Journal of Petrology | Volume 45 | Number 2 | Pages 221-222 | 2004
© Oxford University Press 2004; all rights reserved
Orogenic Lherzolites and Mantle Processes: Editorial
1 DEPARTMENT OF GEOLOGY, ROYAL HOLLOWAY, UNIVERSITY OF LONDON, EGHAM TW20 0EX, UK
2 DEPARTMENT OF GEOLOGY AND MINERALOGY, KYOTO UNIVERSITY, 606 KYOTO, JAPAN
3 DEPARTMENT OF EARTH SCIENCES, KANAZAWA UNIVERSITY, KANAZAWA 820-1192, JAPAN
4 LABORATOIRE DE TECTONOPHYSIQUE, CNRS ET UNIVERSITÉ DE MONTPELLIER 2, PLACE EUGENE BATAILLON, 34095 CEDEX MONTPELLIER, FRANCE
5 DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF MINNESOTA, 310 PILLSBURY DRIVE SE, MINNEAPOLIS, MN 55455, USA
6 DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA 02139-4307, USA
7 DEPARTMENT OF EARTH AND PLANETARY SCIENCE, UNIVERSITY OF TOKYO, TOKYO 113-0033, JAPAN
8 DEPARTMENT OF GEOLOGY AND GEOPHYSICS, WOODS HOLE OCEANOGRAPHIC INSTITUTION, WOODS HOLE, MA 02543, USA
9 FACULTY OF EARTH SCIENCES, UNIVERSITY OF UTRECHT, BUDAPESTLAAN 4, 3584 CD UTRECHT, THE NETHERLANDS
The papers in this thematic issue of Journal of Petrology were delivered at the Fourth International Workshop on Orogenic Lherzolites and Mantle Processes, which was held in Samani, Hokkaido, Japan, between August 26 and September 3, 2002. Fifty oral presentations were given and 45 posters were displayed during the meeting, and the research papers in this issue provide an indication of the range of disciplines. The papers are arranged under four headings: (1) the Horoman orogenic massif; (2) mantle melt dynamics; (3) subduction-related processes; (4) isotopic studies.
HOROMAN OROGENIC PERIDOTITE MASSIF
Obata and Takazawa present a model to explain the compositional variability in the Horoman peridotite. They believe that partial melting and melt segregation best explain whole-rock chemical data, but that chemical discontinuities may be related to melt geometry in partially molten peridotite and to the mode of melt segregation. Morishita et al. report on corundum-bearing mafic rocks found within the Horoman peridotite. Geochemical data confirm their derivation from plagioclase-bearing protoliths, probably subducted oceanic lithosphere that was incorporated into the peridotite. The emplacement history of the Horoman massif is tackled by Ozawa, who argues that the upper part of the massif appears to have resided at a much higher pressure (garnet-facies mantle) than was originally thought. Initial lithosphere thicknesses of 12 km have been thinned to 3 km by the ascent of asthenospheric mantle diapirs, which triggered uplift and deformation of the lithospheric protolith.
MELT DYNAMICS
Using experimental data from a high-pressure, high-temperature deformation apparatus, Zimmerman and Kohlstedt propose that both diffusional and dislocation creep contribute to deformation in partially molten mantle. They stress that, in deformed mantle rocks, it is important to consider melt distribution at both triple junctions and on grain boundaries, and to be aware of melt preferred orientation. Bodinier et al. demonstrate that chromatographic effects lead to the spatial decoupling of isotopic contamination and trace-element enrichment during melt infiltration at Lherz, France. They propose that metasomatic aureoles around pyroxenite veins are better explained by single-stage reactive porous flow models than by invoking discrete influxes of silicate, hydrous and carbonate melts. Xu and Bodinier consider to what extent amphibole-bearing or amphibole-free peridotite xenoliths from Nishan, China, are the result of single or multiple processes. Ionov proposes that the compositional variation of spinel- and garnet-peridotite xenoliths from Vitim, Siberia, are not consistent with varying extents of partial melting or metasomatism. He proposes an important role for near-solidus metamorphic differentiation affecting abundances of elements with a high affinity for garnet. There are important implications for interpretation of Hf isotopic ratios.
SUBDUCTION-RELATED PROCESSES
Arai et al., in a study of xenoliths from the Philippines, show that the mantle wedge comprises coarse- to fine-grained harzburgites. The coarse-grained harzburgites have chemical affinities with arc-type harzburgites and the fine-grained peridotites are believed to have been modified by metasomatic–deformational processes. Green et al. demonstrate that melting of the refractory peridotite mantle wedge, in the presence of fluids, can produce ankaramitic–picritic and boninitic magmas. Michibayashi and Mainprice report the results of structural and fabric work on the Hilti massif, Oman, and show that ridge fabrics determine the location of shear zones that form during obduction or subduction.
Re–Os AND Lu–Hf ISOTOPES
Brueckner et al. describe garnet-peridotites and pyroxenites from Sweden, which may have been derived from an Archaean peridotite protolith with subsequent melt infiltration in the Proterozoic. Pearson and Nowell report data for the Re–Os and Lu–Hf isotopic systems in peridotites and pyroxenites from the Beni Bousera massif in Morocco. The Nd–Sr–Pb–Hf–Os isotopic ratios and stable isotope ratios (C and O) of the pyroxenites are consistent with an origin as recycled oceanic crust, including sediment.
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