Electrical conductivity of omphacite and garnet indicates limited deep water recycling by crust subduction

Abstract

It is usually stated that oceanic crust recycling at subduction zones introduces large quantities of water into Earth's interior. However, it remains clouded how much water is recycled to the deep mantle. This is largely due to the challenge in directly sampling, and analyzing the water content of, a deep subducting crust. The subducting crust below ~30 km depth is dominated by omphacite and garnet in eclogite-facies rocks. Here we have determined the electrical conductivity of omphacite and garnet in representative subduction-related eclogites, each with varying contents of Fe and H2O that are key in electrical conduction. Considering the measured conductivity, the eclogite chemistry by geochemical investigations and the highly resistive property of subducting crusts by geophysical surveys, we demonstrate that, at 70-120 km depths in the subducting crust, the H2O contents of omphacite and garnet are strikingly small, with the maximum value being <400 and <80 ppm in the former and latter, respectively. The very small water contents indicate extremely water-poor conditions, or very low water activity, during the eclogite-facies metamorphism and in the system. This further implies the absence of appreciated amounts of hydrous phases such as amphibole and chlorite in the matrix, because of the strong ability of omphacite and garnet in hosting water as documented in natural samples. We suggest that the recycling of water to the deep mantle by oceanic crust subduction is limited. The results are important for modeling the conductivity of subducting slabs and understanding the deep water circulation.

Water in omphacite fingerprints the thermal history of eclogites

“…Although present at parts-per-million (ppm) levels in NAMs, water has a disproportionate effect on many physical and chemical properties of the host minerals, and on Earth's dynamics. Typically, water lowers melting temperature (Hirose and Kawamoto, 1995;Gaetani and Grove, 1998), promotes ionic diffusion (Hier-Majumder et al, 2005;Costa and Chakraborty, 2008), facilitates plastic deformation (Mackwell et al, 1985;Mei and Kohlstedt, 2000), and enhances electrical conductivity (Karato, 1990;Yang et al, 2011;Yang and McCammon, 2012;Zhang et al, 2012;Liu et al, 2021). Therefore, water in NAMs is key to understanding processes of the deep Earth.…”

Electrical conductivity of melts: implications for conductivity anomalies in the Earth's mantle

“…hydrous olivine and its high-pressure polymorphs. This idea is also supported by the electrical conductivity of granulites [ 123 ], pyroxenites [ 124 ] and eclogites [ 125 , 126 ].…”

Effect of Different Mineralogical Proportions on the Electrical Conductivity of Dry Hot-Pressed Sintering Gabbro at High Temperatures and Pressures

“…As the dominant endmember of constituent mineral in gabbro, the mineralogical content of clinopyroxene has a significant influence on the electrical conductivity of the sample at conditions of 773-1073 K, 1.0 GPa and controlled oxygen fugacity by a Ni-NiO solid buffer. As displayed in Figure 9a, five previously reported results on the electrical conductivity of natural clinopyroxene were employed to compare it in detail [11][12][13][14][15]. Previous results have already confirmed that the iron content and water content are two crucial influential ingredients in the electrical conductivity of Fe-bearing silicate minerals at high temperatures and high pressures [25,[27][28][29][30].…”

“…By virtue of the Wayne Kerr B642 alternating current analyzer at a fixed frequency of 1.6 kHz, electrical conductivities of natural and synthetic polycrystalline diopside were measured at temperatures of 913-1337 K and pressure of 1.0 GPa [13]. Liu et al [14] carried out the impedance spectroscopic measurements on omphacite single crystals containing 7.92 wt% iron and 0.0085 wt% water at temperatures of 623-973 K and 1.0 GPa in a piston-cylinder press. Sun et al [15] researched the electrical conductivities of the clinopyroxene-NaCl-H2O systems with various fluid fractions and salinities at 673-973 K and 1.0 GPa, and they found that the NaClbearing aqueous fluids in clinopyroxene can greatly enhance the electrical conductivity of fluid-bearing sample.…”