Toshisuke Kawasaki

Experimental Study of Fe-Mg Exchange Reaction Between Orthopyroxene and Sapphirine and its Calibration as a Geothermometer

Abstract The exchange of Fe 2+ and Mg 2+ between orthopyroxene and sapphirine has been investigated at pressures 7-16 kbar and temperatures 850-1300°C using a piston cylinder apparatus for the synthetic and natural systems. This reaction is temperature-dependent and would be a good geothermometer. The equilibration temperature T is approximately expressed by the distribution coefficient as follows: T (°C)=1515/(ln K D +0.943)-273. This empirical equation was applied to some Antarctic granulites and associated rocks. The new orthopyroxene-sapphirine geothermometer gives consistent results with those estimated from the Fe-Mg orthopyroxene-garnet geothermometer.

Empirical thermometer of TiO2 in quartz for ultrahigh-temperature granulites of East Antarctica

Abstract Two preliminary experiments, heating of rutilated quartz grains with 0.082 wt% TiO2 on average from Bunt Island, Napier Complex, East Antarctica and a synthetic TiO2–SiO2 (rutile–cristobalite) system in air at 1300 °C for 39 days, showed increasing solubility of TiO2 in silica minerals with temperature. Bunt quartz was converted to cristobalite and traversed by many transparent seams with the disappearence of needles and spots of rutile. Unreacted host grains retained many fine needles of rutile. The seams are homogeneous and slightly enriched in TiO2 up to 0.149 wt% on average, which is about one-fifth lower than that of the synthesized TiO2–SiO2 cristobalite (0.767 wt% on average). Area analyses with an electron beam in the raster mode at a magnification of ×5000 gave 0.308 wt% TiO2 for the bulk composition of the Bunt quartz. This indicates that needles of rutile exsolved from the TiO2-saturated quartz at the cooling stage, or during retrograde metamorphism. Natural examples of quartz in geologically and petrologically well-characterized metamorphic rocks were chemically analysed to examine the temperature controls on the Ti saturation level in quartz. The TiO2 content of quartz in equilibrium with rutile increases sensitively with the metamorphic temperature, which can be expressed as where XTiO2Qtz is the mole fraction of TiO2, or the number of Ti atoms per formula unit based on a two-oxygen atom normalization. This empirical equation is very useful to evaluate the metamorphic temperatures for ultrahigh-temperature granulites. The temperatures calculated by the existing Ti-in-quartz thermometer are about 200 °C higher than those estimated by the present thermometer, potentially because of underestimates of Ti solubility in quartz in the previous calibration.

Palaeogeotherms: Olivine-orthopyroxene-garnet geothermometry and geobarometry

This is the first announcement of the success of an attempt to evaluate the equilibration pressures and temperatures of ultramafic rocks by the application of least-squares solution technique to four independent equilibria involving olivine, orthopyroxene and garnet on a statistically sound basis. The results from the proposed method support the previous suggestion that the palaeogeotherm in the upper mantle beneath northern Lesotho was inflected in late Cretaceous times, but the geotherm given here is significantly displaced to higher temperatures. It is found that inflected palaeogeotherms have rather moderate gradients of about 5–6°C/km in the low-temperature portions and have steep gradients of about 9°C/km on the high-temperature limbs in the upper mantle beneath stable cratonic areas, including Lesotho, Premier, Frank Smith, Siberia. Non-inflected geotherms beneath stable cratonic areas such as Kimberley and Tanzania show gradients similar to those for the low-temperature portions of inflected geotherms. Off-cratonic palaeogeotherms show extremely steep gradients of 10–14°C/km (Namibia, Montana and Navajo). The geotherms deduced for the oceanic regions are very similar to those in previously published results.

Experimental Constraints on the Thermal Peak of a Granulite from McIntyre Island, Enderby Land, East Antarctica

Abstract High-pressure experiments have been carried out at 11-22 kbar and 900-1200°C using a piston cylinder apparatus to constrain the thermal peak condition of a granulite characterized by the mineral assemblage of orthopyroxene+sillimanite+quartz from McIntyre Island, Enderby Land, East Antarctica. The bulk composition of the starting material is 85 wt.% McIntyre granulite+15 wt.% sillimanite. At 11 kbar, orthopyroxene, sillimanite and quartz are stable below 1000°C. At 1050°C sillimanite does not appear, and sapphirine coexists with orthopyroxene and quartz. These experimental results indicate that the McIntyre granulite has undergone the ultra high-temperature metamorphism at 1000-1050°C represented by the diagnostic mineral assemblage of orthopyroxene, sillimanite and quartz.

Possible armalcolite pseudomorph-bearing garnet–sillimanite gneiss from Skallevikshalsen, Lützow-Holm Complex, East Antarctica: Implications for ultrahigh-temperature metamorphism

Abstract A possible armalcolite pseudomorph has been identified in garnet–sillimanite gneiss from Skallevikshalsen, located c. 30 km NE of Rundvågshetta, in a terrane with the highest metamorphic grade in the Lützow-Holm Complex, East Antarctica. It occurs as an Fe–Mg–Ti compositional domain consisting of ilmenite, rutile and pseudorutile, partially mantled by rutile within ilmenite. The domain yields an average XMg of 0.171±0.036 exceeding by 3 wt% TiO2 from armalcolite stoichiometry, while the analysis closest to armalcolite stoichiometry has an XMg value close to 0.202. Host ilmenite with 0.4 mol% hematite is in contact with prismatic sillimanite, quartz, plagioclase and K-feldspar. In run products of annealing experiments performed to investigate the origin of the pseudomorph, armalcolite–ilmenite reaction coronae were developed around relict rutile in rock fragments of quartz eclogite from the Higashi-Akaishi mass of the Sanbagawa belt, central Shikoku, Japan. The experiments were carried out at 1 atm and 960–1050 °C with wüstite–magnetite buffer and imply a minimum temperature of 1290 °C for armalcolite stability when extrapolated to Skallevikshalsen pressures of 1.0 GPa. Mineral chemistry thermobarometry for Skallevikshalsen yields a metamorphic path with P–T peak conditions of 0.88–1.1 GPa and 970–1050 °C, followed by retrograde metamorphism at 0.6 GPa and 780 °C, and finally metasomatic alteration at c. 630 °C. This P–T path matches that for similar ultrahigh-temperature metamorphic rocks from Rundvågshetta and Sri Lanka, and is markedly lower in temperature than the unreasonable estimates based on armalcolite stability. This discrepancy is inferred to reflect chemical impurities in armalcolite that lower its minimum temperature stability by more than 200 °C.

Fe2+-Mg partitioning experiments between orthopyroxene and spinel using ultrahigh-temperature granulite from the Napier Complex, East Antarctica

Abstract Temperature dependence of the Fe2+–Mg exchange between orthopyroxene (Opx) and spinel (Spl), was experimentally determined at 9–13 kbar and 900–1200 °C using an ultrahigh-temperature (UHT) granulite collected from the Napier Complex in Enderby Land, East Antarctica. The Fe2+–Mg distribution coefficient, is empirically obtained as where X is the cationic mole fraction, and pressure P and temperature T are in kbar and Kelvin, respectively. The new geothermometer was applied to various natural UHT and associated high-grade metamorphic rocks from East Antarctica and other regions of the world. The results indicate temperatures between 735 and 902 °C at pressures in the range of 5–14 kbar. This geothermometer utilizing spinel does not give peak metamorphic condition, because it is relatively easy for spinel to re-equilibrate during the cooling stage of metamorphism. Hence, we conclude that the geothermometer is suitable for evaluating the closure temperature for the KD between aluminous orthopyroxene and spinel during retrograde metamorphism rather than the thermal peak.