Electrical conductivity of metasomatized lithology in subcontinental lithosphere

Abstract

A plausible origin of the seismically observed mid-lithospheric discontinuity (MLD) in the subcontinental lithosphere is mantle metasomatism. The metasomatized mantle is likely to stabilize hydrous phases such as amphiboles. The existing electrical conductivity data on amphiboles vary significantly. The electrical conductivity data on hornblendite is much higher than the recent data on tremolite. Thus, if hornblendite truly represents the amphibole varieties in MLD regions, then it is likely that amphibole will cause high electrical conductivity anomalies at MLD depths. However, this is inconsistent with the magnetotelluric observations across MLD depths. Hence, to better understand this discrepancy in electrical conductivity data of amphiboles and to evaluate whether MLD could be caused by metasomatism, we determined the electrical conductivity of a natural metasomatized rock sample. The metasomatized rock sample consists of ~87% diopside pyroxene, ~9% sodium-bearing tremolite amphibole, and ~3% albite feldspar. We This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

Evidence for amorphous calcium carbonate originated mid-lithospheric discontinuities

“…Even if CaCO 3 distributes homogeneously from the surface to the depths of ~150 km, the crystalline-amorphization transition or pressure-induced velocity reduction could guarantee a seismic velocity discontinuity at depths of 80-120 km. This also meet the constraints of global resetting of the age at these regions 21 and magnetolluric observations 19,20 .…”

“…CaCO 3 may work in the same way as hydrous minerals 4,16 . However, the high P-T electrical conductivity measurements of amphibole indicate the maximum fraction of hydrous minerals should be less than 1.5% to reconcile the magnetolluric observations 19,20 . Another consequence of the enrichment of hydrous minerals in the MLDs is the young-age rocks sandwiched by old rocks, while the global resetting of the age at these regions is not observed 21 .…”

Machine Learning Investigation of Clinopyroxene Compositions to Evaluate and Predict Mantle Metasomatism Worldwide

“…Metasomatism modifies the mineralogy and composition of pre‐existing rocks through reaction with melt/fluid at high temperature. This important process produces geochemical and isotopic heterogeneities within Earth's mantle (Aiuppa et al., 2021; Roden & Murthy, 1985; Wang et al., 2022; Zhang et al., 2009), which in turn affect chemical differentiation in the mantle, craton stability, and the physical properties of the lithosphere (Araújo et al., 2009; Dawson, 1984; Liu et al., 2021; Lloyd & Bailey, 1975; Menzies & Murthy, 1980; O’Reilly & Griffin, 2013; Pearson et al., 2021; Peng et al., 2021; Rudnick et al., 1993). Therefore, evaluating mantle metasomatism at the global scale is essential to understanding mantle heterogeneity.…”

Is There a Carbonated Mid‐Lithosphere Discontinuity in Cratons?

“…Similarly, Saha et al (2021) proposed that ∼2 vol.% phlogopite and ∼10 vol.% amphibole could produce 2.0%-3.0% velocity drops. However, the high electrical conductivities of these hydrous minerals are in contradiction with the magnetotelluric observations showing mostly normal electrical conductivities at the MLD depths in many cratonic regions (Peng et al, 2022;Selway, 2018). Moreover, the absence of amphibole/phlogopite-bearing xenoliths in certain cratons, such as the Siberian craton and most of the Slave craton (Selway et al, 2015), raises doubts about interpreting the MLD with enrichment in hydrous minerals.…”