BASALTIC LIQUIDS AND HARZBURGITIC RESIDUES IN THE GARRETT TRANSFORM -A CASE-STUDY AT FAST-SPREADING RIDGES

The peridotite-basalt association in the Garrett Transform, similar to 13 degrees 28'S, East Pacific Rise (EPR), provides a prime opportunit y for examining mantle melting and melt extraction processes from both melts and residues produced in a common environment beneath fast-spre ading ridges. The peridotites are highly depleted, clinopyroxene-poor, harzburgites, Residual spinel, orthopyroxene and clinopyroxene in the se harzburgites are extremely depleted in Al2O3, and plot at the most depleted end of the abyssal peridotite array defined by samples from s low-spreading ridges (including samples from hotspot-influenced ridges ), suggesting that these harzburgites are residues of very high extent s of melting. The residual peridotites from elsewhere at the EPR (i.e. , Hess Deep and the Terevaka Transform) also are similarly depleted. T his suggests that the extent of melting beneath the EPR is similar to, or even higher than, beneath ridges influenced by hotspots (e.g., Azo res hotspot in the Atlantic Ocean and Bouvet hotspot in the Indian Oce an), and is significantly higher than less than or equal to 10%, a val ue 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. Thes e same samples also have various amounts of excess olivine with forste rite contents as low as Fo(85). The total olivine modes correlate inve rsely 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 in compatible element enrichment. These observations contrast with the no tion that melts formed at depth experience little low pressure equilib ration during ascent.