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

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

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

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

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

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.

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

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.

Pore water geochemistry along continental slopes north of the East Siberian Sea: inference of low methane concentrations

Clint M. Miller, Gerald R. Dickens, Martin Jakobsson, Carina Johansson, Andrey Koshurnikov, Matt O’Regan,
Francesco Muschitiello, Christian Stranne, and Carl-Magnus Mörth

Abstract. Continental slopes north of the East Siberian Sea potentially hold large amounts of methane (CH4) in sediments as gas hydrate and free gas. Although release of this CH4 to the ocean and atmosphere has become a topic of discussion, the region remains sparingly explored. Here we present pore water chemistry results from 32 sediment cores taken during Leg 2 of the 2014 joint Swedish–Russian–US Arctic Ocean Investigation of Climate–Cryosphere–Carbon Interactions (SWERUS-C3) expedition. The cores come from depth transects across the slope and rise extending between the Mendeleev and the Lomonosov ridges, north of Wrangel Island and the New Siberian Islands, respectively. Upward CH4 flux towards the seafloor, as inferred from profiles of dissolved sulfate (SO2−4), alkalinity, and the δ13C of dissolved inorganic carbon (DIC), is negligible at all stations east of 143◦ E longitude. In the upper
8 m of these cores, downward SO2−4 flux never exceeds 6.2 mol m−2 kyr−1, the upward alkalinity flux never exceeds
6.8 mol m−2 kyr−1, and δ13C composition of DIC (δ13CDIC) only moderately decreases with depth (−3.6 ‰ m−1 on
average). Moreover, upon addition of Zn acetate to pore water samples, ZnS did not precipitate, indicating a lack of dissolved H2S. Phosphate, ammonium, and metal profiles reveal that metal oxide reduction by organic carbon dominates the geochemical environment and supports very low organic carbon turnover rates. A single core on the Lomonosov Ridge differs, as diffusive fluxes for SO2−4 and alkalinity were 13.9 and 11.3 mol m−2 kyr−1, respectively, the δ13C-DIC gradient was 5.6 ‰ m−1, and Mn2+ reduction terminated within 1.3 m of the seafloor. These are among the first pore water results generated from this vast climatically sensitive region, and they imply that abundant CH4, including gas hydrates, do not characterize the East Siberian Sea slope or rise along the
investigated depth transects. This contradicts previous modeling and discussions, which due to the lack of data are almost entirely based on assumption.

Biogeosciences, 14, 2929–2953, 2017
https://doi.org/10.5194/bg-14-2929-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License

EPSL: High pore fluid pressures stabilize faults

The rupture, localization, and slip of faults in serpentinite were studied under varying pore fluid pressure conditions to understand deformation mechanisms potentially responsible for slow slip in fault zones. Experiments were conducted at a constant effective confining pressure of 10 MPa and under pore fluid pressures from 0 to 120 MPa and at temperatures from 23 to 110◦C. With no fluid pressure, faulting occurs rapidly and audibly, and the duration of failure increases monotonically with increasing fluid pressure and temperature. Although non-dilatant during initial strain hardening, the serpentinite dilates during strain weakening concomitant with fault rupture and slip. Non-dilatant strain hardening occurs by microcracking along serpentine basal planes and grain boundaries and rarely in mode I orientations, consistent with previous studies. Dilatant fault rupture produces a network of transgranular shear fractures in conjugate orientations, generally with one dominant fracture. Structural observations show that as fluid pressure increases, the number of transgranular fractures increases. We propose that when faulting occurs over a distributed zone rather than a pre-existing principal slip surface, dilatant hardening causes deformation to migrate. This process causes an increase in slip weakening distance and fracture energy at elevated fluid pressures that can lead to more stable failure. Further, thermally-activated processes causes deformation at propagating crack tips, which also increases the slip weakening distance and the effective fracture energy with increasing temperature. Given the geologic settings for slow slip, our results indicate that high fluid pressure, distributed deformation, and thermally-activated processes may all contribute to slow fault rupture and slip.

French, Melodie E., and Wenlu Zhu. “Slow fault propagation in serpentinite under conditions of high pore fluid pressure.” Earth and Planetary Science Letters 473 (2017): 131-140. doi: 10.1016/j.epsl.2017.06.009

Bounds on Geologically Current Rates of Motion of Groups of Hotspots

 Chengzu Wang, Richard G. Gordon*, and Tuo Zhang

It is widely believed that groups of hotspots in different regions of the world are in relative motion at rates of 10 to 30 mm a–1 or more. Here we present a new method for analyzing geologically current motion between groups of hotspots beneath different plates. In an inversion of 56 globally distributed, equally weighted trends of hotspot tracks, the dispersion is dominated by differences in trend between different plates rather than differences within plates. Nonetheless the rate of hotspot motion perpendicular to the direction of absolute plate motion, vperp, differs significantly from zero for only three of ten plates and then by merely 0.3 to 1.4 mm a–1. The global mean upper bound on |vperp| is 3.2 ±2.7 mm a–1. Therefore, hotspots move slowly and can be used to define a global reference frame for plate motions.

Link: http://onlinelibrary.wiley.com/doi/10.1002/2017GL073430/full

DOI: 10.1002/2017GL073430