Topotaxic relationships between spinel and pyroxene in kelyphite after garnet in mantle-derived peridotites and their implications to reaction mechanism and kinetics

Kelyphite is a reaction product between garnet and olivine, which was formed by subsolidus reactions upon decompression during the ascent of mantle peridotite. We studied crystallographic relationships among constituent (product) phases of kelyphite ?C orthopyroxene, clinopyroxene, spinel and reactant phases, garnet and olivine, using EBSD and found that, for a relatively high temperature sample (from Czech Moldanubian), spinel and pyroxenes are in a topotaxic relationship in such a way that spinel {111} coincides with pyroxene (100) and spinel {110} coincides with pyroxene (010); while the topotaxy is incomplete or non for a lower-temperature sample (from western Norway). On the basis of the observed microstructural and crystallographic relationships, we propose a hypothesis that the topotaxic relationship may be established at nucleation stages of the onset of the kelyphitization and that the degree of topotaxy may be related to the transformation temperature and the degree of supersaturation of the reaction. The lower the temperature, the higher the supersaturation and, therefore, more rapid the nucleation becomes, resulting in a more disordered state in topotaxic relationship.     

Evaluation of Seasonal Sea Level Variation at Syowa Station, Antarctica, Using GPS Observations

Global Positioning System (GPS) measurements of the vertical displacement of fast ice near Syowa Station, Antarctica, were conducted between April and December 1998 to evaluate measurements of sea level variation derived with a conventional bottom pressure gauge (BPG). The BPG-derived sea level revealed a seasonal variation of about 0.13 m, with a high in April–June and a low in November–December. The GPS-derived sea level, combined with observed sea ice thickness, supported the BPG result, with an RMS error of 0.007 m. Our result also demonstrates that GPS is a powerful technique for monitoring sea level variations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evaluation of olivine-spinel geothermometry as an indicator of thermal history for peridotites

Olivine and spinel in peridotites from the Miyamori ultramafic complex and the Ichinogemata crater of Northeast Japan show a systematic variation in the Mg/ (Mg+Fe) ratio which is correlated mainly with the grain size of spinel. This correlation can be explained by a diffusion model assuming a semi-infinite composite sphere under cooling or heating conditions. In order to obtain absolute temperatures of thermal events, the olivine-spinel geothermometer is applied to pairs of spinel core and olivine core (average composition). The calculated temperatures range over two hundred degrees and have a systematic relationship with the grain size of spinel. In the Miyamori complex, the calculated temperatures decrease monotonically with decrease in grain size of spinel, whereas in the Ichinomegata lherzolite nodule those of spinel smaller than 0.2 mm increase as the grain size decreases and those of spinel larger than 0.2 mm remain constant regardless of further increase in grain size. These observations, in the light of the diffusion model, suggest that the Miyamori complex may have cooled from higher than 800° C to lower than 600° C and that the lherzolite nodule from the Ichinomegata crater may have been in equilibrium at 900° C before it was heated above 1,100° C for less than a few days. These two examples indicate that olivine-spinel pairs of peridotites do not always indicate an appropriate equilibration temperature. We cannot interpret the supposed equilibration temperatures until the existence of isothermal stages in the thermal history of peridotites is established by carefully checking the chemical heterogeneity.     

Boron isotopic composition of fumarolic condensates and sassolites from Satsuma Iwo-jima,Japan

¹¹B/¹⁰B ratios of the high temperature fumarolic gases (>465°C) of this island were found to be constant within the limits of experimental error (¹¹B/¹⁰B = 4.066). This value may represent the ¹¹B/¹⁰B ratio of boron in the andesite magma. ¹¹B/¹⁰B ratios of the low temperature fumarolic gases (<235°C) were found to vary from 4.053 to 4.077. ¹¹B/¹⁰B ratios of some sassolites were approximately equal to that of the fumarolic condensates and the other ones were slightly enriched in ¹⁰B compared to the fumarolic condensates.     

Experimental study of reaction between perovskite and molten iron to 146 GPa and implications for chemically distinct buoyant layer at the top of the core

Partitioning of oxygen and silicon between molten iron and (Mg,Fe)SiO₃ perovskite was investigated by a combination of laser-heated diamond-anvil cell (LHDAC) and analytical transmission electron microscope (TEM) to 146 GPa and 3,500 K. The chemical compositions of co-existing quenched molten iron and perovskite were determined quantitatively with energy-dispersive X-ray spectrometry (EDS) and electron energy loss spectroscopy (EELS). The results demonstrate that the quenched liquid iron in contact with perovskite contained substantial amounts of oxygen and silicon at such high pressure and temperature (P–T). The chemical equilibrium between perovskite, ferropericlase, and molten iron at the P–T conditions of the core–mantle boundary (CMB) was calculated in Mg–Fe–Si–O system from these experimental results and previous data on partitioning of oxygen between molten iron and ferropericlase. We found that molten iron should include oxygen and silicon more than required to account for the core density deficit (<10%) when co-existing with both perovskite and ferropericlase at the CMB. This suggests that the very bottom of the mantle may consist of either one of perovskite or ferropericlase. Alternatively, it is also possible that the bulk outer core liquid is not in direct contact with the mantle. Seismological observations of a small P-wave velocity reduction in the topmost core suggest the presence of chemically-distinct buoyant liquid layer. Such layer physically separates the mantle from the bulk outer core liquid, hindering the chemical reaction between them.