COULD ANORTHITIC PLAGIOCLASE IN MID-OCEAN RIDGE BASALT BE XENOCRYSTIC?

The relationship of anorthitic plagioclase crystals with hosting mid-ocean ridge basalt remains an unanswered question as a variety of investigations have suggested the possibility of an origin distinct from that of the hosting basalt. Adding to the problem is the possibility that the plagioclase composition in these crystals was not controlled according to well-known melting loop characteristics, but rather through pseudo-azeotropic behavior induced by destabilization of anorthite+olivine relative to spinel+liquid.

We have experimentally explored azeotropic phase relations in the anorthitic region (>An₈₅) of the nominally anhydrous plagioclase-olivine system at pressures from 1.0 GPa to 0.35 GPa by conducting both dynamic cooling experiments, which focus on compositional changes in plagioclase, and equilibrium experiments, which focus on the influence of this topology on the thermal stability of plagioclase. These have shown that (a) the pseudo-azeotrope exists in the pressure range 1-0.5 GPa, (b) the pseudo-azeotrope lies at ~An₉₄ at 0.5 GPa and moves slightly towards more albitic compositions with increasing pressure, (c) the solidus temperature on the An side of the pseudo-azeotrope decreases towards pure An, leading to slight An enrichment of plagioclase with decreasing temperature, (d) on the An side of the pseudo-azeotrope, the bulk composition of spinel+liquid is more anorthitic than the co-existing plagioclase, even as the liquid itself is more albitic and pyroxene-normative, and (e) the solidus temperature in this region is reduced with increasing pressure relative to the expected melting loop.

These characteristics allow for the possibility of decompression melting of depleted peridotite (with high An#) at 1 GPa through an incongruent spinel-producing reaction. This assemblage could then separate from the residuum and ascend, undergoing decompression crystallization through the reaction spinel+liquid = olivine+anorthitic plagioclase, but with a higher An# of the plagioclase relative to the source (as dictated by the topology of the pseudo-azeotrope). These crystals could then be brought up with the upwelling mantle and incorporated into rising mid-ocean ridge basalt.