Composition of silicate partial melts of carbonated pelite at 3-5 GPa and genesis of arc magma

The composition of arc magmas reflects a complex process of slab-modified mantle wedge melting and magma differentiation in the crust. The composition of arc magma is distinct among various subduction zones owing to the different thermal structures [1] and perhaps different subduction input. Partial melts of downgoing sediment generated at slab-top or in sedimentary diapir [2] may be key in metasomatizing the mantle wedge. However, the effect of carbonates on the silicate partial melt composition of pelitic sediments is less constrained under the deep sub-arc conditions (~5 GPa). Here we provide silicate partial melt composition of Al-poor carbonated pelite to 5 GPa and discuss whether such melts may be a potential metasomatizing agent for arc source, particularly in Central America, Sunda, and Vanuatu where low-alumina carbonated pelite subduct [3]. We performed piston cylinder (3 GPa) and multianvil (4 and 5 GPa) experiments at 800-1150 °C, using a synthetic pelite with 0.5 and 1.0 wt.% H₂O and 5.0 wt.% CO₂. The experimental procedures and the resulting melting phase relations of this study are described in ref. [4]. The rhyolitic partial melt at 3 GPa evolves to trachydacite at 4 GPa and tephriphonolite at 5 GPa. At 3 GPa silicate partial melt compositions of our study are similar to those derived from hydrous, CO2-free pelite [e.g., 5-7], and are lower in SiO₂ (63-65 wt.%) and higher in TiO₂ (1-2 wt.%), MnO (~0.6 wt.%) and CaO (2-9 wt.%) at 4 GPa. At 5 GPa and 1050-1100 °C immiscible silicate melts, in the presence of carbonatitic melt, are even more distinct in terms of SiO₂ (51-55 wt.%), TiO₂ (~2-3 wt.%), Al₂O₃ (~10-12 wt.%), FeO* (~5-9 wt.%), MnO (0.1-0.3 wt.%), and CaO (~11-14 wt.%) compared to pelite partial melts in the absence of CO₂ (~69-74 wt.% SiO₂, 0.5-1.0 wt.% TiO₂, ~12-15 wt.% Al₂O₃, ~1 wt.% FeO*, ~0.1 wt.% MnO, and 0.3-0.9 wt.% CaO). The compositions of Central America, Sunda, and Vanuatu arc basalts, at 5-15 wt.% MgO, are richer in FeO*, MnO, CaO and K₂O and lower in SiO₂ compared to those at many other arcs, where carbonated sediments do not subduct, and experimental partial melt compositions of hydrous mantle wedge peridotite at 1.2-3.2 GPa [8, 9], which may indicate the need of slab components to the mantle wedge. Relatively CaO, FeO*, and K₂O-rich, and SiO₂-poor, carbonated sediment partial melts from 120-150 km depth generated in our study are plausible metasomatic components required to explain primary arc basalts in these subduction zones. [1] Syracuse, E.M. et al. 2010, PEPI 183, 73-90. [2] Behn, M.D. et al., 2011, NatGeo 4, 641-646. [3] Plank, T. and Langmuir, C. 1998, ChemGeol 145, 325-394. [4] Tsuno, K. et al. 2012, GRL doi:10.1029/2012GL052606. [5] Tsuno, K. and Dasgupta, D. 2012, EPSL 319-320, 185-196. [6] Skora, S. and Blundy, J. 2010, JPetrol 51, 2211-2243. [7] Hermann, J. and Spandler, C. 2008, JPetrol 49, 717-740. [8] Gaetani, G.A. and Grove, T.L. 1998, CMP 131, 323-346. [9] Till, C.B., et al. 2012, CMP 163, 669-688.