Geothermobarometry in Four-phase Lherzolites II. New Thermobarometers, and Practical Assessment of Existing Thermobarometers |
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| Authors: | BREY G P; K?HLER T |
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| Institution: | Max-Planck-Institut f?r Chemie, Abt. Kosmochemie Saarstr. 23, D-6500 Mainz, West Germany |
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| Abstract: | On the basis of experiments presented in Part I of this series,most of the published thermobarometers relevant to four-phaseperidotites are tested here for their ability to reproduce experimentalconditions. They were rejected if any systematic discrepancyin either pressure or temperature was discernible. This testcautions against the use of all published versions of thermometersbasad on the compositions of coexisting ortho- and clinopyroxenesand the use of existing barometers based on the Al content oforthopyroxene axxisting with garnet. Therefore, we formulatednew versions of the two-pyroxene thermometer and the Al-in-opxbarometer:
 with  and  is in degress Kelvin and P is in kilobars. Our new barometer is of the form
 (C1–C3) and site occupancies are given in the text. Temperatures may also be calculated from the Ca content of opxalone:
 This thermometer can be applied both to the CMS and the naturalsystem experiments, which may indicate that Fe and Na have counter-balancingeffects on the Ca content of opx. The partitioning of Na between opx and cpx can also serve asa useful thermometer, and was calibrated from natural rock data:
 where T is in degrees Kelvin, P is in kilobars, and DNa=Naopx/Nacpx. The following three published thermobarometers based on furtherexchange reactions are capable of reprducing experimental conditions:- exchangeof Ca between olivine and clinopyroxene as a barometer(PKB),
- exchange of Fe and Mg between garnet and clinopyroxene asathermometer (TKrogh),
- exchange of Fe and Mg between garnetand olivine as a thermometer(TO'Neiii).
Our tests also show that the most accurate pressure and temperatureestimates arc obtained from the following combinations of thermometersand barometers:- TBKN+PBKN,
- TBKN+PKB,
- TKrogh+PBKN,
- TO'Ne$$$ll+PBKN.
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