Plate tectonics: When ancient continents collide

The geological record preserves scant evidence for early plate tectonics. Analysis of eclogites — metamorphic rocks formed in subduction zones — in the Trans-Hudson mountain belt suggests modern-style subduction may have operated 1,800 million years ago

The effect of eclogitization of crustal rocks on the seismic properties on variable scales: Implications for geophysical imaging of eclogitization at depth

Plate tectonics shapes the face of the earth and subduction and collision zones are among the most important features on Earth. Here, crustal material is recycled into the mantle or integrated into growing orogens. However, the processes active at depth cannot be studied directly and we thus rely on geophysical imaging methods to visualize the geometries that result from the ongoing processes. Additionally, these processes can be studied in fossil subduction and collision zones. However, the scales at which observations from geophysical imaging are made are orders of magnitude larger than those made in field-based studies of fossil subduction and collision zones. This thesis provides insight into how eclogitization modifies the physical properties of deeply buried rocks and what influence the resulting lithologies and their geometrical configuration have on geophysical imaging. In an interdisciplinary approach, I show how structures that are likely representative for those present at depth in subduction and collision zones develop and what their geometries at depth will be. I then derive their petrophysical properties and show how these are modified on various scales, and how this influences the detectability of such associations using geophysical imaging techniques. To do so, the island of Holsnøy in western Norway serves as a natural laboratory that is ideal to study eclogitization of crustal material. Geological mapping on Holsnøy constrains the geometric framework of the constituting lithologies and the scales at which such structures could be expected to establish. Previously, several authors have shown that many of the eclogite occurrences on Holsnøy are produced contemporaneously with ductile deformation forming shear zones at various scales. Our geological mapping aided by photogrammetry using drone images reveals that large parts of this exposed continental sliver were eclogitized statically without associated ductile deformation. This shows that even in domains with ongoing regional deformation, low-strain domains develop within the descending crustal material. Nevertheless, even the major shear zones that are exposed are only a few hundred meters thick, and thus far below the scale that is detectable by geophysical imaging techniques. However, geological mapping of the area suggests that the exposed structures are, at least in a qualitative sense, scale independent, suggesting that the same structural framework could be present at a larger scale in active subduction and collision zones. Measurements of P and S wave velocities of the exposed granulitic protolith and eclogites suggest that eclogitization of the lower crust causes three major changes of the petrophysical properties: (1) increased P and S wave velocities, (2) an increase of the seismic anisotropy, and (3) a decrease of the VP/VS ratio, suggesting distinct variations in the geophysical signal when the descending material is partially eclogitized. Additionally, testing the signal that the exposed shear zones would give in reflection seismic and receiver function studies reveals that the variations in shear zone structure indeed produces variations in the retrieved waveforms. Nevertheless, as the exposed structures are too small for geophysical imaging, the finite element method is used to calculate the effective properties of representative structures acting as an effective medium. The results show that the geometrical configuration of the constituting lithologies only has a minor impact on the P wave velocities and anisotropies of the resulting effective medium. Furthermore, our effective medium calculations on the kilometer scale show that eclogitization of crustal material can indeed produce significant seismic anisotropy. In this case, the calculated anisotropy reaches ~5%, which would produce a dependence of the retrieved signal in, for example, receiver function studies on the backazimuth of the sampled rays. Such backazimuthal dependence is indeed observed in active collision zones such as the Himalaya-Tibet collision system and the results presented here can thus be used to constrain the lithologies at depth, suggesting that the lower crust of India below the Himalaya is partially eclogitized along shear zones similar to those exposed on Holsnøy

Sulfur loss from subducted altered oceanic crust and implications for mantle oxidation

© The Author(s), [year]. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Walters, J. B., Cruz-Uribe, A. M., & Marschall, H. R. Sulfur loss from subducted altered oceanic crust and implications for mantle oxidation. Geochemical Perspectives Letters, 13, (2020): 36-41, doi:10.7185/geochemlet.2011.Oxygen fugacity (fO2) is a controlling factor of the physics of Earth’s mantle; however, the mechanisms driving spatial and secular changes in fO2 associated with convergent margins are highly debated. We present new thermodynamic models and petrographic observations to predict that oxidised sulfur species are produced during the subduction of altered oceanic crust. Sulfur loss from the subducting slab is a function of the protolith Fe3+/ΣFe ratio and subduction zone thermal structure, with elevated sulfur fluxes predicted for oxidised slabs in cold subduction zones. We also predict bi-modal release of sulfur-bearing fluids, with a low volume shallow flux of reduced sulfur followed by an enhanced deep flux of sulfate and sulfite species, consistent with oxidised arc magmas and associated copper porphyry deposits. The variable SOx release predicted by our models both across and among active margins may introduce fO2 heterogeneity to the upper mantle.We thank James Connolly for modelling support and Peter van Keken for providing updated P–T paths for the Syracuse et al. (2010) models. The manuscript benefited from the editorial handling by Helen Williams and from constructive reviews of Maryjo Brounce, Katy Evans, and an anonymous reviewer. JBW acknowledges Fulbright and Chase Distinguished Research fellowships. This work was supported by NSF grant EAR1725301 awarded to AMC

Petrochemical and petrophysical characterization of the lower crust and the Moho beneath the West African Craton, based on Xenoliths from Kimberlites

Additional evidence to the composition of the lower crust and uppermost mantle was presented in the form of xenolith data. Xenoliths from the 2.7-Ga West African Craton indicate that the Moho beneath this shield is a chemically and physically gradational boundary, with intercalations of garnet granulite and garnet eclogite. Inclusions in diamonds indicate a depleted upper mantle source, and zenolith barometry and thermometry data suggest a high mantle geotherm with a kink near the Moho. Metallic iron in the xenoliths indicates that the uppermost mantle has a significant magnetization, and that the depth to the Curie isotherm, which is usually considered to be at or above the Moho, may be deeper than the Moho

Recycling Argon through Metamorphic Reactions: the Record in Symplectites

The 40Ar/39Ar ages of metamorphic micas that crystallized at high temperatures are commonly interpreted as cooling ages, with grains considered to have lost 40Ar via thermally-driven diffusion into the grain boundary network. Recently reported laser-ablation data suggest that the spatial distribution of Ar in metamorphic micas does not always conform to the patterns predicted by diffusion theory and that despite high metamorphic temperatures, argon was not removed efficiently from the local system during metamorphic evolution. In the Western Gneiss Region (WGR), Norway, felsic gneisses preserve microtextural evidence for the breakdown of phengite to biotite and plagioclase symplectites during near isothermal decompression from c. 20–25 to c. 8–12 kbar at ~700°C. These samples provide an ideal natural laboratory to assess whether the complete replacement of one K-bearing mineral by another at high temperatures completely ‘resets’ the Ar clock, or whether there is some inheritance of 40Ar in the neo-crystallized phase. The timing of the high-temperature portion of the WGR metamorphic cycle has been well constrained in previous studies. However, the timing of cooling following the overprint is still much debated. In-situ laser ablation spot dating in phengite, biotite-plagioclase symplectites and coarser, texturally later biotite yielded 40Ar/39Ar ages that span much of the metamorphic cycle. Together these data show that despite residence at temperatures of ~700°C, Ar is not completely removed by diffusive loss or during metamorphic recrystallization. Instead, Ar released during phengite breakdown appears to be partially reincorporated into the newly crystallizing biotite and plagioclase (or is trapped in fluid inclusions in those phases) within a close system. Our data show that the microtextural and petrographic evolution of the sample being dated provides a critical framework in which local 40Ar recycling can be tracked, thus potentially allowing 40Ar/39Ar dates to be linked more accurately to metamorphic history

GEOCHEMICAL CHARACTERISTICS OF ECLOGITES FROM NORTHERN QAIDAM BASIN, CENTRAL CHINA

The occurrence of high-pressure and ultrahigh-pressure eclogites in the northern border of Qaidam basin in central China indicates the existence of a 350 km orogenic belt. These eclogites provide constraints for reconstructing the tectonic evolution history in this region. In this study, we analyzed nine eclogites sampled from the Xitieshan area, for their major and trace element abundances as well as 143Nd/144Nd isotopic ratios to investigate the factors controlling geochemical compositions of these eclogites and to infer the tectonic evolution in this region.The occurrence of high-pressure and ultrahigh-pressure eclogites in the northern border of Qaidam basin in central China indicates the existence of a 350 km orogenic belt. These eclogites provide constraints for reconstructing the tectonic evolution history in this region. In this study, we analyzed nine eclogites sampled from the Xitieshan area, for their major and trace element abundances as well as 143Nd/144Nd isotopic ratios to investigate the factors controlling geochemical compositions of these eclogites and to infer the tectonic evolution in this region

Evidence for a reducing Archean ambient mantle and its effects on the carbon cycle

Chemical reduction-oxidation mechanisms within mantle rocks link to the terrestrial carbon cycle by influencing the depth at which magmas can form, their composition, and ultimately the chemistry of gases released into the atmosphere. The oxidation state of the uppermost mantle has been widely accepted to be unchanged over the past 3800 m.y., based on the abundance of redox-sensitive elements in greenstone belt–associated samples of different ages. However, the redox signal in those rocks may have been obscured by their complex origins and emplacement on continental margins. In contrast, the source and processes occurring during decompression melting at spreading ridges are relatively well constrained. We retrieve primary redox conditions from metamorphosed mid-oceanic ridge basalts (MORBs) and picrites of various ages (ca. 3000–550 Ma), using V/Sc as a broad redox proxy. Average V/Sc values for Proterozoic suites (7.0 ± 1.4, 2s, n = 6) are similar to those of modern MORB (6.8 ± 1.6), whereas Archean suites have lower V/Sc (5.2 ± 0.4, n = 5). The lower Archean V/Sc is interpreted to reflect both deeper melt extraction from the uppermost mantle, which becomes more reduced with depth, and an intrinsically lower redox state. The pressure-corrected oxygen fugacity (expressed relative to the fayalite-magnetite-quartz buffer, DFMQ, at 1 GPa) of Archean sample suites (DFMQ –1.19 ± 0.33, 2s) is significantly lower than that of post-Archean sample suites, including MORB (DFMQ –0.26 ± 0.44). Our results imply that the reducing Archean atmosphere was in equilibrium with Earth’s mantle, and further suggest that magmatic gases crossed the threshold that allowed a build-up in atmospheric O2 levels ca. 3000 Ma, accompanied by the first “whiffs” of oxygen in sediments of that age

Integration of natural data within a numerical model of ablative subduction: A possible interpretation for the Alpine dynamics of the Austroalpine crust

A numerical modelling approach is used to validate the physical and ge- ological reliability of the ablative subduction mechanism during Alpine con- vergence in order to interpret the tectonic and metamorphic evolution of an inner portion of the Alpine belt: the Austroalpine Domain. The model pre- dictions and the natural data for the Austroalpine of the Western Alps agree very well in terms of P-T peak conditions, relative chronology of peak and exhumation events, P-T-t paths, thermal gradients and the tectonic evolu- tion of the continental rocks. These findings suggest that a pre-collisional evolution of this domain, with the burial of the continental rocks (induced by ablative subduction of the overriding Adria plate) and their exhumation (driven by an upwelling flow generated in a hydrated mantle wedge) could be a valid mechanism that reproduces the actual tectono-metamorphic config- uration of this part of the Alps. There is less agreement between the model predictions and the natural data for the Austroalpine of the Central-Eastern Alps. Based on the natural data available in the literature, a critical discus- sion of the other proposed mechanisms is presented, and additional geological factors that should be considered within the numerical model are suggested to improve the fitting to the numerical results; these factors include varia- tions in the continental and/or oceanic thickness, variation of the subduction rate and/or slab dip, the initial thermal state of the passive margin, the oc- currence of continental collision and an oblique convergence.Comment: 11 Figures and 3 Tabe

When do we need pan-global freeze to explain ^(18)O-depleted zircons and rocks?

Rocks with δ^(18)O values of less than 5‰ SMOW (Standard Mean Ocean Water) contain oxygen derived from ∼0‰ seawater or meteoric (rain or melted snow, <0‰) waters. As δ^(18)O_(precipitation) values decrease with increasing latitude, altitude, and toward the interior of continents, the low δ^(18)O values (<5‰) of hydrothermally altered rocks can potentially serve as a proxy for the δ^(18)O values of the altering water and as a proxy for climates (Fig. 1). Hydrothermal exchange of rocks with large quantities of meteoric waters presents the most viable opportunity to imprint low-δ^(18)O water values on the protolith (Fig. 2). Such processes typically require shallow depths of a few kilometers (where water circulates through open cracks and porous rocks), a heat source to drive meteoric-hydrothermal systems, and appropriate hydrogeologic conditions for water refill. These conditions are most commonly found in caldera and rift settings, such as in Yellowstone (Wyoming, United States) and Iceland. Oxygen—as the major element—is not significantly affected by subsequent metamorphism and melting (by more than ~1 ‰), and metamorphism often creates large, refractory metamorphic minerals (garnets, omphacites, zircons) that lock the protolith's oxygen isotopic values permanently in the geologic record