A number of sources of uncertainty are involved in thermobarometric calculations, the most important of which are associated with analytical precision, activity–composition ( a – x ) relationships, and thermodynamic data. Statistical treatment of these uncertainties results in relatively large uncertainties on the calculated values of pressure and temperature. Little can be done, at least in the short term, about the magnitude of such uncertainties, and any thermobarometric calculations in which they are not taken into account should be treated with caution. Given that uncertainties associated with a–x models and thermodynamic data are systematic when applied to multiple samples with the same mineral assemblage, a solution to the problem of imprecise absolute thermobarometry can be obtained via a relative thermobarometric technique referred to as the Δ PT approach. The Δ PT approach offers a major improvement in the precision of thermobarometry if the calculations can be presented in a Δ PT context.
The high‐ P , medium‐ T Pouébo terrane of the Pam Peninsula, northern New Caledonia includes barroisite‐ and glaucophane‐bearing eclogite and variably rehydrated equivalents. The metamorphic evolution of the Pouébo terrane is inferred from calculated P–T and P–T – X H2O pseudosections for bulk compositions appropriate to these rocks in the model system CaO–Na 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O. The eclogites experienced a clockwise P–T path that reached P ≈19 kbar and T ≈600 °C. The eclogitic mineral assemblages are preserved because reaction consequent upon decompression consumed the rocks’ fluid. Extensive reaction occurred only in rocks with fluid influx during decompression of the Pouébo terrane.
Mineral equilibria calculations in the system K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –Fe 2 O 3 (KFMASHTO) using thermocalc and its internally consistent thermodynamic dataset constrain the effect of TiO 2 and Fe 2 O 3 on greenschist and amphibolite facies mineral equilibria in metapelites. The end‐member data and activity–composition relationships for biotite and chloritoid, calibrated with natural rock data, and activity–composition data for garnet, calibrated using experimental data, provide new constraints on the effects of TiO 2 and Fe 2 O 3 on the stability of these minerals. Thermodynamic models for ilmenite–hematite and magnetite–ulvospinel solid solutions accounting for order–disorder in these phases allow the distribution of TiO 2 and Fe 2 O 3 between oxide minerals and silicate minerals to be calculated. The calculations indicate that small to moderate amounts of TiO 2 and Fe 2 O 3 in typical metapelitic bulk compositions have little effect on silicate mineral equilibria in metapelites at greenschist to amphibolite facies, compared with those calculated in KFMASH. The addition of large amounts of TiO 2 to typical pelitic bulk compositions has little effect on the stability of silicate assemblages; in contrast, rocks rich in Fe 2 O 3 develop a markedly different metamorphic succession from that of common Barrovian sequences. In particular, Fe 2 O 3 ‐rich metapelites show a marked reduction in the stability fields of staurolite and garnet to higher pressures, in comparison to those predicted by KFMASH grids.
Garnet glaucophanite and greenschist facies assemblages were formed by the recrystallization of barroisite‐bearing eclogite facies metabasites in northern New Caledonia. The mineralogical evolution can be modelled by calculated P–T and P–X H2O diagrams for appropriate bulk rock compositions in the model system CaO–Na 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O. The eclogites, having developed in a clockwise P–T path that reached P ≈19 kbar and T ≈590 °C, underwent decompression with the consumption of free H 2 O as the volume of hydrous minerals increased. Eclogite is preserved in domains that experienced no fluid influx following the loss of this fluid. Garnet glaucophanite formed at P ≈16 kbar during semi‐pervasive fluid influx. Fluid influx, after further isothermal decompression, was focused in shear zones, and resulted in chlorite–albite‐bearing greenschist facies mineral assemblages that reflect P ≈9 kbar.
Amphibolite facies mafic rocks that consist mainly of hornblende, plagioclase and quartz may also contain combinations of chlorite, garnet, epidote, and, more unusually, staurolite, kyanite, sillimanite, cordierite and orthoamphiboles. Such assemblages can provide tighter constraints on the pressure and temperature evolution of metamorphic terranes than is usually possible from metabasites. Because of the high variance of most of the assemblages, the phase relationships in amphibolites depend on rock composition, in addition to pressure, temperature and fluid composition. The mineral equilibria in the Na 2 O–CaO–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O (NCFMASH) model system demonstrate that aluminium content is critical in controlling the occurrence of assemblages involving hornblende with aluminous minerals such as sillimanite, kyanite, staurolite and cordierite. Except in aluminous compositions, these assemblages are restricted to higher pressures. The iron to magnesium ratio ( X Fe ), and to a lesser extent, sodium to calcium ratio, have important roles in determining which (if any) of the aluminous minerals occur under particular pressure–temperature conditions. Where aluminous minerals occur in amphibolites, the P–T–X dependence of their phase relationships is remarkably similar to that in metapelitic rocks. The mineral assemblages of Fe‐rich amphibolites are typically dominated by garnet‐ and staurolite‐bearing assemblages, whereas their more Mg‐rich counterparts contain chlorite and cordierite. Assemblages involving staurolite–hornblende can occur over a wide range of pressures (4–10 kbar) at temperatures of 560–650 °C; however, except in the more aluminous, iron‐rich compositions, they occupy a narrow pressure–temperature window. Thus, although their occurrence in ‘typical’ amphibolites may be indicative of relatively high pressure metamorphism, in more aluminous compositions their interpretation is less straightforward.
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