Hisayoshi Yurimoto

Hydrous modified spinel, Mg1.75SiH0.5O4: A new water reservoir in the mantle transition region

The water content in modified spinel, synthesized at 15.5 GPa under hydrous conditions, has been measured by means of secondary ion mass spectrometry (SIMSrpar;. We found that the modified spinel crystals contain up to 3.1±0.4 wt % H2O, which is consistent with the amounts estimated from the deficits in the oxide totals of the microprobe analysis. X-ray diffraction analyses for a single crystal showed that the sample containing 2.5±0.3 wt % H2O is of the modified spinel structure with the lattice parameters a=5.663(1), b=11.546(2), c=8.247(4)A and V=539.2(5)A³. The present results suggest that a considerable amount of H2O may exist as hydrous modified spinel in the mantle transition zone, which could have important implications for the constitution and dynamics of the mantle.

Three-Dimensional Structure of Hayabusa Samples: Origin and Evolution of Itokawa Regolith

Laboratory analysis of samples returned from an asteroid establishes a direct link between asteroids and meteorites and provides clues to the complex history of the asteroid and its surface. Regolith particles on the asteroid Itokawa were recovered by the Hayabusa mission. Their three-dimensional (3D) structure and other properties, revealed by x-ray microtomography, provide information on regolith formation. Modal abundances of minerals, bulk density (3.4 grams per cubic centimeter), and the 3D textures indicate that the particles represent a mixture of equilibrated and less-equilibrated LL chondrite materials. Evidence for melting was not seen on any of the particles. Some particles have rounded edges. Overall, the particles’ size and shape are different from those seen in particles from the lunar regolith. These features suggest that meteoroid impacts on the asteroid surface primarily form much of the regolith particle, and that seismic-induced grain motion in the smooth terrain abrades them over time.

Carbon Isotopic Signatures of Individual Archean Microfossils(?) from Western Australia

New types of carbonaceous filamentous microstructures have been identified in silica veins at two new localities in the ∼3.5 Ga North Pole area of Western Australia. Their carbon isotopic compositions were measured in situ by secondary-ion mass spectrometry. The carbonaceous filaments are ∼1μm wide, 10 to 100 μm long, and are permineralized in a fine-grained (∼1 μm) silica matrix. They are morphologically divided into three types (i.e., spiral, threadlike, and branched filaments). Their sizes and morphologies resemble modern and previously reported fossil bacteria. These similarities and their complex three-dimensional geometry suggest that they may represent morphologically preserved fossil bacteria. δ13C values of the carbonaceous filaments range from −42 to −32‰, which strongly suggest that they are composed of biologically fixed organic compounds, possibly via the reductive acetyl-CoA pathway or the Calvin cycle. This is consistent with the hypothesis that autotrophs already existed on the Archean Earth.

Stardust silicates from primitive meteorites

Primitive chondritic meteorites contain material (presolar grains), at the level of a few parts per million, that predates the formation of our Solar System. Astronomical observations and the chemical composition of the Sun both suggest that silicates must have been the dominant solids in the protoplanetary disk from which the planets of the Solar System formed, but no presolar silicates have been identified in chondrites. Here we report the in situ discovery of presolar silicate grains 0.1–1 µm in size in the matrices of two primitive carbonaceous chondrites. These grains are highly enriched in 17O (δ17OSMOW > 100–400‰), but have solar silicon isotopic compositions within analytical uncertainties, suggesting an origin in an oxygen-rich red giant or an asymptotic giant branch star. The estimated abundance of these presolar silicates (3–30 parts per million) is higher than reported for other types of presolar grains in meteorites, consistent with their ubiquity in the early Solar System, but is about two orders of magnitude lower than their abundance in anhydrous interplanetary dust particles. This result is best explained by the destruction of silicates during high-temperature processing in the solar nebula.

Are discontinuous chondrite-normalized REE patterns in pegmatitic granite systems the results of monazite fractionation?

Abstract All 14 stable rare earth elements (REEs) in biotite-muscovite granite and tourmaline-rich granite of the Harney Peak Granite, Black Hills, South Dakota, USA, have been analyzed using inductively coupled plasma mass spectrometry. Chrondrite-normalized REE patterns of the tourmaline-rich granites are discontinuous between Nd and Sm. The discontinuity was modeled successfully by the fractional crystallization of monazite from a biotite-muscovite granite initial composition. This explanation may also apply to the development of such discontinuous patterns in other highly evolved rocks.

Water solubility in Mg-perovskites and water storage capacity in the lower mantle

The water storage capacity of the major constituent of the lower mantle, Mg-perovskite, is a matter of debate. Here we report water solubility of Mg-perovskites with different compositions observed in peridotite and MORB systems. IR spectra of pure MgSiO3-perovskite show bands at 3397, 3423, 3448, and 3482 cm−1 and suggest about 100 ppm H2O. The H2O content in Al-Mg-perovskite (4–7 wt% Al2O3; Mg#=100) is 1000–1500 ppm (major band at 3448 cm−1), whereas Al-Fe-Mg-perovskite in MORB (Al2O3=13–17 wt%; Mg#=58–61) contains 40–110 ppm H2O (major band at 3397 cm−1). The H2O content in Al-Fe-Mg-perovskite observed in peridotite (Al2O3=5–6 wt%; Mg#=88–90) is 1400–1800 ppm (major band at 3397 cm−1). Al-Fe-Mg-perovskite from the MORB system has a high Fe3+ content, Fe3+/∑Fe=0.6, determined by electron energy loss spectroscopy measurements. Water can enter into the perovskite structure with oxygen vacancies originating from the substitution of Si by Al and Fe3+. Oxygen vacancy incorporation is favored for aluminous perovskite synthesized from the MgO-rich peridotite system. The substitution of Si4++Mg2+=2(Al,Fe)3+ prevails however in the Al-Fe-Mg-perovskite from the MORB system (MgO-poor, Al- and Fe-rich), explaining its restricted water solubility. The maximum amount of water stored in the lower mantle is estimated to be 3.42×1021 kg, which is 2.5 times the present ocean mass. Comparison of the phase relations in hydrous pyrolite and hydrous MORB indicates that pyrolite is more important as water container and water carrier in the mantle. Pyrolite contains: (1) dense hydrous magnesium silicates, existing under conditions of subducting slabs, and (2) hydrous wadsleyite, hydrous ringwoodite and water-bearing perovskite under the normal mantle and hotter conditions. Distribution of water to the MORB is restricted at the conditions of the transition zone and lower mantle.

Oxygen Isotopic Compositions of Asteroidal Materials Returned from Itokawa by the Hayabusa Mission

Laboratory analysis of samples returned from an asteroid establishes a direct link between asteroids and meteorites and provides clues to the complex history of the asteroid and its surface. Meteorite studies suggest that each solar system object has a unique oxygen isotopic composition. Chondrites, the most primitive of meteorites, have been believed to be derived from asteroids, but oxygen isotopic compositions of asteroids themselves have not been established. We measured, using secondary ion mass spectrometry, oxygen isotopic compositions of rock particles from asteroid 25143 Itokawa returned by the Hayabusa spacecraft. Compositions of the particles are depleted in 16O relative to terrestrial materials and indicate that Itokawa, an S-type asteroid, is one of the sources of the LL or L group of equilibrated ordinary chondrites. This is a direct oxygen-isotope link between chondrites and their parent asteroid.

Contemporaneous formation of chondrules and refractory inclusions in the early Solar System

Chondrules and calcium-aluminium-rich inclusions (CAIs) are preserved materials from the early history of the Solar System, where they resulted from thermal processing of pre-existing solids during various flash heating episodes which lasted for several million years. CAIs are believed to have formed about two million years before the chondrules. Here we report the discovery of a chondrule fragment embedded in a CAI. The chondrules composition is poor in 16O, while the CAI has a 16O-poor melilite (Ca, Mg, Al-Silicate) core surrounded by a 16O-rich igneous mantle. These observations, when combined with the previously reported CAI-bearing chondrules, strongly suggest that the formation of chondrules and CAIs overlapped in time and space, and that there were large fluctuations in the oxygen isotopic compositions in the solar nebula probably synchronizing astrophysical pulses.

Irradiation History of Itokawa Regolith Material Deduced from Noble Gases in the Hayabusa Samples

Laboratory analysis of samples returned from an asteroid establishes a direct link between asteroids and meteorites and provides clues to the complex history of the asteroid and its surface. Noble gas isotopes were measured in three rocky grains from asteroid Itokawa to elucidate a history of irradiation from cosmic rays and solar wind on its surface. Large amounts of solar helium (He), neon (Ne), and argon (Ar) trapped in various depths in the grains were observed, which can be explained by multiple implantations of solar wind particles into the grains, combined with preferential He loss caused by frictional wear of space-weathered rims on the grains. Short residence time of less than 8 million years was implied for the grains by an estimate on cosmic-ray–produced 21Ne. Our results suggest that Itokawa is continuously losing its surface materials into space at a rate of tens of centimeters per million years. The lifetime of Itokawa should be much shorter than the age of our solar system.

Silicon self-diffusion in MgSiO3 perovskite at 25 GPa

Abstract Silicon self-diffusion coefficients in MgSiO3 perovskite were measured under lower mantle conditions. The MgSiO3 perovskite was synthesized and diffusion annealing experiments were conducted at pressure of 25 GPa and temperature of 1673–2073 K using a MA8 type high-pressure apparatus. The diffusion profiles were obtained by secondary ion mass spectrometry. The lattice and grain boundary diffusion coefficients (D1 and Dgb) were determined to be D1 [m2/s]=2.74×10−10 exp(−336 [kJ/mol]/RT) and δDgb [m3/s]=7.12×10−17 exp(−311 [kJ/mol]/RT), respectively, where δ is the width of grain boundary, R is the gas constant and T is the absolute temperature. These diffusion coefficients play a key role for understanding the rheology of the lower mantle.