Geomorphology: Submerged reef terraces in the Maldivian Archipelago (Indian Ocean)

Authors – Alessio Rovere, Pankaj Khanna, Carlo Nike Bianchi, André W. Droxler, Carla Morri, David F. Naar

Abstract

Sea-level changes have shaped the world’s carbonate platform margins and continental shelves, leaving typical geomorphic imprints, such as drowned reef terraces. In this paper, we present the results of 112 scuba diving transects across seven different Maldivian atolls and one multibeam survey around Malé Island, the capital of Maldives. We report on the occurrence of drowned reef terraces down to 120 m depth. In total, we identified six levels of submerged terraces that we consider as indicative of periods of time with stable or slowly rising sea level that can be attributed either to deceleration of the last deglacial sea-level rise or to Late Quaternary sea-level highstands. We compare our dataset to the depth of reef terraces reported globally, and we discuss the reasons why common global submerged terrace levels are difficult to identify in the field record.

Full text – https://www.sciencedirect.com/science/article/pii/S0169555X1830223X

Chem Geo: A CO2 solubility model for silicate melts from fluid saturation to graphite or diamond saturation

A CO2 solubility model for silicate melts from fluid saturation to graphite or diamond saturation

By: James Eguchi, Rajdeep Dasgupta

Abstract: A model based on a thermodynamic framework for COconcentrations and speciation in natural silicate melts at graphite/diamond-saturated to fluid-saturated conditions is presented. The model is simultaneously calibrated with graphite-saturated and fluid-saturated conditions allowing for consistent model predictions across the CCO buffer. The model was calibrated using water-poor (≤1 wt% H2O) silicate melts from graphite- to CO2-fluid-saturation over a range of pressure (P = 0.05–3 GPa), temperature (T = 950–1600 °C), composition (foidite-rhyolite; NBO = 0.02–0.92; wt% SiO2 ~ 39–77, TiO2 ~ 0.1–5.8, Al2O3 ~ 7.5–18, FeO ~ 0.2–24 MgO ~ 0.1–24, CaO ~ 0.3–14, Na2O~1-5, K2O ~ 0–6), and fO2 (~QFM +1.5 to ~QFM −6). The model can predict CO2 concentrations for a wide range of silicate melt compositions from ultramafic to rhyolitic compositions, i.e., melts that dissolve carbon only as carbonate anions CO32– and those that dissolve carbon both as CO32– and as molecular CO2mol as a function of pressure, temperature, and oxygen fugacity. The model also does a reasonable job in capturing COsolubility in hydrous silicate melts with ≤2–3 wt% H2O. New CO2solubility experiments at pressures >3 GPa suggest that the newly developed CO2 solubility model can be satisfactorily extrapolated to ~4–5 GPa. Above 5 GPa the model poorly reproduces experimental data, likely owing to structural change in silicate melt at pressures above 5 GPa. An Excel spreadsheet and a Matlab function are provided as online supplementary materials for implementing the new CO2 solubility model presented here.

 

 

EPSL: A low-angle detachment fault revealed: Three-dimensional images of the S-reflector fault zone along the Galicia passive margin

Paper Link : https://www.sciencedirect.com/science/article/pii/S0012821X18302073

Received: 19 December 2017

Accepted: 8 April 2018

Published: 17 April 2018

Abstract: A new 3-D seismic reflection volume over the Galicia margin continent–ocean transition zone provides an unprecedented view of the prominent S-reflector detachment fault that underlies the outer part of the margin. This volume images the fault’s structure from breakaway to termination. The filtered time-structure map of the S-reflector shows coherent corrugations parallel to the expected paleoextension directions with an average azimuth of 107◦. These corrugations maintain their orientations, wavelengths and amplitudes where overlying faults sole into the S-reflector, suggesting that the parts of the detachment fault containing multiple crustal blocks may have slipped as discrete units during its late stages. Another interface above the S-reflector, here named S, is identified and interpreted as the upper boundary of the fault zone associated with the detachment fault. This layer, named the S-interval, thickens by tens of meters from SE to NW in the direction of transport. Localized thick accumulations also occur near overlying fault intersections, suggesting either non-uniform fault rock production, or redistribution of fault rock during slip. These observations have important implications for understanding how detachment faults form and evolve over time. 3-D seismic reflection imaging has enabled unique insights into fault slip history, fault rock production and redistribution.

C. Nur Schuba, Gary G. Gray, Julia K. Morgan, Dale S. Sawyer, Donna J. Shillington,
Tim J. Reston, Jonathan M. Bull, Brian E. Jordan

  • Department of Earth, Environmental and Planetary Sciences, Rice University, MS-126 Main Street, 77005, Houston, TX, USA.
  • Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, PO Box 1000, 10964-8000, Palisades, NY, USA.
  • School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK.
  • National Oceanography Centre Southampton, University of Southampton, Southampton, S014 3ZH, United Kingdom.
  • BP America Inc., 501 WestLake Park Blvd., 23.126C, 77079, Houston, TX, USA.

Scientific Reports: Volcanic ash as a driver of enhanced organic carbon burial in the Cretaceous

Volcanic ash as a driver of enhanced organic carbon burial in the Cretaceous

Cin-Ty Lee, Hehe Jiang, Elli Ronay, Daniel Minisini, Jackson Stiles, Matt Neal

Scientific Reportsvolume 8, Article number: 4197 (2018)

doi:10.1038/s41598-018-22576-3

On greater than million year timescales, carbon in the ocean-atmosphere-biosphere system is controlled by geologic inputs of CO2 through volcanic and metamorphic degassing. High atmospheric CO2 and warm climates in the Cretaceous have been attributed to enhanced volcanic emissions of CO2 through more rapid spreading at mid-ocean ridges and, in particular, to a global flare-up in continental arc volcanism. Here, we show that global flare-ups in continental arc magmatism also enhance the global flux of nutrients into the ocean through production of windblown ash. We show that up to 75% of Si, Fe and P is leached from windblown ash during and shortly after deposition, with soluble Si, Fe and P inputs from ash alone in the Cretaceous being higher than the combined input of dust and rivers today. Ash-derived nutrient inputs may have increased the efficiency of biological productivity and organic carbon preservation in the Cretaceous, possibly explaining why the carbon isotopic signature of Cretaceous seawater was high. Variations in volcanic activity, particularly continental arcs, have the potential of profoundly altering carbon cycling at the Earth’s surface by increasing inputs of CO2 and ash-borne nutrients, which together enhance biological productivity and burial of organic carbon, generating an abundance of hydrocarbon source rocks.

 

 

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

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

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 DiscontinuityGeochemistry, Geophysics, Geosystems19https://doi.org/10.1002/2017GC007233

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

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

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