Evaluation of elastic geobarometry of spinel inclusions in olivine and its application to mantle xenoliths
Authors: Hagiwara Yuuki , Angel Ross , Gilio Mattia , Yamamoto Junji , Alvaro Matteo 
Published: March 2022

&lt;p&gt;The determination of the pressure and temperature (&lt;em&gt;P-T&lt;/em&gt;) 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 &lt;em&gt;P-T&lt;/em&gt; conditions of rocks, was applied to spinel inclusions in olivine in a spinel-lherzolite xenolith.&lt;/p&gt;&lt;p&gt;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).&lt;/p&gt;&lt;p&gt;Next, using Raman spectroscopy, we attempted to estimate the residual pressure of spinel inclusions (&lt;em&gt;P&lt;/em&gt;&lt;sub&gt;inc&lt;/sub&gt;) 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&lt;sub&gt;g&lt;/sub&gt; (~410 cm&lt;sup&gt;-1&lt;/sup&gt;) and A&lt;sub&gt;1g&lt;/sub&gt; (~750 cm&lt;sup&gt;-1&lt;/sup&gt;) peak positions could be measured with sufficient accuracy for quantitative analysis of residual pressure. When &lt;em&gt;P&lt;/em&gt;&lt;sub&gt;inc&lt;/sub&gt; 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 &lt;em&gt;P&lt;/em&gt;&lt;sub&gt;inc&lt;/sub&gt; from both A&lt;sub&gt;1g&lt;/sub&gt; and E&lt;sub&gt;g&lt;/sub&gt; peaks, and they agreed within error. However, it is desirable to use the A&lt;sub&gt;1g&lt;/sub&gt; peak for the calculation of &lt;em&gt;P&lt;/em&gt;&lt;sub&gt;inc&lt;/sub&gt; because 1) the E&lt;sub&gt;g&lt;/sub&gt; peak has low Raman scattering intensity, 2) depending on the crystal orientation of the host olivine, the E&lt;sub&gt;g&lt;/sub&gt; peak of spinel could interfere with the B&lt;sub&gt;3g&lt;/sub&gt; peak of olivine, and 3) the E&lt;sub&gt;g&lt;/sub&gt; peak is expected to be sensitive to the differential stress because the &lt;em&gt;P&lt;/em&gt;&lt;sub&gt;inc&lt;/sub&gt; calculated from the E&lt;sub&gt;g&lt;/sub&gt; peak obtained from the edge of the inclusion is unusually higher than that calculated from the A&lt;sub&gt;1g&lt;/sub&gt; peak. Since positive residual pressures were obtained from all the inclusions investigated, by combining the EoS of spinel constrained in this study and measured &lt;em&gt;P&lt;/em&gt;&lt;sub&gt;inc&lt;/sub&gt;, spinel inclusions trapped in olivine can be expected to be a new method for estimating the depth provenance of spinel-bearing peridotite.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Angel et al. (2014) Am Mineral, &lt;strong&gt;99&lt;/strong&gt;, 2146-2149; Angel et al. (2017) Am Mineral, &lt;strong&gt;102&lt;/strong&gt;, 1957-1960; Angel et al. (2018) Phys Chem Miner, &lt;strong&gt;45&lt;/strong&gt;, 95-113, Chopelas and Hofmeister (1991) Phys Chem Miner, &lt;strong&gt;18&lt;/strong&gt;, 279-293; Milani et al. (2017) Am Mineral, &lt;strong&gt;102&lt;/strong&gt;, 851-859; Yamamoto et al. (2012) Tectonophysics, &lt;strong&gt;554-557&lt;/strong&gt;, 74-82.&lt;/p&gt;