Melt–Peridotite Reactions and Fluid Metasomatism in the Upper Mantle, Revealed from the Geochemistry of Peridotite and Gabbro from the Horoman Peridotite Massif, Japan

A highly unradiogenic lead isotopic signature revealed by volcanic rocks from the East Pacific Rise

“…However, the refractory nature of sulphide-bearing abyssal peridotites 11,22 render them unlikely to be involved in the source of EPR 15.4°N basalts. Furthermore, the ULC is more geochemically enriched than ultra-depleted residual peridotites 8,10,23 . Alternatively, sulphides are also common phases in ultramafic xenoliths found in the context of continental magmatic arcs 24,25 and may strongly influence the transfer of chalcophile elements and the composition of the subcontinental lithosphere.…”

“…The trace element compositions used for this source is HR-10 gabbro 10 (Supplementary Table 1), because of its similar trace element pattern and its unradiogenic Pb.…”

“…Ancient peridotites found in the Horoman orogenic massif (Japan) have recently shed light on the existence and properties of such a discrete reservoir 8 . Very low Pb isotope ratios were reported in these rocks and provide physical evidence that unradiogenic Pb could be trapped within the upper mantle as widespread and extremely refractory heterogeneous domains [8][9][10][11] largely untapped by basaltic melts.…”

Orogenic, Ophiolitic, and Abyssal Peridotites

“…The Horoman peridotite has been the subject of detailed geochemical studies devoted to understand the significance of diffusional zoning in minerals and the origin of lithologic heterogeneities and to constrain melt/fluid processes and the nature of the mantle protolith (Frey et al, 1991;Malaviarachchi et al, 2008Malaviarachchi et al, , 2010Morishita and Arai, 2001;Morishita et al, 2004Morishita et al, , 2007Obata and Nagahara, 1987;Obata and Takazawa, 2004;Rehkämper et al, 1999;Saal et al, 2001;Takazawa et al, 1992Takazawa et al, , 1996Takazawa et al, , 1999Takazawa et al, , 2000Yoshikawa and Nakamura, 2000). The massif is well known for its layered structure (Malaviarachchi et al, 2010;Niida, 1974;Obata and Nagahara, 1987;Takahashi, 1992;Takazawa et al, 1992;Toramaru et al, 2001). In the upper part of the massif (the 'upper zone' after Niida, 1974), the layering is underlined by relatively thin (mostly <1 m) bands of pyroxenites alternating with peridotites.…”

The influence of melt infiltration on the Li and Mg isotopic composition of the Horoman Peridotite Massif

“…Yoshikawa and Nakamura 114 (2000) inferred that the LREE-enriched, metasomatic agent is a fluid, possibly derived from 115 the dehydration of the subducting slab based on generally elevated B/Nb and Pb/Ce ratios, 116 although the differences in B/Nb and Pb/Ce ratios between metasomatised and non-117 metasomatised peridotites are not clear. Malaviarachchi et al (2010) also proposed that the 118 metasomatism agent for massive peridotites is fluid because of the enrichment in alkali 119 elements (e.g. Rb).…”