Towards improving the single grain clinopyroxene geobarometer for garnet peridotites
Creators: Elzinga, Connor G
Publisher: name: University of Alberta Library
Year: 2023

<p>Exploration methods for diamonds often employ the geochemistry of single minerals to identify deposits. Due to the scarcity of diamonds, exploration practices use minerals known as “kimberlite indicator minerals” as these minerals are typically more abundant than diamond and may have equilibrated under conditions where diamond may have been stable. One of these minerals is clinopyroxene from garnet lherzolites, because garnet lherzolites are stable in the diamond stability field. There are multiple published discrimination methods to identify clinopyroxenes from garnet lherzolites such as Ramsay (1992), Nimis (1998), Morris et al. (2002) and Grütter (2009). After applying these discrimination methods to identify garnet lherzolite clinopyroxene, the chemistry of clinopyroxenes can be used with geothermobarometers to calculate pressure and temperature estimates for mantle xenoliths or xenocrysts and thereby identify potentially diamond-bearing kimberlite deposits.<br>
Using published major-element compositions of 678 clinopyroxenes from garnet lherzolites, these clinopyroxene discrimination methods were found to accurately identify clinopyroxenes from garnet lherzolites. Trends between cations were also observed in clinopyroxene. Specifically, with the cations Na and Al+Cr having an approximate 1:1 ratio, suggesting that Na, Al and Cr are substituting dominantly as jadeite (NaAlSi2O6) and kosmochlor (NaCrSi2O6) in garnet lherzolite clinopyroxene. Elemental cations are also compared with the calculated pressure and temperature from multiple geothermobarometers with specific focus on the clinopyroxene geothermobarometers. Na, Al and Cr cations were all found to decrease with increasing pressure. Mg and Fe were found to increase with increasing temperature while Ca decreased with increasing temperature.<br>
These natural clinopyroxene samples were compared with clinopyroxenes from high P-T experiments on garnet lherzolites. The experimental clinopyroxenes were noticeably richer in Al and poorer in Na and Cr than natural clinopyroxenes. The differences between natural and experimental clinopyroxenes, specifically with Al, Cr and Na shows a disconnect between the experimental and natural garnet lherzolite clinopyroxenes. The richer Al in experimental samples suggests a higher tschermak component than in the natural clinopyroxenes.<br>
Using the program THERMOCALC, garnet lherzolites were modelled at specific bulk compositions to compare modelled clinopyroxenes to natural clinopyroxenes. The modelled clinopyroxenes, in the garnet lherzolite stability field, did not match well with natural garnet lherzolites as the modelled clinopyroxenes failed garnet lherzolite clinopyroxene discrimination plots. However, the cation trends against pressure and temperature appeared to follow the trends seen in natural samples. Using compositions calculated along geotherms, clinopyroxene appeared more like natural samples based on garnet lherzolite clinopyroxene discrimination methods and followed the cation trends seen in natural samples except for Al cations.<br>
Finally, an updated single-grain clinopyroxene geobarometer was calibrated from natural garnet lherzolites by modifying the Nimis and Taylor (2000) barometer. This barometer was calibrated using the calculated temperature of Taylor’s (1998) two pyroxene thermometer and the calculated pressure of the Nickel and Green (1985) barometer, with Al calculated after Carswell and Gibb (1987) (the recommend thermometer and barometer of Nimis and Grütter 2010). Through multiple linear regression an expression was developed to calculate P (kbar) as a function of T(K), Cr/(Cr+AlM1), Cr/Al, Cr+Al-0.86(Na+K) and Ca/(Ca+Mg+Fe) in clinopyroxene. The expression is:<br>
P(kbar) = 1239.28 – 0.0076 ⋅ T(K) ⋅ ln(Cr/(Cr+AlM1)) + 9.2988 ⋅ ln(Cr/Al) + 0.2056 ⋅ T(K) – 0.0051 ⋅ T(K) ⋅ ln(Cr+Al–0.86(Na+K)) – 201.2262 ⋅ ln(T(K)) + 56.8249 ⋅ ln(Ca/(Ca+Mg+Fe))<br>
With AlM1 calculated as:<br>
AlM1 = Al – 0.5(Al + Cr + 2Ti – Na – K)<br>
Application of this updated barometer was applied to xenoliths from four localities. These localities are Chidliak kimberlite field, Diavik-Ekati kimberlites, Finsch diamond mine and Jagersfontein diamond mine. The updated barometer performs well as pressures calculated from xenoliths from the four localities using the updated barometer are in good agreement with the Nickel and Green (1985) barometer, with Al calculated after Carswell and Gibb (1987). </p>