Chemical composition of fluid inclusions in the Yorii jadeite–quartz rocks from the Kanto Mountains, Japan

Abstract

Jadeite–quartz rocks from the Yorii area, the Kanto Mountains, central Japan occur as tectonic blocks within serpentinite mélange. Primary two–phase (liquid + vapor) aqueous fluid inclusions are observed in jadeite, quartz, albite and zircon. The fluids trapped in jadeite and quartz are H₂O with and without CH₄ and show a wide range of salinity (4.7 ± 1.1 wt% NaCl_eqv in jadeite; 3.4 ± 0.7 and 13.6 ± 2.0 wt% NaCl_eqv in quartz). In this study, we examined the chemical composition of fluid inclusions in quartz from jadeite–quartz rock to understand the characteristics of fluid, which related to the formation of the rocks in the subduction zone. The trace element concentrations of fluid inclusions determined with LA–ICP–MS are enriched in LILE, Li, B, HFSE and transition metals with LILE enriched and relatively HFSE depleted characteristics. These have elemental patterns similar to the HFSE–depleted fluid released from antigorite during subduction, though the orders of magnitude are different. It can be common chemical characteristics of fluid in a subduction zone. There are two possibilities of the chemical composition of the evolved fluid after the formation of jadeite–quartz rocks. HFSE from the fluid can be sequestered into jadeite–quartz rock prior to fluids moving up into a mantle wedge. This process lead the evolved fluids relatively depleted in HFSE and enriched in LILE. Other possibility is that the fluid has same chemical composition of fluid inclusion as the fluid is equilibrated with jadeite–quartz rocks during the jadeite–quartz formation. In both cases, fluid after the metasomatic formation of jadeite–quartz rocks can be relatively enriched in LILE and depleted in HFSE. Magmas generated in subduction zones exhibit distinctive geochemical features of LILE enrichment and HFSE depletion and this characteristic of arc magma can be explained by the behavior of HFSE during the formation of jadeite–quartz rocks.