The Processes of Melt Differentiation in Arc Volcanic Rocks: Insights from OIB-type Arc Magmas in the Central Mexican Volcanic Belt

Abstract

Andesite petrogenesis is inextricably linked to plate processing at convergent margins. The details of andesite formation, however, remain poorly understood because the signatures of the initial arc mantle melts are often modified in the overlying crust. To distinguish initial mantle from crustal signatures in arc magmas, we studied two compositionally zoned Holocene monogenetic volcanoes, Texcal Flow and Volcan Chichinautzin, in the central Mexican Volcanic Belt (MVB). Texcal Flow and V. Chichinautzin erupt ‘ocean island basalt (OIB)-type’, high-Nb (17–36 ppm), olivine-phyric basalts to basaltic andesites (49·4–57·3 wt % SiO2; Mg# = 68–50) that show an arc affinity in their major element oxides. At both volcanoes melt SiO2 increases with time. However, systematic changes of melt SiO2 with 87Sr/86Sr and 143Nd/144Nd, the overall low 87Sr/86Sr = 0·70305–0·70453 and high 143Nd/144Nd = 0·51273–0·51299 relative to continental crust, and the high 3He/4He = 7–8 Ra of olivine phenocrysts preclude melt silica enrichment by crustal assimilation and fractional crystallization. Instead, the data require the existence of silicic initial mantle melts. The high Ni abundances of olivines suggest that the silicic melts originate from segregations of ‘reaction pyroxenites’ that formed in the peridotite mantle wedge following multiple infiltrations of silicic slab components. Sequential melting of zoned silica-deficient to silica-excess pyroxenites can reproduce the time-progressive evolution of melt silica content at Texcal Flow and V. Chichinautzin. As initial melts always have high Mg# > 70 regardless of their SiO2 content, the low-Mg# values of the magmas erupted must reflect loss of moderate amounts (<15%) of olivine and possibly pyroxenes at crustal levels. Fractional crystallization and recharge mixing nearly erase all mantle signatures in the most silicic V. Chichinautzin magmas, so that their origin can only be inferred from their association with the more mafic precursory melts. The pyroxenite model implies that ∼15–18 wt % of the erupted melt mass, and possibly more, is slab-derived. We infer that the elements Fe, Mg, Ca and Ti are principally mantle-derived, whereas significant amounts of the elements Si, K, Na, P and possibly Al may be contributed from slab. As blends of mantle and slab materials, the OIB-type Texcal Flow and V. Chichinautzin magmas provide limited indication of the composition of the sub-arc mantle prior to subduction modification, which is inferred to be similar to primitive mantle, but less enriched than the sources of the intraplate magmas behind the MVB volcanic front.