We present a synthesis of some 20,504 mineral analyses of ~500 Hole 735B gabbros, including 10,236 new analyses conducted for this paper. These are used to construct a mineral stratigraphy for 1.5-km-deep Hole 735B, the only long section of the lower crust drilled in situ in the oceans. At long wavelengths, generally >200 m, there is a good chemical correlation among the principal silicate phases, consistent with the in situ crystallization of three or four distinct olivine gabbro bodies, representing at least two major cycles of intrusion. Initial cooling and crystallization of these bodies must have been fairly rapid to form a crystal mush, followed by subsequent compaction and migration of late iron-titanium-rich liquids into shear zones and fractures through which they were emplaced to higher levels in the lower crust where they crystallized and reacted with the olivine gabbro host rock to form a wide variety of ferrogabbros. At the wave lengths of the individual intrusions, as represented by the several olivine gabbro sequences, there is a general upward trend of iron and sodium enrichment but a poor correlation between the compositions of the major silicate phases. This, together with a wide range in minor incompatible and compatible element concentrations in olivine and pyroxene at a given Mg#, is con-., 2002.Primary silicate mineral chemistry of a 1.5-km section of very slow spreading lower ocean crust: ODP Hole 735B, Southwest Indian Ridge. In Natland, H.J.B. DICK ET AL. PRIMARY SILICATE MINERAL CHEMISTRY 2 sistent with widespread permeable flow of late melt through these intrusions, in contrast to what has been documented for a 600-m section of reputedly fast-spreading ocean crust in the Oman Ophiolite. This unexpected finding could be related to enhanced compaction and deformation-controlled late-stage melt migration at the scale of intrusion at a slow-spreading ocean ridge, compared to the relatively static environment in the lower crust at fast-spreading ridges. H.J.B. DICK ET AL. PRIMARY SILICATE MINERAL CHEMISTRY 3basis of clear textural and mineralogic differences. This breakdown did not generally include intervals of less than ~4 cm, and further subdivision of the core could be made. With the analytical data set estimated to represent ~500 discrete samples, it remains inadequate to describe the core in anywhere near its entirety. ANALYTICAL METHODSThe data sets reported in Tables T1, T2, T3, T4, and T5, other than those collected by the first author and his colleagues at Massachusetts Institute of Technology (MIT), were collected by investigators using different analytical schemes, techniques, and standards. They describe these in papers in the Leg 118 and Leg 176 volumes Hebert et al., 1991;Natland et al., 1991; Niu et al., Chap. 8, this volume;Ozawa et al., 1991; Robinson et al., Chap. 9, this volume). The new data collected at the MIT Electron Microprobe Facility used a JEOL JXA-733 Superprobe. The operating conditions included a 15-keV accelerating voltage, 10-nA probe current, 10-µm spot si...

The Hayachine and Miyamori hydrous ultramafic complexes, northeastern Japan, exhibit not only diverse lithological variations from very primordial spinel lherzolites (olivine forsterite mol % = 89 (Fo89), spinel 100Cr/(Cr+Al) (Cr #) =10) to refractory harzburgites (Fo93, spinel Cr #=70) but also show diverse trace element characteristics in amphiboles and clinopyroxenes correlated with the major element behavior. Some of the most primitive lherzolites are from the Hayachine complex and contain a clinopyroxene with strongly light rare earth element (REE) depleted abundances. The slightly refractory aluminous spinel peridotites of the Miyamori complex forming kilometric blocks in chromian spinel peridotites, exhibit flat to weakly V‐shaped REE patterns with lower heavy REE (HREE) abundances. The chromite‐bearing peridotites of the Miyamori complex exhibit light REE (LREE) enriched patterns with further lower HREE abundances than the aluminous spinel peridotites. The most refractory clinopyroxene‐free harzburgite (Fo93) contains amphibole with the highest LREE/HREE ratio among the Miyamori‐Hayachine peridotites. The observed covariance between major and trace elements together with geological and petrological data for the complex is reproduced with an open‐system melting model, in which continuous supply of LREE‐enriched melt, melting, and melt segregation are assumed to be coupled. Nonmodal melting with specified amounts of trapped melt is also taken into consideration. The result shows that REE and trace element data for less refractory peridotites can be reproduced fairly well by the model. Accounting for the REE patterns of more refractory peridotites, however, proved less tractable, indicative of the importance of a melting and melt separation process involving melt migration and reactions during the decompression of the mantle material.

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We report a new type of ultramafic pseudotachylyte that forms a fault- and injection-vein network hosted in the mantle-derived Balmuccia peridotite (Italy). In the fault vein the pseudotachylyte is now deformed and recrystallized into a spinel-lherzolite facies ultramylonite, made of a fine (< 2 mu m) aggregate of olivine, orthopyroxene, clinopyroxene, and spinel, with small amounts of amphibole and dolomite. Electron backscattered diffraction study of the ultramylonite shows a clear crystallographic preferred orientation (CPO) of olivine. The fault vein pseudotachylyte overprints a spinel-lherzolite facies amphibole-bearing mylonite, indicating that shear localization accompanying chemical reaction had taken place in the peridotite before seismic slip produced frictional melting. The occurrence of amphibole in the host mylonite and that of dolomite as well as amphibole in the matt-ices of ultramylonite and pseudotachylyte may indicate that fluid was present and had evolved in its composition from H2O-rich to CO2-rich during ductile deformation with metamorphic reactions, which may account for the observed rheological transition from ductile to brittle behavior. The spinel-lherzolite facies assemblage in mylonites, P-T estimations from pyroxene geothermometry and carbonate reactions, and the type of olivine CPO in deformed pseudotachylyte indicate that both the preseismic and the postseismic ductile deformations occurred at similar to 800 degrees C and 0.7-1.1 GPa