed by magnetic circular X-ray dichroism

Abstract For transition metal-platinum compounds with Cu3Au-type superstructure, the Pt 5d electronic states are studied by magnetic circular X-ray dichroism at the Pt L2,3-edges. The derived Pt 5d spin, orbital and total magnetic moments are in agreement with neutron scattering data and band calculations. The Pt 2p dichroism acts as a probe of magnetic states in transition metals across the series.

Magnetic circular X-ray dichroism at Fe K-edge in ferrimagnetic Fe-oxides☆

Abstract Magnetic circular X-ray dichroism at the Fe K-edge has been recorded from ferrimagnetic Fe-oxides. The characteristic pre-peak structure is specified to the 1s → 3d transitions relating to the Fe 3+ ion at tetrahedral sites. Main absorption depends on local symmetry around the Fe ion lying at octahedral sites.