Experimental constraints on the origins of primitive potassic lavas from the Trans-Mexican Volcanic Belt

Abstract

Primitive magmas in the Trans-Mexican Volcanic Belt (TMVB) span a wide geochemical range that includes calc-alkaline basalt and basaltic andesite, potassic shoshonites, and intraplate alkaline basalts, indicating that the subarc mantle wedge is chemically heterogeneous. The aim of this study is to experimentally constrain the origins of potassic lavas that have erupted along the volcanic front in the TMVB. We used a piston-cylinder apparatus to determine the P–T–H2O near-liquidus phase relations for two primitive potassic lavas: a hornblende trachybasalt (shoshonite) from Cerro La Pilita in the central TMVB and a high-K calc-alkaline basalt from Ayutla in the western TMVB. Experiments were conducted at mantle pressures (0.8–2.5 GPa) and temperatures (1,100–1,400 °C) with 1.5–6 wt% H2O. Results show that both samples were last equilibrated with an olivine + clinopyroxene assemblage at upper mantle pressures. Integrating our results with trace element characteristics, we conclude that the potassic magmas formed by a complex, multistage process in which melts from the hottest part of the mantle wedge either reequilibrated with clinopyroxene-rich veins in the shallow upper mantle or caused melting of such veins by advective heating. We combine our results with previous experiments on TMVB lavas to provide an along-arc perspective of melt equilibration depths in the mantle wedge. The results suggest that although melts may initially form deep in the wedge, they commonly reequilibrate with heterogeneous mantle at shallower depths. Primitive, medium-K basaltic andesites in the TMVB form by reequilibration with harzburgite, which we infer to be a common lithology in the upper mantle, whereas some potassic magmas like the ones studied here form through reequilibration with or melting of veins of olivine + clinopyroxene ± phlogopite. Though somewhat rare at the volcanic front relative to the more abundant medium-K volcanic rocks, the potassic magmas are an important lava type for revealing mantle chemical heterogeneities.