- One of the most persistent questions regarding the phase equilibria of mid-ocean ridge basalts
(MORB) pertains to the petrogenesis of the anorthitic plagioclase phenocrysts (>An₉₀) that are
characteristic of the more primitive members of such suites. Anorthitic phenocrysts are present in many if
not most MORB suites in spite of the fact that no naturally occurring MORB glasses have ever been
discovered to be in equilibrium with plagioclase more calcic than An₈₅. We have addressed this paradox
by attempting to saturate natural basalts with anorthite in a series of 1 atm experiments using three
different natural basaltic starting compositions: an N-MORB, an E-MORB, and a continental highalumina
basalt. To ensure duplication of the olivine and anorthite saturation observed in natural anorthitebearing
basalt, the experiments were run in An₉₃₋₆ capsules with Fo₉₂ olivine added to the starting glass.
The compositions of experimental liquids are generally colinear with the trends observed in the lava
suites used as the source material for the starting glasses. Significantly, aluminous spinel (Al₂O₃ contents
of 61–68 wt%) was produced at 1290°C in all compositions and chromites (Al₂O₃ contents of 33–
42 wt%) at lower temperatures in N-MORB-derived liquids despite no spinel having been added to the
starting mixture. In addition, the experiments produced basaltic liquid in equilibrium with both >Fo₈₉
olivine and >An₈₅ feldspar at temperatures of 1230° and 1210°. These liquids have compositions with
Mg# (at% Mg/Mg + Feᵀ*100) that range from 63 to >85. The TiO₂-MgO correlation indicates large
(~16–23%) amounts of crystallization for each percent decrease in MgO. These results suggest the
possibility that dry, anorthite-bearing basaltic magmas are the product of the interaction between primary
melt and Al-spinel-bearing upper mantle. In addition, the results indicate that MORB magmas can
undergo a large amount (>50%) of crystallization prior to reaching 8% MgO. Further, although anorthitebearing
magmas have characteristics consistent with their being a significant volumetric component of
MORB ‘‘parent’’ magmas, the reaction mechanism suggested for their petrogenesis indicates that they are
not necessarily primary magmas.
- Kohut, E. J., and R. L. Nielsen (2003), Low-pressure phase equilibria of anhydrous anorthite-bearing mafic magmas, Geochemistry, Geophysics, Geosystems, 4(7), 1057, doi:10.1029/2002GC000451.
|Funding Statement (additional comments about funding)
- Phaseequilibria experiments funded by NSF grant EAR 9903137.
- Vol. 4 no. 1057
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