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At least 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Stability of Fe-bearing hydrous phases and element partitioning in the system MgO–Al 2 O 3 –Fe 2 O 3 –SiO 2 –H 2 O in Earth’s lowermost mantle

We performed high pressure–temperature ( P-T ) experiments on a model composition of hydrous subducted slabs in the MgO–Al 2 O 3 –Fe 2 O 3 –SiO 2 –H 2 O system using laser-heated diamond anvil cells. The phase assemblages were characterized combining in-situ synchrotron X-ray diffraction and ex-situ transmission electron microscope techniques. The hydrous δ-phase AlOOH–FeOOH–MgSiO 2 (OH) 2 –SiO 2 coexists with bridgmanite (Bdg), post-perovskite (pPv), or both in a broad P-T range of 104–126 GPa and 1750–2500 K. The hydrous pyrite-type FeOOHx phase was observed in-situ over a P-T range of 112–123 GPa and 1750–2300 K, coexisting with the pPv phase. Chemical analysis on recovered samples showed that considerable amount of Fe 2 O 2 (OH) 2 (8–13 mol%) and SiO 2 (9–13 mol%) in the δ-phase does not reduce its thermal stability compared to the Al-endmember, indicating that the Fe-bearing δ-phase can transport water to the lowermost mantle along the mantle geotherm. In this hydrous system, we observed that Al depletion in both the Bdg and pPv phases can significantly reduce the width of the Bdg to pPv transition in contrast to a wide two-phase coexistence region in a dry Al-rich system. Meanwhile, the Fe enrichment in the pPv phase relative to the coexisting Bdg phase lowers the transition pressure to the depth of the D" discontinuity. Accordingly, the depth and thickness of the Bdg to pPv transition in subducted basaltic crustal materials can explain the seismically detected D" discontinuity. Partial melting could be triggered by dehydration of the Fe-bearing hydrous phases due to a steep temperature gradient at the core mantle boundary (CMB), and therefore the ultralow-velocity zones (ULVZs) might be the regions where partial melting occurs at the lowermost mantle.

58 GEOSCIENCES↗
DOE OSTI2017

Partitioning of uranyl between ferrihydrite and humic substances at acidic and circum-neutral pH

As part of a larger study of the reactivity and mobility of uranyl (U(VI)O 2 2+ ) cations in subsurface environments containing natural organic matter (NOM) and hydrous ferric oxides, we have examined the effect of reference humic and fulvic substances on the sorption of uranyl on 2-line ferrihydrite (Fh), a common, naturally occurring nano-Fe(III)-hydroxide. Uranyl was reacted with Fh at pH 4.6 and 7.0 in the presence and absence of Elliott Soil Humic Acid (ESHA) (0–835 ppm) or Suwanee River Fulvic Acid (SRFA) (0–955 ppm). Here, no evidence was found for reduction of uranyl by either form of NOM after 24 h of exposure.

58 GEOSCIENCES↗