1 research outputs found
Subduction metamorphism of serpentiniteāhosted carbonates beyond antigorite-serpentinite dehydration (NevadoāFilĆ”bride Complex, Spain)
I. MartĆnez Segura and M. J. RomĆ”n Alpiste are thanked for their kind assistance during sample preparation and SEM operation, and M. T. GĆ³mezāPugnaire 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 MarieāCurie Initial Training Network ABYSS under REA Grant Agreement no. 608001 in the framework of M.D.M.'s PhD project, the Spanish āAgencia Estatal de InvestigaciĆ³nā (AEI) grants no. CGL2016ā75224āR to V.L.S.āV and CGL2016ā81085āR to C.J.G and C.M and grant no. PCINā2015ā053 to C.J.G. The āJunta de AndalucĆaā is also thanked for funding under grants no. RNMā131, RNMā374 and P12āRNMā3141. C.M. thanks MINECO for financing a RamĆ³n y Cajal fellowship no. RYCā2012ā11314 and K.H. for a Juan de la Cierva Fellowship no. FPDIā2013ā16253 and a research contract under grant no. CGL2016ā81085āR. 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 subāarc depths, the release of carbon from subducting slab lithologies is mostly
controlled by fluid released by devolatilization reactions such as dehydration of antigorite (Atgā) serpentinite to prograde peridotite. Here we investigate carbonateāsilicate rocks hosted in Atgāserpentinite and prograde chlorite (Chlā) harzburgite in the
Milagrosa and Almirez ultramafic massifs of the palaeoāsubducted NevadoāFilĆ”bride
Complex (NFC, Betic Cordillera, S. Spain). These massifs provide a unique opportunity to study the stability of carbonate during subduction metamorphism at PāT
conditions before and after the dehydration of Atgāserpentinite in a warm subduction
setting. In the Milagrosa massif, carbonateāsilicate rocks occur as lenses of Tiāclinohumiteādiopsideācalcite marbles, diopsideādolomite marbles and antigoriteādiopsideādolomite rocks hosted in clinopyroxeneābearing Atgāserpentinite. In Almirez,
carbonateāsilicate rocks are hosted in Chlāharzburgite and show a highāgrade assemblage composed of olivine, Tiāclinohumite, diopside, chlorite, dolomite, calcite, Crā
bearing magnetite, pentlandite and rare aragonite inclusions. These NFC
carbonateāsilicate 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 Caācarbonate (i.e., ophicarbonates). Thermodynamic
modelling shows that the carbonateāsilicate rocks attained peak metamorphic conditions similar to those of their host serpentinite (Milagrosa massif; 550ā600Ā°C and
1.0ā1.4 GPa) and Chlāharzburgite (Almirez massif; 1.7ā1.9 GPa and 680Ā°C).
Microstructures, mineral chemistry and phase relations indicate that the hybrid carbonateāsilicate bulk rock compositions formed before prograde metamorphism,
likely during seawater hydrothermal alteration, and subsequently underwent subduction metamorphism. In the CaOāMgOāSiO2 ternary, these processes resulted in a
compositional variability of NFC serpentiniteāhosted carbonateāsilicate rocks along
the serpentineācalcite mixing trend, similar to that observed in serpentiniteāhosted
carbonateārocks in other palaeoāsubducted metamorphic terranes. Thermodynamic modelling using classical models of binary H2OāCO2 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 carbonābearing species occur only in minor amounts in equilibrium with carbonateāsilicate 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 H2OāCO2 fluids,
but does not affect the topology of phase relations in serpentiniteāhosted carbonateā
rocks. Phase relations, mineral composition and assemblages of Milagrosa and
Almirez (meta)āserpentiniteāhosted carbonateāsilicate rocks are consistent with local
equilibrium between an infiltrating fluid and the bulk rock composition and indicate
a limited role of infiltrationādriven decarbonation. Our study shows natural evidence
for the preservation of carbonates in serpentiniteāhosted carbonateāsilicate rocks beyond the Atgāserpentinite breakdown at subāarc depths, demonstrating that carbon
can be recycled into the deep mantle.Funding from the European Union FP7 MarieāCurie Initial Training Network ABYSS under REA Grant Agreement no. 608001Spanish āAgencia Estatal de InvestigaciĆ³nā (AEI) grants no. CGL2016ā75224āR to V.L.S.āV and CGL2016ā81085āR to C.J.G and C.M and grant no. PCINā2015ā053 to C.J.GJunta de AndalucĆa Funding under grants no. RNMā131, RNMā374 and P12āRNMā3141MINECO for financing a RamĆ³n y Cajal fellowship no. RYCā2012ā11314 and K.H. for a Juan de la Cierva Fellowship no. FPDIā2013ā16253 and a research contract under grant no. CGL2016ā81085ā