Masaaki Obata

Whole rock compositional variations in an upper mantle peridotite (Horoman, Hokkaido, Japan): Are they consistent with a partial melting process?

Abstract Whole rock major and trace element abundances of the Horoman peridotites were used to understand processes forming lithological and compositional variations in the upper mantle. Similar to other orogenic peridotites, Horoman peridotites range from fertile lherzolites (3 to 4% Al2O3 and CaO) to depleted harzburgites (∼0.5% Al2O3 and CaO). Abundances of major oxides and compatible to moderately incompatible elements vary systematically with variations in MgO content. Such trends are commonly interpreted as indicating that the peridotites formed as residues from varying degrees of partial melting. The fertile end of these trends coincides with estimates of primitive mantle composition. Because of a mismatch between experimental melting trends for spinel peridotite, especially the Na2O-MgO trend, the compositional variations of Horoman peridotites are not consistent with formation as residues from partial melting of spinel peridotite. Non-linear trends in minor and trace element versus major element abundance diagrams also preclude a two-component mixing model. Recent melting experiments on garnet peridotite demonstrate that at 3 GPa the near-solidus peridotite has a large amount of subcalcic clinopyroxene (ca. 27%) coexisting with small amount of garnet (ca. 2%). Residues from polybaric melting of such garnet peridotite are consistent with the abundance variations of major and moderately incompatible elements, such as Na and heavy rare-earth elements, in the Horoman peridotites. A similar conclusion is applicable to other orogenic peridotites such as the Ronda peridotite because their major element compositional variations are similar to the Horoman peridotite.

Mantle earthquakes frozen in mylonitized ultramafic pseudotachylytes of spinel-lherzolite facies

F. Seifert and Bayerisches Geoinstitut (University of Beyreuth); Japan Society for the Promotion of Science grant 17340159; Progetti di RilevanteInteresse Nazionale grant 2005044945 and a Progetti di Eccellenza Fondazione Cassa di Risparmio di Padova e Rovigo (CARIPARO) grant.

Some comments on the rheologically critical melt percentage

Abstract The concept of rheologically critical melt percentage (RCMP) originally proposed by Arzi [Tectonophysics 44 (1978) 173–184] for partially molten granitic rocks is re-examined. It is shown that there is no experimental support to show the presence of RCMP. The published experimental data suggest that the effective viscosity of partially molten granitic rocks is reduced rapidly and continuously with increasing melt fraction. It is also shown that the experimental data may be modeled by means of the upper bound behavior (the Voigt bound) of two-phase material by assuming a melt localization, which implies that there is no partitioning of strain between the solid and the melt.

Isochemical breakdown of garnet in orogenic garnet peridotite and its implication to reaction kinetics

An isochemical kelyphite (orthopyroxene+spinel+plagioclase) that has nearly the same bulk chemical composition as the precursor garnet was found within a matrix of ordinary kelyphites (orthopyroxene+clinopyroxene+spinel±amphibole) in garnet peridotites from the Czech part of the Moldanubian Zone. It was shown that the kelyphitization of garnet took place in three stages: (1) the garnet-olivine reaction, accompanied by a long-range material transfer across the reaction zone, and (2) the isochemical breakdown of garnet, essentially in a chemically-closed system, and finally, (3) an open-system hydration reaction producing a thin hydrous zone (amphibole+spinel+plagioclase), which is located between the isochemical kelyphite and relict garnet. The presence of relict garnet suggests that this breakdown reaction of the second stage did not proceed to a completion probably being hindered by the formation of the hydrous zone at the reaction front. It was found by electron back-scattered diffraction method that orthopyroxene and spinel do not show any topotaxic relationship in the first type of kelyphite; whereas they show locally topotaxic relationship in the isochemical kelyphite. The transition from the first type to the second type of kelyphite is discussed on the basis of the detailed observations in the transition zone between the two kelyphites. More widespread occurrence of isochemical kelyphite is expected to occur in orogenic peridotites as well as from xenoliths brought by volcanics.

A New Statistical Description of the Spatial Distribution of Minerals in Rocks

We propose a new method to describe quantitatively rock textures with a special emphasis on the spatial distributions of minerals. The method employs a statistical approach and examines probabilities for particular mineral pairs to occur for variable interpoint distances. The probability is measured by counting the number of point-pairs on digitized images of rock textures that are obtained with a computer-aided digital image processor. Several problems in obtaining textural images and processing the data that were encountered in the actual procedures are reviewed. The method is applied to two rock samples, granite and contact metamorphic hornfels. The method is useful in examining the extent of homogeneity and randomness of mineral distributions and spatial correlations among minerals of different kinds.

Spinel inclusions in olivine and plagioclase crystals in a layered gabbro: a marker and a tracer for primary phenocrysts in a differentiating magma reservoir

The problem of whether cumulate rocks were formed by crystal settling or by in situ crystallization after magma emplacement is an important issue concerning the mechanisms of magmatic differentiation. However, it is hard to distinguish these two processes for plutonic rocks because the primary texture and chemical composition have generally been modified by postcumulus processes. To contribute this problem, we studied the distribution and compositions of Cr-spinel inclusions hosted in olivine and plagioclase in the Murotomisaki Gabbroic Intrusion (MGI), SW Japan. It is shown that the olivine-hosted inclusions are restricted to specific horizons where accumulation of olivine phenocrysts is thought to have occurred and that the compositional variations of the Cr-spinel are explained by a secondary compositional modification that probably took place after the magma emplacement. It is also shown that the Cr-spinel inclusions in a chilled margin have suffered the least compositional modification and nearly retains the primary composition. Those in the interior of the intrusion, on the contrary, have been significantly modified by re-equilibration with residual melt driven by cation diffusions through the host phases. Those in plagioclase have been less modified. It is shown that all the spinel inclusions had primarily the same and common composition at the time of magma emplacement. This implies that all the inclusion-bearing crystals, olivine and plagioclase, represent primary phenocrysts that had already existed in the emplaced magma. In this way, spinel inclusion in the MGI may be regarded to be a useful petrographic “marker” for identifying intratelluric phenocrysts and also as a “tracer” to trace the motion of the primary phenocrysts after the magma emplacement.

Zoning and resorption of plagioclase in a layered gabbro, as a petrographic indicator of magmatic differentiation

ABSTRACT: The Murotomisaki Gabbroic Intrusion is a sill-like layered gabbro emplaced insedimentary strata of Tertiary age in southwest Japan. The zoning (including resorption structures)and the compositional variations of plagioclase from throughout the intrusion were studied, and itwas found that the zoning pattern may be classified into four types, which may well correlated withthe hosting rock types, the mode of occurrences and their stratigraphic positions in the intrusion.The plagioclase zoning was successfully decoded, and the sequence of events that took place duringthe magmatic differentiation was deduced and further interpreted in the context of a stratified basalboundary layer slowly ascending in a solidifying magma body. It was shown that various layeredstructures – modal layering, podiform gabbroic pegmatites and anorthositic layers – observed in theMurotomisaki Gabbro were formed within the moving basal boundary layer by flushing of H 2 O-richfluid and fractionated silicate melts from below. By the fluxing of hydrous fluids, plagioclase crystalspreferentially dissolved and then melt fraction increased in the basal boundary layer. Under thesecircumstances, plagioclase-rich fractionated melts diapirically segregated from the crystal pile. Calcicplagioclases, which are out of equilibrium in the central part of the intrusion, may have originatedfrom the basal boundary layer in this manner.KEY WORDS: Anorthosite, boundary layer, crystal mush, diapir, fluid fluxing, magma chamberMagma reservoirs are subjected to crystallisation and associ-ated fractionation that are driven by the various physico-chemical processes such as gravitational settling or flotation ofcrystals, compositional convection and compaction. As ther-mal gradient is created near the marginal zones of the magmareservoir (e.g. Brandeis & Jaupart 1986; Turner