EPSL: An imbalance in the deep water cycle at subduction zones: The potential importance of the fore-arc mantle

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An imbalance in the deep water cycle at subduction zones: The potential importance of the fore-arc mantle

Julia Ribeiro and Cin-Ty Lee

Earth and Planetary Science Letters

Volume 479, 1 December 2017, Pages 298-309

The depth of slab dehydration is thought to be controlled by the thermal state of the downgoing slab: cold slabs are thought to mostly dehydrate beneath the arc front while warmer slabs should mostly dehydrate beneath the fore-arc. Cold subduction zone lavas are thus predicted to have interacted with greater extent of water-rich fluids released from the downgoing slab, and should thus display higher water content and be elevated in slab-fluid proxies (i.e., high Ba/Th, H2O/Ce, Rb/Th, etc.) compared to hot subduction zone lavas. Arc lavas, however, display similar slab-fluid signatures regardless of the thermal state of the slab, suggesting more complexity to volatile cycling in subduction zones. Here, we explore whether the serpentinized fore-arc mantle may be an important fluid reservoir in subduction zones and whether it can contribute to arc magma generation by being dragged down with the slab. Using simple mass balance and fluid dynamics calculations, we show that the dragged-down fore-arc mantle could provide enough water (∼7–78% of the total water injected at the trenches) to account for the water outfluxes released beneath the volcanic arc. Hence, we propose that the water captured by arc magmas may not all derive directly from the slab, but a significant component may be indirectly slab-derived via dehydration of dragged-down fore-arc serpentinites. Fore-arc serpentinite dehydration, if universal, could be a process that explains the similar geochemical fingerprint (i.e., in slab fluid proxies) of arc magmas.

 

G-cubed: High-Pressure Phase Relations of a Depleted Peridotite Fluxed by CO2-H2O-Bearing Siliceous Melts and the Origin of Mid-Lithospheric Discontinuity

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High-Pressure Phase Relations of a Depleted Peridotite Fluxed by CO2-H2O-Bearing Siliceous Melts and the Origin of Mid-Lithospheric Discontinuity

Abstract:

We present phase equilibria experiments on a depleted peridotite (Mg# 92) fluxed with variable proportions of a slab-derived rhyolitic melt (with 9.4 wt.% H2O, 5 wt.% CO2), envisaging an interaction that could occur during formation of continents by imbrication of slabs/accretion of subarc mantles. Experiments were performed with 5 wt.% (Bulk 2) and 10 wt.% (Bulk 1) melt at 950–1175°C and 2–4 GPa using a piston-cylinder and a multi-anvil apparatus, to test the hypothesis that volatile-bearing mineral-phases produced during craton formation can cause reduction in aggregate shear-wave velocities (VS) at mid-lithospheric depths beneath continents. In addition to the presence of olivine, orthopyroxene, clinopyroxene, and garnet/spinel, phlogopite (Bulk 1: 3–7.6 wt.%; Bulk 2: 2.6–5 wt.%) at 2–4 GPa, and amphibole (Bulk 1: 3–9 wt.%; Bulk 2: 2–6 wt.%) at 2–3 GPa (≤1050°C) are also present. Magnesite (Bulk 1: ∼1 wt.% and Bulk 2: ∼0.6 wt.%) is present at 2–4 GPa (<1000°C at 3 and < 1050°C at 4 GPa) and its thermal breakdown coincides with the visual appearance of trace-melt. However, an extremely small fraction of melt is inferred at all experiments based on the knowledge of fluid-saturated peridotite solidus and the difference between bulk H2O and total H2O stored in the hydrous phases. Calculated mineral end-member volume-proportions were used to calculate VS of the resulting assemblage at experimental conditions and along representative continental geotherms (surface heat flow of 40–50 mWm−2). We note that reactive crystallization of phlogopite ± amphibole by infiltration of 3–10% slab-derived hydrous-silicic melt can cause up to 6% reduction in VS and that the estimated reduction in VS increases with increasing melt:rock ratio. The presence of phlogopite limits amphibole-stability, making phlogopite a more likely candidate for MLDs at >100 km depth.

Saha, S., Dasgupta, R., & Tsuno, K. (2018). High pressure phase relations of a depleted peridotite fluxed by CO2-H2O-bearing siliceous melts and the origin of Mid-Lithospheric Discontinuity. Geochemistry, Geophysics, Geosystems, 19. https://doi.org/10.1002/2017GC007233

AJS: Volatile-bearing partial melts beneath oceans and continents – Where, how much, and of what compositions?

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Volatile-bearing partial melts beneath oceans and continents – Where, how much, and of what compositions?

Abstract: Besides depth and temperature, CO2 and H2O, are the two most important variables in stabilizing partial melts in the Earth’s mantle. However, despite decades of experimental studies on the roles of these two volatile species in affecting mantle melting, ambiguity remains in terms of the stability, composition, and proportion of volatile-bearing partial melts at depths. Furthermore, the difference in the influence of H2O versus CO2 in production of mantle melts is often inadequately discussed. Here I first discuss how as a function of depth and concentration of volatiles, the peridotite + H2O versus peridotite + CO2 near-solidus melting conditions differ – discussing specifically the concepts of saturation of volatile-bearing phases and how the mode of storage of ‘water’ and carbon affects the near solidus melting relations. This analysis shows that for the Earth’s mantle beneath oceans and continents, deep, volatile-induced melting is influenced mostly by carbon, with water-bearing carbonated silicate melt being the key agent. A quantitative framework that uses the existing experimental data, allows calculation of the loci, extent of melting, and major element compositions of volatile-bearing partial melts beneath oceans and continents. How the domains of volatile-bearing melt stability are affected when possible oxygen fugacity variation at depths in the mantle is taken into account is also discussed. I show that trace amount hydrous carbonated silicate melt is likely stabilized at two or more distinct depths in the continental lithospheric mantle, at depths ranges similar to where mid-lithospheric discontinuity (MLD) and lithosphere-asthenosphere boundary (LAB) have been estimated from seismology. Whereas beneath oceans, hydrous carbonated silicate melt likely remain continuously stable from the base of the thermal boundary layer to at least 200 km or deeper depending on the prevailing oxygen fugacity at depths. Hotter mantles, such as those beneath oceans, prevent sampling strongly silica-undersaturated, carbonated melts such as kimberlites as shallower basaltic melt generation dominates. Thick thermal boundary layers, such as those in cratonic regions, on the other hand allow production of kimberlitic to carbonatitic melt only. Therefore, the increasing frequency of occurrence of kimberlites starting at the Proterozoic may be causally linked to cooling and growth of sub-continental mantles through time.

 

Dasgupta, R. (2018). Volatile bearing partial melts beneath oceans and continents – where, how much, and of what compositions? American Journal of Science 318 (1), 141-165. doi:10.2475/01.2018.06

Nature Communication: Platinum-bearing chromite layers are caused by pressure reduction during magma ascent

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Platinum-bearing chromite layers are caused by pressure reduction during magma ascent

Rais Latypov 1, Gelu Costin 2, Sofya Chistyakova 1, Emma J. Hunt 1, Ria Mukherjee 1 & Tony Naldrett 3

1: School of Geosciences, University of the Witwatersrand, Johannesburg, 2050, South Africa; 2: Department of Earth, Environmental and Planetary Sciences, Rice University, Houston, 6100, TX, 77005, USA; 3: Department of Earth, Environmental and Planetary Sciences, University of Toronto, Toronto, 1066, Canada

Platinum-bearing chromitites in mafic-ultramafic intrusions such as the Bushveld Complex are key repositories of strategically important metals for human society. Basaltic melts saturated in chromite alone are crucial to their generation, but the origin of such melts is controversial. One concept holds that they are produced by processes operating within the magma chamber, whereas another argues that melts entering the chamber were already saturated in chromite. Here we address the problem by examining the pressure-related changes in the topology of a Mg2SiO4–CaAl2Si2O8–SiO2–MgCr2O4 quaternary system and by thermodynamic modelling of crystallisation sequences of basaltic melts at 1–10 kbar pressures. We show that basaltic melts located adjacent to a so-called chromite topological trough in deep-seated reservoirs become saturated in chromite alone upon their ascent towards the Earth’s surface and subsequent cooling in shallow-level chambers. Large volumes
of these chromite-only-saturated melts replenishing these chambers are responsible for monomineralic layers of massive chromitites with associated platinum-group elements.

Citation:

Latypov, R., Costin, G., Chistyakova, S., Hunt, E. J., Mukherjee, R. & Naldrett, T. (2018). Platinum-bearing chromite layers are caused by pressure reduction during magma ascent. Nature Communications. Springer US 9, 462, doi:10.1038/s41467-017-02773-w

https://www.nature.com/articles/s41467-017-02773-w

 

 

 

GCA: New high pressure experiments on sulfide saturation of high-FeO* basalts with variable TiO2 contents – Implications for the sulfur inventory of the lunar interior

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New high pressure experiments on sulfide saturation of high-FeO* basalts with variable TiO2 contents – Implications for the sulfur inventory of the lunar interior

Shuo Ding, Taylor Hough, Rajdeep Dasgupta

Abstract: In order to constrain sulfur concentration in intermediate to high-Ti mare basalts at sulfide saturation (SCSS), we experimentally equilibrated FeS melt and basaltic melt using a piston cylinder at 1.0–2.5 GPa and 1400–1600 °C, with two silicate compositions similar to high-Ti (Apollo 11: A11, ∼11.1 wt.% TiO2, 19.1 wt.% FeO∗, and 39.6 wt.% SiO2) and intermediate-Ti (Luna 16, ∼5 wt.% TiO2, 18.7 wt.% FeO∗, and 43.8 wt.% SiO2) mare basalts. Our experimental results show that SCSS increases with increasing temperature, and decreases with increasing pressure, which are similar to the results from previous experimental studies. SCSS in the A11 melt is systematically higher than that in the Luna 16 melt, which is likely due to higher FeO∗, and lower SiO2 and Al2O3 concentration in the former. Compared to the previously constructed SCSS models, including those designed for high-FeO∗ basalts, the SCSS values determined in this study are generally lower than the predicted values, with overprediction increasing with increasing melt TiO2 content. We attribute this to the lower SiO2 and Al2O3 concentration of the lunar magmas, which is beyond the calibration range of previous SCSS models, and also more abundant FeTiO3 complexes in our experimental melts that have higher TiO2 contents than previous models’ calibration range. The formation of FeTiO3 complexes lowers the activity of FeO∗, , and therefore causes SCSS to decrease. To accommodate the unique lunar compositions, we have fitted a new SCSS model for basaltic melts of >5 wt.% FeO∗ and variable TiO2 contents. Using previous chalcophile element partitioning experiments that contained more complex Fe-Ni-S sulfide melts, we also derived an empirical correction that allows SCSS calculation for basalts where the equilibrium sulfides contain variable Ni contents of 10–50 wt.%. At the pressures and temperatures of multiple saturation points, SCSS of lunar magmas with compositions from picritic glasses, mare basalts, to young lunar meteorites vary from 2600 to 4800 ppm for basalt equilibration with a pure FeS melt and from 1400 to 2600 ppm for basalt equilibration with a Fe-rich sulfide melt containing 30 wt.% Ni. The measured S contents in these proposed near-primary lunar magmas are lower than the predicted SCSS at the conditions of their last equilibration with the lunar mantle, indicating no sulfide retention in the lunar mantle source during partial melting. Sulfide exhaustion during partial melting in the lunar mantle also supports the notion that the bulk silicate moon is depleted in highly siderophile elements. Based on the measured S contents and the estimated degree of melting, the estimated S contents for the mantle source of A15 green glass and A15 mare basalts is 10–23 ppm; for A17 orange glass is 25–62 ppm, for A12 mare basalts is 27–92 ppm, and for A11 basalt is 35–120 ppm. Consideration of SCSS decrease due to the presence of Ni in the sulfide melt does not change these mantle S abundance estimates for <30 wt.% Ni in the sulfide. The inferred S contents suggest that the lunar mantle is heterogeneous in terms of S. Although variable among different groups, the inferred S abundance of up to 120 ppm in the lunar mantle falls near the lower end of the S content of the depleted terrestrial mantle such as the MORB source.

 

Ding, S., Hough, T., and Dasgupta, R. (2018). New high pressure experiments on sulfide saturation of high-FeO* basalts with variable TiO2 contents – Implications for the sulfur inventory of the lunar interior. Geochimica et Cosmochimica Acta 222, 319-339. doi:10.1016/j.gca.2017.10.025

Science Advances: Extreme enrichment in atmospheric 15N15N

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Laurence Y. Yeung, Shuning Li, Issaku E. Kohl, Joshua A. Haslun, Nathaniel E. Ostrom, Huanting Hu, Tobias P. Fischer, Edwin A. Schauble and Edward D. Young

Abstract

Molecular nitrogen (N2) comprises three-quarters of Earth’s atmosphere and significant portions of other planetary atmospheres. We report a 19 per mil (‰) excess of 15N15N in air relative to a random distribution of nitrogen isotopes, an enrichment that is 10 times larger than what isotopic equilibration in the atmosphere allows. Biological experiments show that the main sources and sinks of N2 yield much smaller proportions of 15N15N in N2. Electrical discharge experiments, however, establish 15N15N excesses of up to +23‰. We argue that 15N15N accumulates in the atmosphere because of gas-phase chemistry in the thermosphere (>100 km altitude) on time scales comparable to those of biological cycling. The atmospheric 15N15N excess therefore reflects a planetary-scale balance of biogeochemical and atmospheric nitrogen chemistry, one that may also exist on other planets.

doi: 10.1126/sciadv.aao6741

EPSL: Low oxygen and argon in the Neoproterozoic atmosphere at 815 Ma

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Laurence Y. Yeung

Abstract

The evolution of Earth’s atmosphere on >106-yr timescales is tied to that of the deep Earth. Volcanic degassing, weathering, and burial of volatile elements regulates their abundance at the surface, setting a boundary condition for the biogeochemical cycles that modulate Earth’s atmosphere and climate. The atmosphere expresses this interaction through its composition; however, direct measurements of the ancient atmosphere’s composition more than a million years ago are notoriously difficult to obtain. Gases trapped in ancient minerals represent a potential archive of the ancient atmosphere, but their fidelity has not been thoroughly evaluated. Both trapping and preservation artifacts may be relevant. Here, I use a multi-element approach to reanalyze recently collected fluid-inclusion data from halites purportedly containing snapshots of the ancient atmosphere as old as 815 Ma. I argue that those samples were affected by the concomitant trapping of air dissolved in brines and contaminations associated with modern air. These artifacts lead to an apparent excess in O2 and Ar. The samples may also contain signals of mass-dependent fractionation and biogeochemical cycling within the fluid inclusions. After consideration of these artifacts, this new analysis suggests that the Tonian atmosphere was likely low in O2, containing ≤10% present atmospheric levels (PAL), not ∼50% PAL as the data would suggest at face value. Low concentrations of O2 are consistent with other geochemical constraints for this time period and further imply that the majority of Neoproterozoic atmospheric oxygenation occurred after 815 Ma. In addition, the analysis reveals a surprisingly low Tonian Ar inventory—≤60% PAL—which, if accurate, challenges our understanding of the solid Earth’s degassing history. When placed in context with other empirical estimates of paleo-atmospheric Ar, the data imply a period of relatively slow atmospheric Ar accumulation in the Paleo- and Meso-Proterozoic, followed by extensive degassing in the late Neoproterozoic or early Cambrian, before returning to a relatively quiescent state by the Devonian. This two-step structure resembles that for the evolution of atmospheric O2, hinting at a common driving force from the deep Earth. Some caution is warranted, however, because still more enigmatic contaminations than the ones presented here may be relevant. Gases trapped in minerals may offer important constraints on the evolution of Earth’s surface, climate, and atmosphere, but potential contaminations and other confounding factors need to be considered carefully before these records can be considered quantitative.

10.1016/j.epsl.2017.09.044

Nature Comm: Coralgal reef morphology records punctuated sea-level rise during the last deglaciation

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Paper Link : http://www.nature.com/articles/s41467-017-00966-x

Received: 24 November 2016

Accepted: 9 August 2017

Published: 19 October 2017

Abstract: Coralgal reefs preserve the signatures of sea-level fluctuations over Earth’s history, in particular since the Last Glacial Maximum 20,000 years ago, and are used in this study to indicate that punctuated sea-level rise events are more common than previously observed during the last deglaciation. Recognizing the nature of past sea-level rises (i.e., gradual or stepwise) during deglaciation is critical for informing models that predict future vertical behavior of global oceans. Here we present high-resolution bathymetric and seismic sonar data sets of 10 morphologically similar drowned reefs that grew during the last deglaciation and spread 120 km apart along the south Texas shelf edge. Herein, six commonly observed terrace levels are interpreted to be generated by several punctuated sea-level rise events forcing the reefs to shrink and backstep through time. These systematic and common terraces are interpreted to record punctuated sea-level rise events over timescales of decades to centuries during the last deglaciation, previously recognized only during the late Holocene.

Pankaj Khanna 1, André W. Droxler 1, Jeffrey A. Nittrouer 1, John W. Tunnell Jr 2 & Thomas C. Shirley 3

1 Department of Earth, Environmental and Planetary Sciences, Rice University, 6100 Main St, Houston, TX 77005, USA.

2 Harte Research Institute for Gulf of Mexico Studies TAMU-CC, 6300 Ocean Dr., Corpus Christi, TX 78412, USA.

3 Department of Life Sciences, TAMU-CC, 6300 Ocean Dr., Corpus Christi, TX 78412, USA. Correspondence and requests for materials should be addressed to P.K. (email: pankaj89@gmail.com) or to A.W.D. (email: andre@rice.edu)

“Data report: reanalysis of interstitial water barium, iron, and sulfur concentrations at Sites U1426 and U1427”, by Clint Miller and Gerald Dickens, in Volume 346 of the Proceedings of the Integrated Ocean Drilling Program.

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Abstract

Within the south of the marginal sea between Japan and Korea, interstitial water (IW) profiles exhibit a prominent sulfate–methane transition (SMT) in the upper few meters of sediment. As the SMT has become a focus of attention, IW samples were collected at high spatial resolution within shallow sediment at Sites U1426 and U1427 and examined on board the R/V JOIDES Resolution, under the auspices of the Integrated Ocean Drilling Program, for a wide range of dissolved species. However, irregularities were noted for the sulfate (SO42–), Ba, and Fe concentration profiles, each of importance to understanding the SMT. Splits of 134 IW samples, prepared with HNO3 during the expedition, were therefore reanalyzed at Rice University for S, Ba, and Fe, with S as a proxy for SO42–. Results of 134 samples included 29 duplicates with low percent difference (0.01%–34.69%, 0.01%–14.90%, and 0.03%–35.19%) and 6 spiked blanks with low percent error relative to stock solution concentration (1.59%, 2.41%, and 4.11%). The shore-based S and Ba profiles have trends similar to those determined on ship but with obvious offsets. The remeasured Fe profiles are comparable to those measured on ship, albeit with more data points. Although the IW samples were measured between 95 and 113 days after the expedition, the new results have high data reproducibility, render smooth profiles, and give more expected chemistry across the SMTs. For these three elements, we suggest the new results should replace the shipboard data.

1 Miller, C., and Dickens, G., 2017. Data report: reanalysis of interstitial water barium, iron, and sulfur concentrations at Sites U1426 and U1427. In Tada, R., Murray, R.W., Alvarez Zarikian, C.A., and the Expedition 346 Scientists, Proceedings of the Integrated Ocean Drilling Program, 346: College Station, TX (Integrated Ocean Drilling Program).
doi:10.2204/iodp.proc.346.203.2017

2 Department of Earth Sciences, Rice University, Houston, TX, 77005, USA. Correspondence author: clint.m.miller@rice.edu

Initial receipt: 21 April 2017
Acceptance: 3 August 2017
Publication: 3 October 2017
MS 346-203

Regional background O3 and NOx in the Houston–Galveston–Brazoria (TX) region: a decadal-scale perspective

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Loredana G. Suciu1, Robert J. Griffin2, and Caroline A. Masiello11Department of Earth Science, Rice University, Houston, 77005, USA
2Department of Civil and Environmental Engineering, Rice University, Houston, 77005, USA

Abstract. Ozone (O3) in the lower troposphere is harmful to people and plants, particularly during summer, when photochemistry is most active and higher temperatures favor local chemistry. Local precursor emissions, such as those of volatile organic compounds (VOCs) and nitrogen oxides (NOx), together with their chemistry contribute to the O3 and NOx mixing ratios in the Houston–Galveston–Brazoria (HGB) region. In addition to local emissions, chemistry and transport, larger-scale factors also contribute to local O3 and NOx. These additional contributions (often referred to as regional background) are not well quantified within the HGB region, impeding more efficient controls on precursor emissions to achieve compliance with the National Ambient Air Quality Standards for O3. In this study, we estimate ground-level regional background O3 and NOx in the HGB region and quantify their decadal-scale trends.

We use four different approaches based on principal component analysis (PCA) to quantify background O3 and NOx. Three of these approaches consist of independent PCA on both O3 and NOx for both 1 and 8 h levels to compare our results with previous studies and to highlight the effect of both temporal and spatial scales. In the fourth approach, we co-varied O3, NOx and meteorology.

Our results show that the estimation of regional background O3 has less inherent uncertainty when it was constrained by NOx and meteorology, yielding a statistically significant temporal trend of −0.68 ± 0.27 ppb yr−1. Likewise, the estimation of regional background NOxtrend constrained by O3 and meteorology was −0.04 ± 0.02 ppb yr−1 (upper bound) and −0.03 ± 0.01 ppb yr−1 (lower bound). Our best estimates of the 17-year average of season-scale background O3 and NOx were 46.72 ± 2.08 ppb and 6.80 ± 0.13 ppb (upper bound) or 4.45 ± 0.08 ppb (lower bound), respectively. Average background O3 is consistent with previous studies and between the approaches used in this study, although the approaches based on 8 h averages likely overestimate background O3 compared to the hourly median approach by 7–9 ppb. Similarly, the upper bound of average background NOx is consistent between approaches in this study (A–C) but overestimated compared to the hourly approach by 1 ppb, on average. We likely overestimate the upper-bound background NOx due to instrument overdetection of NOx and the 8 h averaging of NOx and meteorology coinciding with MDA8 O3.

Regional background O3 and NOx in the HGB region both have declined over the past 2 decades. This decline became steadier after 2007, overlapping with the effects of controlling precursor emissions and a prevailing southeasterly–southerly flow.

Citation: Suciu, L. G., Griffin, R. J., and Masiello, C. A.: Regional background O3 and NOx in the Houston–Galveston–Brazoria (TX) region: a decadal-scale perspective, Atmos. Chem. Phys., 17, 6565-6581, https://doi.org/10.5194/acp-17-6565-2017, 2017.