The oxidation state of iron in Earth's mantle is well known to depths of ~200km, but has not been measured in samples from the lowermost upper mantle (200-410 km depth) or the transition zone (410-660 km). Here we use Synchrotron Mössbauer Source spectroscopy complemented by single crystal X-ray diffraction to make the first measurements of the oxidation state of Fe in inclusions of ultrahigh pressure majoritic garnet in diamond. The garnets show a pronounced increase in oxidation state with depth, with Fe 3+ /(Fe 3+ + Fe 2+) increasing from 0.08 at ~240 km depth to 0.30 at ~500 km depth. The latter majorites, which come from pyroxenitic bulk compositions, are twice as rich in Fe 3+ as the most oxidised garnets from the shallow mantle. Corresponding oxygen fugacities are above the upper stability limit of Fe metal. This observation implies that the increase in oxidation state is not linked to the putative disproportionation of Fe 2+ to Fe 3+ plus Fe metal. Instead, the Fe 3+ increases with depth are consistent with the hypothesis that carbonated fluids or melts are the oxidising agents responsible for the high Fe 3+ contents of the inclusions.

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A portable double-sided pulsed laser heating system for diamond anvil cells has been developed that is able to stably produce laser pulses as short as a few microseconds with repetition frequencies up to 100 kHz. In situ temperature determination is possible by collecting and fitting the thermal radiation spectrum for a specific wavelength range (particularly, between 650 nm and 850 nm) to the Planck radiation function. Surface temperature information can also be time-resolved by using a gated detector that is synchronized with the laser pulse modulation and space-resolved with the implementation of a multi-point thermal radiation collection technique. The system can be easily coupled with equipment at synchrotron facilities, particularly for nuclear resonance spectroscopy experiments. Examples of applications include investigations of high-pressure high-temperature behavior of iron oxides, both in house and at the European Synchrotron Radiation Facility using the synchrotron Mössbauer source and nuclear inelastic scattering.