Description: Publication date: 1 September 2018 Source: Earth and Planetary Science Letters, Volume 497 Author(s): J.D. Toner, D.C. Catling Oxychlorine salts (chlorates and perchlorates) are globally important components of surface soils on Mars, and could form liquid water in concentrated salt solutions despite prevailing cold and dry conditions. Although perchlorate salts are well-characterized, basic thermodynamic properties of chlorate solutions, such as water activity ( a w ) and even solubility, are poorly known. To address this knowledge-gap, we measured water activities and solubilities in the Na–Ca–Mg–ClO 3 system at 25 °C using the isopiestic method, and fit the data to an aqueous ion-interaction Pitzer model. We find that chlorate solutions have extremely low water activities that could allow liquid water to form on the surface of Mars. Compared to perchlorates, chlorates generally have higher water activities at the same concentration; however, saturated Mg(ClO 3 ) 2 solutions, in particular, are extremely concentrated (7.59 mol kg −1 ) and have a w = 0.2 at 25 °C, substantially below saturated Mg(ClO 4 ) 2 solutions ( a w = 0.4 ) . If Mg(ClO 3 ) 2 salts are present on Mars' surface, then our results suggest a much greater potential for liquid water formation in soils due to freezing point depression or deliquescence than with perchlorates.

Description: Publication date: 1 September 2018 Source: Earth and Planetary Science Letters, Volume 497 Author(s): Martijn Klaver, Jon D. Blundy, Pieter Z. Vroon The generation of continental crust of intermediate composition occurs predominantly in convergent margin settings, yet the mechanisms by which felsic, calc-alkaline arc magmas are generated remain poorly understood. Magma mixing appears to be a common process in voluminous intermediate arc rocks but the composition of the felsic mixing endmember is typically obscured by the mixing process. We investigate a suite of porphyritic (rhyo)dacitic magmas (65–72 wt.% SiO 2 ) from Nisyros, a young stratovolcano in the Aegean arc, Greece. These magmas are not affected by shallow process such as hybridisation or crystal-melt segregation and thus offer a valuable insight into the origin of felsic melts at convergent margins. We find that the Nisyros (rhyo)dacites form through a reaction in which earlier-formed wehrlite cumulates in the deep arc crust react with melts to form amphibole. This implies that melt major element compositions are effectively buffered by a low-variance mineral assemblage to follow this peritectic boundary such that the silica content of melts extracted from the deep crustal hot zone is controlled by the amount of amphibole crystallised. The resorption of cumulates is pivotal in imparting a distinct trace element signature that is decoupled from major element systematics. For example, high compatible element contents and a strong amphibole signature (low Y and Dy/Yb) cannot be captured by simple crystallisation models and require cumulate resorption. Variable radiogenic isotope systematics indicate minor crustal contamination although assimilation is not proportional to silica content and hence not a main driving force behind the generation of felsic melts. Instead, the Nisyros (rhyo)dacites formed through melt-cumulate reaction processes prior to emplacement as mush bodies at shallow depth and partial eruption. Magma mixing only becomes an important process in the youngest unit on Nisyros. On a global scale, peritectic boundary melts are rarely sampled in the whole rock or melt inclusion record. Conversely, peritectic boundary melts do form a suitable felsic mixing endmember for the generation of voluminous “ monotonous intermediate ” magmas.

Description: Publication date: 1 September 2018 Source: Earth and Planetary Science Letters, Volume 497 Author(s): J. Yamamoto, T. Kawano, N. Takahata, Y. Sano We measured noble gas isotopic compositions of quenched lavas sampled from seamounts, so-called petit-spot volcanoes, on the 160-million-year-old northwestern Pacific Plate. The samples 3 He/ 4 He and 40 Ar/ 36 Ar ratios were, respectively, 2.5–8.3 Ra and up to 1735, where Ra stands for atmospheric 3 He/ 4 He, which are analogous to or lower than those of MORB. Considering narrow sampling regions, a secondary effect might be responsible for variation of the data. During ascent and subsequent cooling of magma in the oceanic lithosphere, chemical components in the magma will be assimilated with those in the lithosphere. Correlation between CO 2 / 3 He ratios and carbon isotopic ratios suggests that carbon was affected by the incorporation of seafloor carbonate. The same would be true of noble gases. The mixing of noble gases among a mantle source, an atmospheric component dissolved in seawater and a radiogenic component can explain the data distribution. No 3 He/ 4 He ratio exceeds the MORB-like value. The mantle source of the petit-spot magma was likely to have had a MORB-like 3 He/ 4 He ratio originally. The eruption of petit-spot magma shows a close relation with the bending of subducting oceanic plates. The MORB-like 3 He/ 4 He ratio supports the hypothesis that the petit-spot magma is derived from the lithosphere–asthenosphere boundary.

Description: Publication date: 1 September 2018 Source: Earth and Planetary Science Letters, Volume 497 Author(s): Hiroyuki Kurokawa, Julien Foriel, Matthieu Laneuville, Christine Houser, Tomohiro Usui The hydrogen isotopic (D/H) ratio reflects the global cycling and evolution of water on Earth as it fractionates through planetary processes. We model the water cycle taking seafloor hydrothermal alteration, chemical alteration of continental crust, slab subduction, hydrogen escape from the early Earth, and degassing at mid-ocean ridges, hot spots, and arcs into account. The differences in D/H ratios between present-day oceans, oceanic and continental crust, and mantle are thought to reflect isotopic fractionation through seafloor alteration, chemical alteration, and slab dehydration. However, if the speed of plate tectonics has been nearly constant throughout Earth's history, the degassing and regassing rates are too small to reach the present-day D/H ratios. We show that (a) hydrogen escape from reduced early atmosphere, (b) secular net regassing, or (c) faster plate tectonics on early Earth is needed to reproduce the present-day D/H ratios of the water reservoirs. The low D/H ratio of Archean seawater at 3.8 Ga has previously been interpreted as a signature of (a) hydrogen escape, but we find it can also be explained either by (b) secular net degassing or by (c) faster plate tectonics on early Earth. The rates of hydrogen escape from early Earth and secular regassing on present-day Earth are constrained to be lower than 2.1 × 10 11 kg/yr and 3.9 × 10 11 kg/yr. Consequently, the volume of water in the present-day mantle could result entirely from the regassing through Earth's history. In that case, the volume of initial oceans could be 2 to 3 times larger than that of current Earth. We suggest that, in addition to the D/H ratio of Archean seawater, identifying the D/H ratios of both seawater and mantle throughout Earth's history would allow to distinguish these evolutionary scenarios.

Description: Publication date: 1 September 2018 Source: Earth and Planetary Science Letters, Volume 497 Author(s): Yibing Dong, Sidao Ni, David A. Yuen, Zhiwei Li In order to investigate earthquakes in the lower crust of North China Basin (NCB), we develop a new method of resolving very accurately the focal depth for local earthquakes in the sedimentary regions by using P and S to P converted wave (Sp) at the sedimentary interface. Theoretical analysis shows clearly that the travel-time difference between the Sp and P wave almost linearly correlates with the focal depth. This finding provides tight constraints on the depth. With this method, we obtain well-constrained depths of 44 events in the NCB, with uncertainties in the depth of about 2 km. Such a fine resolution can have great potential in asking questions regarding the crustal rheology. The depth distribution shows abundance of earthquakes in depth interval of ∼20 km, with some events in the lower crust, but also reveals the absence of seismicity deeper than 25 km. We find a good fit between the depth-frequency distribution in this region and the Yield Strength Envelope (YSE) in the Baikal Rift Systems (BRS). We infer that, the seismogenic thickness is ∼25 km in the NCB and the main deformation mechanism is brittle fracture. We further hypothesize that: (1) the rheological layering of dominant rheology in the NCB is similar to that of the BRS, which can be explained with a quartz rheology at 0–10 km depth and a diabase rheology at 10–35 km depth; (2) the temperature is moderate in the seismogenic zone of crust. We emphasize that many accurately resolved earthquake locations can shed light on the local nature of the crustal rheology, and this strategic method can be employed in other sedimentary regions, which are seismically active.

Description: Publication date: 15 August 2018 Source: Earth and Planetary Science Letters, Volume 496

Description: Publication date: 1 September 2018 Source: Earth and Planetary Science Letters, Volume 497 Author(s): Joanna L.O. Hall, Robert J. Newton, James D. Witts, Jane E. Francis, Stephen J. Hunter, Robert A. Jamieson, Elizabeth M. Harper, J. Alistair Crame, Alan M. Haywood The shell material of marine benthic bivalves provides a sensitive archive of water chemistry immediately above the sediment–water interface, which in turn is affected by sedimentary geochemistry and redox reactions. Sulfate has a major controlling effect on sedimentary carbon cycling, particularly the processes of methane production and oxidation, with lower concentrations of sulfate likely resulting in an increase in sedimentary methane production. Whilst it is accepted that ocean sulfate varied markedly across the Phanerozoic, evidence of changes in methane production in sediments has so far been lacking. There is potential for the oxidation products of sedimentary methane to be preserved and detected in marine fossils. Here we present the results of high resolution carbonate isotope records from two taxa of well-preserved shallow-infaunal bivalve ( Lahillia and Cucullaea ) collected from the marine shelf succession across the Cretaceous–Paleogene (K–Pg) boundary in Seymour Island, Antarctica. The succession has pre-existing subtle indications of more abundant methane, and the time period is characterized by much lower marine sulfate concentrations than modern. These shell carbonate–carbon isotope records vary widely: at one extreme, shells have typical average values and small ranges compatible with a contemporaneous marine dissolved inorganic carbon (DIC) source and modern-style sedimentary carbon cycling. At the other, the shells have large-amplitude annual cycles of carbon isotopic variability of up to 23.8‰ within a single year of growth and shell carbonate δ 13 C compositions as negative as −34‰. Shells with these increased ranges and unusually negative values are found at discrete intervals and across both bivalve taxa. The contribution of methane required to explain the most negative carbonate–carbon isotopic values in the bivalve shells is extremely high (between 30 to 85% of bottom-water DIC based on mass balance calculations). Records of organic-carbon isotopes from the same succession remained between −26.1 and −21.7‰ throughout, suggesting that methane influence was restricted to bottom-waters. A lack of authigenic carbonate in the section indicates that methane oxidation progressed aerobically and may have provided a significant driver for transient bottom water de-oxygenation. Where methane addition is indicated, the seasonal sensitivity precludes control by methane hydrates. We argue that these data represent the increased importance and sensitivity of methanogenesis in the sediments, enabled by lower ocean sulfate concentrations during the Late Cretaceous. The tendency towards a more dynamic role for marine methane production and oxidation is likely to apply to other times of low marine sulfate in Earth's history.

Description: Publication date: 1 September 2018 Source: Earth and Planetary Science Letters, Volume 497 Author(s): Xu Wang, Ling Chen, Yinshuang Ai, Tao Xu, Mingming Jiang, Yuan Ling, Yifan Gao The present-day Tibetan crust records the shallow response of the Cenozoic continental collision between the Indian and Eurasian plates. An analysis of the deep crustal structure beneath eastern and northeastern Tibet is of vital significance for studying the geodynamic processes of crustal thickening and expansion of the Tibetan Plateau. We herein provide detailed images of the crustal structure of eastern and northeastern Tibet and the adjacent Sichuan Basin using teleseismic P-wave receiver function (P-RF) data from a NW–SE-trending linear seismic array. Our P-RF imaging result reveals distinct structural features of the study region, including marked lateral variations in the depth to basement beneath the Songpan–Ganzi block and the Sichuan Basin, a seismically slow mid-lower crust beneath the Songpan–Ganzi block and a low-velocity anomaly just above the Moho around the easternmost Kunlun fault area, and obvious Moho offsets near the boundaries of tectonics blocks. These structural features may reflect various crustal responses within the continental interior to the India–Eurasia collision at the plate margin. The rigid crust of the Sichuan Basin might have wedged into the Tibetan crust in the Longmenshan area, which probably facilitated crustal thickening and enabled channelized mid-lower crustal flow in the Songpan–Ganzi block to the west. Being a pre-existing tectonic boundary, the Kunlun fault could have acted as a focus of heating and hot mantle upwelling associated with the deep processes of the Indian plate underthrusting and subduction, possibly resulting in localized weakening and modification of the lower crust around this fault area. The observed significant differences in the crustal structure of eastern and northeastern Tibet suggest that crustal shortening in this region may have been absorbed by not only vertical thickening in the interiors of the tectonic blocks but also complex local deformation along the boundary zones.

Description: Publication date: 1 September 2018 Source: Earth and Planetary Science Letters, Volume 497 Author(s): Alexander R. Blanchette, Simon L. Klemperer, Walter D. Mooney, Hani M. Zahran We use earthquake geothermometry, measured heat flow, and structural constraints from P-wave receiver functions to model the thermal evolution of the lithosphere beneath Harrat Lunayyir. We suggest that the lithosphere thinned to its present 60-km thickness in a second stage of lithospheric thinning at 15–12 Ma following initial Red Sea extension at ∼27 Ma. Harrat Lunayyir is an active volcanic field located in the Arabian Shield >150 km east of the Red Sea rift axis. In the lithospheric mantle beneath Harrat Lunayyir we locate 64 high-frequency earthquakes at depths of 42–48 km, all with m L ≤ 2.5 . These brittle-failure earthquakes must have nucleated at relatively low temperatures, based upon global maximum nucleation depths and temperature-dependent-deformation experimental results. Therefore, the mantle earthquakes show that the upper mantle lithosphere is not in thermal equilibrium with the shallow (60 km) underlying asthenosphere. Our thermal modeling indicates that the lithosphere beneath Harrat Lunayyir thinned to its current 60-km thickness at 12 ± 2 Ma, as constrained by thermal modeling of: (1) surface heat-flow; (2) the depth to the mid-crustal brittle–ductile transition, and (3) the depth to the upper-mantle brittle–ductile transition. Graphical abstract

Description: Publication date: 1 September 2018 Source: Earth and Planetary Science Letters, Volume 497 Author(s): Yu-Yan Sara Zhao, Scott M. McLennan, W. Andrew Jackson, Suniti Karunatillake Widely distributed perchlorate on the Martian surface and over three orders of magnitude variation in bromine abundances in surface samples are difficult to explain solely by chloride and bromide aqueous geochemistry. New experiments show that photochemical oxidation (ultraviolet wavelength 254 nm) of chloride- and bromide-bearing evaporative brines in the presence of silica beads produces substantial perchlorate (ClO − 4 ), chlorate (ClO − 3 ), and bromate (BrO − 3 ) under conditions relevant to Mars. Neutral to alkaline aqueous environments result in the dominance of chlorate over perchlorate. Preferential atmospheric recycling of Br over Cl causes variable Br/Cl ratios, consistent with numerous in-situ measurements of Cl and Br abundances on Mars. Bromate reacts with organics more readily than chlorate or perchlorate, and its presence in subsurface brines could challenge habitability in the Martian subsurface.