Thermal history of iherzolite xenoliths—I. Petrology of iherzolite xenoliths from the ichinomegata crater, oga peninsula, northeast Japan

Abstract

The thermal history of four spinel lherzolites (Lhz-13. Lhz-28, Lhz-29 and Lhz-53) from tuff breccia of the Ichinomegata crater, northeast Japan, has been studied in detail. Lhz-13 and Lhz-53 showed nearly perfect chemical homogeneity of the constituent minerals, and increase of Ca near the rim of olivine is the only disequilibrium evidence observed. In addition to the Ca zoning in olivine, Lhz-28 and Lhz-29 revealed compositional zoning in the Mg/Mg + Fe ratio and Ca content in ortho- and clinopyroxenes. Lhz-13 and Lhz-53 equilibrated at about 800°C in the upper mantle, based on Fe/Mg partitioning between olivine/spinel and olivine/clinopyroxene, and on the mutual solubility of Ca between olivine and pyroxenes. Lhz-28 and Lhz-29 also equilibrated originally at about 800°C, but were preheated at about 1000°C prior to their entrapment by the ascending host magma. The Fe/Mg partitioning between olivine /spinel and olivine/clinopyroxene reequilibrated during the preheating event: however, the Ca solubility did not reequilibrate. Olivine alone has rehomogenized with a high-Ca content but pyroxenes were compositionally zoned with Ca. The preheating event, indicated by the high-Ca content in the core of olivine, is recognized from about a half of the Ichinomegata Iherzolites (50 xenoliths were studied). The duration of heating during the transport of the xenolith by the magma (estimated from the width of the Ca zoning in the rim of olivine) ranges between several hours to a year depending on the rock specimen. From the requirement to reset olivine core compositions, the duration of the preheating event was estimated as greater than 1000 yr.