Evaluation of elastic geobarometry of spinel inclusions in olivine and its application to mantle xenoliths

The determination of the pressure and temperature (P-T) history experienced by mantle xenoliths, especially the pressure, is essential for elucidating the physicochemical layering structure of the uppermost mantle. However, the lack of continuous reactions between solid solution minerals with large volume changes in spinel-lherzolites makes it difficult to apply conventional geobarometry based on mineral chemistry. Here, elastic geobarometry (Angel et al., 2014; Angel et al., 2017), a complementary technique for determining equilibrium P-T conditions of rocks, was applied to spinel inclusions in olivine in a spinel-lherzolite xenolith.

To utilize elastic geobarometry, reliable equations of state (EoS) for the host mineral and inclusion are essential. Although the EoS for mantle olivine is well constrained by Angel et al. (2018), detailed studies on the EoS for spinel are scarce. Therefore, we firstly conducted a comprehensive review of previous studies investigating the temperature and/or pressure dependence of volume, bulk modulus, and heat capacity, and then determined the EoS for end member spinel using EoSfit7c (Milani et al., 2017).

Next, using Raman spectroscopy, we attempted to estimate the residual pressure of spinel inclusions (P_inc) trapped in olivine in a mantle xenolith from Ennokentiev, Sikhote-Alin, Far Eastern Russia (see Yamamoto et al. (2012) for the chemical composition of the sample). As a result, the peaks of the spinel inclusions were always shifted to higher wavenumbers than those of the unstrained reference spinel crystal from the same xenolith, but only E_g (~410 cm^-1) and A_1g (~750 cm^-1) peak positions could be measured with sufficient accuracy for quantitative analysis of residual pressure. When P_inc was estimated using relation between spinel peak position and pressure reported by Chopelas and Hofmeister (1991), the data obtained from the center of the inclusion showed positive P_inc from both A_1g and E_g peaks, and they agreed within error. However, it is desirable to use the A_1g peak for the calculation of P_inc because 1) the E_g peak has low Raman scattering intensity, 2) depending on the crystal orientation of the host olivine, the E_g peak of spinel could interfere with the B_3g peak of olivine, and 3) the E_g peak is expected to be sensitive to the differential stress because the P_inc calculated from the E_g peak obtained from the edge of the inclusion is unusually higher than that calculated from the A_1g peak. Since positive residual pressures were obtained from all the inclusions investigated, by combining the EoS of spinel constrained in this study and measured P_inc, spinel inclusions trapped in olivine can be expected to be a new method for estimating the depth provenance of spinel-bearing peridotite.

 

References

Angel et al. (2014) Am Mineral, 99, 2146-2149; Angel et al. (2017) Am Mineral, 102, 1957-1960; Angel et al. (2018) Phys Chem Miner, 45, 95-113, Chopelas and Hofmeister (1991) Phys Chem Miner, 18, 279-293; Milani et al. (2017) Am Mineral, 102, 851-859; Yamamoto et al. (2012) Tectonophysics, 554-557, 74-82.