Abstract
The pressure dependence of the exchange of Cr between clinopyroxene and garnet in peridotite is applicable as a geobarometer for mantle-derived Cr-diopside xenocrysts and xenoliths. The most widely used calibration (Nimis and Taylor Contrib Miner Petrol 139: 541–554, 2000; herein NT00) performs well at pressures below 4.5 GPa, but has been shown to consistently underestimate pressures above 4.5 GPa. We have experimentally re-examined this exchange reaction over an extended pressure, temperature, and compositional range using multi-anvil, belt, and piston cylinder apparatuses. Twenty-nine experiments were completed between 3–7 GPa, and 1100–1400 °C in a variety of compositionally complex lherzolitic systems. These experiments are used in conjunction with several published experimental datasets to present a modified calibration of the widely-used NT00 Cr-in-clinopyroxene (Cr-in-cpx) single crystal geobarometer. Our updated calibration calculates P (GPa) as a function of T (K), CaCr Tschermak activity in clinopyroxene left( {a_{{{text{CaCrTs}}}}^{{{text{cpx}}}} } right), and Cr/(Cr + Al) (Cr#) in clinopyroxene. Rearranging experimental results into a 2n polynomial using multiple linear regression found the following expression for pressure:Pleft( {{text{GPa}}} right) = 11.03 + left( { - T{ }left( {text{K}} right){text{ ln}}(a_{{{text{CaCrTs}}}}^{{{text{cpx}}}} ) times 0.001088{ }} right) + left( {1.526 times {text{ln}}left( {frac{{{text{Cr}}#^{{{text{cpx}}}} }}{{T{ }left( {text{K}} right)}}} right)} right){ }where {text{Cr}}#^{{{text{cpx}}}} = left( {frac{{{text{Cr}}}}{{{text{Cr}} + {text{Al}}}}} right), a_{{{text{CaCrTs}}}}^{{{text{cpx}}}} = {text{Cr}} - 0.81 cdot {text{Cr}}#^{{{text{cpx}}}} cdot left( {{text{Na}} + {text{K}}} right), with all mineral components calculated assuming six oxygen anions per formula unit in clinopyroxene.Temperature (K) may be calculated through a variety of geothermometers, however, we recommend the NT00 single crystal, enstatite-in-clinopyroxene (en-in-cpx) geothermometer. The pressure uncertainty of our updated calibration has been propagated by incorporating all analytical and experimental uncertainties. We have found that pressure estimates below 4 GPa, between 4–6 GPa and above 6 GPa have associated uncertainties of 0.31, 0.35, and 0.41 GPa, respectively. Pressures calculated using our calibration of the Cr-in-cpx geobarometer are in good agreement between 2–7 GPa, and 900–1400 °C with those estimated from widely-used two-phase geobarometers based on the solubility of alumina in orthopyroxene coexisting with garnet. Application of our updated calibration to suites of well-equilibrated garnet lherzolite and garnet pyroxenite xenoliths and xenocrysts from the Diavik-Ekati kimberlite and the Argyle lamproite pipes confirm the accuracy and precision of our modified geobarometer, and show that PT estimates using our revised geobarometer result in systematically steeper paleogeotherms and higher estimates of the lithosphere‒asthenosphere boundary compared with the original NT00 calibration.
Highlights
Experimental studies on garnet lherzolites have demonstrated that the solubility of Cr in clinopyroxene is pressure (P) dependent (Brey et al 1990)
An updated high-P experimental dataset that includes mineral data from new experiments conducted between 3–7 GPa and 1050–1400 °C, and published data from Brey et al (1990), Taylor (1998) and Walter (1998) has been used to re-examine the solubility of Cr in cpx in garnet-bearing lherzolites
The empirical recalibration of NT00 geobarometer proposed by Nimis et al (2020) appears to provide a closer approximation for pressures in the 4.5–6 GPa interval but still carries significant uncertainties at higher pressure due to the small number of xenoliths equilibrated at and above 6 GPa
Summary
Experimental studies on garnet lherzolites have demonstrated that the solubility of Cr in clinopyroxene (cpx) is pressure (P) dependent (Brey et al 1990) This relationship has previously been used empirically to calibrate the NT00. Cr-in-cpx geobarometer for chrome-rich (> 0.50 wt% Cr2O3) cpx derived from garnet lherzolites Their geobarometer was calibrated using previously published data (Nickel 1986; Brey et al 1990; Taylor 1998) obtained from experimental studies on a variety of synthetic fertile to refractory lherzolite starting compositions over a pressure–temperature (PT) range of 2–6 GPa and 900–1400 °C. This correction results in an overall improved agreement with Al-in-orthopyroxene P estimates, it has larger uncertainties, especially at high P (> 4.5 GPa)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
