Mineral equilibria in the MgO-SiO 2 -H 2 O system; II, Talc-antigorite-forsterite-anthophyllite-enstatite stability relations and some geologic implications in the system

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ID: 294090
1977
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Abstract
Mineral equilibria in the MgO--SiO/sub 2/--H/sub 2/O system were investigated, with emphasis on the stability relations of talc, antigorite, forsterite, anthophyllite, and enstatite. Thermal dehydration temperatures at 1 kb H/sub 2/O are at 514/sup 0/ +- 12/sup 0/C (Ta--Antig--Fo), 627/sup 0/ +- 7/sup 0/C (Ta--Fo--Anth), 670/sup 0/ +- 7/sup 0/C (Anth--Fo--En), 675/sup 0/ +- 7/sup 0/C (Ta--Anth--Q), 643/sup 0/ +- 8/sup 0/C (Ta--Fo--En), 690/sup 0/ +- 8/sup 0/C (Ta--En--Q), 729/sup 0/ +- 8/sup 0/C (Anth--En--Q). Derived G/sup 0//sub f/ values in kcal/gf are -928.756 +- 1.1 (Antig, based on Mg/sub 2/./sub 825/Si/sub 2/O/sub 3/ (OH)/sub 3/./sub 65/), -2711.320 +- 1.7 (Anth) and -348.955 +- 0.6 (EN). Anthophyllite has a relatively narrow temperature stability range at 1 kb, but it increases as P/sub H/sub 2/O/ increases. The Ta--Fo--En and Ta--En--Q reactions are low-pressure changes, metastable at 1 kb. The (Q) and (Fo) invariant points are at approximately 603/sup 0/C, 500 bars, and 601/sup 0/C, 175 bars, P/sub H/sub 2/O/, respectively. Higher P/sub total/ raises and spreads the P/sub H/sub 2/O/ values of the invariant points. The log SiO/sub 2/ /sub aq/ versus 1/T diagrams utilized in this study are applied to metasomatic and isochemical processes involving this system. Aspects of contactmore » metamorphism and metasomatism, the process of serpentinization, and compositional controls and relative stabilities of the serpentine phases are discussed. Isochemical cooling in the subsolidus at finite, though not necessarily constant, P/sub H/sub 2/O/ results in a mineral reaction series analogous to that of the classical reaction series of silicate melts. The chemical constraints revealed in the various equilibria studied help to clarify and more rigorously to define the isochemical PT relationships of the system. The geological application of activity-activity or chemical potential diagrams is also discussed, and the necessity of considering reactions that correspond to or approximate the true process involved is illustrated.« less
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Authors J. J. Hemley, J. W. Montoya, Donnie R. Shaw, Robert W. Luce
Journal american journal of science
Year 1977
DOI
10.2475/ajs.277.4.353
URL
https://ajsonline.org/article/59906.pdf
Keywords Keywords not found

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