Na, Al(Iv) and Al(Vi) in Clinopyroxenes of Subcontinental and Suboceanic Ridge Peridotites - a Clue to Different Melting Processes in the Mantle

The concentrations of Na, Al(IV) and Al(VI) in clinopyroxene from plagioclase-free, spinel-lherzolites and harzburgites recovered from the mid-Atlantic ridge are compared with data from subcontinental and pre-oceanic, plagioclase-free, spinel-peridotites. The concentration of Al2O3 in clinopyroxenes varies within the same range for both suboceanic and subcontinental peridotite groups, from highly aluminous to Al-poor. However, most clinopyroxenes in suboceanic peridotites have Na contents (0.02-0.6 wt% Na2O) and Al(VI)/Al(IV) ratios (0.3-1) lower than Na contents (0.7 to > 2 wt% Na2O) and Al(VI)/Al(IV) ratios (> 1) of subcontinental peridotite clinopyroxenes. These differences appear to be unrelated to both the degree of melting undergone by the two peridotite groups, and to their post-melting evolution (e.g., late metasomatism and magma/peridotite interactions). The clinopyroxene compositional variations within the oceanic peridotite group result from variable degrees of melting due to adiabatic mantle upwelling below mid-ocean ridges, starting in the garnet stability field. In contrast, the range of clinopyroxene compositions observed in subcontinental peridotites cannot be explained only by melting during similar adiabatic decompression. The data suggest that the subcontinental upper mantle was enriched in a jadeite component before partial melting, and that change in pressure and/or temperature was not the major cause of melting. Partial melting in subcontinental upper mantle could have been brought about by the introduction of water-rich fluids in the hot ultramafics.