Olivine-rich veins in high-pressure serpentinites: A far-field paleo-stress snapshot during subduction

Field observations within the Atg-serpentinite domain of the subducted ultramafic massif from Cerro del Almirez (SE Spain) reveal the existence of two generations of abundant olivine-rich veins formed as open, mixed mode and shear fractures during prograde metamorphism. Type I veins were synchronous with the development of the serpentinite main foliation (S1) and shearing, whereas Type II veins post-date the S1 surfaces. These structural relationships indicate that, while the Atgserpentinites underwent ductile plastic deformation at temperatures of 450◦-600 ◦C and pressures of 0.7–1.7 GPa, they also experienced punctuated brittle behaviour events. The brittle fractures were most likely due to fluid overpressures formed by release of H2O during the brucite breakdown reaction for the case of Type I veins (2 vol % H2O) and due to a combination of minor dehydration reactions related to continuous compositional and structural changes in antigorite (0.3 vol % H2O) for Type II veins. Type II olivine-rich veins were formed by brittle failure in a well-defined paleo-stress field and were not significantly deformed after their formation. Comparison of the principal paleo-stress orientation inferred from Type II veins with those formed at peak metamorphic conditions in the ultramafic rocks at Cerro del Almirez shows a relative switch in the orientation of the maximum and minimum principal paleo-stress axes. These relative changes can be attributed to the cyclic evolution of shear stress, fluid pressure and fault-fracture permeability allowing for stress reversal.MICIN/AEI PID2019-105192GB-I00Junta de Andalucia RNM-208 RNM-141 RNM-145 RNM-131 RNM-374FEDER program "una manera de hacer Europa"Spanish Government RYC2018-024363-IUniversidad de Granada/ CBU

Mantle wedge oxidation due to sediment-infiltrated deserpentinisation

This work is part of the project DESTINE (PID2019-105192GB-I00) funded by MICIN/AEI/10.13039/501100011033 and the FEDER program “una manera de hacer Europa”. J.A.P.N. acknowledges a Ramón y Cajal contract (RYC2018-024363-I) funded by MICIN/AEI/10.13039/501100011033 and the FSE program “FSE invierte en tu futuro”. This research is part of the Junta de Andalucia research group RNM-131.The Earth's mantle is oxygen-breathing through the sink of oxidised tectonic plates at convergent Margins. Ocean floor serpentinisation increases the bulk oxidation state of iron relative to dry oceanic mantle and results in a variable intake of other redox-sensitive elements such as sulphur. The reversibility of seafloor oxidation in subduction zones during high-pressure dehydration of serpentinite (“deserpentinisation”) at subarc depths and the capacity of the resulting fluids to oxidise the mantle source of arc basalts are highly contested. Thermodynamic modelling, experiments, and metaperidotite study in exhumed highpressure terrains result in differing estimates of the redox state of deserpentinisation fluids, ranging from low to highly oxidant. Here we show that although intrinsic deserpentinisation fluids are highly oxidant, the infiltration of small fractions of external fluids equilibrated with metasedimentary rocks strongly modulates their redox state and oxidation-reduction capacity explaining the observed discrepancies in their redox state. Infiltration of fluids equilibrated with graphite-bearing sediments reduces the oxidant, intrinsic deserpentinisation fluids to oxygen fugacities similar to those observed in most graphite-furnace experiments and natural metaperidotites. However, infiltration of CO2-bearing fluids equilibrated with modern GLOSS generates sulphate-rich, highly oxidising deserpentinisation fluids. We show that such GLOSS infiltrated deserpentinisation fluids can effectively oxidise the mantle wedge of cold to hot subduction zones potentially accounting for the presumed oxidised nature of the source of arc basalts.MICIN/AEI/10.13039/501100011033 (PID2019-105192GB-I00) (RYC2018-024363-I)FEDERFSEJunta de Andalucia research group RNM-131

Rasgos de la geología de Sierra Nevada

Este documento forma parte de: "Martín, J.M., Braga, J.C. y Gómez-Pugnaire, M.T. (2011). Itinerarios geológicos por Sierra Nevada. Guía de campo por el Parque Nacional y el Parque Natural de Sierra Nevada. Consejería de Medio Ambiente, Junta de Andalucía. ISBN: 9788496776524" [http://www.juntadeandalucia.es/medioambiente/portal_web/servicios_generales/doc_tecnicos/2011/itinerarios_geologicos_snevada/itinerarios_geologicos_sierra_nevada.pdf]Morfológicamente en Sierra Nevada se distinguen tres grandes zonas: la “Alta Montaña” (el núcleo metamórfico, de edad Paleozoica o más antigua), la “Baja Montaña” (la orla carbonatada, de edad Triásica) y las “Colinas Periféricas” (los sedimentos detríticos marginales, del Neógeno-Cuaternario). El origen de esta zonación está íntimamente ligado al de la historia de levantamiento reciente de la Sierra. Cada una de estas zonas tiene sus peculiaridades geológicas distintivas en lo que se refiere a estructura, litología, mineralizaciones y paisaje. En la “Alta Montaña” se distinguen dos grandes unidades tectónicas superpuestas: la Unidad Veleta, la inferior, y la Unidad Mulhacén, la superior, que muestran litologías, estructura y grado de metamorfismo diferente. Las mineralizaciones de la Unidad Veleta son de origen filoniano y de ellas se ha beneficiado el cobre. Las de la Unidad Mulhacén son de tipo masivo y se ha extraído el hierro. El paisaje que domina en la “Alta Montaña”, de origen glacial, no está en equilibrio con las condiciones climáticas actuales. En la “Baja Montaña” afloran esencialmente carbonatos triásicos con restos fósiles que indican su origen marino. La estructura tectónica es compleja, diferenciándose una serie de mantos y escamas. Las mineralizaciones son de fluorita, plomo y zinc. Como rasgos geomorfológicos más destacados están los cañones fluviales y los “ríos de grava”. Los sedimentos de las “Colinas Periféricas” son mayoritariamente conglomerados, depositados al pie de la montaña conforme esta fue levantando en los últimos 10 Ma. Inicialmente son marinos y luego continentales. En los del Plioceno inferior (Formación Alhambra) se encuentran finas partículas de oro. Este oro fue explotado en mina a cielo abierto en la Época Romana y ha sido también objeto de bateo en los ríos que atraviesan dichos conglomerados. Los paisajes más espectaculares en estos sedimentos periféricos son los “badlands” de la zona oriental del macizo montañoso.Three major zones are distinguished in Sierra Nevada: the “Alta Montaña” (the Paleozoic and/or older metamorphic core), the “Baja Montaña” (the Triassic carbonate fringe) and the “Colinas Periféricas” (the Neogene to Recent, marginal detrital sediments). The genesis of this zonation is closely linked to that of the mountain uplifting. Each of these zones has its own, distinct geological features concerning structure, lithology, ore deposits and landscape. Two major superimposed tectonic units make up the core: the Veleta Unit (the lower one) and the Mulhacén Unit (the upper one), both exhibiting different lithology, structure, and type, degree and age of metamorphism. Ore deposits in the Veleta Unit are copper-rich vein infillings, while in the Mulhacén Unit are massive iron-ore deposits. A fossil, glacial, Quaternary landscape is still preserved in the “Alta Montaña”. In the Triassic carbonates from the “Baja Montaña” some marine fossils can be found. The tectonic structure consists of a series of over-thrusted, superimposed nappes and tectonic slices. Fluorite, lead and zinc ore deposits are present. River canyons and “gravel rivers” are the most outstanding geomorphological features. Sediments from the “Colinas Periféricas” are mostly conglomerates that were deposited at the foot of the mountain as it was being uplifted during the last 10 Ma. They were marine first and then continental in origin. Those from the Lower Pliocene (the “Alhambra conglomerate”) contain some fine-grained, alluvial gold particles. This gold was mined by the Romans and has also been panned in rivers cutting across the conglomerate. Some spectacular badlands have been recently sculptured in the easternmost conglomerates

Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite-serpentinite dehydration (Nevado‐Filábride Complex, Spain)

I. Martínez Segura and M. J. Román Alpiste are thanked for their kind assistance during sample preparation and SEM operation, and M. T. Gómez‐Pugnaire and A. Jabaloy for early work on Almirez ophicarbonates. We are grateful to the Sierra Nevada National Park for providing permits for fieldwork and sampling at the Almirez massif. We further acknowledge the editorial handling by D. Whitney and D. Robinson and the reviews of M. Galvez and T. Pettke, whose comments and constructive criticism helped to improve the manuscript. We acknowledge funding from the European Union FP7 Marie‐Curie Initial Training Network ABYSS under REA Grant Agreement no. 608001 in the framework of M.D.M.'s PhD project, the Spanish ‘Agencia Estatal de Investigación’ (AEI) grants no. CGL2016‐75224‐R to V.L.S.‐V and CGL2016‐81085‐R to C.J.G and C.M and grant no. PCIN‐2015‐053 to C.J.G. The ‘Junta de Andalucía’ is also thanked for funding under grants no. RNM‐131, RNM‐374 and P12‐RNM‐3141. C.M. thanks MINECO for financing a Ramón y Cajal fellowship no. RYC‐2012‐11314 and K.H. for a Juan de la Cierva Fellowship no. FPDI‐2013‐16253 and a research contract under grant no. CGL2016‐81085‐R. This work and the research infrastructure at the IACT have received (co)funding from the European Social Fund and the European Regional Development Fund.At sub‐arc depths, the release of carbon from subducting slab lithologies is mostly controlled by fluid released by devolatilization reactions such as dehydration of antigorite (Atg‐) serpentinite to prograde peridotite. Here we investigate carbonate–silicate rocks hosted in Atg‐serpentinite and prograde chlorite (Chl‐) harzburgite in the Milagrosa and Almirez ultramafic massifs of the palaeo‐subducted Nevado‐Filábride Complex (NFC, Betic Cordillera, S. Spain). These massifs provide a unique opportunity to study the stability of carbonate during subduction metamorphism at P–T conditions before and after the dehydration of Atg‐serpentinite in a warm subduction setting. In the Milagrosa massif, carbonate–silicate rocks occur as lenses of Ti‐clinohumite–diopside–calcite marbles, diopside–dolomite marbles and antigorite–diopside–dolomite rocks hosted in clinopyroxene‐bearing Atg‐serpentinite. In Almirez, carbonate–silicate rocks are hosted in Chl‐harzburgite and show a high‐grade assemblage composed of olivine, Ti‐clinohumite, diopside, chlorite, dolomite, calcite, Cr‐ bearing magnetite, pentlandite and rare aragonite inclusions. These NFC carbonate–silicate rocks have variable CaO and CO2 contents at nearly constant Mg/ Si ratio and high Ni and Cr contents, indicating that their protoliths were variable mixtures of serpentine and Ca‐carbonate (i.e., ophicarbonates). Thermodynamic modelling shows that the carbonate–silicate rocks attained peak metamorphic conditions similar to those of their host serpentinite (Milagrosa massif; 550–600°C and 1.0–1.4 GPa) and Chl‐harzburgite (Almirez massif; 1.7–1.9 GPa and 680°C). Microstructures, mineral chemistry and phase relations indicate that the hybrid carbonate–silicate bulk rock compositions formed before prograde metamorphism, likely during seawater hydrothermal alteration, and subsequently underwent subduction metamorphism. In the CaO–MgO–SiO2 ternary, these processes resulted in a compositional variability of NFC serpentinite‐hosted carbonate–silicate rocks along the serpentine‐calcite mixing trend, similar to that observed in serpentinite‐hosted carbonate‐rocks in other palaeo‐subducted metamorphic terranes. Thermodynamic modelling using classical models of binary H2O–CO2 fluids shows that the compositional variability along this binary determines the temperature of the main devolatilization reactions, the fluid composition and the mineral assemblages of reaction products during prograde subduction metamorphism. Thermodynamic modelling considering electrolytic fluids reveals that H2O and molecular CO2 are the main fluid species and charged carbon‐bearing species occur only in minor amounts in equilibrium with carbonate–silicate rocks in warm subduction settings. Consequently, accounting for electrolytic fluids at these conditions slightly increases the solubility of carbon in the fluids compared with predictions by classical binary H2O–CO2 fluids, but does not affect the topology of phase relations in serpentinite‐hosted carbonate‐ rocks. Phase relations, mineral composition and assemblages of Milagrosa and Almirez (meta)‐serpentinite‐hosted carbonate–silicate rocks are consistent with local equilibrium between an infiltrating fluid and the bulk rock composition and indicate a limited role of infiltration‐driven decarbonation. Our study shows natural evidence for the preservation of carbonates in serpentinite‐hosted carbonate–silicate rocks beyond the Atg‐serpentinite breakdown at sub‐arc depths, demonstrating that carbon can be recycled into the deep mantle.Funding from the European Union FP7 Marie‐Curie Initial Training Network ABYSS under REA Grant Agreement no. 608001Spanish ‘Agencia Estatal de Investigación’ (AEI) grants no. CGL2016‐75224‐R to V.L.S.‐V and CGL2016‐81085‐R to C.J.G and C.M and grant no. PCIN‐2015‐053 to C.J.GJunta de Andalucía Funding under grants no. RNM‐131, RNM‐374 and P12‐RNM‐3141MINECO for financing a Ramón y Cajal fellowship no. RYC‐2012‐11314 and K.H. for a Juan de la Cierva Fellowship no. FPDI‐2013‐16253 and a research contract under grant no. CGL2016‐81085‐

High-P metamorphism of rodingites during serpentinite dehydration (Cerro del Almirez, Southern Spain): Implications for the redox state in subduction zones

The transition between antigorite-serpentinite and chlorite-harzburgite at Cerro del Almirez (Betic Cordillera, Southern Spain) exceptionally marks in the field the front of antigorite breakdown at high pressure (~16–19 kbar) and temperature (~650°C) in a paleosubducted serpentinite. These ultramafic lithologies enclose three types of metarodingite boudins of variable size surrounded by metasomatic reaction rims. Type 1 Grandite-metarodingite (garnet+chlorite+diopside+titanite±magnetite±ilmenite) mainly crops out in the antigorite-serpentinite domain and has three generations of garnet. Grossular-rich Grt-1 formed during rodingitization at the seafloor (10 kbar, ~350–650°C, ~FMQ buffer) to influx events of oxidizing fluids (fO ~HM buffer) released by brucite breakdown in the host antigorite-serpentinite. Type 2 Epidote-metarodingite (epidote+diopside+titanite±garnet) derives from Type 1 and is the most abundant metarodingite type enclosed in dehydrated chlorite-harzburgite. Type 2 formed by increasing μSiO (from −884 to −860 kJ/mol) and decreasing μCaO (from −708 to −725 kJ/mol) triggered by the flux of high amounts of oxidizing fluids during the high-P antigorite breakdown in serpentinite. The growth of Grt-4, with low-grandite and high-pyralspite components, in Type 2 metarodingite accounts for progressive reequilibration of garnet with changing intensive variables. Type 3 Pyralspite-metarodingite (garnet+epidote+amphibole+chlorite±diopside+rutile) crops out in the chlorite-harzburgite domain and formed at peak metamorphic conditions (16–19 kbar, 660–684°C) from Type 2 metarodingite. This transformation caused the growth of a last generation of pyralspite-rich garnet (Grt-5) and the recrystallization of diopside into tremolitic amphibole at decreasing fO and μCaO (from −726 to −735 kJ/mol) and increasing μMgO (from −630 to −626 kJ/mol) due to chemical mixing between the metarodingite and the reaction rims. The different bulk Fe/Fe ratios of antigorite-serpentinite and chlorite-harzburgite, and of the three metarodingite types, reflect the highly heterogeneous oxidation state of the subducting slab and likely point to the transfer of localized oxidized reservoirs, such as metarodingites, into the deep mantle.“Ministerio de Economía, Industria y Competitividad” (MINECO), Grant/Award Number: CGL2012-32067, CGL201675224-R; Junta de Andalucía, Grant/ Award Number: RNM-145, P12-RNM3141; Ramón y Cajal, Grant/Award Number: RYC-2012-11314; MINECO, Grant/Award Number: CGL2016-81085-R, PCIN-2015-05

Where do seedlings go? A spatio-temporal analysis of seedling mortality in a semi-arid gypsophyte

Studies of seedling population dynamics often focus on survival because it provides an integrated measure of seedling performance. However, this approach involves a substantial loss of information because survival is the net result of a wide range of mechanisms. The present study overcomes these shortcomings by investigating spatial and temporal patterns in the causes of plant mortality in a population of Helianthemum squamatum seedlings. We use new point pattern analyses based on K functions combined with a new null model (‘‘independent labeling’’). A total of 871 seedlings of H. squamatum were mapped and regularly monitored over an 18-month period. More than 60% of seedlings died during this period. Causes of mortality were spatially structured, and these structures shifted through time. Small differences in either the time of emergence or the environment surrounding H. squamatum seedlings had profound influences on their fate. Seedlings emerging late in the season under the canopy of adult plants died from drought more often than expected, whereas those emerging earlier in the same microsite survived more than expected. The identity of neighbors also affected the spatio-temporal dynamics of mortality causes. Our results show that seedling-adult interactions cannot be easily predicted from simple models, and that the time of seedling emergence, its age and the identity of its neighbors determine the sign and the spatial scale of these interactions. The new methods introduced in this article open an avenue for the detailed analyses of the spatio-temporal dynamics of plant mortality and can help to disentangle the complexity of biotic interactions along environmental severity gradients

11B-rich fluids in subduction zones: the role of antigorite dehydration in subducting slabs and boron isotope heterogeneity in the mantle

Serpentinites form by hydration of mantle peridotite and constitute the largest potential reservoir of fluid-mobile elements entering subduction zones. Isotope ratios of one such element, boron, distinguish fluid contributions from crustal versus serpentinite sources. Despite 85% of boron hosted within abyssal peridotite being lost at the onset of subduction at the lizardite-to-antigorite transition, a sufficient cargo of boron to account for the composition of island arc magma is retained (c. 7 μg g− 1, with a δ11B of + 22‰) until the down-going slab reaches the antigorite-out isograd. At this point a 11B-rich fluid, capable of providing the distinctive δ11B signature of island arc basalts, is released. Beyond the uniquely preserved antigorite-out isograd in serpentinites from Cerro del Almirez, Betic Cordillera, Spain, the prograde lithologies (antigorite–chlorite–orthopyroxene–olivine serpentinite, granofels-texture chlorite-harzburgite and spinifex-texture chlorite-harzburgite) have very different boron isotope signatures (δ11B = − 3 to + 6‰), but with no significant difference in boron concentration compared to the antigorite-serpentinite on the low P–T side of the isograd. 11B-rich fluid, which at least partly equilibrated with pelagic sediments, is implicated in the composition of these prograde lithologies, which dehydrated under open-system conditions. Serpentinite-hosted boron lost during the early stages of dehydration is readily incorporated into forearc peridotite. This, in turn, may be dragged to sub-arc depths as a result of subduction erosion and incorporated in a mélange comprising forearc serpentinite, altered oceanic crust and pelagic sediment. At the antigorite-out isograd it dehydrates, thus potentially providing an additional source of 11B-rich fluids

Late orogenic doming in the Eastern Betics : final exhumation of the Nevado-Filabride complex and its relation to basin genesis.

The geometry, timing, and kinematics of late orogenic extension in the Betic Cordilleras pose the problem of a decoupling of upper crustal and lower crustal deformation regimes. Perpendicular directions of extension in metamorphic domes and nearby sedimentary basins remain unexplained. This paper puts kinematic constraints on the final exhumation of the Nevado-Filabride complex, focusing on the formation of metamorphic domes and their relations with the adjacent basins. Structural fabrics and kinematic indicators below the main shear zones as well as their relations with both published changing metamorphic P-T conditions and geochronological data were studied. Our approach describes (1) a consistent top-to-the-west shear parallel to dome axes of during D2 (i.e., during decompression) with distributed ductile flow and the onset of strain localization along major shear zones, (2) further strain localization along the major shear zones under greenschist facies conditions, during D3 leading to S-C′ mylonites formation accompanied with a rock strong thickness reduction, (3) the divergence of shear direction on either limbs of domes during D3 showing the appearance of the dome geometry, and (4) a local evolution toward N-S brittle extension (D4) in the upper plate and formation of sedimentary basins. Continuous ductile to brittle top-to-the-west shear is compatible with the slab retreat hypothesis from the Miocene; the formation of domes which adds gravitational forces responsible for the final stages of exhumation is thus characterized by important kinematics changes necessary to explain coeval N-S opened basins. Later, from the upper Tortonian, a contractional event (D5) amplified the earlier domal structures forming the present north vergent folds