Magnetic petrology of arc xenoliths from Japan and Aleutian Islands

Abstract

Xenoliths from Japan and the Aleutian Islands were studied for the purpose of identifying source rocks with sufficient magnetization to account for positive subduction zone Magsat anomalies. Two types of magnetic lithologies are identified: (1) amphibolites and hornblende gabbros containing up to 8.6% titanomagnetite, mostly as large, unoxidized, multidomain crystals, and (2) pyroxene gabbros and metagabbros with up to 20.3% titanomagnetite, usually in association with primary ilmenite. The first type is represented by xenoliths from Ichinomegata, Japan and Adak Island in the Aleutians. The second, restricted to Iki Island, Japan and Kanaga Island, Aleutians, has abundant coarse‐grained titanomagnetite which in most samples has been oxidized, producing secondary, trellis‐type ilmenite lamellae; a few Iki Island xenoliths show more advanced oxidation. Arc xenoliths show 3 orders of magnitude range in natural remanent magnetization and saturation remanence and about 2 orders of magnitude range in magnetic susceptibility. Nearly all Iki Island rocks and many from Ichinomegata and the Aleutians possess induced magnetization 2 A m−1 and greater, with the expectation of additional magnetization due to viscous remanence at elevated temperature. The TiO2 content of titanomagnetite, together with the presence of appreciable Al2O3 and MgO, ensure that most xenoliths have Curie points between 400° and 550°C. Remanence acquisition curves exhibit a range reflecting varying degrees of magnetic hardening due to oxidation. Correspondingly, each xenolith site groups distinctly on a plot of magnetic hysteresis ratios. This paper indicates that lithologies with total magnetizations of many A m−1 exist in lower crust arc settings. Their volumetric distribution is uncertain based on the limited xenolith population reported herein.