Composition of peridotites of ODP Hole 109-670A

The Ocean Drilling Program's Hole 670A is located ~45 km south of the Kane Fracture Zone. Non-constructive (amagmatic) axial divergence ~10 km east of Hole 670A exposes residual mantle peridotites at the seafloor. The average peridotite in the group of 18 studied has 68% of its original mineralogy replaced by serpentine and related minerals. Average reconstructed (preserpentinization) modal proportions are 81.6% OL, 14.7% OPX, 2.4% CPX, and 1.3% SPINEL. Average chemical parameters are OL Mg# = 90.5, OL NiO = 0.31 wt%, OPX Mg# = 90.5, OPX A12O3 = 4.21 wt%, CPX Mg# = 90.9, CPX Na2O = 0.19 wt%, and spinel Cr# = 24.7. Chemical and mineralogical compositions of most samples fall in a restricted range, suggesting the rocks record a similarly narrow range of melting percentages. Major and trace element modeling indicate 14%-16% melting of a partially depleted source like the Tinaquillo lherzolite can account for most of the sample compositions. One harzburgite collected 30 m away from the other samples contains minerals with more refractory compositions; calculated melting percentages for this harzburgite are 18%-20%. The difference in calculated melting percentages between the refractory harzburgite and the rest of the samples may indicate that the percent of melting (or the efficiency of melt extraction) in the sub-oceanic upper mantle melting varies on a scale of tens of meters. Comparison of the Hole 670A peridotite compositions with peridotites from the KFZ reveals no significant differences. Therefore, there is no support in this small sample set for a variation of residual peridotite compositions correlative with the transform fault effect documented from basalt compositional variations.