ALBERT

Description: Majoritic garnet, characterized by an excess of silicon (〉3 Si per formula unit), is considered one of the major phases of the Earth’s transition zone from 410–660 km depth. Quantifying the H 2 O content of nominally anhydrous mantle minerals is necessary to evaluate their water storage capacity from experiments and modeling the Earth’s deep water cycle. We present mineral-specific infrared absorption coefficients for the purpose of quantifying the amount of water incorporated into majorite as hydroxyl point defects. A suite of majoritic garnet samples with varying proportions of Si, Fe, Al, Cr, and H 2 O was synthesized at conditions of 18–19 GPa and 1500–1800 °C. Single-crystals were characterized using X-ray diffraction, electron microprobe analysis, secondary ion mass spectrometry (SIMS), IR, Raman, and Mössbauer spectroscopy. We utilize SIMS and Raman spectroscopy in combination with IR spectroscopy to provide IR absorption coefficients for water in majoritic garnets with the general mineral formula (Mg,Fe) 3 (Si,Mg,Fe,Al,Cr) 2 [SiO 4 ] 3 . The IR absorption coefficient for majoritic garnet in the OH stretching region is frequency dependent and ranges from 10 470 ± 3100 Lmol –1 cm –2 to 23 400 ± 2300 Lmol –1 cm –2 .

Description: The amount of insoluble macromolecular organic matter in the Earth’s crust, commonly referred to as kerogen, far exceeds the mass of living organic matter. The fraction of kerogen in sediments subducted into the mantle remains poorly constrained and will vary depending on the physical-chemical properties of kerogen along different slab geotherms. We studied the pressure-temperature evolution of carbon vibrational frequencies in isolated kerogen, previously not subjected to metamorphism, using Raman spectroscopy in a sapphire optical cell up to 3.2 GPa and 450 °C, corresponding to colder subduction geotherms. For blue-green laser excitation, we find optical irradiance exceeding ~3 kW/cm 2 induces changes in spectral features of the primary graphitic (G-band) and two main disordered modes (D1 and D2) that might otherwise be mistaken for thermal maturation. Whereas previous in situ studies have investigated the changes in these molecular vibrations of kerogen at high temperature or high pressure, we collected Raman spectra of isolated kerogen at simultaneous high P-T conditions. Although instantaneous and irreversible changes in band ratios of isolated kerogen were observed above ~350 °C at room-pressure, long-duration (2–8 h) heating experiments at 450 °C and 2.7–3.0 GPa reveal no permanent change in band structure. The reduction in vibrational frequencies of the disordered carbon modes with temperature (d/d T ) at pressures 〉1 GPa is slightly less than found at room pressure, further indicating that pressure effectively increases the thermal stability of kerogen. Our results suggest that kerogen reaching depths of 60 km where the temperature is below ~450 °C may subduct into the mantle, providing a potential source for the organic-rich component of carbon recently detected in certain lower-mantle diamonds.

Description: As the most abundant solid phase at depths of 410–525 km, wadsleyite constitutes a large geochemical reservoir in the Earth. To better understand the implications of minor element substitution and cation ordering in wadsleyite, we have synthesized wadsleyites coexisting with pyroxenes with 2–3 wt% of either TiO 2 , Cr 2 O 3 , V 2 O 3 , CoO, NiO, or ZnO under hydrous conditions in separate experiments at 1300 °C and 15 GPa. We have refined the crystal structures of these wadsleyites by single-crystal X-ray diffraction, analyzed the compositions by electron microprobe, and estimated M3 vacancy concentration from b / a cell-parameter ratios. According to the crystal structure refinements, Cr and V show strong preferences for M3 over M1 and M2 sites and significant substitution up to 2.9 at% at the tetrahedral site (T site). Ni, Co, and Zn show site preferences similar to those of Fe with M1 M3 〉 M2 〉 T. The avoidance of Ni, Co, and Fe for the M2 site in both wadsleyite and olivine appears to be partially controlled by crystal field stabilization energy (CFSE). The estimated CFSE values of Ni 2+ , Co 2+ , and Zn 2+ at three distinct octahedral sites show a positive correlation with octahedral occupancy ratios [M2/(M1+M3)]. Ti substitutes primarily into the M3 octahedron, rather than M1, M2, or T sites. Ti, Cr, and V each have greater solubility in wadsleyite than in olivine. Therefore these transition metal cations may be enriched in a melt or an accessory phase if hydrous melting occurs on upward convection across the wadsleyite-olivine boundary and may be useful as indicators of high-pressure origin.