27Al and 29Si NMR spectroscopy of MgSiO3 perovskite: mechanisms of Al and Fe incorporation

The boundary between the Earth's upper and lower mantle is generally attributed to the decomposition of (Mg,Fe)₂SiO₄ ringwoodite into ferropericlase and perovskite-structured (Mg,Fe)SiO₃, hereafter perovskite. Concomitant with this phase change is the more gradual disappearance of a separate Al-bearing phase (majoritic garnet) and dissolution of Al into perovskite. The mechanism of Al incorporation is strongly affected by the presence of Fe due to the co-substitution of Al³⁺ and Fe³⁺ for Si⁴⁺ and Mg²⁺, respectively. However, questions about specific substitution mechanisms, site occupancy, and long- and short-range ordering of Fe and Al are still not fully understood. These are questions that NMR is particularly suited to address. We are applying Mossbauer, and ²⁷Al and ²⁹Si MAS-NMR spectroscopy to a series of Al- and Fe-bearing MgSiO₃ perovskites of nominal composition Mg_1-xFe_xSi_1-xAl_xO₃ (x = 0.01, 0.025, 0.05). These were synthesized in a multi-anvil cell press at 26 GPa and 1900°C from either glassy starting material or Fe-bearing, aluminous pyroxene synthesized at 8-10 GPa and 1200°C. ²⁹Si spectra show the presence of only a narrow perovskite peak at -191.7 ppm and a broad peak at -81 ppm from an amorphous phase likely formed when crushing the multi-anvil run products. ²⁷Al spectra contain one narrow peak at 6.4 ppm and one broad peak at -14.6 ppm, matching those seen in previous NMR studies of aluminous perovskite and corresponding to Al³⁺ substituting for Si⁴⁺ and Mg²⁺ respectively. The ratio of observable Al³⁺ in Si⁴⁺ vs. that in Mg²⁺ sites ranges from 5.5 to 6.3 when the signal to noise of the -14.6 ppm peak is sufficient for quantitation. NMR spectra of materials containing paramagnetic species (i.e. Fe²⁺ or Fe³⁺) are often complicated and difficult to interpret due to line broadening, signal loss, "contact" shifts, etc. However, with an understanding of paramagnetic interactions in NMR spectroscopy, valuable information can be gained about some Fe-bearing geological materials. For the pyroxene starting materials used here, NMR signal loss increases linearly with increasing Fe content, an expected result given the likely uncoupled substitution of Fe²⁺ and Al³⁺ in these materials. However, ²⁷Al signal loss for the perovskite samples is non-linear with a nearly constant value going from Fe/(Fe + Mg) = 0.01 to 0.025 indicating strong ordering of Fe³⁺ and Al³⁺ onto adjacent crystallographic sites. Complete signal loss at Fe/(Fe + Mg) = 0.05 suggests the upper limit of Fe²⁺ and Fe³⁺ concentration at which useful NMR data can be obtained for this system. The results of this study are a start to a better understanding of the mechanism of solution of Fe and Al in perovskite and continued work on this problem could provide valuable information on this important lower mantle phase. In addition, this study has potential to give us more general insights into paramagnetic interactions in NMR spectroscopy of geological materials.