Search for: All records

Abstract Clinopyroxene (Cpx) is a major constituent of terrestrial planetary interiors, thus constraining its elasticity across key solid solutions is essential for interpreting geophysical observations. While the effects of coupled NaAl–CaMg substitution along the diopside (Di)–jadeite (Jd) join are well characterized, the influence of Fe–Mg substitution along the Di–hedenbergite (Hd) join remains less certain. We measured the single-crystal elasticity of three Fe-rich natural Cpx spanning the Di–Hd join using Brillouin spectroscopy. We found that the elastic moduli of Cpx vary linearly with Fe–Mg substitution: longitudinal and shear moduli decrease with Fe (or Hd) content, whereas most off-diagonal elastic moduli increase. Increasing Fe content has a negligible effect on the bulk modulus but produces a pronounced reduction in the shear modulus. Combining our results with previous studies, we developed an empirical compositional model for the single-crystal elastic moduli of Cpx at ambient conditions, which predicts that each 10 mol% increase in Hd decreases VP by ∼1.1% and VS by ∼1.8%. These results provide improved constraints on the seismic signatures of Fe-enriched terrestrial upper mantle lithologies. 

Abstract Understanding the natural petrologic and tectonic processes, such as slab subduction, requires accurate determination of the metamorphic pressure (P) and temperature (T) conditions of the exhumed rocks. Recent development of elastic thermobarometry based on the difference between the thermoelastic properties of mineral inclusions and their hosts allows constraining the formation P-T conditions of the inclusion/host system without assuming thermodynamic equilibrium. However, the reliability of elastic thermobarometry heavily depends on the accurate determination of the high P-T single-crystal elastic properties of relevant minerals. In this study, we performed the first high P-T single-crystal elasticity measurements of zircon, which is one of the most common inclusion/host minerals in metamorphic rocks, using Brillouin spectroscopy at pressures up to 7 GPa and temperatures up to 700 K. The single-crystal elastic properties of zircon at ambient conditions were determined as: ρ0 = 4.6674(4) g/cm3; KS = 231(3) GPa; G = 108(11) GPa; C11 = 428(2) GPa; C33 = 484(3) GPa; C44 = 113.5(8) GPa; C66 = 46.9(8) GPa; C12 = 75(2) GPa; and C13 = 151(1) GPa. We also obtained the P and T derivatives of the elastic moduli as ∂KS0/∂P = 5.0(2), ∂KS0/∂T = −0.019(2) GPa/K, ∂G0/∂P = 0.73(6), and ∂G0/∂T = −0.011(1) GPa/K. Compared with other common rock-forming minerals (e.g., quartz, pyroxene, olivine), zircon is extremely incompressible (high KS) and its stiffness (KS) has relatively small T dependence, making it an ideal mineral for the application of elastic thermobarometry. Finally, we utilized the high P-T single-crystal elasticity data of zircon and garnet to evaluate the potential applications of zircon-in-garnet in elastic thermobarometry via elastic modeling. 

Geophysical detection of subducted mid–ocean ridge basalt (MORB) in the lower mantle is hindered by uncertainties in the elasticity of Fe,Al,Mg,Ti–bearing davemaoite, a key MORB component. Using Brillouin spectroscopy and x-ray diffraction, we determined the elasticity of a Ca_0.906(1)Fe²⁺_0.027(1)Fe³⁺_0.042(1)Mg_0.033(1)Al_0.072(1)Ti_0.020(1)Si_0.912(1)O₃davemaoite up to 113 gigapascals and 2294 K. We found that it exhibited a shear wave velocity 10 to 20% slower than end-member davemaoite, making it the slowest phase among major lower-mantle minerals. Our models show that MORB, containing 20 to 25 volume percent davemaoite, potentially contributes to large low-shear-velocity provinces (LLSVPs), whereas a cumulate layer enriched in davemaoite crystallized from basal magma ocean may comprise ultralow-velocity zones (ULVZs). Davemaoite’s ability to host incompatible and heat-producing elements possibly links LLSVPs and ULVZs to mantle plume initiation and geochemical signatures of ocean island basalts. 

Abstract Metasomatized mantle xenoliths containing hydrous minerals, such as amphiboles, serpentine, and phlogopite, likely represent the potential mineralogical compositions of the metasomatized upper mantle, where low seismic velocities are commonly observed. This study presents the first experimentally determined single‐crystal elasticity model of an Fe‐free near Ca, Mg‐endmember amphibole tremolite at high pressure and/or temperature conditions (maximum pressure 7.3(1) GPa, maximum temperature 700 K) using Brillouin spectroscopy. We found that sound velocities of amphiboles strongly depend on the Fe content. We then calculated the sound velocities of 441 hydrous‐mineral‐bearing mantle xenoliths collected around the globe, and quantitatively evaluated the roles that amphiboles, phlogopite and serpentine played in producing the low velocity anomalies in the metasomatized upper mantle.