Abstract
Phase relations for the magnesio-hornblende bulk composition, 2 CaO·4 MgO·Al2O3·7 SiO2+ excess H2O, have been investigated to 10 kb employing hydrothermal and piston-cylinder techniques. The low-temperature limit of amphibole in this system lies at 519° C, 1,000 bars, 541° C, 2,000 bars, and 718° C, 10 kb. The low-T assemblage consists of an+chl+di+tc(+f), and is related to the adjacent high-T equilibrium assemblage, amph+an+chl+f, by the solid-solid reaction (A): 2 di+tc=tr. Small amounts of aluminum, hypothesized to be preferentially dissolved in the cpx (and in the tc) relative to amph, may account for the broad P-T stability range of the di+tc assemblage in the synthetic work relative to systems involving stoichiometric tr, ∘Ca2Mg5Si8O22(OH)2, such as are common in natural, Al-poor calc-silicate parageneses. Alternatively, the low-temperature assemblage produced in the experiments may be metastable. For the investigated bulk composition, synthetic tremolitic-cummingtonitic amphibole contains relatively modest amounts of ts, ∘Ca2Mg3Al2 IVSi6-Al2 IVO22(OH)2; at pressures of 1,000–3,000 bars, solid solution extends from near tremolite only to about cu11tr69ts20, analogous to most analyzed natural magnesio-hornblendic specimens. At 10 kb fluid pressure, the solid solution reaches approximately cu06tr53ts41 for the investigated bulk composition, and appears to be virtually independent of temperature. Amphibole and 14 Å chl react within the amphibole stability field, along curve (B), at about 704° C and 2,000 bars, to produce an, en, fo and f (ΔH=40.9 kcal/ mole); at pressures greater than approximately 7kb, due to the incompatibility of an and fo, the higher temperature assemblage consists of amph, an, en, sp and f. Above P fluid− T curve (B), the amphibole coexists with an+en+fo+f at low pressures; at higher pressures, the amphibole, which is in equilibrium with an+en+sp+f, is relatively more aluminous. The high-T stability limit of aluminous tr+fo lies approximately 20–25° C below the dehydration curve for stoichiometric tremolite on its own bulk composition. Reaction (C), tr+fo=2 di+5 en+f (ΔH = 39.4 kcal/mole), produces an+di+en+f, the highest temperature subsolidus assemblage investigated for the tr50ts50 bulk composition. Hydrous melt is encountered at temperatures at least as low as 900° C at 10 kb, and at that fluid pressure coexists with amphibole over an interval of more than 60° C. Limited solid solution observed between tr and ts in nature (tr100-70) is accounted for by the restricted range of amphibole compositions produced in the present study. Such amphiboles, moreover, appear to have both high- and low-temperature stability limits, as demonstrated by the experimental results.
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Institute of Geophysics and Planetary Physics Publication No. 2811
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Rong-Iong, C., Ross, C. & Ernst, W.G. Experimental studies to 10 kb of the bulk composition tremolite50-tschermakite50+excess H2O. Contr. Mineral. and Petrol. 93, 160–167 (1986). https://doi.org/10.1007/BF00371317
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DOI: https://doi.org/10.1007/BF00371317