An optical image showing the Tenham Chondrite thin section USNM 7703 where bridgmanite was identified in a shock vein. The shock melt veins are dark brown to black over the thickness of the thin section of about 30 µm.
However, until a recent study by researchers from the COMPRES member institutions UNLV, Caltech, and CARS-University of Chicago this important phase has never been established as a mineral. (Mg,Fe)SiO3 in the perovskite structure is now called bridgmanite [Tschauner, O. et al, Science 346, 1100 (2014), see also: Sharp, T., Science 346, 1057 (2014)].
The elusiveness of bridgmanite is result of its limited metastability at ambient conditions – about 24 GPa lower than the pressures of its stability field: Any terrestrial material originating from the lower mantle is transformed to low pressure minerals upon upwelling to shallower regions in Earth. However, not only the Earth’s interior but also shocked meteorites are a repository of high-pressure minerals. Previous attempts of detecting bridgmanite with electron microscopy were insufficient to establish this phase as a mineral through structural and chemical analysis. Finally, the use of micro-focused high energy X-ray synchrotron beams in combination with SEM-imaging and EPMA chemical analysis provided sufficient evidence for establishing bridgmanite as a mineral occuring in the highly shocked Tenham L6 Chondrite.
The mineral was named after 1964 Nobel laureate and pioneer of high-pressure experimental research Percy W. Bridgman. The naming does more than remove a vexing gap in petrological terminology. The crystal chemistry of natural bridgmanites also will aid our understanding of the deep Earth.
Brandon Schmandt, Steven D. Jacobsen, Thorsten W. Becker, Zhenxian Liu, Kenneth G. Dueker; Science 344, 1265-1268 doi: 10.1126/science.1253358
(A) Single-crystal of hydrous ringwoodite (blue crystal) containing 1 wt % H2O inside a diamond-anvil cell at 30 GPa. The sample was laser heated to 1600°C in several spots (orange circles) to perform direct transformation to bridgmanite and (Mg,Fe)O. Laser heating was conducted at Sector 13 (GSECARS) of the APS. (B) Synchrotron-FTIR spectra of the recovered sample were collected at beamline U2A of the NSLS. Spectrum 1 is an unheated spot, characteristic of hydrous ringwoodite. Spectra 2 and 3 from within the laser heated spots exhibit modified IR-absorption spectra in the OH region, with a broad and asymmetric band at 3400 cm-1 (characteristic of OH in quenched glass) and a sharp peak (3680 cm-1) associated with brucite. On conversion to bridgmanite plus (Mg,Fe)O, dehydration melting occurred as intergranular melt, viewed by TEM in panel C. In this study, dehydration melting was detected just beneath the mantle transition zone from P-to-S converted phases using seismic data from NSF-Earthscope, US-Array.
The high water storage capacity of minerals in Earth’s mantle transition zone (410- to 660-kilometer depth) implies the possibility of a deep H2O reservoir, which could cause dehydration melting of vertically flowing mantle. We examined the effects of downwelling from the transition zone into the lower mantle with high-pressure laboratory experiments, numerical modeling, and seismic P-to-S conversions recorded by a dense seismic array in North America. In experiments, the transition of hydrous ringwoodite to perovskite and (Mg,Fe)O produces intergranular melt. Detections of abrupt decreases in seismic velocity where downwelling mantle is inferred are consistent with partial melt below 660 kilometers. These results suggest hydration of a large region of the transition zone and that dehydration melting may act to trap H2O in the transition zone.
Posted July 2, 2014