Melt-Rock Reaction in the Mantle Wedge

Abstract:

As H₂O-rich melts ascend from the base of the mantle wedge into the hotter overlying mantle, reaction occurs between melts and the surrounding mantle. These mantle melt-mantle wall rock reactions have been little studied under the wet conditions of subduction zone settings. Here, we present the results of an experimental approach to quantify the effect of such reactions on melt composition. We have conducted a series of experiments that react deeper melts from 1.6 GPa with shallower mantle at 1.2 GPa under H₂O-undersaturated conditions. The ratio of melt:mantle is varied to determine the influence of percent reaction on phase stability and on the compositions of the resulting melts.

Experiments conducted using 70% deeper melt and 30% shallower mantle contain glass (gl) + olivine (ol) + clinopyroxene (cpx) + spinel (sp). Decreasing the melt:mantle ratio to 20% melt and 80% mantle stabilizes orthopyroxene (opx) in the crystallizing assemblage, as demonstrated by the following crystallization sequence produced using a melt:mantle ratio of 20:80 over a temperature range of 1180-1120 °C: gl + ol + sp; gl + ol + opx + sp; and gl + ol + opx + cpx + sp. For all ratios investigated, high SiO₂, high MgO melts are produced that contain ~6 wt% H₂O. Decreasing the melt:mantle ratio from 70:30 to 20:80 causes small increases in SiO₂ (55-56 wt% to 58-61 wt%) but significant decreases in Al₂O₃ (15-18 wt% to 10-14 wt%).

Using the experimental results, we develop a quantitative model to predict changes in melt composition as a function of reaction with the surrounding mantle. The experimental and model results presented determine how reaction within the mantle can alter the range of melts that erupt into the crust at convergent margins.