Paired stable isotopes (O, C) and clumped isotope thermometry of magnesite and silica veins in the New Caledonia Peridotite Nappe

Abstract

The stable isotope compositions of veins provide information on the conditions of fluid-rock interaction and on the origin of fluids and temperatures. In New Caledonia, magnesite and silica veins occur throughout the Peridotite Nappe. In this work, we present stable isotope and clumped isotope data in order to constrain the conditions of fluid circulation and the relationship between fluid circulation and nickel ore-forming laterization focusing on the Koniambo Massif. For magnesite veins occurring at the base of the nappe, the high d18O values between 27.8‰ and 29.5‰ attest to a low temperature formation. Clumped isotope analyses on magnesite give temperatures between 26°C and 42°C that are consistent with amorphous silica -magnesite oxygen isotope equilibrium. The meteoric origin of the fluid is indicated by calculated d18Owater values between -3.4‰ to +1.5‰ Amorphous silica associated with magnesite or occurring in the coarse saprolite level displays a narrow range of d18O values between 29.7‰ and 35.3‰. For quartz veins occurring at the top of the bedrock and at the saprolite level, commonly in association with Ni-talc-like minerals, the d18O values are lower, between 21.8‰ and 29.0‰ and suggest low-temperature hydrothermal conditions (~40-95°C). Thermal equilibration of the fluid along the geothermic gradient before upward flow through the nappe and/or influence of exothermic reactions of serpentinization could be the source(s) of heat needed to form quartz veins under such conditions. Dear Editor, Please find attached the revised version of manuscript #GCA-D-15-00678, entitled "Paired stable isotope (O, C) and clumped isotope thermometry of magnesite and silica veins in the New Caledonia Peridotite Nappe", by B. Quesnel, P. Boulvais , P. Gautier, M. Cathelineau, C.M. John, M. Dierick, P. Agrinier and M. Drouillet, which required moderate revisions before final acceptance for publication in Geochimica et Cosmochimica Acta. You can find on this file our answers (in red below) to reviewer's comments. Accordingly to these comments, the text and some figures have been modified. For the text, we have let these changes visible on the manuscript to make easier your lecture. Note also that we have taken into account the minor editorial suggestions of reviewers (for example those noticed in yellow boxes in pdf files); these changes do not appear in the new version. We have also built new tables that could be useful for the geochemists community; these calculations have been suggested by reviewers 1 and reviewer A. Gilg (we now present a temperature range for quartz formation of 40-95°C, instead of 40-80°C in the previous version; conclusions remain unchanged). These tables are introduced as electronic supplements to preserve the ms as short and clear as it was.Best regards, Benoit Quesnel and co-authors *Cover LetterDear Editor, Please find attached the revised version of manuscript #GCA-D-15-00678, entitled "Paired stable isotope (O, C) and clumped isotope thermometry of magnesite and silica veins in the ...

Stable isotope evidence for near-surface, low-temperature formation of Mg-(hydro)carbonates in highly altered Greek Mesozoic serpentinites

“…While we cannot be definitive regarding fractionation and re-equilibration, our data agree well with recent studies where a near-surface, low-temperature, meteoricwater origin for ultramafic-hosted magnesites has been proposed (Oskierski et al 2012;Quesnel et al 2013Quesnel et al , 2016 and also in the classic work of O'Neil & Barnes (1971). Data fields from these studies are shown in Figure 9 for comparison with our values.…”

“…The hydrothermal signature may be pre-and/or co-genetic with weathering, and while we cannot constrain temperatures attained in the past, the isotope data show that most of the Mg-(hydro)carbonates in this study formed (or re-equilibrated) at temperatures at or below 30 ˚C. This interpretation, supported by similar recent field case studies (Oskierski et al 2012;Quesnel et al 2013Quesnel et al , 2016) conflicts with earlier laboratory work (Hänchen et al 2008;Möller 1989;Sayles & Fyfe 1973;Stevula et al 1978) where magnesite does not precipitate until fluid temperatures are in the range 60-120 ˚C.…”

“…In parallel to the carbon-storage context and often stimulated by economic interest in magnesite for industrial end-uses, there has been growing consensus that most ultramafic-hosted magnesites form under near surface conditions with involvement of meteoric water (see recent overviews in Keleman et al 2011;Oskierski et al 2013;Quesnel et al 2013Quesnel et al , 2016. While 'low' (typically <60 ˚C) temperatures of magnesite mineralization are often inferred (Keleman et al 2011) -usually from oxygen isotope data -precise temperatures of carbonation are difficult to ascribe, partly because the parent fluid is often not well-constrained, and because the magnesite-water oxygen isotope fractionation factor is still not fully resolved (Chako & Deines 2008).…”

Kapcsolt izotópok (clumped isotopes) a földtudományi kutatásokban

“…Karbonátkiválások vizsgálatával meghatározható a törések és a vetők mentén mozgó fluidumok hőmérséklete, illetve a fluidum eredeti oxigénizotópos-összetétele (SWANSON et al 2012, BERGMAN et al 2013, LOYD et al 2013, CRUSET et al 2016, QUESNEL et al 2016. A töréskitöltő karbonátok ∆ 47 értékei geotermális rezervoárok hőmérsék -letének meghatározására is használhatóak (SUMNER et al 2015).…”

The emplacement of the Peridotite Nappe of New Caledonia and its bearing on the tectonics of obduction

“…Due to its position right above the presumed major thrust system, the frontal part of the Peridotite Nappe may have undergone renewed tangential shear on this occasion. In line with this hypothesis, top‐to‐SW shearing along the serpentine sole of the Koniambo Massif involves synkinematic magnesite veins [ Quesnel et al ., , ] formed at temperatures of only ~30°C [ Quesnel et al ., ], which suggests that the thickness of the Peridotite Nappe had already been much reduced when this deformation occurred.…”