Silica-enriched mantle sources of subalkaline picrite-boninite-andesite island arc magmas

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Benard, Antoine, Arculus, Richard, Nebel, Oliver, Ionov, Dmitri A., McAlpine, Sarlae
• Format: Article in Journal/Newspaper
• Language: English
• Published: Pergamon Press Ltd.
• Subjects: Silica enrichment; Peridotite; Subduction; Primary melt; Picrite; Boninite; Bismarck; Pacific; Kamchatka
• Online Access: http://hdl.handle.net/1885/247915; https://doi.org/10.1016/j.gca.2016.09.030; https://openresearch-repository.anu.edu.au/bitstream/1885/247915/3/01_Benard_Silica-enriched_mantle_sources_2017.pdf.jpg

Primary arc melts may form through fluxed or adiabatic decompression melting in the mantle wedge, or via a combination of both processes. Major limitations to our understanding of the formation of primary arc melts stem from the fact that most arc lavas are aggregated blends of individual magma batches, further modified by differentiation processes in the sub-arc mantle lithosphere and overlying crust. Primary melt generation is thus masked by these types of second-stage processes. Magma-hosted peridotites sampled as xenoliths in subduction zone magmas are possible remnants of sub-arc mantle and magma generation processes, but are rarely sampled in active arcs. Published studies have emphasised the predominantly harzburgitic lithologies with particularly high modal orthopyroxene in these xenoliths; the former characteristic reflects the refractory nature of these materials consequent to extensive melt depletion of a lherzolitic protolith whereas the latter feature requires additional explanation. Here we present major and minor element data for pristine, mantle-derived, lava-hosted spinel-bearing harzburgite and dunite xenoliths and associated primitive melts from the active Kamchatka and Bismarck arcs. We show that these peridotite suites, and other mantle xenoliths sampled in circum-Pacific arcs, are a distinctive peridotite type not found in other tectonic settings, and are melting residues from hydrous melting of silica-enriched mantle sources. We explore the ability of experimental studies allied with mantle melting parameterisations (pMELTS, Petrolog3) to reproduce the compositions of these arc peridotites, and present a protolith (‘hybrid mantle wedge’) composition that satisfies the available constraints. The composition of peridotite xenoliths recovered from erupted arc magmas plausibly requires their formation initially via interaction of slab-derived components with refractory mantle prior to or during the formation of primary arc melts. The liquid compositions extracted from these hybrid sources are ...