Liquid line of descent of a basanitic liquid at 1.5 Gpa: constraints on the formation of metasomatic veins

Byline: Sebastien Pilet (1), Peter Ulmer (2), Samuel Villiger (2) Keywords: Experimental petrology; Basanite; Metasomatism; High-pressure crystallization; Alkaline lavas Abstract: The metasomatism observed in the oceanic and continental lithosphere is generally interpreted to represent a continuous differentiation process forming anhydrous and hydrous veins plus a cryptic enrichment in the surrounding peridotite. In order to constrain the mechanisms of vein formation and potentially clarify the nature and origin of the initial metasomatic agent, we performed a series of high-pressure experiments simulating the liquid line of descent of a basanitic magma differentiating within continental or mature oceanic lithosphere. This series of experiments has been conducted in an end-loaded piston cylinder apparatus starting from an initial hydrous ne-normative basanite at 1.5 GPa and temperature varying between 1,250 and 980degC. Near-pure fractional crystallization process was achieved in a stepwise manner in 30degC temperature steps and starting compositions corresponding to the liquid composition of the previous, higher-temperature glass composition. Liquids evolve progressively from basanite to peralkaline, aluminum-rich compositions without significant SiO.sub.2 variation. The resulting cumulates are characterized by an anhydrous clinopyroxene + olivine assemblage at high temperature (1,250--1,160degC), while at lower temperature (1,130--980degC), hydrous cumulates with dominantly amphibole + minor clinopyroxene, spinel, ilmenite, titanomagnetite and apatite (1,130--980degC) are formed. This new data set supports the interpretation that anhydrous and hydrous metasomatic veins could be produced during continuous differentiation processes of primary, hydrous alkaline magmas at high pressure. However, the comparison between the cumulates generated by the fractional crystallization from an initial ne-normative liquid or from hy-normative initial compositions (hawaiite or picrobasalt) indicates that for all hydrous liquids, the different phases formed upon differentiation are mostly similar even though the proportions of hydrous versus anhydrous minerals could vary significantly. This suggests that the formation of amphibole-bearing metasomatic veins observed in the lithospheric mantle could be linked to the differentiation of initial liquids ranging from ne-normative to hy-normative in composition. The present study does not resolve the question whether the metasomatism observed in lithospheric mantle is a precursor or a consequence of alkaline magmatism however, it confirms that the percolation and differentiation of a liquid produced by a low degree of partial melting of a source similar or slightly more enriched than depleted MORB mantle could generate hydrous metasomatic veins interpreted as a potential source for alkaline magmatism by various authors. Author Affiliation: (1) Institute of Mineralogy and Geochemistry, University of Lausanne, Anthropole, 1015, Lausanne, Switzerland (2) Department of Earth Sciences, ETH-Zurich, Clausiusstrasse 25, 8092, Zurich, Switzerland Article History: Registration Date: 01/09/2009 Received Date: 23/01/2009 Accepted Date: 31/08/2009 Online Date: 21/09/2009 Article note: Communicated by J. Blundy.