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Sulfur in the slab: a sulfur-isotopes and thermodynamic-modeling perspective from exhumed terranes
(2022)
Sulfur is a key element in the subduction zone-volcanic arc system; however, the mechanism(s) that recycle sulfur from the slab into the overlying volcanic arc are debated. Here we summarize recent advances in quantifying this component of the deep sulfur cycle. First, primary metamorphic or inherited sulfides in oceanic-type eclogites are only rarely observed as inclusions and are typically absent from the rock matrix. Additionally, sulfides are relatively common in rocks metasomatized at the slab-mantle interface by slab-derived fluids during exhumation. Combined, these two observations suggest that sulfur loss from subducted mafic crust is relatively efficient. Thermodynamic modeling in Perple_X using the Holland and Powell (2011) database combined with the Deep Earth Water model suggests that the efficiency and speciation of sulfur loss varies depending on the degree of seafloor alteration prior to subduction and the geothermal gradient of the slab. In relatively cold subduction zones, such as Honshu, slab-fluids derived from subducted mafic crust are predicted to exhibit elevated concentrations of HSO4-, SO42-, HSO3-, and CaSO4(aq), whereas hot subduction zones, such as Cascadia, are predicted to produce slab fluids enriched in HS- and H2S at lower pressures. The oxidation of sulfur expelled from subducted pyrite is balanced by the reduction of Fe3+ to Fe2+, consistent with the low Fe3+/SFe of exhumed eclogites relative to blueschists and altered oceanic crust. Where oxidized S-bearing fluids are produced, they are anticipated to interact with more reduced rocks at the slab-mantle interface and within the mantle wedge, resulting in sulfide precipitation and significant isotopic fractionation. The δ34S values of slab fluids are estimated to fall between -11 and +8 ‰. Rayleigh fractionation during progressive fluid-rock interaction results in fractionations of tens of per mil as oxidized species are depleted and sulfides are precipitated, resulting in δ34S values of sulfides that easily span the -21.7 to +13.9 ‰ range observed in metasomatic sulfides in exhumed high-pressure rocks. However, in subduction zones where reduced species prevail, the S isotopic signature of slab fluids is expected to reflect their source and will exhibit a narrower range in δ34S values. As a result, the δ34S values measured in arc magmas may not always be a reliable indicator of the contribution of different components of the slab, such as sediments vs. AOC. Additionally, the impact of S recycling on the oxygen fugacity of arc magmas is expected to vary both spatially and temporally throughout Earth history.
This article presents a multiscale approach for detecting and monitoring soil erosion phenomena (i.e. gully erosion) in the agro-industrial area around the city of Taroudannt, Souss basin, Morocco. The study area is characterized as semi-arid with an annual average precipitation of 200 mm. Water scarcity, high population dynamics and changing land use towards huge areas of irrigation farming present numerous threats to sustainability. The agro-industry produces citrus fruits and vegetables in monocropping, mainly for the European market. Badland areas strongly affected by gully erosion border the agricultural areas as well as residential areas. To counteract the significant loss of land, land-leveling measures are attempted to create space for plantations and greenhouses. In order to develop sustainable approaches to limit gully growth the detection and monitoring of gully systems is fundamental. Specific gully sites are monitored with unmanned aerial vehicle (UAV) taking small-format aerial photographs (SFAP). This enables extremely high-resolution analysis (SFAP resolution: 2-10 cm) of the actual size of the gully channels as well as a detailed continued surveillance of their growth. Transferring the methodology on a larger scale using Quickbird satellite data (resolution: 60 cm) leads to the possibility of a large-scale analysis of the whole area around the city of Taroudannt (Area extent: ca. 350 km²). The results will then reveal possible relationships of gully growth and agro-industrial management and may even illustrate further interdependencies. The main objective is the identification of areas with high gully-erosion risk due to non-sustainable land use and the development of mitigation strategies for the study area.
Garnet xenocrysts from kimberlites provide unique insights into the composition, structure and evolution of the subcontinental lithospheric mantle (SCLM). For example, different metasomatic events in the SCLM are reflected in compositional differences between garnet xenocrysts. As mantle metasomatism largely controls the physical and chemical properties of the SCLM, it exerts first order control over the genesis of kimberlitic magmas and diamond formation. However, dating mantle lithologies and processes is complicated by high ambient temperatures that allow the equilibration of most isotopic systems up to the time of kimberlite eruption. As a consequence, the temporal connection between metasomatic events in the mantle and kimberlite genesis is commonly ambiguous.
In this study, we applied LA-ICPMS U-Pb dating to 43 harzburgitic, lherzolithic and megacrystic garnet xenocrysts from the ~376 Ma diamondiferous V. Grib kimberlite, Russia, in order to investigate the link between different types of mantle metasomatism and kimberlite genesis.
Our results indicate that, with two possible exceptions, only harzburgitic garnet overlaps in age with the kimberlite eruption, whereas lherzolitic and megacrystic garnet crystals are ~20 to 130 million years older. Furthermore, garnet U-Pb ages and Ni-in-garnet temperatures of ~820 to 1200 °C do not correlate. This, and the high closure temperature of U-Pb in garnet (≥900 °C) suggests that the garnet U-Pb ages indeed reflect metasomatic events in the SCLM. However, the U-Pb ages could also reflect cooling ages. In this case, the metasomatic events recorded in the garnet crystals must still have occurred up to ~130 million years prior to the eruption of the V. Grib kimberlite.
These findings have far-reaching implications for the genesis of (diamondiferous) kimberlites, as they clearly show that the time lag between metasomatic events in the SCLM, as recorded in kimberlitic garnet xenocrysts, and kimberlite eruption may extend to tens of millions of years.
The Ceboruco is a 2280 m high stratovolcano located in Nayarit State, Mexico. Despite its last eruption which occurred in 1870, it is the most active volcano in the area, showing volcanicearthquake activity together with ongoing vapor emissions. The magnetotelluric survey was carried out in November 2016. It was part of a geothermal project (CeMIEGeo-P24) and focused on the determination of the electrical conductivity distribution in the subsurface of the volcano.
The Magnetotelluric Apparent Resistivity Tensor, as introduced by Brown (2016), can be decomposed into an amplitude and a phase tensor. The fundamental physics behind those new tensors were presented in Hering et al. (2019), using canonical models in 1-D (isotropic and anisotropic) and 2-D resistivity environments. Here, the tensors are introduced for a high-quality data set, where their interpretational benefits become very obvious. Additionally, results from an isotropic 3-D inversion are presented and compared to an alternative 3-D anisotropic forward model.
The 99th Annual Meeting of the Geologische Vereinigung (GV) and International Conference on Earth Control on Planetary Life and Environment, held in October 2009 at the Geosciences Centre of the Georg-August-Universität Göttingen, brings together researchers from all fields of Earth Sciences and beyond to shape an attractive interdisciplinary program on the geological history of Planet Earth and its control over and interaction with biological evolution, development of habitats, environmental and climate change as well as history and culture of Homo sapiens. This volume contains the abstracts of invited keynote lectures as well as all oral and poster presentations.