EPSL: Coupled magmatism-erosion in continental arcs: reconstructing the history of the Cretaceous Peninsular Ranges batholith, southern California through detrital hornblende barometry in forearc sediments

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Hehe Jiang, Cin-Ty A. Lee

Continental magmatic arcs are characterized by voluminous flare-ups accompanied by rapid arc unroofing and sedimentation in the forearc basin. Such magmatism and erosion may be dynamically linked and influence the long-term evolution of crustal thickness. To evaluate these effects, we conducted a case study in the Peninsular Ranges batholith (PRB) in southern California, where mid-Late Cretaceous (125-75 Ma) emplacement of felsic plutons coincided with a major pulse of arc-derived sediments into the adjacent forearc basin. We compiled zircon U-Pb ages in the PRB plutons and estimated magmatic addition rates from exposed areas of plutons with different ages. To obtain erosion rates, sandstone samples of known depositional age from the PRB forearc basin were investigated. Major element compositions of detrital hornblendes were determined by electron probe microanalysis and used to calculate emplacement depths of eroded plutons using Al-in-hornblende barometry. These results were combined with laser ablation ICPMS based U-Pb ages of accompanying detrital zircons to estimate the integrated erosion rate by dividing the detrital hornblende emplacement depth by the lag time between peak detrital zircon age and depositional age. Both magmatic addition and erosion rates are between 0.1-2 km/Myr. Magmatic addition peaked at 100-90 Ma, followed by a long, protracted period of erosion between 90-50 Ma. Mass balance and isostatic modeling suggests that due to high magmatic influx, more than 30 km integrated crustal growth and 5 km elevation increase was achieved shortly after peak magmatism. The data and models suggest that erosion was driven by magma-induced crustal thickening and subsequent surface uplift, with an erosional response time of 3-6 Myr. Prolonged erosion after the cessation of magmatism resulted in gradual smoothing of the topography and significant removal of the excess crustal thickness by late Eocene time. The short erosional response times inferred from this study suggest that erosion and magmatism are intimately linked, begging the question of whether the thermal state, metamorphism and rheology of crust in continental arcs is controlled in part by the interplay between erosion and magmatism. We speculate that syn-magmatic erosion, through its effects on the thermal structure of the crust, may also play a role in modulating the depth of pluton emplacement.

Earth and Planetary Science Letters, Volume 472, 15 August 2017, Pages 69-81. doi: 10.1016/j.epsl.2017.05.009

Sci.Adv.: The exceptional sediment load of fine-grained dispersal systems: Example of the Yellow River, China

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Hongbo Ma, Jeffrey A. Nittrouer, Kensuke Naito, Xudong Fu, Yuanfeng Zhang, Andrew J. Moodie, Yuanjian Wang, Baosheng Wu and Gary Parker

http://advances.sciencemag.org/content/3/5/e1603114

Sedimentary dispersal systems with fine-grained beds are common, yet the physics of sediment movement within them remains poorly constrained. We analyze sediment transport data for the best-documented, fine-grained river worldwide, the Huanghe (Yellow River) of China, where sediment flux is underpredicted by an order of magnitude according to well-accepted sediment transport relations. Our theoretical framework, bolstered by field observations, demonstrates that the Huanghe tends toward upper-stage plane bed, yielding minimal form drag, thus markedly enhancing sediment transport efficiency. We present a sediment transport formulation applicable to all river systems with silt to coarse-sand beds. This formulation demonstrates a remarkably sensitive dependence on grain size within a certain narrow range and therefore has special relevance to silt-sand fluvial systems, particularly those affected by dams.

EPSL: Effects of crustal thickness on magmatic differentiation in subduction zones

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Rise of Earth’s atmospheric oxygen controlled by efficient subduction of organic carbon

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Duncan, M. S. and Dasgupta, R.

Abstract: The net flux of carbon between the Earth’s interior and exterior, which is critical for redox evolution and planetary habitability, relies heavily on the extent of carbon subduction. While the fate of carbonates during subduction has been studied, little is known about how organic carbon is transferred from the Earth’s surface to the interior, although organic carbon sequestration is related to sources of oxygen in the surface environment. Here we use high pressure–temperature experiments to determine the capacity of rhyolitic melts to carry carbon under graphite-saturated conditions in a subducting slab, and thus to constrain the subduction efficiency of organic carbon, the remnants of life, through time. We use our experimental data and a thermodynamic model of CO2 dissolution in slab melts to quantify organic carbon mobility as a function of slab parameters. We show that the subduction of graphitized organic carbon, and the graphite and diamond formed by reduction of carbonates with depth, remained efficient even in ancient, hotter subduction zones where oxidized carbon subduction probably remained limited. We suggest that immobilization of organic carbon in subduction zones and deep sequestration in the mantle facilitated the rise (~103–5 fold) and maintenance of atmospheric oxygen since the Palaeoproterozoic and is causally linked to the Great Oxidation Event. Our modelling shows that episodic recycling of organic carbon before the Great Oxidation Event may also explain occasional whiffs of atmospheric oxygen observed in the Archaean.

Duncan, M. S. & Dasgupta, R. (2017). The rise of Earth’s atmospheric oxygen controlled by efficient subduction of organic carbon. Nature Geoscience. doi:10.1038/NGEO2939

Experimental determination of CO2 content at graphite saturation along a natural basalt-peridotite melt join: Implications for the fate of carbon in terrestrial magma oceans

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Megan S. Duncan, Rajdeep Dasgupta, Kyusei Tsuno

Abstract

Knowledge of the carbon carrying capacity of peridotite melt at reducing conditions is critical to constrain the mantle budget and planet-scale distribution of carbon set at early stage of differentiation. Yet, neither measurements of CO2 content in reduced peridotite melt nor a reliable model to extrapolate the known solubility of CO2 in basaltic (mafic) melt to solubility in peridotitic (ultramafic) melt exist. There are several reasons for this gap; one reason is due to the unknown relative contributions of individual network modifying cations, such as Ca2+ versus Mg2+, on carbonate dissolution particularly at reducing conditions. Here we conducted high pressure, temperature experiments to estimate the CO2 contents in silicate melts at graphite saturation over a compositional range from natural basalts toward peridotite at a fixed pressure (P) of 1.0 GPa, temperature (T) of 1600 °C, and oxygen fugacity (logfO2 ~ IW+1.6). We also conducted experiments to determine the relative effects of variable Ca and Mg contents in mafic compositions on the dissolution of carbonate. Carbon in quenched glasses was measured and characterized using Fourier transform infrared spectroscopy (FTIR) and Raman Spectroscopy and was found to be dissolved as carbonate (CO32-). The FTIR spectra showed CO32- doublets that shifted systematically with the MgO and CaO content of silicate melts. Using our data and previous work we constructed a new composition-based model to determine the CO2 content of ultramafic (peridotitic) melt representative of an early Earth, magma ocean composition at graphite saturation. Our data and model suggest that the dissolved CO2 content of in reduced, peridotite melt is significantly higher than that of basaltic melt at shallow magma ocean conditions; however, the difference in C content between the basaltic and peridotitic melts may diminish with depth as the more depolymerized peridotite melt is more compressible. Using our model of CO2 content at graphite saturation as a function of P–T-fO2-melt composition, we predict that a superliquidus shallow magma ocean should degas CO2. Whereas if the increase of fO2 with depth is weak, a magma ocean may ingas a modest amount of carbon during crystallization. Further, using the carbon content of peridotite melt at logfO2 of IW and the knowledge of C content of Fe-rich alloy melt, we also consider the core-mantle partitioning of carbon, showing that DCmetal/peridotite of a shallow magma ocean is generally higher than previously estimated.

 

Duncan, M. S., Dasgupta, R., Tsuno, K. (2017). Experimental determination of CO2 content at graphite saturation along a natural basalt-peridotite melt join: Implications for the fate of carbon in terrestrial magma oceans. Earth and Planetary Science Letters 466, 115-128. doi:10.1016/j.epsl.2017.03.008

JGR-ES: Morphodynamic modeling of fluvial channel fill and avulsion time scales during early Holocene transgression, as substantiated by the incised valley stratigraphy of the Trinity River, Texas

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Kaitlin E. Moran , Jeffrey A. Nittrouer, Mauricio M. Perillo, Jorge Lorenzo-Trueba, and John B. Anderson

Journal of Geophysical Research: Earth Surface (2016) doi:10.1002/2015JF003778

The Trinity River system provides a natural laboratory for linking fluvial morphodynamics to stratigraphy produced by sea-level rise, because the sediments occupying the Trinity incised valley are well constrained in terms of timing of deposition and facies distribution. Herein, the Trinity River is modeled for a range of base-level rise rates, avulsion thresholds, and water discharges to explore the effects of backwater-induced in-channel sedimentation on channel avulsion. The findings are compared to observed sediment facies to evaluate the capability of a morphodynamic model to reproduce sediment deposition patterns. Base-level rise produces mobile locations of in-channel sedimentation and deltaic channel avulsions. For scenarios characteristic of early Holocene sea-level rise (4.3 mm yr1 ), the Trinity fluvial-deltaic system progrades 13 m yr1 , followed by backstepping of 27 m yr1 . Avulsion is reached at the position of maximum sediment deposition (located 108 km upstream of the outlet) after 3,548 model years, based on sedimentation filling 30% of the channel. Under scenarios of greater base-level rise, avulsion is impeded because the channel fill threshold is never achieved. Accounting for partitioning of bed-material sediment between the channel and floodplain influences the timing and location of avulsion over millennial time scales: the time to avulsion is greatly increased. Sedimentation patterns within the valley, modeled and measured, indicate preference toward sandy bed material, and the rates of deposition are substantiated by previous measurements. Although the results here are specific to the Trinity River, the analysis provides a framework that is adaptable to other lowland fluvial-deltaic systems.

CO2 content of andesitic melts at graphite-saturated upper mantle conditions with implications for redox state of oceanic basalt source regions and remobilization of reduced carbon from subducted eclogite

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James Eguchi, Rajdeep Dasgupta

DOI: 10.1007/s00410-017-1330-8

We have performed experiments to determine the effects of pressure, temperature and oxygen fugacity on the CO2 contents in nominally anhydrous andesitic melts at graphite saturation. The andesite composition was specifically chosen to match a low-degree partial melt composition that is generated from MORB-like eclogite in the convective, oceanic upper mantle. Experiments were performed at 1–3 GPa, 1375–1550 °C, and fO2 of FMQ −3.2 to FMQ −2.3 and the resulting experimental glasses were analyzed for CO2 and H2O contents using FTIR and SIMS. Experimental results were used to develop a thermodynamic model to predict CO2 content of nominally anhydrous andesitic melts at graphite saturation. Fitting of experimental data returned thermodynamic parameters for dissolution of CO2 as molecular CO2: ln(K0) = −21.79 ± 0.04, ΔV0 = 32.91 ± 0.65 cm3mol−1, ΔH0 = 107 ± 21 kJ mol−1, and dissolution of CO2 as CO32−: ln(K0) = −21.38 ± 0.08, ΔV0 = 30.66 ± 1.33 cm3 mol−1, ΔH0 = 42 ± 37 kJ mol−1, where K0 is the equilibrium constant at some reference pressure and temperature, ΔV0 is the volume change of reaction, and ΔH0 is the enthalpy change of reaction. The thermodynamic model was used along with trace element partition coefficients to calculate the CO2 contents and CO2/Nb ratios resulting from the mixing of a depleted MORB and the partial melt of a graphite-saturated eclogite. Comparison with natural MORB and OIB data suggests that the CO2 contents and CO2/Nb ratios of CO2-enriched oceanic basalts cannot be produced by mixing with partial melts of graphite-saturated eclogite. Instead, they must be produced by melting of a source containing carbonate. This result places a lower bound on the oxygen fugacity for the source region of these CO2-enriched basalts, and suggests that fO2 measurements made on cratonic xenoliths may not be applicable to the convecting upper mantle. CO2-depleted basalts, on the other hand, are consistent with mixing between depleted MORB and partial melts of a graphite-saturated eclogite. Furthermore, calculations suggest that eclogite can remain saturated in graphite in the convecting upper mantle, acting as a reservoir for C.

How habitable zones and super-Earths lead us astray

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William B. Moore, A. Lenardic, A. M. Jellinek, C. L. Johnson, C. Goldblatt and R. D. Lorenz

“The English language … becomes ugly and inaccurate because our thoughts are foolish, but the slovenliness of our language makes it easier for us to have foolish thoughts.” – George Orwell

The field of exoplanet research is currently experiencing a period of integration among researchers from the planetary exploration, astrobiology, and astronomical observation communities (among others). In addition, exoplanet discoveries generate strong interest from the public and a need for frequent communication of scientific discoveries to non-scientists. Together, these factors have put significant pressure on the field to develop an easily digested scientific shorthand for frequently used concepts. While useful, such terminology is often imprecise and can ultimately mislead the very audience it was designed to reach.

Nature Astronomy Volume 1, Feb 2, 2017 DOI: 10.1038/s41550-017-0043

http://rdcu.be/o3yL

Episodic nature of continental arc activity since 750 Ma: A global compilation

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Episodic nature of continental arc activity since 750 Ma: A global compilation

Wenrong Cao, Cin-Ty Lee, Jade Star Lackey

Earth and Planetary Science Letters (2017) 461:85-95

http://dx.doi.org/10.1016/j.epsl.2016.12.044

Continental arcs have been recently hypothesized to outflux large amounts of CO2 compared to island arcs so that global flare-ups in continental arc magmatism might drive long-term greenhouse events. Quantitative testing of this hypothesis, however, has been limited by the lack of detailed studies on the spatial distribution of continental arcs through time. Here, we compile a worldwide database of geological maps and associated literature to delineate the surface exposure of granitoid plutons, allowing reconstruction of how the surface area addition rate of granitoids and the length of continental arcs have varied since 750 Ma. These results were integrated into an ArcGIS framework and plate reconstruction models. We find that the spatial extent of continental arcs is episodic with time and broadly matches the detrital zircon age record. Most vigorous arc magmatism occurred during the 670–480 Ma and the 250–50 Ma when major greenhouse events are recognized. Low continental arc activity characterized most of the Cryogenian, middle–late Paleozoic, and Cenozoic when climate was cold. Our results indicate that plate tectonics is not steady, with fluctuations in the nature of subduction zones possibly related in time to the assembly and dispersal of continents. Our results corroborate the hypothesis that variations in continental arc activity may play a first order role in driving long-term climate change. The dataset presented here provides a quantitative basis for upscaling continental arc processes to explore their effects on mountain building, climate, and crustal growth on a global scale.

The relative abundances of resolved 12CH2D2 and 13CH3D and mechanisms controlling isotopic bond ordering in abiotic and biotic methane gases

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E. D. Young, I. E. Kohl, B. Sherwood Lollar, G. Etiope, D. Rumble III, S. Li (李姝宁), M. A. Haghnegahdar, E. A. Schauble, K. A. McCain, D. I. Foustoukos, C. Sutclife, O. Warr, C. J. Ballentine, T. C. Onstott, H.  Hosgormez, A. Neubeck, J. M. Marques, I. Pérez-Rodríguez, A. R. Rowe, D. E. LaRowe, C. Magnabosco, Laurence Y. Yeung, Jeanine L. Ash, L. T. Bryndzia. Geochim. Cosmochim. Acta (2017).

doi: 10.1016/j.gca.2016.12.041

Abstract

We report measurements of resolved 12CH2D2 and 13CH3D at natural abundances in a variety of methane gases produced naturally and in the laboratory. The ability to resolve 12CH2D2 from 13CH3D provides unprecedented insights into the origin and evolution of CH4. The results identify conditions under which either isotopic bond order disequilibrium or equilibrium are expected. Where equilibrium obtains, concordant Δ12CH2D2 and Δ13CH3D temperatures can be used reliably for thermometry. We find that concordant temperatures do not always match previous hypotheses based on indirect estimates of temperature of formation nor temperatures derived from CH4/H2 D/H exchange, underscoring the importance of reliable thermometry based on the CH4 molecules themselves. Where Δ12CH2D2 and Δ13CH3D values are inconsistent with thermodynamic equilibrium, temperatures of formation derived from these species are spurious. In such situations, while formation temperatures are unavailable, disequilibrium isotopologue ratios nonetheless provide novel information about the formation mechanism of the gas and the presence or absence of multiple sources or sinks. In particular, disequilibrium isotopologue ratios may provide the means for differentiating between methane produced by abiotic synthesis versus biological processes. Deficits in 12CH2D2 compared with equilibrium values in CH4 gas made by surface-catalyzed abiotic reactions are so large as to point towards a quantum tunneling origin. Tunneling also accounts for the more moderate depletions in 13CH3D that accompany the low 12CH2D2 abundances produced by abiotic reactions. The tunneling signature may prove to be an important tracer of abiotic methane formation, especially where it is preserved by dissolution of gas in cool hydrothermal systems (e.g., Mars). Isotopologue signatures of abiotic methane production can be erased by infiltration of microbial communities, and Δ12CH2D2 values are a key tracer of microbial recycling.