Mantle melting as a function of water content beneath back‐arc basins

Abstract

[1] Subduction zone magmas are characterized by high concentrations of H 2 O, presumably derived from the subducted plate and ultimately responsible for melting at this tectonic setting. Previous studies of the role of water during mantle melting beneath back-arc basins found positive correlations between the H 2 O concentration of the mantle (H 2 O o ) and the extent of melting (F), in contrast to the negative correlations observed at mid-ocean ridges. Here we examine data compiled from six back-arc basins and three mid-ocean ridge regions. We use TiO 2 as a proxy for F, then use F to calculate H 2 O o from measured H 2 O concentrations of submarine basalts. Back-arc basins record up to 0.5 wt % H 2 O or more in their mantle sources and define positive, approximately linear correlations between H 2 O o and F that vary regionally in slope and intercept. Ridge-like mantle potential temperatures at back-arc basins, constrained from Na-Fe systematics (1350°-1500°C), correlate with variations in axial depth and wet melt productivity ($30-80% F/wt % H 2 O o ). Water concentrations in back-arc mantle sources increase toward the trench, and back-arc spreading segments with the highest mean H 2 O o are at anomalously shallow water depths, consistent with increases in crustal thickness and total melt production resulting from high H 2 O. These results contrast with those from ridges, which record low H 2 O o (<0.05 wt %) and broadly negative correlations between H 2 O o and F that result from purely passive melting and efficient melt focusing, where water and melt distribution are governed by the solid flow field. Back-arc basin spreading combines ridge-like adiabatic melting with nonadiabatic mantle melting paths that may be independent of the solid flow field and derive from the H 2 O supply from the subducting plate. These factors combine significant quantitative and qualitative differences in the integrated influence of water on melting phenomena in back-arc basin and mid-ocean ridge settings.

Petrogenesis of early Jurassic basalts in southern Jiangxi Province, South China: Implications for the thermal state of the Mesozoic mantle beneath South China

“…Samples with MgO b 8 wt.% were given the Fe 8.0 and Na 8.0 expressions of Klein and Langmuir (1987) to correct the MgO, FeO and Na 2 O concentrations. Kelley et al (2006) showed that most melts with MgO b 8.5 wt.% are still on the olivine + plagioclase cotectic and adding olivine at this point would lead to a false high FeO content. To further remove the effect of plagioclase, expressions of Kelley et al (2006) were employed to make an additional fractionation correction to 8.5 wt.% MgO.…”

“…Kelley et al (2006) showed that most melts with MgO b 8.5 wt.% are still on the olivine + plagioclase cotectic and adding olivine at this point would lead to a false high FeO content. To further remove the effect of plagioclase, expressions of Kelley et al (2006) were employed to make an additional fractionation correction to 8.5 wt.% MgO. Samples with MgO between 8.0 and 8.5 wt.% were corrected to 8.5 wt.% MgO only along this linear olivine + plagioclase cotectic (Kelley et al, 2006).…”

“…To further remove the effect of plagioclase, expressions of Kelley et al (2006) were employed to make an additional fractionation correction to 8.5 wt.% MgO. Samples with MgO between 8.0 and 8.5 wt.% were corrected to 8.5 wt.% MgO only along this linear olivine + plagioclase cotectic (Kelley et al, 2006). These 8.5 wt.% MgO-equivalent compositions and samples with initial MgO N8.5 wt.% were chosen as starting compositions.…”

The Fina Nagu volcanic complex: Unusual submarine arc volcanism in the rapidly deforming southern Mariana margin

“…(b) A plot of TiO 2 /Y ratios versus Na 2 O contents for Fina Nagu volcanic chain lavas and MalaguanaGadao ridge lavas. The black line shows the trajectory of a melt produced by bath melting a normal MORB composition mantle from Kelley et al [2006]. Arrows show the general effects of increasing melt fraction and increasing source depletion on the composition of melts.…”

A petrological and geochemical study on time-series samples from Klyuchevskoy volcano, Kamchatka arc

“…The degrees of melting of the DMM were estimated for the lower-K primary magma based on the distribution of Ti between the source mantle and melt following the method of Kelley et al (2006). Using the bulk distribution coefficient for Ti of 0.04 and a TiO 2 content of the DMM of 0.133 wt.% (Salters and Stracke 2004) Fig.…”