Basaltic liquids and harzburgitic residues in the Garrett Transform: a case study at fast-spreading ridges

Abstract

The peridotite-basalt association in the Garrett Transform, ∼ 13°28′S, East Pacific Rise (EPR), provides a prime opportunity for examining mantle melting and melt extraction processes from both melts and residues produced in a common environment beneath fast-spreading ridges. The peridotites are highly depleted, clinopyroxene-poor, harzburgites. Residual spinel, orthopyroxene and clinopyroxene in these harzburgites are extremely depleted in Al2O3, and plot at the most depleted end of the abyssal peridotite array defined by samples from slow-spreading ridges (including samples from hotspot-influenced ridges), suggesting that these harzburgites are residues of very high extents of melting. The residual peridotites from elsewhere at the EPR (i.e., Hess Deep and the Terevaka Transform) also are similarly depleted. This suggests that the extent of melting beneath the EPR is similar to, or even higher than, beneath ridges influenced by hotspots (e.g., Azores hotspot in the Atlantic Ocean and Bouvet hotspot in the Indian Ocean), and is significantly higher than ≤ 10%, a value that has been advocated to be the average extent of melting beneath global ocean ridges. Many of these harzburgite samples, however, show whole-rock incompatible element abundances higher than expected. These same samples also have various amounts of excess olivine with forsterite contents as low as Fo85. The total olivine modes correlate inversely with olivine forsterite contents, and positively with whole-rock incompatible element abundances. These correlations suggest that the excess olivine and incompatible element enrichment are both the result of melt-solid re-equilibration. The buoyant melts that ascend through previously depleted residues crystallize olivine at shallow levels as a result of cooling. Entrapment of these melts leads to whole-rock incompatible element enrichment. These observations contrast with the notion that melts formed at depth experience little low pressure equilibration during ascent.