- View All
For Reporting Year
Submitted by Jay J. Ague, September 2015
Fluid-mediated decarbonation of subducted altered oceanic crust: Classic experimental and thermodynamic studies and recent field-based budgets have suggested that the altered oceanic crust does not significantly contribute to the deep carbon fluxes at forearc depths (Kerrick and Connolly, 2001; Poli et al., 2009; Collins et al., 2015). However, most of these works do not account for (i) the mode of occurrence of carbon in altered metabasalts (localized or diffuse) and its effect on carbonate stability, (ii) the role of metastable assemblages during high-pressure/low-temperature metamorphism, and (iii) the role of flushing by external fluids, and the associated redox variations.
Because Alpine Corsica offers world-class exposures of high-pressure metamorphic rocks, it is a prime target for field-based research focused on these issues. Key questions about the fate of carbon in subduction zones may be investigated there, and processes like carbonate devolatilization or dissolution and graphite formation or oxidation may be studied in situ. Moreover, redox phenomena involving serpentinite and methane production could have relevance for both the Deep Energy and Deep Life communities. A ~10-day field season involving the personnel listed above was completed in fall 2014. Our current research and preliminary findings are summarized below.
The study focuses on the petrologic evolution of carbonate-bearing metabasalts subducted to eclogite-facies conditions in Alpine Corsica. The studied unit consists of a thick (ca. 200m) sequence of pillow metabasalts and pillow metabreccias showing little or no deformation. We focused our study on primary carbonate-bearing hydrothermal structures in undeformed pillow metabasalts (e.g. carbonate-filled cooling cracks in pillow cores) that are exceptionally well preserved in Alpine Corsica owing to the little retrograde deformation. The petrographic study was done in Paris by means of optical and electron microscopy and Raman spectroscopy. The rock mainly consists of carbonate (both calcite and relict aragonite) + Ca-silicates, typically lawsonite, epidote and garnet, part of which likely represent relicts of the primary seafloor assemblage (epidote). The rocks exhibit clear evidence of interaction between carbonates and silicates. The microstructural study allowed identifying a main reaction involving consumption of carbonate + lawsonite + epidote to form garnet. This redox reaction and the related time-integrated fluid fluxes were modeled in Yale. The estimated fluid composition and fluxes are surprisingly rich in carbon, especially those required to activate the redox reaction. This result indicates that carbonic fluids percolating the subducting oceanic crust enhances additional carbonate devolatilization.
We also performed in-situ garnet δ18O measurement in order to define the possible isotopic signature of fluid-induced decarbonation on new forming phases. The results, however, do not show any significant carbonate contribution to the garnet δ18O, which was most likely buffered by the external fluid.
A manuscript is in preparation and should be submitted by the end of the year (Vitale Brovarone et al., in prep.).
Rock carbonation at high-pressure conditions (PhD Francesca Piccoli, co-funded DCO):
Recent works have shown the fundamental contribution of carbonate dissolution into the carbon fluxes at subduction zones (Ague and Nicolescu, 2014; Frezzotti et al., 2011; Kelemen and Manning, 2015). Owing to the relatively recent diffusion of this concept in the Earth Science communities, still little is known on the fate of carbonic fluids produced by carbonate dissolution. Migration towards volcanic arc and volcanic degassing has been proposed in the attempt of reconciling the carbon input and output budgets of subduction zones. However, interaction of these fluids with shallower rocks may potentially generate new carbon reservoirs.
The PhD work of Francesca Piccoli (continuity with her MSc) focuses on peculiar carbonate-bearing rocks found along major lithologic boundaries in Alpine Corsica. Structural, textural and whole rock δ18O and δ13C suggest that these rocks formed by precipitation of carbonate by external fluids bearing considerable amount of dissolved carbon. During her first year of PhD (based in Paris), Francesca has been doing a detailed geochemical characterization of these rocks including whole rock and in-situ major and trace element analysis, C-O stable isotopes and Raman fluid inclusion study. Francesca found presence of carbonic species such as CH4 and CO2 in her fluid inclusions, and will dedicate the next months to microthermometric measurements in order to constrain the conditions of entrapment. Whole rock strontium isotope analyses will be performed by the end of the year in order to better constrain the source of Ca associated with the rock carbonation, and possibly the carbon source.
Francesca did a rough mass balance of her rocks and noticed that the amount of carbon that could be sequestrated in the rock by high-pressure carbonation is directly comparable to some estimated carbon fluxes produced by carbonate dissolution. The preliminary results of Francesca’s work are promising and have the potential to identify a new important piece of the deep carbon cycling at subduction zones. However, additional and complementary studies, most notably thermodynamic calculations, are needed to better quantify these fluxes. We are currently searching for a financial support to ensure Francesca mobility to Yale in the next years. This part of the work aims at providing her with computational skills in thermodynamics in order to quantify the carbon fluxes in her rocks under the supervision of Jay Ague.
Initial results were presented at Goldschmidt2015 and a first paper is in preparation to be submitted soon (Piccoli et al., in prep.).