Variations in Earth's 1D viscosity structure in different tectonic regimes

Earth’s long-wavelength geoid provides insights into the thermal, structural, and compositional evolution of the mantle. Historically, most estimates of mantle viscosity using the long-wavelength geoid have considered radial variations with depth in a symmetric Earth. Global estimates of this kind suggest an increase in viscosity from the upper mantle to lower mantle of roughly 2 -- 3 orders of magnitude. Using a spatio-spectral localization technique with the geoid, here we estimate a series of locally constrained viscosity-depth profiles covering two unique regions, the Pacific and Atlantic hemispheres, which show distinct rheological properties. The Pacific region exhibits the conventional Earth's 1D rheology with a factor of roughly 80-100 increase in viscosity occurring at transition zone depths (400 - 800 km). The Atlantic region in contrast does not show significant viscosity jumps with depth, and instead has a near uniform viscosity in the top 1000~km. The inferred viscosity variations between our two regions could be due to the prevalence of present-day subduction in the Pacific and the infrequence of slabs in the Atlantic, combined with a possible hydrated transition zone and mid-mantle of the Atlantic region by ancient subduction during recent supercontinent cycles. Rigid slab material within the top 800 km, with about 90\% Majoritic garnet in the form of subducted oceanic crust, coupled with unique regional mantle structures, may be generating a strong transition zone viscosity interface for the Pacific region. These effective lateral variations in mantle viscosity could play a role in the observed deformation differences between the Pacific and Atlantic hemispheres.

Investigating influences on the Pb pseudo-isochron using three-dimensional mantle convection models with a continental reservoir

For mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs), measurements of Pb isotope ratios show broad linear correlations with a certain degree of scatter. In 207Pb/204Pb - 206Pb/204Pb space, the best fit line defines a pseudo-isochron age (τPb) of ~1.9 Gyr.Previous modelling suggests a relative change in the behaviours of U and Pb between 2.25-2.5 Ga, resulting in net recycling of HIMU (high U/Pb) material in the latter part of Earth's history, to explain the observed τPb. However, simulations in which fractionation is controlled by a single set of partition coefficients throughout the model runs fail to reproduce τPb and the observed scatter in Pb isotope ratios. We build on these models with 3D mantle convection simulations including parameterisations for melting, U recycling from the continents and preferential removal of Pb from subducted oceanic crust.We find that both U recycling after the great oxygenation event (GOE) and Pb extraction after the onset of plate tectonics, are required in order to fit the observed gradient and scatter of both the 207Pb/204Pb - 206Pb/204Pb and 208Pb/204Pb - 206Pb/204Pb arrays. Unlike much previous work, our model does not require accumulations of subducted oceanic crust to persist at the CMB for long periods of time in order to match geochemical observations.

Pervasive fluid-rock interaction in subducted oceanic crust revealed by oxygen isotope zoning in garnet

Dehydration reactions in the subducting slab liberate fluids causing major changes in rock density, volume and permeability. Although it is well known that the fluids can migrate and interact with the surrounding rocks, fluid pathways remain challenging to track and the consequences of fluid-rock interaction processes are often overlooked. In this study, we investigate pervasive fluid-rock interaction in a sequence of schists and mafic felses exposed in the Theodul Glacier Unit (TGU), Western Alps. This unit is embedded within metaophiolites of the Zermatt-Saas Zone and reached eclogite-facies conditions during Alpine convergence. Chemical mapping and in situ oxygen isotope analyses of garnet from the schists reveal a sharp chemical zoning between a xenomorphic core and a euhedral rim, associated to a drop of ~ 8‰ in δ18O. Thermodynamic and δ18O models show that the large amount of low δ18O H2O required to change the reactive bulk δ18O composition cannot be produced by dehydration of the mafic fels from the TGU only, and requires a large contribution of the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is 1.1 × 105 cm3/cm2, which is above the open-system behaviour threshold and argues for pervasive fluid flow at kilometre-scale under high-pressure conditions. The transient rock volume variations caused by lawsonite breakdown is identified as a possible trigger for the pervasive fluid influx. The calculated schist permeability at eclogite-facies conditions (~ 2 × 10–20 m2) is comparable to the permeability determined experimentally for blueschist and serpentinites.

Across-arc variations in Mo isotopes and implications for subducted oceanic crust in the source of back-arc basin volcanic rocks

Molybdenum (Mo) isotope ratios provide a potential means of tracing material recycling involved in subduction zone processes. However, the geochemical behavior of Mo in subducted oceanic crust remains enigmatic. We analyzed Mo isotope ratios of arc and back-arc basin lavas from the Mariana subduction zone (western Pacific Ocean), combining newly obtained element and Sr-Nd-Pb-Li isotope data to investigate subduction zone geochemical processes involving Mo. The Mo isotope ratios (δ98/95MoNIST3134; U.S. National Institute of Standards and Technology [NIST] Mo standard) of the volcanic rocks showed clear across-arc variations, decreasing with increasing depth to the Wadati-Benioff zone. The high δ98/95Mo values in the Mariana Islands (–0.18‰ to +0.38‰) correspond to high 87Sr/86Sr, low 143Nd/144Nd, and radiogenic Pb isotope ratios, suggesting that altered upper oceanic crust played an important role in the magma source. The low δ98/95Mo values in the Central Mariana Trough (–0.65‰ to –0.17‰) with mantle-like Sr-Nd-Pb but slightly low δ7Li values provide direct evidence for the contribution of deep recycled oceanic crust to the magma source of the back-arc basin lavas. The isotopically light Mo magmas originated by partial melting of a residual subducted slab (eclogite) after high degrees of dehydration and then penetrated into the back-arc mantle. This interpretation provides a new perspective with which to investigate the deep recycling of subducted oceanic lithosphere and associated magma petrogenesis.

Supplemental Material: Across-arc variations in Mo isotopes and implications for subducted oceanic crust in the source of back-arc basin volcanic rocks

Sample information, methods, and supplemental figures and tables.<br>

Supplemental Material: Across-arc variations in Mo isotopes and implications for subducted oceanic crust in the source of back-arc basin volcanic rocks

Sample information, methods, and supplemental figures and tables.<br>

Petrogenesis of (meta-) basalts from the North Qilian Orogenic Belt, NW China: implications for the Palaeoproterozoic–Mesoproterozoic tectonic evolution of the North Qilian Block

The North Qilian Orogenic Belt is surrounded by the Tarim Craton to the NW and the North China Craton to the NE. The Precambrian continental crust remnants that are distributed in the North Qilian Orogenic Belt are termed the North Qilian Block (NQB), and their tectonic evolution has profound implications for the evolution of the Columbia Supercontinent. Here we present major- and trace-element and Sr–Nd–Hf isotope data for (meta-) basalts from the Beidahe Group (BDHG) and Zhulongguan Group (ZLGG) in the western North Qilian Orogenic Belt, to investigate the tectonic evolution of the NQB during the Proterozoic Eon. The protoliths of Palaeoproterozoic amphibole gneisses and plagioclase amphibolites from the BDHG are calc-alkaline series basalts. These metabasalts show island-arc-basalt affinities with variable Nd and Hf isotopes (ϵNd(t) = −5.0–0.6 and 2.7–4.3; ϵHf(t) = −14.2–2.0 and 6.9–8.8) and were generated by partial melting of the asthenospheric mantle that was metasomatized by aqueous fluid and sediment melt in a continental-arc setting. The early Mesoproterozoic ZLGG basalts show features of shoshonite-series basalts and are geochemically similar to ocean-island basalts. These basalts show variable (87Sr/86Sr)i, ϵNd(t) and ϵHf(t) values of 0.70464–0.70699, −1–2.6 and −1.5–5.7, and are products of mantle plume magmatism that participated with subducted oceanic crust in an intracontinental rift setting. This study suggests that the NQB underwent tectonic evolution from palaeo-oceanic subduction to intracontinental rifting during the Palaeoproterozoic–Mesoproterozoic eras. Furthermore, the above tectonomagmatic events were in response to convergence–splitting events of the Columbia Supercontinent during the Palaeoproterozoic–Mesoproterozoic eras.