Éditeur(s) : HAL CCSD Royal Society of Chemistry
Résumé : International audience
Low-level water measurements of geological materials are fundamental in understanding the volatile inventories of the Earth from the mantle to crustal reservoirs. Here we describe the development of microanalytical techniques using the new SHRIMP SI ion microprobe to measure the abundances of OH− (as a proxy for water) in volcanic glass and nominally anhydrous minerals (NAMs). Samples were first analysed at the Carnegie Institute of Washington (CIW) on their Cameca ims-6f with calibrations based on previous FTIR and manometry analyses. SHRIMP SI is a large geometry ion microprobe and is currently mainly used for O and S isotope analyses. The analytical protocol used here incorporates multiple collection of 16O− and 16O1H− allowing rapid measurements. A single calibration line incorporating all glasses and NAMs for the SHRIMP SI allows calibration of 16O1H−/16O− to H2O over a wide range in concentration (50 to 15 000 ppm H2O). This calibration line has around a 10% uncertainty, which appears to be limited only by sample heterogeneity. The current background for SHRIMP analysis is between 20–40 ppm but this is expected to improve with improved pumping on the source chamber. A current limitation to water analysis of NAM samples, by any technique, is having a range of standard materials to enable OH− calibration to absolute H2O concentrations. Data are presented for 7 NAM samples (2 olivines, 2 orthopyroxenes and 3 clinopyroxenes) that appear to be promising as potential standards for international laboratory H2O measurements. These NAM samples have been analysed and characterised here by SHRIMP SI, FTIR, EMP and the Cameca ims-6f ion microprobe at CIW. Four of these samples have previously been measured by manometry to determine absolute H2O concentrations.
ISSN: 0267-9477

hal-01243514
https://hal.archives-ouvertes.fr/hal-01243514
DOI : 10.1039/c5ja00047e

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
The southwestern Tianshan (China) metamorphic belt records high-pressure (HP) to ultrahigh-pressure (UHP) conditions corresponding to a cold oceanic subduction-zone setting. Serpentinites enclosing retrogressed eclogite and rodingite occur as lenses within metapelites in the UHP unit, which also hosts coesite-bearing eclogites. Based on the petrology and petrography of these serpentinites, five events are recognized: (1) formation of a wehrlite–harzburgite–dunite association in the mantle; (2) retrograde metamorphism and partial hydration during exhumation of the mantle rocks close to the seafloor; (3) oceanic metamorphism leading to the first serpentinization and rodingitization; (4) UHP metamorphism during subduction; (5) retrograde metamorphism during exhumation together with a second serpentinization. The peak metamorphic mineral assemblage of the serpentinized wehrlite comprises Ti-chondrodite + olivine + antigorite + chlorite + magnetite + brucite. A computed pseudosection for this serpentinized wehrlite shows that the Al content in antigorite is mostly sensititive to temperature but can also be used to constrain pressure. The average XAl = 0·204 ± 0·026 of antigorite (XAl = Al (a.p.f.u.)/8, where Al is in atoms per formula unit for a structural formula M48T34O85(OH)62, and M and T are octahedral and tetrahedral sites, respectively) included in Ti-chondrodite and average XAl = 0·203 ± 0·019 of antigorite in the matrix result in a well-constrained peak metamorphic temperature of 510–530°C. Peak pressures are less precisely constrained at 37 ± 7 kbar. The Tianshan serpentinites thus record UHP metamorphic conditions and represent the deepest subducted serpentinites discovered so far. The retrograde evolution occurs within the stability field of brucite + antigorite + olivine + chlorite and formation of Ti-clinohumite at the expense of Ti-chondrodite has been observed, suggesting isothermal decompression. The resulting P–T path is in excellent agreement with the metamorphic evolution of country rocks, indicating that the UHP unit in Tianshan was subducted and exhumed as a coherent block. To refine the metamorphic path of the ultramafic rocks, we have investigated the stability fields of Ti-chondrodite and Ti-clinohumite using piston-cylinder experiments. A total of 11 experiments were conducted at 25–55 kbar and 600–750°C in a F-free natural system. Combined with previous experiments and information from natural rocks we constructed a petrogenetic grid for the stability of Ti-chondrodite and Ti-clinohumite in F-free peridotite compositions. The formation of Ti-chondrodite in serpentinites requires a minimum pressure of about 26 kbar, whereas in Ti-rich systems it can form at considerably lower pressures. A key finding is that at UHP conditions, F-free Ti-chondrodite or Ti-clinohumite breaks down in the presence of orthopyroxene between 700 and 750°C, at temperatures that are significantly lower than those of the terminal breakdown reactions of these humite minerals. These breakdown reactions are an additional source of fluid during prograde subduction of serpentinites.
ISSN: 0022-3530

hal-01243507
https://hal.archives-ouvertes.fr/hal-01243507
DOI : 10.1093/petrology/egv042

Éditeur(s) : HAL CCSD [London] : Nature Pub. Group Nature Publishing Group
Résumé : International audience
The deeply eroded West Gondwana Orogen is a major continental collision zone that exposes numerous occurrences of deeply subducted rocks, such as eclogites. The position of these eclogites marks the suture zone between colliding cratons, and the age of metamorphism constrains the transition from subduction-dominated tectonics to continental collision and mountain building. Here we investigate the metamorphic conditions and age of high-pressure and ultrahigh-pressure eclogites from Mali, Togo and NE-Brazil and demonstrate that continental subduction occurred within 20 million years over at least a 2,500-km-long section of the orogen during the Ediacaran. We consider this to be the earliest evidence of large-scale deep-continental subduction and consequent appearance of Himalayan-scale mountains in the geological record. The rise and subsequent erosion of such mountains in the Late Ediacaran is perfectly timed to deliver sediments and nutrients that are thought to have been necessary for the subsequent evolution of sustainable life on Earth.
ISSN: 2041-1723

hal-01122789
https://hal.archives-ouvertes.fr/hal-01122789
DOI : 10.1038/ncomms6198