The Discovery of Davemaoite, the CaSiO3 Perovskite, in a Diamond from Earth’s Lower Mantle
Common silicate minerals such as olivine and garnet convert to other phases when brought to great depths in the planet, where they are subjected to extreme pressures. It has long been thought that two of the most common phases at depth in Earth have the perovskite structure. They have long been known from experimental studies in which they have been called magnesium silicate perovskite (MgSiO3) and calcium silicate perovskite (CaSiO3). Only recently has the magnesium silicate perovskite been found in nature, in a meteorite, where it was characterized and named bridgmanite (O. Tschauner et al., “Discovery of bridgmanite, the most abundant mineral in Earth, in a shocked meteorite,” Science, Vol. 346, 2014, pp. 1100–1102). More recently, the calcium silicate perovskite has been found in a diamond and sufficiently characterized to be given a mineral species name (O. Tschauner et al., “Discovery of davemaoite, CaSiO3-perovskite, as a mineral from the lower mantle,” Science, Vol. 374, 2021, pp. 891–894).
Diamonds are one of the few minerals that bring us samples of the minerals found in the deep earth. Diamonds serve as chemically inert and nearly volume-conserving hosts for these minerals, thereby allowing them to retain their composition and in some cases even their original structure. The examination of the inclusions in an 81 mg (~0.405 ct) octahedral diamond from Orapa, Botswana, has resulted in the discovery of the high-pressure form of CaSiO3 in the perovskite structure occurring as a natural mineral in the diamond (figure 1). This phase has been approved by the International Mineralogical Association with the name davemaoite, named in honor of Dave (Ho-kwang) Mao, for his contributions to understanding the deep-mantle geophysics and petrology of our planet.
Experimental studies in the laboratory have indicated that CaSiO3 assumes the perovskite structure at depths between 420 and ~2700 km. This phase, first synthesized in 1975 (L. Liu and T. Ringwood, “Synthesis of a perovskite-type polymorph of CaSiO3,” Earth and Planetary Science Letters, Vol. 28, 1975, pp. 209–211), is only thermodynamically stable at pressures greater than 200,000 atmospheres (20 GPa). Furthermore, the rate of back-conversion to low-pressure polymorphs is rapid if the pressure is lowered. Consequently, the natural mineral is unlikely to survive transport from the deep earth to the surface.
Davemaoite was first identified and characterized within a diamond studied at the Advanced Photon Source at Argonne National Laboratory in Illinois, where an X-ray beam was focused to a 0.5 × 0.5 μm spot. Calcium-rich inclusions in the diamond were first located. Two inclusions, 4 × 6 μm and 4 × 16 μm in size, were examined with X-ray diffraction. They both produced a perovskite pattern. Their identity was further confirmed by infrared spectroscopy. Theoretically, a cubic perovskite has no Raman pattern, and none was obtained from these inclusions. The inclusions were next excavated from the surrounding diamond and analyzed with laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) (figure 2). Their composition corresponded to the calcium silicate perovskite.
Davemaoite is significant in Earth’s deep mantle because it acts as a host to many elements that are incompatible with upper-mantle minerals, fulfilling a role that garnet plays in the upper mantle. Davemaoite may be the most geochemically important phase in the lower mantle because it will act as a repository in which the radioactive, heat-generating elements thorium and uranium become concentrated. Calculations suggested that davemaoite was entrapped in the host diamond at pressures of at least 290,000 atmospheres and temperatures of at least 1400 K. These diamonds from Orapa are also noteworthy because in addition to davemaoite, a high-pressure form of ice was found as a solid inclusion within them where the diamond’s internal pressure was great enough to maintain the ice in the solid form, allowing it to be characterized as the new mineral species ice-VII (O. Tschauner et al., “Ice-VII inclusions in diamonds: Evidence for aqueous fluid in Earth’s deep mantle,” Science, Vol. 359, 2018, No. 6380, pp. 1136–1139).