Olivines Fo-Ni-MnO Systematics in the Trans-Mexican Volcanic Belt: Evidence for Cool and Silicic Primary Arc Magmas

The discovery of systematic differences in the trace element composition of forsteritic olivines in primitive magmas from within-plate, arc and mid-ocean ridge volcanoes engendered much debate about a causal link to the recycling of oceanic crust into the mantle sources of within-plate and arc magmas. Here we address this problem using Cr-spinel bearing, forsteritic (~Fo80–91) olivines from high-Mg# =50 = 73 [Mg# = molar ratio of Mg/(Mg + Fe2+)*100] arc magmas from the Trans-Mexican Volcanic Belt (TMVB). The TMVB arc front olivines have similar high Ni, low MnO, and low Mn/Fe as forsteritic olivines from within-plate basalts erupting through thick lithosphere (=WPB-thick). However, the olivines in TMVB arc front primary melts crystallize at much lower temperatures of Tcrystᵒˡⁱᵛ~1119 ± 38°C (calculated with olivine-spinel aluminum exchange thermometry) in hydrous (~4–9 wt% H2O), silicic, less magnesian (≤10 wt% MgO) mantle melts from mostly garnet-free mantle sources. Model calculations suggest that the primary arc front melt last equilibrated in the mantle at pressures of ~1.4 to ~1.9 GPa (~51–69 km depth), and low temperatures (Tsource = 1150 ± 45°C) that are only slightly higher than the olivine crystallization temperatures. While the Kdoliv/ meltNi increases in the cooler and silicic melts, such modulation cannot account for the full range of Ni concentration in TMVB magmatic olivines. A small population of very high-Ni olivines (>4000–5500 μg/g Ni) is best explained by crystallization in Ni-rich components melt which formed by melt rock reaction processes in the mantle wedge. Unlike Ni, olivine MnO is not sensitive to melt temperature and only moderately to melt composition, and able retains mantle source characteristics. In the TMVB, olivine Fo-MnO-Mn/Fe systematics record an ambient mantle wedge (= mantle without slab component) that is similar to WPB source and that is variably depleted by slab-flux driven melt extraction. Overall, the olivine Fo-Ni-MnO systematics confirm with greater detail than possible by bulk rock studies that the TMVB primary melts are hydrous and silicic, and originate from a mantle wedge that is strongly and variably modified by the slab-flux. These results reaffirm a strong genetic link between slab recycling and the genesis of silicic arc magmas.