JGR Solid Earth: Phase relations of a depleted peridotite fluxed by a CO2-H2O fluid – Implications for the stability of partial melts versus volatile-bearing mineral phases in the cratonic mantle

Phase relations of a depleted peridotite fluxed by a CO2-H2O fluid – Implications for the stability of partial melts versus volatile-bearing mineral phases in the cratonic mantle
Sriparna Saha and Rajdeep Dasgupta

Abstract

We present phase‐equilibria experiments of a K‐bearing, depleted peridotite (Mg# 92) fluxed with a mixed CO2‐H2O fluid (0.5 wt.% CO2 and 0.94 wt.% H2O in the bulk) to gain insight into the stability of volatile‐bearing partial melts versus volatile‐bearing mineral phases in a depleted peridotite system. Experiments were performed at 850–1150 °C and 2–4 GPa using a piston‐cylinder and a multianvil apparatus. Olivine, orthopyroxene, clinopyroxene, and spinel/garnet are present at all experimental conditions. Textural confirmation of partial melt is made at temperatures as low as 1000 °C at 2 GPa, 950 °C at 3 GPa, and 1000 °C at 4 GPa marking the onset of melting at 900–1000 °C at 2 GPa, 850–950 °C at 3 GPa, and 950–1000 °C at 3 GPa. Phlogopite and magnesite breakdown at 900–1000 °C at 2 GPa, 950–1000 °C at 3 GPa, and 1000–1050 °C at 4 GPa. Comparison with previously published experiments in depleted peridotite system with identical CO2‐H2O content introduced via a silicic melt show that introduction of CO2‐H2O as fluid lowers the temperature of phlogopite breakdown by 150–200 °C at 2–4 GPa and stabilizes partial melts at lower temperatures. Our study thus, shows that the volatile‐bearing phase present in the cratonic mantle is controlled by bulk composition and is affected by the process of volatile addition during craton formation in a subduction zone. In addition, volatile introduction via melt versus aqueous fluid, leads to different proportion of anhydrous phases such as olivine and orthopyroxene. Considering the agent of metasomatism is thus critical to evaluate how the bulk composition of depleted peridotite is modified, leading to potential stability of volatile‐bearing phases as the cause of anomalously low shear wave velocity in mantle domains such as mid lithospheric discontinuities beneath continents.

Saha, S. & Dasgupta, R. (2019). Phase relations of a depleted peridotite fluxed by a CO2-H2O fluid – Implications for the stability of partial melts versus volatile-bearing mineral phases in the cratonic mantle. Journal of Geophysical Research: Solid Earth 124. doi:10.1029/2019JB017653

 

Cambridge University Press: A framework for understanding whole Earth carbon cycling

A Framework for Understanding Whole-Earth Carbon Cycling

Summary

This chapter explores how the cycling of carbon in subduction zones and orogenic belts varies with supercontinent cycles and mountain building. It discusses the processes that link short-term and long-term carbon cycling and the timescales of these processes, such as the response times of weathering and atmospheric drawdown at periods of enhanced volcanism. This chapter covers topics of potential fluctuations in the long-term CO2 content of Earth’s atmosphere because of mantle–climate feedback.

Lee, C-T. A., Jiang, H., Dasgupta, R. & Torres, M. (2019). A framework for understanding whole Earth carbon cycling. In Orcutt, B., Daniel, I., and Dasgupta, R. (Eds.) Deep Carbon: Past to Present. Cambridge University Press, Cambridge, pp. 313-357. doi:10.1017/9781108677950.011

 

Cambridge University Press: Origin and Early Differentiation of Carbon and Associated Life-Essential Volatile Elements on Earth

Rajdeep Dasgupta, Damanveer S. Grewal

Summary

This chapter reviews what is known about the fate of carbon during early differentiation of inner solar system planets. It reviews the nature of carbon fractionation in a magma ocean as compared to the core, mantle, and atmosphere, and how this may have varied between planetary bodies in the solar system. It discusses whether magma ocean processes could have established the present-day budget of carbon in Earth’s bulk silicate, and also reviews possibilities for the early temporal evolution of the mantle carbon budget through core formation, later veneer addition, and magma ocean crystallization processes.

Dasgupta, R., & Grewal, D. (2019). Origin and Early Differentiation of Carbon and Associated Life-Essential Volatile Elements on Earth. In B. Orcutt, I. Daniel, & R. Dasgupta (Eds.), Deep Carbon: Past to Present (pp. 4-39). Cambridge: Cambridge University Press. doi:10.1017/9781108677950.002

EPSL: Recycling reduced iron at the base of magmatic orogens

Ming Tang, Cin-Ty A. Lee, Gelu Costin, Heidi E. Höfer

Abstract

The Earth’s continental crust is thought to originate from melting of the mantle, but it is too felsic and depleted in Fe relative to a primary mantle melt. This depletion in Fe is also commonly found in continental arc magmas and is often attributed to magnetite crystallization. However, Fe depletion in arc magmas coincides with an enrichment in ferric Fe relative to ferrous Fe, which cannot be explained by removal of a ferric Fe-rich mineral like magnetite. Deep-seated garnet pyroxenite arc cumulates (arclogites) have Fe-rich compositions that complement the Fe-depleted nature of the continental crust and continental arc magmas, and are likely candidates for the “missing link” between basaltic mantle magmas and the felsic continental crust. To test this suggestion, we present high precision in-situ Fe valence data for garnets in arclogites and reconstruct whole rock Fe valence states. We show that arclogites have low bulk Fe3+/ΣFe due to the low Fe3+/ΣFe of garnets and the lack of magnetite. At high pressures, garnet crystallizes, but magnetite does not, the former causing preferential accumulation of ferrous Fe at the base of magmatic orogens. Arclogite fractionation thus leads to the formation of oxidized felsic residual liquids (Fe3+/ΣFe of 0.2-0.4). Such oxidation may profoundly influence the speciation of magmatic volatiles as well as the oxidative weathering capacity of the crust.

Tang, M., Lee, C. A., Costin, G., Höfer, H. E., (2019). Recycling reduced iron at the base of magmatic orogens. Earth and Planetary Science Letters. Elsevier B.V. 528, 115827. doi.org/10.1016/j.epsl.2019.115827

 

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