74 research outputs found
Control of serpentinisation rate by reaction-induced cracking
Serpentinisation of mantle rocks requires the generation and maintenance of transport pathways for water. The solid volume increase during serpentinisation can lead to stress build-up and trigger cracking, which ease fluid penetration into the rock. The quantitative effect of this reaction-induced cracking mechanism on reactive surface generation is poorly constrained, thus hampering our ability to predict serpentinisation rate in geological environments. Here we use a combined approach with numerical modelling and observations in natural samples to provide estimates of serpentinisation rate at mid-ocean ridges. We develop a micromechanical model to quantify the propagation of serpentinisation-induced cracks in olivine. The maximum crystallisation pressure deduced from thermodynamic calculations reaches several hundreds of megapascals but does not necessary lead to crack propagation if the olivine grain is subjected to high compressive stresses. The micromechanical model is then coupled to a simple geometrical model to predict reactive surface area formation during grain splitting, and thus bulk reaction rate. Our model reproduces quantitatively experimental kinetic data and the typical mesh texture formed during serpentinisation. We also compare the model results with olivine grain size distribution data obtained on natural serpentinised peridotites from the Marum ophiolite and the Papuan ultramafic belt (Papua New Guinea). The natural serpentinised peridotites show an increase of the number of olivine grains for a decrease of the mean grain size by one order of magnitude as reaction progresses from 5 to 40%. These results are in agreement with our model predictions, suggesting that reaction-induced cracking controls the serpentinisation rate. We use our model to estimate that, at mid-ocean ridges, serpentinisation occurs up to 12km depth and reaction-induced cracking reduces the characteristic time of serpentinisation by one order of magnitude, down to values comprised between 10 and 1000yr. The increase of effective pressure with depth also prevents cracking, which positions the peak in serpentinisation rate at shallower depths, 4km above previous predictions
Subduction metamorphism of serpentiniteâhosted carbonates beyond antigorite-serpentinite dehydration (NevadoâFilĂĄbride Complex, Spain)
I. MartĂnez Segura and M. J. RomĂĄn Alpiste are thanked for their kind assistance during sample preparation and SEM operation, and M. T. GĂłmezâPugnaire 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 MarieâCurie Initial Training Network ABYSS under REA Grant Agreement no. 608001 in the framework of M.D.M.'s PhD project, the Spanish âAgencia Estatal de InvestigaciĂłnâ (AEI) grants no. CGL2016â75224âR to V.L.S.âV and CGL2016â81085âR to C.J.G and C.M and grant no. PCINâ2015â053 to C.J.G. The âJunta de AndalucĂaâ is also thanked for funding under grants no. RNMâ131, RNMâ374 and P12âRNMâ3141. C.M. thanks MINECO for financing a RamĂłn y Cajal fellowship no. RYCâ2012â11314 and K.H. for a Juan de la Cierva Fellowship no. FPDIâ2013â16253 and a research contract under grant no. CGL2016â81085âR. 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 subâarc depths, the release of carbon from subducting slab lithologies is mostly
controlled by fluid released by devolatilization reactions such as dehydration of antigorite (Atgâ) serpentinite to prograde peridotite. Here we investigate carbonateâsilicate rocks hosted in Atgâserpentinite and prograde chlorite (Chlâ) harzburgite in the
Milagrosa and Almirez ultramafic massifs of the palaeoâsubducted NevadoâFilĂĄbride
Complex (NFC, Betic Cordillera, S. Spain). These massifs provide a unique opportunity to study the stability of carbonate during subduction metamorphism at PâT
conditions before and after the dehydration of Atgâserpentinite in a warm subduction
setting. In the Milagrosa massif, carbonateâsilicate rocks occur as lenses of Tiâclinohumiteâdiopsideâcalcite marbles, diopsideâdolomite marbles and antigoriteâdiopsideâdolomite rocks hosted in clinopyroxeneâbearing Atgâserpentinite. In Almirez,
carbonateâsilicate rocks are hosted in Chlâharzburgite and show a highâgrade assemblage composed of olivine, Tiâclinohumite, diopside, chlorite, dolomite, calcite, Crâ
bearing magnetite, pentlandite and rare aragonite inclusions. These NFC
carbonateâsilicate 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 Caâcarbonate (i.e., ophicarbonates). Thermodynamic
modelling shows that the carbonateâsilicate rocks attained peak metamorphic conditions similar to those of their host serpentinite (Milagrosa massif; 550â600°C and
1.0â1.4 GPa) and Chlâharzburgite (Almirez massif; 1.7â1.9 GPa and 680°C).
Microstructures, mineral chemistry and phase relations indicate that the hybrid carbonateâsilicate bulk rock compositions formed before prograde metamorphism,
likely during seawater hydrothermal alteration, and subsequently underwent subduction metamorphism. In the CaOâMgOâSiO2 ternary, these processes resulted in a
compositional variability of NFC serpentiniteâhosted carbonateâsilicate rocks along
the serpentineâcalcite mixing trend, similar to that observed in serpentiniteâhosted
carbonateârocks in other palaeoâsubducted metamorphic terranes. Thermodynamic modelling using classical models of binary H2OâCO2 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 carbonâbearing species occur only in minor amounts in equilibrium with carbonateâsilicate 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 H2OâCO2 fluids,
but does not affect the topology of phase relations in serpentiniteâhosted carbonateâ
rocks. Phase relations, mineral composition and assemblages of Milagrosa and
Almirez (meta)âserpentiniteâhosted carbonateâsilicate rocks are consistent with local
equilibrium between an infiltrating fluid and the bulk rock composition and indicate
a limited role of infiltrationâdriven decarbonation. Our study shows natural evidence
for the preservation of carbonates in serpentiniteâhosted carbonateâsilicate rocks beyond the Atgâserpentinite breakdown at subâarc depths, demonstrating that carbon
can be recycled into the deep mantle.Funding from the European Union FP7 MarieâCurie Initial Training Network ABYSS under REA Grant Agreement no. 608001Spanish âAgencia Estatal de InvestigaciĂłnâ (AEI) grants no. CGL2016â75224âR to V.L.S.âV and CGL2016â81085âR to C.J.G and C.M and grant no. PCINâ2015â053 to C.J.GJunta de AndalucĂa Funding under grants no. RNMâ131, RNMâ374 and P12âRNMâ3141MINECO for financing a RamĂłn y Cajal fellowship no. RYCâ2012â11314 and K.H. for a Juan de la Cierva Fellowship no. FPDIâ2013â16253 and a research contract under grant no. CGL2016â81085â
Coupled phengite 40Arâ39Ar geochronology and thermobarometry: P-T-t evolution of Andros Island (Cyclades, Greece)
Andros is a key island for understanding both the timing of high-pressureâlow-temperature (HP-LT) metamorphism and the dynamics of crustal-scale detachment systems exhuming high-grade units in the Cyclades (Greece). Using phengite 40Arâ39Ar geochronology coupled with thermobarometry, as well as data from literature, we constrain the pressureâtemperatureâtime (P-T-t) paths of the Makrotantalon and AtticâCycladic Blueschist units on Andros. Peak conditions of the HP-LT episode in the Makrotantalon unit are 550°C and 18.5 kbar, dated at 116 Ma. We correlate this episode with Early Cretaceous blueschist facies metamorphism recognized in the Pelagonian zone of continental Greece. This is a new argument favouring a Pelagonian origin for the Makrotantalon unit. In the AtticâCycladic Blueschist unit, the P-T-t path is characterized by: (1) exhumation after peak conditions in HP-LT conditions between 55 and 35 Ma; (2) isobaric heating at 7 kbar until 30 Ma; and (3) isothermal decompression until 21 Ma. This thermal evolution and timing are similar to those of the neighbouring Tinos Island, emphasizing major thermal re-equilibration at the transition between stable and retreating subduction. Modifications of the crustal thermal state played a major role in the evolution of the North Cycladic Detachment System, below which Andros HP-LT units were exhumed
The iron record of asteroidal processes in carbonaceous chondrites
International audienceThe valence of iron has been used in terrestrial studies to trace the hydrolysis of primary silicate rocks. Here, we use a similar approach to characterize the secondary processes, namely thermal metamorphism and aqueous alteration, that have affected carbonaceous chondrites. X-ray absorption near-edge structure spectroscopy at the Fe-K-edge was performed on a series of 36 CM, 9 CR, 10 CV, and 2 CI chondrites. While previous studies have focused on the relative distribution of Fe0 with respect to oxidized iron (Feox = Fe2+ + Fe3+) or the iron distribution in some specific phases (e.g., Urey-Craig diagram; Urey and Craig 1953), our measurements enable us to assess the fractions of iron in each of its three oxidation states: Fe0, Fe2+, and Fe3+. Among the four carbonaceous chondrites groups studied, a correlation between the iron oxidation index (IOI = [2(Fe2+) + 3(Fe3+)]/[FeTOT]) and the hydrogen content is observed. However, within the CM group, for which a progressive alteration sequence has been defined, a conversion of Fe3+ to Fe2+ is observed with increasing degree of aqueous alteration. This reduction of iron can be explained by an evolution in the mineralogy of the secondary phases. In the case of the few CM chondrites that experienced some thermal metamorphism, in addition to aqueous alteration, a redox memory of the aqueous alteration is present: a significant fraction of Fe3+ is present, together with Fe2+ and sometimes Fe0. From our data set, the CR chondrites show a wider range of IOI from 1.5 to 2.5. In all considered CR chondrites, the three oxidation states of iron coexist. Even in the least-altered CR chondrites, the fraction of Fe3+ can be high (30% for MET 00426). This observation confirms that oxidized iron has been integrated during formation of fine-grained amorphous material in the matrix (Le Guillou and Brearley 2014; Le Guillou et al. 2015; Hopp and Vollmer 2018). Last, the IOI of CV chondrites does not reflect the reduced/oxidized classification based on metal and magnetite proportions, but is strongly correlated with petrographic types. The valence of iron in CV chondrites therefore appears to be most closely related to thermal history, rather than aqueous alteration, even if these processes can occur together (Krot et al. 2004; Brearley and Krot 2013)
Supplementary material for "Measurement of volume change and mass transfer during serpentinisation: insights from the Oman Drilling Project" (JGR Solid Earth)
The three supplementary files include one figure displaying the results of X-ray microtomography (File S1), two compilations of figures dealing with the volume change measurement (Files S2 and S3) and a table with the measured compositions in major and trace elements (Table S1).
File S1 (Malvoisin_ds01.pdf). Result of X-ray microtomography. Olivine and Fe-brucite have a similar attenuation. The grains segmented by selecting this attenuation are displayed with a circle with a size proportional to the radius of a sphere of equivalent volume. The color of each circle depends on e, the aspect ratio (ratio of the length of the minimum and maximum axes of an ellipsoid fitted to each grain). The white arrow indicates the location of the main central vein. Grains near the main central vein have a lower e attributed to the platy habitus of Fe-brucite. The other grains are considered as olivine and used for the calculation of the extent of reaction.
File S2 (Malvoisin_ds02.pdf). Optical photomicrographs used to calculate volume change during reaction (see main text for details about the calculation). Olivine grains (white) are surrounded by the serpentine + brucite mixture (light green to yellowish). The magnetite/clinopyroxene platelets are mapped in blue in olivine and in red in the serpentine + brucite mixture. The results of the model fitting the best the average orientation of the platelets in olivine are shown in green. The title of each photomicrograph provides the minimum and maximum estimates for volume change calculated with this model (see details about the calculation in the main text), and the size of the bottom right scale bar.
File S3 (Malvoisin_ds03.pdf). Histograms of magnetite/clinopyroxene platelet orientation (in degree). Each histogram corresponds to a photomicrograph of File S1. The order of the histograms and the photomicrographs is the same. The orientations in the olivine grains, the serpentine + brucite mixture and for the model fitting the best the average orientation in olivine are displayed in blue, red and green, respectively. The title of the histograms provides the standard deviation of the platelet orientation in the olivine grains (STDoli), the serpentine + brucite mixture (STDori) and calculated with the model fitting the best the orientation in olivine (STDcal).
Table S1 (Malvoisin_ds01.xlsx). Composition in major and trace elements measured with LA-ICPMS along five profiles perpendicular to the main vein. The composition as a function of the distance to the main vein is provided. The name of the tabs indicates if the profile was acquired in spot or in continuous mode (the two modes are described in the main text)
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