Is upper-mantle phosphorus contained in sodic garnet?

Abstract

Garnets crystallized experimentally from within the anhydrous melting ranges of an olivine tholeiite, a tholeiitic andesite and an augite leucitite at pressures between 18 and 45 kbars contain up to 0.4% Na 2O and 0.6% P 2O 5. The Na and P are thought to form a substitution couple, replacing Ca and Si in the garnet structure; representing limited solid solution between grossular (Ca 3Al 2Si 3O 12) and the phosphate Na 3Al 2P 3O 12. This substitution is enhanced by increasing pressure and by falling temperature (increasing degree of crystallization) at constant pressure. Current knowledge of the crystalline site of P in the upper mantle is hampered by lack of data on the stability of apatite and other phosphates at appropriate pressures and temperatures. If all samples of garnetiferous upper mantle brought to the surface by magmatic processes have been depleted to some extent by previous escape of a partial-melt fraction, P 2O 5 concentrations below 0.1% in their garnets could nevertheless signify that this phase was the sole predepletion host for P in the upper mantle, at the depths from which such inclusions are derived. If garnet and apatite are the principal minerals containing P in the upper mantle, it may be possible to use covariances between P and rare-earth elements in mafic liquids to detect which of these phases was the dominant host for P at the site of magma genesis. This approach confirms the widely-held opinion that strongly alkalic mafic magmas are products of upper-mantle partial fusion in the presence of residual garnet. It also leads to a contrasting proposal that mid-ocean ridge basalts may be generated by upper-mantle partial fusion at comparatively small depths, in the presence of residual apatite.