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The composition of diopside solid solutions, and of liquids, in equilibrium with forsterite, plagioclase, and liquid in the system Na2O-CaO-MgO-Al2O3-SiO2 and in remelted rocks from 1 bar to 12 kbar

  • Gordon M. Biggar (a1)

Abstract

Subsolidus diopside compositions and coexisting diopside and liquid compositions which are also in equilibrium with anorthite and forsterite in the system CaO-MgO-Al2O3-SiO2 were determined by X-ray diffraction and by electron microprobe in samples equilibrated at 1 bar and at 7 kbar. Along with previous data from the literature and using a recently published grid relating diffraction peaks to composition, results from both techniques are satisfactorily reconciled. At 1 bar, diopside composition (moles, Di = CaMgSi2O6, En = Mg2Si2O6, CaTs = CaAl2Si2O6) in equilibrium with anorthite, forsterite, and spinel (analogous to alkali basalts) are close to Di79CaTs20En1 and those in equilibrium with anorthite, forsterite, and pigeonite (analogous to tholeiitic basalt) are close to Di75CaTs3En22. At 7 kbar the equivalent compositions are Di69CaTs28En3 and Di69CaTs26En5 respectively.

In the system CaO-Na2O-MgO-Al2O3-SiO2, electron microprobe analyses of augites and liquids at 1 bar confirm the changes expected in the loci of liquids and show that the low Na2O (< 0.50 wt.%) augites are similar to those in CaO-MgO-Al2O3-SiO2. At 7 kbar the orthopyroxene field has expanded sufficiently and augite was not encountered in the limited range of samples studied.

In remelted rocks the augite compositions at 1 bar are similar to the compositions of augites in low pressure effusive rocks. At 2–15 kbar, the available data in the literature for dry basalts show that experimentally recrystallized augites are very sub-calcic and very aluminous, and no natural equivalents exist in plutonic gabbros or in ophiolitic rocks. This discrepancy is not resolved but either the experimental pyroxenes are metastable or the pyroxenes presently in the rocks are not the pristine compositions.

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