Title: Wet and Dry Mantle Melting and Fractional Crystallization Processes at Newberry Volcano, Oregon

Abstract: Newberry Volcano in central Oregon, USA is a large (600 km3) back arc volcano behind the Cascades. It contains a 500,000 year history of volcanism. The basalts at Newberry fall into two distinctive groups. The first group is tholeiitic high-alumina olivine basalt and these lavas are diktytaxitic and aphyric. These lavas are dry (< 0.5 wt. % pre-eruptive H2O) with relatively flat primitive chondrite-normalized incompatible element abundance patterns. Similar basalts are erupted widely at rear-arc volcanoes along the Cascades and across the High Lava Plains to the East of Newberry. These basalts evolve by fractional crystallization along tholeiitic iron-enrichment trends. The second group is calc-alkaline and contains 2-4 vol. % olivine, calcic plagioclase and high-Ca clinopyroxene as phenocrysts. These lavas contained 2 to 4 wt. % H2O prior to eruption and show strong enrichment in the light rare earth elements, Ba, and Sr, and deficits in high-field strength elements such as Nb, Ta, Hf and Zr. Basalts with these compositional characteristics evolve by fractional crystallization along calc-alkaline trends of increasing SiO2 and decreasing FeO. Such compositional characteristics are reflecting a contribution from the subducted lithosphere transported to the mantle wedge by an H2O-rich fluid or melt. The tholeiitic basalts are produced by anhydrous adiabatic decompression melting of convecting mantle asthenosphere. Depths and temperatures of tholeiite melt segregation are 50 – 55 km and 1350 oC. The calc-alkaline basalts are produced by hydrous flux melting. Depths of melting for the calc-alkaline primitive melts are near the seismically defined moho (35 – 60) km depth) and 1175 – 1310 oC. Shallow crustal level fractional crystallization of primitive tholeiitic and calc-alkaline basalts creates the compositional spectrum of Newberry lavas. 

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