CMP: Bushveld symplectic and sieve textured chromite is a result of coupled dissolution reprecipitation: a comparison with xenocrystic chromite reactions in arc basalt

Marina A. Yudovskaya, Gelu Costin, Vladimir Shilovskikh, Ilya Chaplygin, Matthew McCreesh, Judith Kinnaird

Abstract

Textures of Bushveld chromite from thin seams and accessory disseminations in the Platreef and the northernmost Waterberg Project area were compared with textures of xenocrystic chromite from mantle xenoliths found in Neogene basalt in the Kurile Island Arc. The sieve-textured to symplectic rims around the resorbed chromite in the Kurile samples resulted from the reaction between chromite and chromite-undersaturated basaltic melt, with the inclusions in chromite being entrapped during episodes of chromite primary growth, chemical dissolution, and reprecipitation or secondary growth. The relics of the lattice-oriented etch tunnels suggest that the dissolution preferentially developed along the crystallographic planes and defects. The Bushveld chromites exhibiting similar textures are interpreted as reaction-textured chromites, by analogy with the Kurile samples. The Bushveld sieve-textured, fsh-hook to symplectic and amoeboidal to atoll-like chromites, are believed to have been formed due to coupled dissolution-reprecipitation of the earlier cumulus or xenocrystic chromite during interaction with chromite-undersaturated evolved melt. The electron backscattered difraction data confrm the same singlecrystal crystallographic orientation of all domains of the reaction-textured chromites as well as their clustered semi-dissolved relics. Therefore, Bushveld inclusion-rich chromite might have captured diferent populations of melt inclusions during its discontinuous out-of-equilibrium growth with fast episodic resorption and regeneration. The occurrence of reaction-textured chromites indicates a zone of interaction between dynamic magmatic infuxes where chemical equilibrium was not achieved whereas a complete re-equilibration between chromite and the stagnant and sequestered interstitial liquid was attained during the formation of the massive chromitites.

 

Yudovskaya, M.A., Costin, G., Shilovskikh, V. et al. Bushveld symplectic and sieve‑textured chromite is a result of coupled dissolution‑reprecipitation: a comparison with xenocrystic chromite reactions in arc basalt. Contrib Mineral Petrol (2019) 174: 74. https://doi.org/10.1007/s00410-019-1613-3

Tectonophysics: Fracture-induced pore fluid pressure weakening and dehydration of serpentinite

Melodie E. French, Greg Hirth, and Keishi Okazaki

Tectonophysics 767 (2019) 228168

doi: 10.1016/j.tecto.2019.228168

Abstract

We investigate the strength, deformation processes, and pore fluid weakening during localized shear of antigorite serpentine. Recent work has shown that some phyllosilicates, including antigorite, undergo a reverse transition from ductile to localized deformation at the pressure-temperature conditions of deep slow slip and tremor in subduction zones. Here, we investigate the processes that lead to and occur during localized deformation. Because high pore fluid pressure is hypothesized to control the location and style of fault slip at these conditions, we investigate the role of pore fluids on these deformation processes. We present the results of undrained general shear experiments on antigorite-rich serpentinite deformed to varying strains at 500°C, 1 GPa pressure, and with 0 to 2 wt.% added pore water. At all fluid conditions, the serpentinite exhibits strain hardening during distributed deformation and subsequent strain weakening associated with the formation of a prominent shear fracture zone. The magnitude of strain weakening correlates with increasing pore water content. We evaluate two end-member scenarios for how the effective stress influences strength during localized deformation and find that either an increase in fluid pressure or increase in the parameter α in the effective stress law can explain the weakening. At all fluid conditions, we also find evidence for localized dehydration of antigorite within the fracture zones, at pressures and temperatures where antigorite is considered stable. Although the extent of the reaction did not measurably affect fault strength in our experiments, at the time scales of in-situ deformation in the Earth, reaction weakening and associated pore fluid pressurization may occur.

JGR-Solid Earth: Upper Crustal Structure and Magmatism in Southwest Washington: Vp, Vs, and Vp/Vs Results From the iMUSH Active‐Source Seismic Experiment

Eric Kiser, Alan Levander, Colin Zelt, Brandon Schmandt, and Steven Hansen

J. Geophys. Res.-Solid Earth 124 (2019) 7067-7080.

10.1029/2018JB016203

Abstract

Structural details of the crust play an important role in controlling the distribution of volcanic activity in arc systems. In southwest Washington, several different regional structures associated with accretion and magmatism have been invoked to explain the broad distribution of Cascade volcanism in this region. In order to image these regional structures in the upper crust, Pg and Sg travel times from the imaging Magma Under St. Helens (iMUSH) active‐source seismic experiment are inverted for Vp, Vs, and Vp/Vs models in the region surrounding Mount St. Helens. Several features of these models provide new insights into the regional structure of the upper crust. A large section of the Southern Washington Cascades Conductor is imaged as a low Vp/Vs anomaly that is inferred to represent a broad sedimentary/metasedimentary sequence that composes the upper crust in this region. The accreted terrane Siletzia is imaged west of Mount St. Helens as north/south trending high Vp and Vp/Vs bodies. The Vp/Vs model shows relatively high Vp/Vs regions near Mount St. Helens and the Indian Heaven Volcanic Field, which could be related to the presence of magmatic fluids. Separating these two volcanic regions below 6‐km depth is a northeast trending series of high Vp and Vs bodies. These bodies have the same orientation as several volcanic/magmatic features at the surface, including Mount St. Helens and Mount Rainier, and it is argued that these high‐velocity features are a regional‐scale group of intrusive bodies associated with a crustal weak zone that focuses magma ascent.

GRL: Constraining crustal properties using receiver functions and the autocorrelation of earthquake‐generated body waves

Jonathan Delph, Fenglin Niu, and Alan Levander

Geophys. Res. Lett. (2019)

10.1029/2019JB017929

Abstract

Passive seismic methods for imaging the discontinuity structure of Earth have primarily focused on differences in vertically and radially‐polarized energy in the coda of earthquake‐generated body waves (e.g., receiver functions). To convert the timing of scattered wave arrivals to depth, 3 parameters must be known or inferred: depth or layer thickness (H), P‐wave velocity (VP), and S‐wave velocity (VS). A common way to solve for these parameters is through H‐κ stacking analysis, in which layer thickness and the ratio between VP and VS is calculated while holding one of the velocity parameters constant. However, this assumption biases estimates of layer properties and leads to uncertainties that are not appropriately quantified. As these results are commonly used as starting models for more complex seismic or geodynamic analyses, these assumptions can propagate much further than the initial study. In this study, we introduce independent observations from body‐wave autocorrelations that can help constrain this underdetermined problem. P‐wave autocorrelation allows for the recovery of the Moho‐reflected P‐wave phase from teleseismic earthquakes, which is removed during deconvolution in the calculation of receiver functions. As the Moho‐reflected P‐wave is independent of VS, this constraint allows us to create a system of equations that better quantifies the thickness, VP, and VS of a layer and produces a more appropriate estimation of associated uncertainties. We apply this to 88 seismic stations that are spatially distributed throughout the United States to obtain a model of crustal variability that is unbiased by a priori assumptions of velocity structure.

Nature: Isotopic constraint on the twentieth-century increase in tropospheric ozone

 

Rice University researchers and collaborators used ice cores, like the one shown here from Antarctica, in combination with atmospheric chemistry models to establish an upper limit for the increase in ozone levels in the lower atmosphere since 1850. (Photo by Jeff Fitlow/Rice University)

Isotopic constraint on the twentieth-century increase in tropospheric ozone

Laurence Y. Yeung, Lee. T. Murray, Patricia Martinerie, Emmanuel Witrant, Huanting Hu, Asmita Banerjee, Anaïs Orsi & Jérôme Chappellaz

Nature 570 (2019) 224-227

Abstract

Tropospheric ozone (O3) is a key component of air pollution and an important anthropogenic greenhouse gas. During the twentieth century, the proliferation of the internal combustion engine, rapid industrialization and land-use change led to a global-scale increase in O3 concentrations; however, the magnitude of this increase is uncertain. Atmospheric chemistry models typically predict an increase in the tropospheric O3 burden of between 25 and 50 per cent since 1900, whereas direct measurements made in the late nineteenth century indicate that surface O3 mixing ratios increased by up to 300 per cent over that time period. However, the accuracy and diagnostic power of these measurements remains controversial. Here we use a record of the clumped-isotope composition of molecular oxygen (18O18O in O2) trapped in polar firn and ice from 1590 to 2016 ad, as well as atmospheric chemistry model simulations, to constrain changes in tropospheric O3 concentrations. We find that during the second half of the twentieth century, the proportion of 18O18O in O2 decreased by 0.03 ± 0.02 parts per thousand (95 per cent confidence interval) below its 1590–1958 ad mean, which implies that tropospheric O3 increased by less than 40 per cent during that time. These results corroborate model predictions of global-scale increases in surface pollution and vegetative stress caused by increasing anthropogenic emissions of O3 precursors. We also estimate that the radiative forcing of tropospheric O3 since 1850 ad is probably less than +0.4 watts per square metre, consistent with results from recent climate modelling studies.

DOI: 10.1038/s41586-019-1277-1

 

 

GRL: Evaluating indices of blocking anticyclones in terms of their linear relations with surface hot extremes

Evaluating Indices of Blocking Anticyclones in Terms of Their Linear Relations With Surface Hot Extremes

RevPalBot: Sedimentary organic matter characterization of the Whitehill shales (Karoo Basin, South Africa): An integrated quantitative approach using FE-EPMA and LA-ICP-MS

Sedimentary organic matter characterization of the Whitehill shales (Karoo Basin, South Africa): An integrated quantitative approach using FE-EPMA and LA-ICP-MS

Gelu Costin, Annette E. Götz, Katrin Ruckwied

Permian black shales of South Africa’s Karoo Basin have been classified as potential unconventional gas resources, the Whitehill Formation of the southern basin parts being the main target for future shale gas exploration and production. Here, we present a novel approach of SOM characterization integrating routine palynofacies analysis and high-resolution BSE imaging, modal mineral analysis, quantitative carbon analysis, EPMA trace element analysis, WDS mapping of carbon particles and carbon peak shift method using field emission microprobe as well as LA-ICP-MS trace element analysis. Black shales of the Whitehill Formation intersected in two deep boreholes of the southern Karoo Basin were studied: (1) massive carbonaceous siltstone (borehole KZF-1, southwestern basin) and (2) laminated shale made of alternating silty and clayey–silty laminae (borehole KWV-1, southeastern basin). Palynofacies indicates an outer shelf setting in the southwestern part (KZF-1) and a stratified basin in the southeastern part (KWV-1). The two shale types reveal striking differences in mineralogy, major and trace element concentrations, and shape and texture of organic particles. The EPMA trace element data of SOM show that the concentration of the elements in the organic particles does not correlate with the chemistry of the black shale when SOM is thermally untransformed. However, in the case of thermally affected and mobilized SOM, the organic acid-rich fluids liberated during the transformation and deformation of SOM can differentially dissolve chemical elements from the surrounding minerals. The LA-ICP-MS data show that the Whitehill shales are enriched in S, Mn, Mo, Ag, Cd, Re, Bi and U relative to the standardmarinemudMAG-1. Shales from the southeastern basin (KWV-1) are strongly depleted in Cu, Zn, As and Ba. The granulometric and mineralogical properties of the shales suggest an outer shelf setting with low-energy sediment fallout and sediment reworking, consistent with the paleoenvironmental interpretation based on palynofacies analysis. Different burial depths are inferred from different organic textures: KZF-1 shales show organic particles which preserve organic textures, randomly distributed throughout the siltstone, whereas KWV-1 shales show deformed, thermally transformed and mobilized organic particles, located in between distinct laminae.WDS quantitative element mapping allows for identifying various carbon concentrations within the organic particles. This variability of carbon at grain scale may indicate different carbon–hydrogen molecule speciations. The observed peak shift of carbon in EPMA on different textural types of SOM allowed a correlation between the peak shift and the degree of SOM maturation, indicating that this method potentially works as a vitrinite reflectance proxy. Ultimately, the evaluation of the diagenetic conditions of organic-rich shales from different parts of the basin based on textural criteria of the detrital material and organic particles combined with the carbon peak shift in different organic particles contributes to the assessment of the hydrocarbon potential.

Costin, G., Götz, A. E. & Ruckwied, K. (2019). Reviw of Palaeobotany and Palynology Sedimentary organic matter characterization of the Whitehill shales ( Karoo Basin , South Africa ): An integrated quantitative approach using. Review of Palaeobotany and Palynology. Elsevier B.V. 268, 29–42. DOI:  /10.1016/j.revpalbo.2019.05.008

 

Chem. Geol.: Effect of sulfate on the basaltic liquidus and Sulfur Concentration at Anhydrite Saturation (SCAS) of hydrous basalts – Implications for sulfur cycle in subduction zones.

Effect of sulfate on the basaltic liquidus and Sulfur Concentration at Anhydrite Saturation (SCAS) of hydrous basalts- Implications for sulfur cycle in subduction zones

Proteek Chowdhury, Rajdeep Dasgupta

To add to our understanding of sulfur cycle in subduction zones in general and constrain the effects of slab-released sulfate (SO42−) on the magma genesis and sulfur transport from sub-arc mantle to the arc volcanoes in particular we carried out an experimental study. High pressure-temperature, piston-cylinder experiments were carried out in Au-Pd capsules at 0.5–3 GPa and 1050–1325 °C to investigate (a) the effect of variable sulfur concentration from 0 to ~2 wt%, dissolved as SO42− on the stability field of a primary arc basalt with ~4 wt% H2O and determine (b) the sulfur content at anhydrite saturation (SCAS) of hydrous mafic magmas. Speciation of sulfur in the silicate melt was confirmed to be SO42− by S Kα X-ray peak position using electron microprobe. S-free hydrous clinopyroxene liquidus at 2 GPa is ~25 °C hotter than the hydrous clinopyroxene liquidus with ~0.1 wt% S in the liquidus melt as SO42− and the liquidus depression with further S-enrichment to anhydrite saturation (~2 wt% S) can be fitted by a power function ΔT(°C) = 26.52(±3.48)(Smelt in wt%)0.24(±0.06). Anhydrite-saturated experiments show that SCAS increases with increasing temperature and CaO content of melt and decreases with increasing SiO2 content of the melt. Previous SCAS models based mostly on lower PT experiments and/or on silicic melt compositions can’t capture our new experimental SCAS data. A new SCAS parameterization was developed using previous and our new experimental data. Calculations using our new parameterization and assuming 200–500 ppm S in the arc mantle show that <10% hydrous melting of mantle wedge would exhaust anhydrite, if present. Therefore, anhydrite even if present is expected to be exhausted by partial melting in the mantle wedge and parental basaltic melt will extract similar amount of S via mantle melting irrespective of the presence of sulfide or sulfate at subsolidus conditions. The S content, as dissolved SO42−, of hydrous arc basalts produced by 10–30% melting will be 500–4000 ppm, which is comparable to the melt inclusion S contents from various arcs. The sulfate undersaturated basalts may assimilate crustal sulfate and lead to high observed SO2 flux at the arcs, known as the “excess S”.

 

Chowdhury, P. and Dasgupta, R., 2019. Effect of sulfate on the basaltic liquidus and Sulfur Concentration at Anhydrite Saturation (SCAS) of hydrous basalts–Implications for sulfur cycle in subduction zones. Chemical Geologyhttps://doi.org/10.1016/j.chemgeo.2019.05.020

PNAS: Climate models can correctly simulate the continuum of global-average temperature variability

Feng Zhu, Julien Emile-Geay, Nicholas P. McKay, Gregory J. Hakim, Deborah Khider, Toby R. Ault, Eric J. Steig, Sylvia Dee, and James W. Kirchner
Proc. Natl. Acad. Sci. USA 116 (2019) 8728-8733.

DOI: 10.1073/pnas.1809959116

Significance

Climate models are foundational to formulations of climate policy and must successfully reproduce key features of the climate system. The temporal spectrum of observed global surface temperature is one such critical benchmark. This spectrum is known to obey scaling laws connecting astronomical forcings, from orbital to annual scales. We provide evidence that the current hierarchy of climate models is capable of reproducing the increase in variance in global-mean temperature at low frequencies. We suggest that successful climate predictions at decadal-to-centennial horizons hinge critically on the accuracy of initial and boundary conditions, particularly for the deep ocean state.

Abstract

Climate records exhibit scaling behavior with large exponents, resulting in larger fluctuations at longer timescales. It is unclear whether climate models are capable of simulating these fluctuations, which draws into question their ability to simulate such variability in the coming decades and centuries. Using the latest simulations and data syntheses, we find agreement for spectra derived from observations and models on timescales ranging from interannual to multimillennial. Our results confirm the existence of a scaling break between orbital and annual peaks, occurring around millennial periodicities. That both simple and comprehensive ocean–atmosphere models can reproduce these features suggests that long-range persistence is a consequence of the oceanic integration of both gradual and abrupt climate forcings. This result implies that Holocene low-frequency variability is partly a consequence of the climate system’s integrated memory of orbital forcing. We conclude that climate models appear to contain the essential physics to correctly simulate the spectral continuum of global-mean temperature; however, regional discrepancies remain unresolved. A critical element of successfully simulating suborbital climate variability involves, we hypothesize, initial conditions of the deep ocean state that are consistent with observations of the recent past.

QSR: Evidence of Ice Age humans in eastern Beringia suggests early migration to North America

Richard S. Vachula, Yongsong Huang, William M. Longo, Sylvia G. Dee, William C. Daniels, and James M. Russell

Quant. Sci. Rev. 205 (2019) 35-44.

DOI: 10.1016/j.quascirev.2018.12.003

 

Abstract

Our understanding of the timing and pathway of human arrival to the Americas remains an important and polarizing topic of debate in archaeology and anthropology. Traditional consensus, supported by archaeological and paleoenvironmental data, favors a ‘swift peopling’ of the Americas from Asia via the Bering Land Bridge during the last Glacial termination. More recent genetic data and archaeological finds have challenged this view, proposing the ‘Beringian standstill hypothesis’ (BSH), wherein a population of proto-Americans migrated from Asia during, or even prior to the Last Glacial Maximum (LGM) and lived in Beringia for thousands of years before their eventual spread across the American continents. Using a sediment archive from Lake E5 (68.641667° N, 149.457706° W), located on Alaska’s North Slope, we present new data supporting the BSH and shedding new light on the environmental impact of these early American populations. Fecal biomarkers support human presence in the environs of the lake, and our data demonstrate elevated biomass burning in this region during the last Glacial. Elevated burning defies the expectation that natural fires would be less frequent in the Arctic during the last Glacial, thereby suggesting human ignition as the likely culprit. Our data shed new light on the pathway and timing of human migration to the Americas and demonstrate the possibility of the sustainable coexistence of humans and the Ice Age megafauna in Beringia prior to their extinction.