gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1980.043.329.05 Petrogenesis of AlnÖitic Rocks from Malaita, Solomon Islands, Melanesia Nixon Peter H. Geology Department, P.O. Box 4820, University, Papua New Guinea Mitchell Roger H. Geology Department, Lakehead University, Thunder Bay, Ontario, Canada Rogers Nicholas W. University of London Reactor Centre, Sunninghill, Ascot, Berks, U.K. 3 1980 43 329 587 596 28 6 1979 17 9 1979 Copyright © 1980, The Mineralogical Society 1980 Summary

Aphanitic and fragmental alnöitic rocks from Malaita contain ultrabasic xenoliths and discrete nodules (megacrysts) of pyroxenes and garnets. Primary minerals in the alnöites are olivine (Fo85), clinopyroxene (diopside-sahlite), natro-melilite (1/3 melilite-2/3 åkermanite), phlogopite (1–9% TiO2), perovskite, spinel (ulvö spinel-magnetite series) and accessory nepheline, melanite, and apatite. Alnöite olivines and clinopyroxenes are compositionally different from those phases in the xenoliths and megacrysts. Rare earth element distribution patterns are linear and indicate strong enrichment in the light rare earths (La/Yb = 42−49). The alnöites are possible primary melts of a pyrolite-type mantle formed by approximately 4% partial melting at depths greater than 120 km, under high carbon dioxide pressures. Despite containing a mantle xenolith assemblage similar to that found in kimberlites, the host Malaita rocks are minera-logically and geochemically different from kimberlite.