Development and Evolution of the Lithospheric Mantle: A Case Study from the Horoman Peridotite, Japan

Abstract

The Horoman peridotite, Hokkaido, Japan is a layered upper mantle peridotite that is a piece of heterogeneous mantle created by melt formation, segregation, migration and waUrock/melt interaction. (Obata and Nagahara, 1987; Takahashi, 1991, 1992; Takazawa et al., 1992b). In order to understand the scale and timing of these processes we studied 60 samples from a 40 m stratigraphie section (the Bozu section). This section is well exposed and several rock types as layers, 10's of meters thick. Abundances of REE, Sr, Zr, Ti, Y, Cr and V in clinopyroxenes (cpxs) from 60 peridotites were analyzed by a secondary ion mass spectrometer at WHOI. Abundances of REE, Sr, Zr, Nb, Hf, Th and REE in 34 whole rocks were measured by ICP-MS at Universit6 de Montpellier II, France. Plagioclase lherzolites are generally depleted in incompatible elements (Fig. 1). It is noted that Zr fractionates from Hf as the bulk rocks becomes more depleted in incompatible elements. Fig. 2 shows variation of chondrite-normalized La/Nd ratio [(La/Nd)N] in cpxs in terms of stratigraphic height. Plagioelase lherzolites have the lowest values (< 1), except in a subsection of LREE enriched plagioclase lherzolite (~2) . In the lherzolite layer the ratio increases from near unity to 10 towards the boundary with the harzburgite, then decreases gradually within 3 meters from the boundary because abundances of Nd in the cpxs increase. Lherzolites near the phlogopite-bearing zone have (La/Nd)N ratio higher than unity. Harzburgite has constant (La/Nd)N ratio of 1 ~ 2 while some harzburgite near a mafic layer has (La/Nd)N ratio lower than unity. Generally the lithological boundary is associated with steep compositional gradient. Fig. 3 shows variation of chondrite normalized patterns of REE, Sr, Zr and Ti along a traverse in the porphyroclastic cpx in a ptagiodase lherzolite. The rim is enriched in REE, especially in HREE, and depleted in Sr relative to the core. It is noted that the core is relatively depleted in Zr but this depletion decreases toward the rim. The geochemical data provide evidence for five sequential events: Event 1. Formation as asthenospheric mantle. The plagioclase lherzolites are fertile peridotites, 34 % CaO and A1203, which have been variably depleted in incompatible elements by segregation of small and variable extents of partial melting (Fig. I). Because the plagioclase lhcrzolite is the most