Rheology Dependent on the Distance to the Propagating Thrust Tip—(Ultra‐)Mylonites and Pseudotachylytes of the Silvretta Basal Thrust

To evaluate how the presence of pseudotachylytes affects the strength of crustal rocks, deformed pseudotachylytes and their relationship with pristine pseudotachylytes at the base of the Silvretta nappe are analyzed. Pseudotachylytes formed associated with high-stress crystal plasticity (σd > 400 MPa), as indicated by twinned amphiboles in gneisses. Mylonitic quartz clasts enclosed within deformed pseudotachylytes and mylonitic vein-quartz, hosting folded pseudotachylyte injection veins, reflect creep at lower stresses (ca. 100 MPa) after seismic rupturing. Deformed pseudotachylytes can be crosscut by pristine pseudotachylytes, indicating a second, independent stage of coseismic rupturing after creep. The evidence of dynamic dislocation creep of quartz and the presence of stilpnomelane and epidote associated with all fault rocks indicate similar ambient greenschist facies conditions during all deformation stages. Whereas the intermediate stage of creep is interpreted to represent deformation at large distance to the propagating thrust tip, the pristine pseudotachylytes represent the last stage of rupturing eventually leading to nappe decoupling from its basement. Gneiss clasts in an ultramylonitic matrix (i.e., deformed pseudotachylyte) reveal that pseudotachylytes have a lower strength during creep in relation to the hosting gneisses. In contrast, during coseismic high-stress crystal plasticity, the coarse gneisses accumulate a higher amount of strain. This strength-relationship explains that only those rocks rupture, which have not been previously deformed before. The study demonstrates the importance of different strengths of crustal rocks at specific stress- and strain-rate conditions in dependence on the distance to the propagating fault tip

The Eichstädt meteorite, a historical fall in Bavaria from 1785 -revisited

第6回極域科学シンポジウム[OA] 南極隕石11月16日（月）　国立極地研究所１階交流アトリウ

Comparing the magnetic signatures of Neuschwanstein (EL6) and E-chondritic lithologies of Almahata Sitta (polymict ureilite).

第2回極域科学シンポジウム/第34回南極隕石シンポジウム 11月17日（木） 国立国語研究所 2階講

News on the Machtenstein H5 ordinary chondrite

第6回極域科学シンポジウム[OA] 南極隕石11月16日（月）　国立極地研究所１階交流アトリウ

New Results of Micro Raman Spectroscopy and Magnetism on Martian Meteorites.

第3回極域科学シンポジウム/第35回南極隕石シンポジウム 11月29日（木）、30日（金） 国立国語研究所 2階講

Almahata Sitta Meteorite: Combination of Raman Spectroscopy and ElectronMicroprobe analysis within the ureilitic lithologies.

第2回極域科学シンポジウム/第34回南極隕石シンポジウム 11月17日（木） 国立国語研究所 2階講

Distribution of mantle and atmospheric argon in mantle xenoliths from the Western Arabian peninsula: Constraints on timing and composition of metasomatizing agents in the lithospheric mantle

To investigate the geochemical behaviour of argon isotopes during mantle metasomatism and to obtain chronological information on the age of metasomatic events under the Arabian Shield, we analyzed mantle xenoliths and hornblende megacrysts from Saudi Arabian volcanic fields (Uwayrid, Al Birk) applying the 40Ar-39Ar dating technique. Two hornblende megacrysts yield plateau ages indicating formation or total resetting of the K/Ar system 1-2 Myr ago. The ultramafic xenoliths trapped mantle-derived and atmospheric argon in different proportions, resulting in variable isotopic compositions: 40Ar/36Ar ratios range from 296 (i.e. atmospheric) to 10 500, reflecting interactions with isotopically and genetically different fluids and/or melts during recent mantle metasomatism. One such episode of metasomatism led to the formation of Ba-rich phlogopite, which yielded a saddle-shaped age spectrum with a maximum age estimate of 18 Ma. Another episode, inducing formation of secondary pargasite in the lithospheric mantle, was dated to 4 Ma. In the mantle xenoliths the concentration of mantle argon is clearly related to the intensity of metasomatism. Argon extraction by high-resolution stepwise heating allowed us to deconvolve various argon components distributed heterogeneously within single xenoliths and ascribe them to specific carrier phases. Pyroxenes generally preserve much higher 40Ar/36Ar ratios than olivine, as they contain up to 100 times higher concentrations of mantle argon, which also correlates with a higher fluid inclusion content in pyroxenes. Hydrous phases (phlogopite/amphibole) have more variable 40Ar/36Ar ratios. K and Cl concentrations and the argon isotope compositions of the Uwayrid xenoliths indicate distinct metasomatic agents, causing elemental and isotopic disequilibrium on a local scale. On the basis of correlations between Ar isotope composition and K and Cl concentration in the samples most strongly affected by the late metasomatic fluids, we suggest that metasomatic processes in the local mantle occurring simultaneously with the opening of the Red Sea were accompanied by the introduction of saline-water saturated fluids into deep lithospheric zones

Mineralogy and crystallography of vein metals in the Almahata Sitta ureilite.

第2回極域科学シンポジウム/第34回南極隕石シンポジウム 11月18日（金） 国立国語研究所 2階講

Minerology and Crystallography of Some Itokawa Particles Returned by the Hayabusa Mission

JAXA Hayabusa mission success-fully returned particles of the asteroid Itokawa to the earth in 2010. The recovered particles were carefully studied by the preliminary examination (PE) team and the obtained results are providing significant and unique information about the formation and evolution of meteorite parent bodies [1-6]. These particles further revealed that their mineral compositions and oxygen isotopes are close to those of equilibrated LL chon-drites [1,2], which matches with the observation at the orbit [7]. After the PE, JAXA distributed the samples as international AO study and we received 4 new and 3 PE particles. The new samples may contain some exot-ic components that have not been found by the PE study, and are of special interest. We have performed a detailed mineralogical and crystallographic study on these particles and here report the results