A terrestrial brine-seepage analog for Martian slope streaks near Salar de Pedernales in the Atacama Desert, Chile

Martian slope streaks are elongated down-slope, low-albedo surface features that currently form within sub-annual time scales in the high-albedo (“dusty”) regions of equatorial Mars. These km-scale streaks, which can persist up to several decades on the Martian surface, present one of the most enigmatic and dynamic suites of active surface features on present-day Mars. Two categories of explanations remain in debate regarding their nature: 1) “Dry” formation models, in which surface darkening is associated with mass wasting processes, such as dust avalanches; and 2) “Wet” formation models, in which surface darkening is associated with transient wetting of the surface by either seepage of sub-surface brines or deliquescence of atmospheric moisture. Here, we report recently discovered dark slope streaks that occur in the high Atacama Desert in Chile and display a close geomorphic resemblance to the Martian streaks. Field examination of the Atacama slope streaks revealed that they formed through down-slope seepage of groundwater brines sourced from the Salar de Pedernales located 500 m away. Chemical and mineralogical analyses demonstrate that salts deposited from the Pedernales brines combined with detrital input from soils/dust are responsible for surface darkening in the Atacama case. Field-based spectral measurements in the 0.4-2.5 micron wavelength range compare to those obtained from orbit for the Martian slope streaks. In addition, high-resolution topography derived from drone imagery revealed that the Atacama streaks are rougher than their surroundings at the decimeter-scale roughness of the entire hillslope they occur on. A similar distinction was previously established between Martian slope streaks and their surrounding slopes, although on Mars these roughness variations appear to occur at lower-range sub-centimeter scales. Our study of the unique Atacama slope streaks may support “wet” formation as a viable hypothesis for some of the Martian slope streaks.Fil: Mushkin, Amit. Geological Survey of Israel; IsraelFil: Sletten, Ronal. University of Washington; Estados UnidosFil: Trombotto, Dario Tomas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; ArgentinaFil: Jigjidsurengiin, Batbaatar. University of Washington; Estados UnidosFil: Amit, Rivka. Geological Survey of Israel; IsraelFil: Halevy, Itay. Weizmann Institute Of Science.; IsraelFil: Morag, Navot. Geological Survey of Israel; IsraelFil: Gillespie, Alan R.. University of Washington; Estados UnidosThe Geological Society of America: Connects 2021OregonEstados UnidosThe Geological Society of Americ

Crustal evolution and recycling in the northern Arabian-Nubian Shield : new perspectives from zircon Lu-Hf and U-Pb systematics

This paper presents laser-ablation (MC)-ICP-MS single-zircon Hf isotopic data coupled with U-Pb geochronology from a unique rock inventory in southern Israel, recording ca. 300m.y. of Neoproterozoic crustal evolution in the northern Arabian-Nubian Shield (ANS). The early, island-arc magmatic cycle in this area is recorded by detrital zircons from arc-derived metasediments, dated between 870 and 760Ma, and by ca. 790 and 740Ma orthogneisses. Zircon e{open}Hf(t) values from these rocks are all positive, supporting previous isotopic evidence for the overall juvenile nature of northern ANS crust. Nonetheless, detrital zircons from the metasedimentary units display a wide range of e{open}Hf(t) values between +1 and +13. The lower values in this range suggest some contribution from an older crustal component. The involvement of pre-Neoproterozoic crustal component is also indicated by the occurrence of rare zircons with pre-Neoproterozoic ages and/or negative e{open}Hf(t) values in younger (post-collisional) magmas that intrude the arc sequence. Arc accretion was followed by a period of ∼60m.y. of relative igneous tranquility interrupted only by the emplacement of local basic dike swarm at ca. 705Ma. Average zircon e{open}Hf (t) value of +10 found in one of these dikes indicates a depleted mantle source possibly implying a mid-Neoproterozoic extensional phase. The later, post-collisional magmatic cycle marks the transition of the accreted arc complexes into a mature continental crust. It consists of voluminous 680-600Ma granitoid plutons of calc-alkaline affinity, and subordinate 610-580Ma intrusions of alkaline affinity. Average zircon e{open}Hf(t) values in these post-collisional rocks are all positive and range between +6 and +9, compatible with both mantle source and juvenile crust reworking.16 page(s

Long-distance transport of North Gondwana Cambro-Ordovician sandstones : evidence from detrital zircon Hf isotopic composition

A voluminous Early Paleozoic sequence of quartz-rich sandstones was deposited in northern Gondwana following its assembly during the Neoproterozoic-Cambrian Pan-African Orogeny. Field evidence for the sense of transport indicate that sediments were carried from Gondwana hinterland towards the supercontinent margins in the North (present coordinates). Derivation from Pan-African terranes is evident from the ubiquity of detrital zircons with Neoproterozoic U-Pb ages, but the exact provenance of these siliciclastic deposits remains unclear. Herein we present new Hf isotopic data from U-Pb dated detrital zircons of the Cambro-Ordovician sandstone that top the juvenile Neoproterozoic basement of the Arabian-Nubian Shield in Israel and Jordan. Remarkably, the detrital zircon Hf isotopic signal stands in marked contrast with Nd and Hf isotopic signature of the underlying basement. A preponderance (61%) of the Neoproterozoic-aged detrital zircons from the Cambro-Ordovician sandstones in Israel and Jordan yielded negative epsilon (super Hf(t) ) values incompatible with a juvenile source. Therefore, rather than from the adjacent Arabian-Nubian Shield, most of the detrital zircons were derived from distant terrane(s), comprising pre-Neoproterozoic crust reworked during Pan-African orogeny. Because our sampling sites are situated at the northern tip of the Arabian-Nubian Shield, sand must have been transported several thousand kilometers before deposition. This finding also implies that the Arabian-Nubian Shield and other Pan-African orogens of NE Africa were completely warned down by the onset of Cambro-Ordovician deposition and that vast areas in the northern part of Gondwana wer e then low-lying such as to allow transfer of sand across the continent.1 page(s

Drainage response to Arabia-Eurasia collision: Insights from provenance examination of the Cyprian Kythrea flysch (Eastern Mediterranean Basin)

The Cenozoic geodynamics of the north-eastern Mediterranean Basin have been dominated by the subduction of the African Plate under Eurasia. A trench-parallel crustal-scale thrust system (Misis-Kyrenia Thrust System) dissects the southern margin of the overriding plate and forms the structural grain and surface expression of northern Cyprus. Late Eocene to Miocene flysch of the Kythrea (Degirmenlik) Group is exposed throughout northern Cyprus, both at the hanging-wall and foot-wall of the thrust system, permitting access to an extensive Cenozoic sedimentary record of the basin. We report the results of a combined examination of detrital zircon and rutile U-Pb geochronology (572 concordant ages), coupled with Th/U ratios, Hf isotopic data and quantitative assessment of grain morphology of detrital zircon from four formations (5 samples) from the Kythrea flysch. These data provide a line of independent evidence for the existence of two different sediment transportation systems that discharged detritus into the basin between the late Eocene and late Miocene. Unique characteristics of each transport system are defined and a sediment unmixing calculation is demonstrated and explained. The first system transported almost exclusively North Gondwana-type, Precambrian-aged detrital zircon sourced from siliciclastic rock units in southern Anatolia. A different drainage system is revealed by the middle to late Miocene flysch sequence that is dominated by Late Cretaceous-Cenozoic-aged detrital zircon, whose age range is consistent with the magmatic episodicity of southeast Anatolia, along the Arabia-Eurasia suture zone. Deposition of these late Miocene strata took place thereupon closure of the Tethyan Seaway and African-Eurasian faunal exchange, and overlap in time with a pronounced uplift of eastern Anatolia. Our analytical data indicate the onset of prominent suture-parallel sediment transport from the collision zone of south-eastern Anatolia into the Kyrenia Range of northern Cyprus, marking the drainage response to the continental collision between Arabia and Eurasia

Detrital zircon Hf isotopic composition indicates long-distance transport of North Gondwana Cambrian-Ordovician sandstones

A voluminous Cambrian-Ordovician sequence of quartz-rich sandstones was deposited in northern Gondwana following its assembly by a series of Neoproterozoic-Cambrian orogenic events. Paleocurrent markers indicate that the sediments were carried from Gondwana hinterland toward the supercontinent margins in the north (present coordinates). Derivation from Neoproterozoic terranes is evident from the ubiquity of detrital zircons with Neoproterozoic U-Pb ages, but the exact provenance of these siliciclastic deposits remains unclear. Herein we present new Hf isotopic data from U-Pb dated detrital zircons of the Cambrian-Ordovician sandstone that tops the juvenile Neoproterozoic basement of the Arabian-Nubian Shield in Israel and Jordan. It is remarkable that the detrital zircon Hf isotopic signal is in marked contrast to the Nd and Hf isotopic signature of the underlying basement. A preponderance (61%) of the Neoproterozoic-aged detrital zircons from the Cambrian-Ordovician sandstones yielded negative εHf(t) values incompatible with a juvenile source. Therefore, most of the detrital zircons were derived from distant terranes comprising pre-Neoproterozoic crust reworked during the assembly of Gondwana, rather than from the adjacent Arabian-Nubian Shield. Because our sampling sites are situated at the northern tip of the Arabian-Nubian Shield, sand must have been transported several thousand kilometers before deposition. This finding also implies that the Arabian-Nubian Shield and other Neoproterozoic orogens of northeast Africa were completely worn down by the onset of Cambrian deposition and that vast areas in the northern part of Gondwana were low lying at that time.4 page(s

Cadomian (ca. 550 Ma) magmatic and thermal imprint on the North Arabian-Nubian Shield (south and central Israel): New age and isotopic constraints

The Arabian-Nubian Shield (ANS) is a province of mostly juvenile Neoproterozoic continental crust in NE Africa and Arabia, forming the northern extension of the East-African orogen. Subsequent to widespread, mostly late- to post-orogenic early-Ediacaran calc-alkaline granitoid emplacement, the latest magmatic phase in the ANS saw the intrusion of alkaline shallow plutons alongside volcanic massifs and dikes of variable compositions. While some of these intrusions were dated to 550–530 Ma by Rb-Sr and Ar-Ar, U-Pb zircon data has mostly limited ANS magmatism to no younger than ca. 580 Ma. Here we report new age and isotopic data from latest Neoproterozoic intrusions along a N-S transect within the northern ANS and its periphery, substantiating the record of a ca. 550 Ma igneous-thermal event in the area. In the northern Negev of Israel, mafic igneous rocks intruded into the subsurface late Neoproterozoic clastic wedge of the Zenifim Formation produced apatite U-Pb isochron ages between 557 and 545 Ma. In the Amram alkaline massif, 150 km to the south, an alkali-granite was dated to 605 ± 2.5 Ma by zircon U-Pb, but apatite U-Pb ages from all rocks of the massif concentrate at ca. 550 Ma indicating thermal resetting. Apatites from the zircon dated 611 ± 5 Ma Roded quartz diorite in the Elat area, south of Amram, produced a 600 ± 4 Ma U-Pb age, demonstrating that southern segments were not thermally reset at ca. 550 Ma. Nd isotopes in apatite and whole rock show the ca. 550 Ma igneous rocks to be variable in their isotopic composition, ranging between εNd(t)=(-4) – (+2), far less juvenile than typical ANS magmas. The ca. 550 Ma intrusions resemble in their age and isotopic properties Cadomian granites and volcanic rocks that are known from southern Turkey, Iran and Europe. We interpret the ca. 550 Ma igneous overprint and thermal resetting documented in the northernmost ANS as related to the superposition of Cadomian orogeny on the ANS periphery. This can be related to the late Ediacaran subduction of proto-Tethys accompanying the accretion of the Cadomian basement of the Tauride block to NE Africa and Arabia, before the area was overstepped by Cambrian platform sedimentation

Precise determination of dolomite content in marine sediments

Dolomite (CaMg[CO3](2)) is a common rock-forming mineral. Nevertheless, its mechanisms of formation and the factors that cause dolomite concentration variations within the sedimentary records constitute long-standing geochemical questions. In addition, the flux of Mg2+ leaving the ocean by the formation of dolomite is a controversial question, with some studies arguing that dolomite formation is a negligible Mg2+ sink in the modern ocean, while others show that it constitutes more than 50% of the total Mg2+ removal rate. An important factor that impedes the resolution of the dolomite Mg2+ flux is the lack of analytical methods with adequate precision and detection limit to directly measure minute quantities of authigenic dolomite in marine sediments. Here, we present a new analytical method for direct, precise measurement of dolomite content in marine sediments. The method is based on sequential leaching of carbonate minerals in acid and tracks the CO2 emitted by the dissolution. Based on the measurement of gravimetric standards of calcite and dolomite, the method's detection limit and precision were determined as better than 0.2 and +/- 0.2 dry wt% of dolomite, respectively. The method out-performed dolomite quantification made by x-ray diffraction and by inductive coupled plasma mass-spectrometry, which provided precision of +/- 2 and +/- 1 dry wt%, respectively. Measurements of the dolomite content in modern sediments from the seafloor below the oligotrophic Eastern Mediterranean and the eutrophic Mississippi plume, and in clayey-silty alluvial soil from south-eastern Israel, demonstrated that the aforementioned precisions are also valid for natural samples.ISSN:1541-585

Microstructure-specific carbon isotopic signatures of organic matter from ∼3.5 Ga cherts of the Pilbara Craton support a biologic origin.

21 pagesInternational audienceThe ∼3.5 Ga Dresser Formation from the North Pole Dome of the Pilbara Craton (Western Australia) contains some of the oldest evidence for life on Earth. Here, we present a detailed study of microstructure-specific carbon isotopic composition of organic matter (OM) preserved in Dresser Formation bedded cherts and hydrothermal chert vein using in situ Secondary-Ion Mass Spectrometry (SIMS). The OM in these rocks occurs mainly as clots that, together with minor fine OM layers and laminae, are considered primary textures formed prior to host rock lithification. Other than rare OM-rich stylolites, no evidence was found for later OM migration beyond the micrometer scale. Average δ13C(OM) values in specific microstructural types range between −33.6‰ and −25.7‰. No correlation is seen between measured δ13C values and H/C ratios in the studied OM microstructures. This lack of correlation and the low metamorphic grade of the rocks studied argue against significant modification of OM isotopic composition by later metamorphic alteration. It is thus concluded that the range of δ13C values found in the samples represents primary OM isotopic variability. Within some individual samples variable δ13C(OM) values are correlated with specific microstructural types. This observation is not consistent with solely abiotic OM formation via Fisher-Tropsch type reactions. When compared with associated δ13C(ankerite) values, average δ13C(OM) values indicate C isotopic fractionation [Δ13C(Ank–OM)] of 25–33‰, which translates to dissolved CO2–OM isotopic fractionation [Δ13C(CO2–OM)] of 20–30‰. This range of Δ13C(CO2–OM) is consistent with enzymatic C fixation via the Calvin cycle utilized by photoautotrophs and the reductive acetyl-CoA pathway utilized by chemolithoautotrophs. Photosynthetic OM formation is supported by the relatively shallow water depth inferred for the Dresser environment and the restricted occurrence of stromatolites to shallow water deposits in this unit, whereas chemolithosynthesis is supported by the abundance of OM in sub-seafloor hydrothermal chert veins. The range of δ13C(OM) values observed in the samples may therefore represent the remains of different organisms utilizing different C-fixation pathways. Other biologic effects, such as the growth rate and density of microbial communities, and further heterotrophic overprinting of the autotrophic biomass may have also contributed to the observed range of δ13C(OM) values