ALBERT

Description: Apatite is a ubiquitous accessory mineral in crustal rocks. The Sr-isotope record of apatite has a wide range of applications in earth science studies. However, apatite has been documented to be easily altered by certain fluids. Currently, the impact of metasomatic alteration on Sr-isotopic abundances in apatite is not well known. In order to better understand this issue, well-characterized fluorapatite grains have been metasomatized experimentally at a temperature of 800 or 600 °C and a pressure of 200 MPa. Fluids used included a CO2-H2O mixture and NaF-, CaCl2-, and HCl-bearing solutions, all of which were doped with a standard solution with a known 87Sr/86Sr ratio. In the fluorapatite + CO2-H2O experiments, the fluorapatite grains were not altered by the fluids; thus, their Sr isotope compositions were generally kept unchanged. However, the other fluids induced partial to complete alteration of fluorapatite. In experiments involving the NaF- or HCl-bearing solutions, the Sr content remained constant or was increased in altered areas of the fluorapatite, and the Sr isotopes underwent changes with partial isotopic signature of the reacting solutions. In experiments involving CaCl2, the Sr content was decreased in the altered fluorapatite because high Ca activity in the solution caused Ca to replace Sr on the Ca site. Notably, the Sr isotopic ratios are still changed, although to a relatively small extent. The efficient Sr isotopic exchange between fluid and apatite is attributed to the rapid transport rate of Sr between the reaction-interface fluid and bulk fluid surrounding the apatite. This experimental study demonstrates that the response of apatite Sr isotopes to metasomatic alteration is mainly controlled by the chemistry of fluids. Overall, Sr isotopes become susceptible to hydrothermal alteration once the apatite is chemically reactive with the fluids. Therefore, it is important to evaluate the fluid-rock history, especially the conjectured fluid composition before using Sr isotopes from apatite as a geochemical tracer.

Description: Dense hydrous magnesium silicates (DHMSs) with large water content and wide stability fields are a potential H2O reservoir in the deep Earth. Al-bearing superhydrous phase B (shy-B) with a wider stability field than the Al-free counterpart can play an important role in understanding H2O transport in the Earth’s transition zone and topmost lower mantle. In this study, a nominally Al-free and two different Al-bearing shy-B with 0.47(2) and 1.35(4) Al atoms per formula unit (pfu), were synthesized using a rotating multi-anvil press. The single-crystal structures were investigated by X-ray diffraction (XRD) complemented by Raman spectroscopy, and Fourier-transform infrared spectroscopy (FTIR). Single-crystal XRD shows that the cell parameters decrease with increasing Al-content. By combining X-ray diffraction and spectroscopy results, we conclude that the Al-poor shy-B crystallizes in the Pnn2 space group with hydrogen in two different general positions. Based on the results of the single crystal X-ray diffraction refinements combined with FTIR spectroscopy, three substitutions mechanisms are proposed: 2 Al3+ = Mg2+ + Si4+; ☐Mg2+ = 2H+; Si4+ = Al3+ + H+. Thus, in addition to the two general H positions, hydrogen is incorporated into the hydrous mineral via point defects. The elastic stiffness coefficients were measured for the Al-shy-B with 1.35 pfu Al by Brillouin scattering (BS). Al-bearing shy-B shows lower C11, higher C22 and similar C33 when compared to Al-free shy-B. The elastic anisotropy of Al-bearing shy-B is also higher than that of the Al-free composition. Such different elastic properties are due to the effect of lattice contraction as a whole and the specific chemical substitution mechanism that affect bonds strength. Al-bearing shy-B with lower velocity, higher anisotropy and wider thermodynamic stability can help to understand the low velocity zone and high anisotropy region in the subducted slab located in Tonga.

Description: Water reinjection into the formation is an indispensable operation in many energy engineering practices. This operation involves a complex hydromechanical (HM) coupling process and sometimes even causes unpredictable disasters, such as induced seismicity. It is acknowledged that the relative magnitude and direction of the principal stresses significantly influence the HM behaviors of rocks during injection. However, due to the limitations of current testing techniques, it is still difficult to comprehensively conduct laboratory injection tests under various stress conditions, such as in triaxial extension stress states. To this end, a numerical study of HM changes in rocks during injection under different stress states is conducted. In this model, the saturated rock is first loaded to the target stress state under drainage conditions, and then the stress state is maintained and water is injected from the top to simulate the formation injection operation. Particular attention is given to the difference in HM changes under triaxial compression and extension stresses. This includes the differences in the pore pressure propagation, mean effective stress, volumetric strain, and stress-induced permeability. The numerical results demonstrate that the differential stress will significantly affect the HM behaviors of rocks, but the degree of influence is different under the two triaxial stress states. The HM changes caused by the triaxial compression stress states are generally greater than those of extension, but the differences decrease with increasing differential stress, indicating that the increase in the differential stress will weaken the impact of the stress state on the HM response. In addition, the shear failure p otential of fracture planes with various inclination angles is analyzed and summarized under different stress states. It is recommended that engineers could design suitable injection schemes according to different tectonic stress fields versus fault occurrence to reduce the risk of injection-induced seismicity.

Description: In this study, we investigated the phase stability of Al-free and Al-bearing superhydrous phase B (shy-B) up to 55 GPa and 2500 K. In comparison with Al-free shy-B, the incorporation of 11.7 wt.% Al2O3 in shy-B expands the stability by ∼400-800 K at 20-30 GPa. The determined dehydration boundary for Al-bearing phase D indicates that it could be present even at normal mantle geotherm conditions at 30-40 GPa. Up to 23.8 mol.% Al2O3 can be dissolved into the structures of akimotoite and bridgmanite as a result of the decomposition reactions of Al-bearing shy-B and phase D between 20-40 GPa. Results of further experiments indicate that δ-AlOOH is the stable hydrous phase coexisting with Al-depleted bridgmanite at pressures above 52 GPa. This study shows that the incorporation of Al in dense hydrous magnesium silicates can have a profound impact on our picture of the water cycle in the deep Earth.

Description: The WHU-GRACE-GPD01s models are the latest monthly gravity field solutions recovered from GRACE intersatellite geopotential difference (GPD) data processed at the School of Geodesy and Geomatics, Wuhan University, China. The intersatellite GPDs are estimated from GRACE Level-1B (RL03) data based on the improved energy balance equation and remove-compute-restore (RCR) technique, and the background models are consistent with GRACE Level-2 processing standards document (RL06). Further details are presented in Zhong et al. (2020, 2022). The WHU-GRACE-GPD01s models include two sets of GRACE monthly solutions: one is the unconstrained monthly solutions with the maximum degree and order of 60, the other is the constrained monthly solutions up to the maximum degree and order 96 with Kaula regularization constraint, and the optimal regularization parameter is determined using variance component estimation (VCE). This work is supported by the National Natural Science Foundation of China (No. 41974015, 41474019, 42061134007) and the Project Supported by the Special Fund of Hubei Luojia Laboratory (Grant No. 220100004).

Description: In order to understand the effect of fluid-induced alteration on the Sm-Nd isotope systematic in apatite, a series of fluid/apatite reaction experiments, which involve reacting the well-characterized Durango fluorapatite with CO2 ± CaCO3-, NaF- or HCl ± CaCl2-bearing solutions with a known 143Nd/144Nd ratio, were conducted at 800 or 600 °C at 200 MPa. In experiments involving CO2-H2O ± CaCO3, the fluorapatite grains did not react with the solution, such that the Sm-Nd isotopic system was undisturbed. In experiments involving NaF, the fluorapatite grains were partially to completely altered. During the alteration process, REE mobilization was retarded via the coupled substitution Na+ + REE3+ = 2Ca2+ due to the high activity of Na in the fluid. Because the REE were not mobilized, the 147Sm/144Nd ratios remained constant. However, the 143Nd/144Nd ratios were slightly altered due to small degrees of Nd isotopic exchange between the fluid and fluorapatite. In experiments involving HCl ± CaCl2, the fluorapatite grains were partially altered, and the REE were variably leached from the altered fluorapatite. Leaching of REE was accompanied by an increase in the 147Sm/144Nd ratio, which is related to the higher compatibility of Sm in the fluorapatite structure and the lower mobility of Sm in Cl-bearing fluids. Although the 147Sm/144Nd ratios were strongly affected, the 143Nd/144Nd ratios experienced only a small change, which is related to the slow rate of transport for Nd between reaction-front fluid and bulk fluid. In general, the experimental results indicate that fluid chemistry is the main factor controlling the response of the Sm-Nd isotopic system to fluid-induced alteration. The 147Sm/144Nd ratio of apatite can be highly modified, and in turn the Sm-Nd isotopic system is disturbed when the fluids are rich in ligands that are able to facilitate REE mobilization and fractionation. Therefore, a thorough evaluation of the fluid-rock history, along with the conjectured fluid chemistry, is necessary before using apatite Sm-Nd isotopes as geological indicators.