ANORTHITE IN MID-OCEAN RIDGE BASALTS: IMPLICATIONS OF THE PSEUDO-AZEOTROPE IN THE PLAGIOCLASE-OLIVINE SYSTEM

The presence of highly An-rich plagioclase (up to An₉₈) hosting Mg-rich olivine and Al-rich spinel in MORB-like lavas from seamounts and mid-ocean ridges remains enigmatic, as phase equilibria of the hosting lavas show that bytownite should be the highest temperature plagioclase. The recent discovery of a “pseudo”-azeotrope (PA), specifically, a negative azeotrope beneath a field containing a crystalline product of incongruent melting, in the anorthitic plagioclase-olivine system extending to pressures below 1 GPa (Nekvasil et al. 2015, GRL) provides new insights into the potential behavior of feldspar components during partial melting of upwelling MORB source material. Loss of sodic early formed melt formed in the spinel lherzolite pressure regime (e.g., Baker and Stolper, 1994) brings the An# of the residuum into the region where potentially both the peritectic behavior of Al-rich spinel and the PA dictate plagioclase stability, amount of melt produced, and melt composition. Using the plagioclase-olivine system for the first step towards understanding partial melting of anorthitic residua, the low melting temperature of anorthitic plagioclase at 1 GPa produces a large amount of melt at the plagioclase lherzolite boundary. This melting would be incongruent, producing an aluminous spinel-bearing assemblage with a higher An# than the plagioclase in the residuum. If this L+finely disseminated spinel separated from the residuum and ascended, the spinel would react with the liquid, crystallizing olivine+highly anorthitic plagioclase due to the initial rise in solidus temperature during ascent and could undergo complete crystallization. If the resulting troctolite was dragged upward with mantle flow, the shift of the PA towards An with decreasing pressure indicates that it could begin partial melting. Separation of such second-stage liquid and its crystallization would produce crystals that were highly anorthitic, easily achieving An₉₈ compositions.