Experimental determination of trace element partitioning between garnet and silica‐rich liquid during anhydrous partial melting of MORB‐like eclogite

Abstract

We present new experimentally determined trace element partition coefficients for nine garnet/melt and two clinopyroxene/melt pairs at 2.9–3.1 GPa and 1325°–1390°C, applicable to anhydrous partial melting of MORB‐like eclogite in the upper mantle. Phase compositions are similar to those documented in partial melting experiments of eclogite at these conditions: garnets with 16–25% grossular component and 0.4–2.0 wt % TiO2 coexist with siliceous partial melts having 52.4–57.1 wt % SiO2 and 1.1–6.7 wt % TiO2. Observed garnet/melt partitioning depends on TiO2 concentrations in garnet. Among the high‐TiO2 garnets (1.4–2.0 wt % TiO2), partition coefficients (Dgt/melt) increase with increasing grossular content of garnet (16–24%), but for the low‐TiO2 garnets (0.4–0.6 wt % TiO2) there is no difference in partitioning behavior at 19–24% grossular. In general, partition coefficients for the low‐TiO2 garnets tend to be higher than for the high‐TiO2 garnets. DZr and DHf increase with higher grossular content, as does DZr/DHf, but DZr and DHf always remain smaller than unity, and TiO2 in garnet appears to have little effect on DZr/DHf. DTh and DU have average values of 0.0012 and 0.0111 for the high‐TiO2 garnets and 0.0074 and 0.0376 for the low‐TiO2 garnets (DU/DTh of 9.1 and 5.1, respectively), affirming that partial melting of eclogite in the upper mantle can produce liquids with significant (230Th)/(238U) excess. D values of highly charged cations (Nb, Ta, Th, U) are lower in high‐TiO2 garnets. This is likely because Ti4+ occupying the garnet Y site requires charge balance by 2+ cations in Y or 3+ cations in Z, thereby limiting the availability of these charge‐balancing substitutions to accommodate other highly charged cations. The overall range of our new garnet/melt partition coefficients, regardless of Ti and grossular content, is similar to literature garnet/melt partitioning data applicable to garnet peridotite melting, and therefore bulk partition coefficients for eclogite and peridotite depend largely on mineral modes and choice of partitioning data for clinopyroxene. Using our new data and previously published data from the literature, we calculate bulk partition coefficients appropriate for two MORB‐like eclogites (82% clinopyroxene + 18% garnet and 75% clinopyroxene + 25% garnet) and a garnet peridotite (60% olivine + 17% orthopyroxene + 13% clinopyroxene + 10% garnet). Bulk partition coefficients are generally much higher for MORB‐like eclogite when compared to garnet peridotite, with the exception of Th and U. However, the ratios of bulk partition coefficients of eclogite and garnet peridotite are similar for pairs such as DSm/DYb, DZr/DHf and DU/DTh, indicating that both lithologies can induce similar trace element fractionations during partial melting in the presence of garnet.