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- Title
The Missing Magmas of MOR: Insights From Phase Equilibrium Experiments on Plagioclase Ultraphyric Basalts.
- Authors
Ustunisik, Gokce K.; Nielsen, Roger L.; Walker, David
- Abstract
Plagioclase ultraphyric basalts (PUBs) are a class of mid‐ocean ridge (MOR) lavas found in a variety of ocean floor environments, are characterized by abundant (15–40 volume %) plagioclase megacrysts and a diverse trace element and isotopic signature. Paradoxically, we never see lavas erupted on the seafloor that are in equilibrium with these PUB megacrysts. Based on petrographic evidence, melt inclusion composition, and new data on depth of entrapment calculated from CO2 contents in plagioclase‐hosted inclusions, many of the megacrysts formed at upper mantle pressures (∼3–7 kbars). To constrain the composition of the parent magmas of the plagioclase megacrysts, we conducted a series of experiments at 5 and 10 kbars using mid‐ocean ridge basalts glasses as starting materials. The experimental results were consistent with the presence of a pseudoazeotrope in the anorthitic segment of the plagioclase + basalt pseudobinary. This has the effect of dropping the anorthitic end of the feldspar loop, lowering the solidus for upper mantle conditions, and driving evolving magmas toward higher Ca. As magmas rise and pressure drops, the pseudoazeotrope disappears, and the feldspar loop at the high‐An end rises, causing those magmas to undergo decompression crystallization of plagioclase and resorption of olivine. Therefore, the conditions which generated the magmas from which the megacrysts form disappear as the magmas rise and magmas evolve toward lower Ca, Mg (as we normally assume during plagioclase + olivine crystallization). In effect, the phase equilibria conditions that allow for the generation of such liquids also prevent them from being erupted as lavas. Plain Language Summary: Much of our understanding of the Earth's mantle is based on study of the magmas erupted from the mid‐ocean ridge (MOR) system. This investigation involves a class of MOR lavas, plagioclase ultraphyric basalts (PUB), characterized by abundant anorthitic (high‐Ca) megacrysts. The question we address relates to the fact that few naturally occurring MOR lavas are in equilibrium with such anorthitic compositions. Understanding what the geochemical signal of the megacrysts means requires we understand what their parental magma types are. We conducted a series of experiments at mantle pressures on a pseudobinary in the plagioclase‐MOR liquid system. Our results document the presence of a thermal maximum (pseudoazeotrope) on the anorthitic end of the pseudobinary. The effect of this pseudoazeotrope is to lower (in temperature) the anorthitic end of the feldspar loop. Magmas evolving under those conditions evolve to higher Ca and lower Mg at depths above 15 km. As magmas rise, the feldspar loop at the high‐An end rises, causing those magmas to undergo decompression crystallization, preventing such liquids from being erupted on the seafloor. In effect, the conditions that generate the unique composition of anorthitic megacrysts prevent their parental liquids from being erupted on the ocean floor as lavas. Key Points: Our results document the presence of a thermal maximum (pseudoazeotrope) on the anorthitic end of the pseudobinaryThe conditions generate unique composition of anorthitic megacrysts prevent their parental liquids from being erupted on the ocean floorPhase equilibria at high P and change in high An plagioclase melting loop cause formation of magmas that form plagioclase megacrysts
- Subjects
MAGMAS; IGNEOUS rocks; PLAGIOCLASE; FELDSPAR; OCEAN bottom
- Publication
Geochemistry, Geophysics, Geosystems: G3, 2022, Vol 23, Issue 1, p1
- ISSN
1525-2027
- Publication type
Article
- DOI
10.1029/2021GC009943