Single clinopyroxene thermobarometry for garnet peridotites. Part I. Calibration and testing of a Cr-in-Cpx barometer and an enstatite-in-Cpx thermometer |
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| Authors: | Paolo Nimis Wayne R Taylor |
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| Institution: | (1) Research School of Earth Sciences, Australian National University, Canberra 0200, Australia e-mail: paolon@dmp.unipd.it, AU |
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| Abstract: | Experimental clinopyroxenes synthesized at 850–1500 °C and 0–60 kbar in the CMS and CMAS-Cr systems and in more complex lherzolitic
systems have been used to calibrate a Cr-in-Cpx barometer and an enstatite-in-Cpx thermometer for Cr-diopsides derived from
garnet peridotites. The experiments cover a wide range of possible natural peridotitic compositions, from fertile pyrolite to refractory, high-Cr lherzolite. The barometer is based on the Cr exchange between clinopyroxene and garnet. Pressure is formulated as
a function of temperature and clinopyroxene composition:
where a
CaCrTs
Cpx=Cr−0.81·Cr#·(Na+K) and Cr#= , with elements in atoms per 6 oxygens. This formulation reproduces the experimental pressures to ±2.3 kbar (1σ) and has a
temperature dependence (1.2–2.4 kbar/50 °C, varying with composition) that is weaker than that of the widely used Al-in-Opx
barometer (2–3 kbar/50 °C). The enstatite-in-Cpx thermometer includes corrections for the effect of minor components and is
formulated as
where K)). The thermometer reproduces the experimental temperatures to ±30 °C (1σ).
The uncertainties of the present formulations are comparable to, or better than, those of the most widely used thermobarometers
for garnet peridotites. P-T estimates obtained for diamond-bearing and graphite-bearing lherzolite xenoliths and peridotitic clinopyroxene inclusions
in kimberlitic and lamproitic diamonds confirm the reliability of the thermobarometer. Cr-diopside thermobarometry appears
to be a potential tool for obtaining information on the thermal state of the upper mantle and the extent of mantle sampling
by deep-seated magmas. We consider the Cr-in-Cpx barometer to be the best alternative to the Al-in-Opx barometer for the evaluation
of pressure conditions of equilibration of natural garnet lherzolites. P-T conditions of equilibration can be directly retrieved from the composition of Cr-diopside alone, thus allowing application
to partially altered xenoliths, inclusions in diamonds, and loose grains from sediments. We foresee application of the present
thermobarometer to evaluation of the diamond potential of kimberlite and lamproite provinces and in diamond exploration where
Cr-diopside from deep mantle sources is preserved in the surficial weathering environment.
Received: 16 August 1999 / Accepted: 17 March 2000 |
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