Abstract In situ oxygen isotope compositions of white mica and quartz have been used to characterize the interplay of metamorphism and fluid events between a metasomatic whiteschist and its granite protolith in the Monte Rosa nappe, Western Alps. New natural muscovite and phengite reference materials were calibrated for in situ Secondary Ion Mass Spectrometry (SIMS) oxygen isotope measurement. White mica and quartz oxygen isotope compositions were measured in situ in one whiteschist and two metagranites. Based on microtextural observation, phengite composition of white mica, and phase petrology modeling, it is possible to identify two events of fluid infiltration and one event of fluid expulsion, all of which were responsible for forming this unique whiteschist occurrence and for tracing its metamorphic evolution from late Permian intrusion to Alpine subduction and finally to the present day, exhumed whiteschist. Metagranite samples contain three generations of white mica: igneous, high-P metamorphic, and late Alpine, retrograde compositions. In the whiteschist samples, we distinguish two distinct Alpine white mica generations: (1) prograde to peak generation and (2) retrograde generation. The δ18OVSMOW values of white mica and quartz from a whiteschist of 5.3 to 7.3‰ and 9.1 to 10.6‰ are significantly lower than in the metagranites, with 9.1 to 10.8‰ and 13.2 to 14.6‰, respectively. This indicates a complete recrystallization of the whiteschist protolith during intense fluid-rock interaction. Subsequent Alpine metamorphism transformed the protolith into the whiteschist. The isotopic composition of the whiteschist, fine-grained, retrograde white mica (5.3 to 6‰) is lower than that of the high-pressure phengite (6.2 and 7.5‰). The low δ18O values could be explained by infiltration of external fluids with δ18O values of 2 to 6‰. Such fluids would carry the isotopic signature of the serpentinites of the Piemonte-Liguria Ocean by either equilibration of fluids with or dehydration of serpentinites. Another, more simple explanation would be the infiltration of very small quantities of fluids leading to the breakdown of chloritoid. Local inheritance of the oxygen composition would then hide the origin of the fluids. Isotope exchange temperatures calculated from high-P phengite-quartz pairs in whiteschist give an average temperature of 440 ± 50 °C. These are lower than the best T-estimates from phase petrology of 570 °C, at 2.2 GPa. Igneous muscovite-quartz pairs in the metagranite yield 400 ± 40 °C. Only one high-P phengite-quartz pair was analyzed, resulting in 350 ± 40 °C. Greenschist facies, low silica phengites give an average temperature of 310 ± 10 °C. Propagation of analytical uncertainty suggests large errors of 60 to 120 °C, due to the relatively small T-dependence of the quartz-white mica fractionation factor for oxygen isotopes.
Two quartz samples of igneous origin, UNIL ‐Q1 (Torres del Paine Intrusion, Chile) and BGI ‐Q1 (Shandong province, China), were calibrated for their oxygen isotope composition for SIMS measurements. UNIL ‐Q1 and BGI ‐Q1 were evaluated for homogeneity using SIMS . Their reference δ 18 O values were determined by CO 2 laser fluorination. The average δ 18 O value found for UNIL ‐Q1 is 9.8 ± 0.06‰ and that for BGI ‐Q1 is 7.7 ± 0.11‰ (1 s ). The intermediate measurement precision of SIMS oxygen isotope measurements was 0.32–0.41‰ (2 s ; UNIL ‐Q1) and 0.40–0.48‰ (2 s ; BGI ‐Q1), respectively. While less homogeneous in its oxygen isotope composition, BGI ‐Q1 is also suitable for SIMS trace element measurements.
Serpentinite replacement by carbonates in the seafloor is one of the main carbonation processes in nature providing insights into the mechanisms of CO2 sequestration; however, the onset of this process and the conditions for the reaction to occur are not yet fully understood. Preserved serpentine rim with pseudomorphs of carbonate after serpentine and lobate-shaped carbonate grains are key structural features for replacement of serpentinite by carbonates. Cathodoluminescence microscopy reveals that Ca-rich carbonate precipitation in serpentinite is associated with a sequential assimilation of Mn. Homogeneous δ18O values at the µm-scale within grains and host sample indicate low formation temperature (<20 °C) from carbonation initiation, with a high fluid to rock ratio. δ13C (1–3 ± 1‰) sit within the measured values for hydrothermal systems (−3–3‰), with no systematic correlation with the Mn content. δ13C values reflect the inorganic carbon dominance and the seawater source of CO2 for carbonate. Thermodynamic modeling of fluid/rock interaction during seawater transport in serpentine predicts Ca-rich carbonate production, at the expense of serpentine, only at temperatures below 50 °C during seawater influx. Mg-rich carbonates can also be produced when using a model of fluid discharge, but at significantly higher temperatures (150 °C). This has major implications for the setting of carbonation in present-day and in fossil margins.
We compare quartz zonation and diffusion timescales of crystal-rich rhyolitic ignimbrites and crystal-poor rhyolitic lava flows from the Jurassic Chon Aike Province as exposed in Patagonia (Argentina). The timescales are assessed by using diffusion modelling based on nanoscale secondary ion mass spectrometry (NanoSIMS) analysis of titanium (Ti) concentration profiles in quartz crystals oriented by image analysis using micro-tomography. Quantitative Ti-data were acquired by SIMS to estimate crystallization temperatures. The textural and geochemical analysis revealed clear differences between crystal-poor rhyolitic lava flows and crystal-rich rhyolitic ignimbrites. Quartz crystals from rhyolitic lava flows display simple oscillatory cathodoluminescence (CL) zoning interpreted to be magmatic and diffusion chronometry suggest a short timescale for quartz crystallization from 5.6 ± 2.2 yr to 41.6 ± 9.8 yr. Resorption textures are rare, and hence crystals in rhyolitic lava flows recorded a simple, rapid extraction, transport and eruption history for these crystal-poor melts. Rhyolitic ignimbrites, in contrast, reveal complex zoning patterns, reflecting several episodes of partial resorption and growth throughout their crystallization history. The complex quartz zoning textures together with longer diffusion times (< 350 yr), rather suggest a storage in a mush with fluctuating pressure and temperature conditions leading to intermittent resorption. Yet, a final quartz overgrowth rim occurred over a much shorter timescale in the order of years (< 3 yr), which implies that crystal-rich ignimbrites can be re-mobilized very fast.
To help understand bioapatite microstructures and related chemical variations, their impact on O‐isotope compositions measured and give insights on sample preparation, this study analysed conodonts and shark teeth prepared in different orientations through microanalytical and bulk sampling techniques: scanning electron microscopy (SEM); electron probe microanalysis (EPMA); continuous‐flow and high‐temperature reduction – isotope ratio mass spectrometry; and secondary ion mass spectrometry (SIMS). The SEM and EPMA measurements in conodonts allowed to distinguish the tissues commonly analysed by SIMS, which included albid and hyaline crowns but given their often small‐scale intergrowth, mixtures of these are difficult to avoid. In situ SIMS O‐isotope analyses provided different δ 18 O values: lower values with higher variance (16 ± 1‰ n = 13, 15.7 ± 1.9‰ n = 11) for mixed albid‐hyaline tissues, and higher, homogeneous values (17.1 ± 0.2‰, n = 13) for mainly hyaline tissues. Recent shark teeth δ 18 O SIMS value for dentine of the same tooth was 10‰ lower than the mean δ 18 O SIMS value for enameloid whereas the δ 18 O PO4 values measured for enameloid and dentine using the HTR method were identical. The variation of δ 18 O seems sensitive to analytical artefacts related to sample textures, caused during the sample preparation over more porous biomineral surfaces.
Melt inclusions are minute melts trapped during crystal growth [e.g., 1]. When trapped in euhedral olivine they are believed to represent the melt in which olivine grew[1], despite some post-entrapment modifications [e.g., 2, 3]. In this study, we analysed olivine hosted melt inclusions (OHMIs) in Mid-Ocean Ridge Basalts (MORBs) from two different locations on the Mid-Atlantic Ridge: FAMOUS and the 14° MAR triple junction (respectively Normal-MORB and Enriched-MORB).Silicon, as oxygen,is a major element inolivines and it diffusesslowly in dry basaltic melt [4]. Both isotope systems (O and Si) do not fractionate during melting [e.g., 5]. The isotopic ranges reported for bulk MORBs are narrow(<0.5‰)[e.g., 6, 7].Thus, we do not expectδ30Siand δ18Ovariability in OHMIs. However, previous analyses ofδ18Oin these OHMIs showup to 2.5‰ variation (8 times the analytical error)in each sample[8]. Any difference or similarity betweenthe δ30Si and δ18O signaturesinthe OHMIs would give valuable insight on the process responsible for the δ18Ovariation in the OHMIs.Silicon isotopes in OHMIs were measured by ion probe. OHMIs from the two samples displaysimilar δ30Sivalues, rangingfrom 0.08±0.23‰to 2.8±0.32‰for the N-MORB, and from 0.51±0.34‰to 2.8±0.31‰for the E-MORB(2se).The variation is larger than the accuracy (~0.4‰).δ18Oand δ30Sidisplay a slight positive trend. While theaverage δ18Ovalue in the OHMIs from the N-MORB sample is similar to the values previoulsy reported for bulk MORBs [6], the average δ30Siis muchhigher, and similar for both of our samples.Such high valuesare not representative of any high temperature terrestrial reservoirs. The large δ30Sioffset with bulk MORBsand the large variation within the OHMI population thus most likelyreflect either a localizedor OHMIs-specific process(e.g. boundary layer or local dissolution).
