Selected Phase Relationships in the System Al-Mn-Fe-Si-O-H: A Model for Garnet Equilibria

Abstract

The bulk compositions 3FeOx.Al2O3.3SiO2 $ excess H2O and 3MnO.Al2O3.3SiO2 $ excess H2O were investigated employing conventional hydrothermal techniques. Almandine and spessartine were synthesized and stability relationships determined in terms of temperature, fluid pressure, and oxygen fugacity. Synthetic almandine has unit cell edge, a0 = 11.528 ± 0.001 Å index of refraction, ND = 1.829 ± 0.003. No systematic variations of these values with respect to temperature, fluid pressure, and oxygen fugacity were observed. Spessartine, synthesized at high temperatures, has average values of a0 = 11.614 ± 0.001 Å and ND = 1.799 ± 0.003. However, below about 600° C a0 gradually increases to 11.635 ± 0.001 Å and ND decreases to 1.772 ± 0.003 with decreasing temperature, irrespective of fluid pressure and oxygen fugacity. These changes appear to reflect the production of hydrospessartine below about 600° C. The stability of almandine strongly depends on the oxygen fugacity. It is stable up to the vicinity of oxygen fugacities defined by the fayalite−magnetite$quartz buffer; the low fo2, range has not been determined, but lies at oxygen fugacities less than those defined by the iron±quartz−fayalite buffer. The stability field of almandine$fluid is bounded by the following Pfluid-T values. At low oxidation states, the low temperature hydrous assemblage of equivalent composition consists of quartz$iron chlorite ($magnetite)$fluid and the high temperature equivalent assemblage consists of fayalite$iron cordierite$hercynite88$fluid. Where fO2 approximates or is in excess of that defined by the fayalite−magnetite$quartz buffer the low temperature hydrous assemblages consist of quartz$iron chlorite$magnetite$fluid, iron chlorite$pyrophyllite$magnetite$fluid, magnetite$mullite$pyrophyllite$fluid, and hematite$mullite$pyrophyllite$fluid; the anhydrous equivalent assemblages consist of quartz$hercynite88, $magnetite88$fluid, quartz$mullite$magnetite$fluid, and quartz$mullite$hematite$fluid, both in order of increasing oxygen fugacity. The stability of spessartine, in contrast to that of almandine, is essentially independent of oxygen fugacity at least up to that defined by the magnetite-hematite buffer. Spessartine is stable up to the highest temperature, 930° C, employed in this investigation at Pfluid = 500 bars. However, it decomposes to a hydrous assemblage consisting of quartz$manganese chlorite$fluid at the following Pfluid-T values: 414° ± 5° C and 3000 bars; 405° ± 5°C and 2000 bars; 386° ± 10° C and 1000 bars; 364°±5° C and 500 bars. Garnets are rare constituents of igneous rocks; those which do occur are predominantly spessartine-rich, and are virtually confined to felsic magmas. Garnets are absent from mafic igneous rocks because the thermal stability ranges of iron-rich members are below the solidus. The near absence of almandine in contact metamorphosed pelitic rocks may reflect a relatively high oxidation state in the aureoles rather than inappropriate P-T conditions. It is argued that the compositions of pyralspite garnets in pelitic schists are subject to various physical and chemical factors, including fO2. With appropriate provisions, the Mn/Fe ratios of garnet coexisting with chlorite and quartz might be used as a temperature indicator. The rarity of spessartine in igneous and metamorphic rocks apparently stems from the departure of rock bulk composition from Mn-rich values rather than from the absence of appropriate physical conditions.