ALBERT

Description: Key Points We developed the first serpentine reference materials for precise and accurate in situ oxygen isotope determination by ion microprobe. Matrix bias effects up to +3‰ and ‐1‰ were observed between antigorite/olivine and antigorite/lizardite. Crystal orientation effects up to 1‰ were observed only in chrysotile. We present the first investigation of in situ oxygen isotopes in serpentine minerals by sensitive high‐resolution ion microprobe (SHRIMP). Chemically homogeneous samples of antigorite (δ18O = 8.30 ± 0.12‰), chrysotile (δ18O = 4.37 ± 0.02‰) and lizardite (δ18O = 5.26 ± 0.20‰) analysed by laser fluorination are identified as potential reference materials. They were analysed by SHRIMP to assess their homogeneity compared with the San Carlos olivine, as well as for potential matrix bias and crystal orientation effects. The reproducibility achieved for all samples was ± 0.30–0.55‰ (95% confidence level). Matrix bias between antigorite/olivine and antigorite/lizardite was up to ~ +3‰ and −1‰, respectively. Crystal orientation effects were identified only in chrysotile, and no matrix bias was observed over the investigated compositional range within each serpentine mineral. The new reference materials were used to measure the oxygen isotope composition of serpentines in an ultrahigh‐pressure metamorphic belt from Tianshan (China). By combining oxygen isotopes and trace element microanalyses, several stages of serpentinisation were recognised: (a) seafloor alteration, (b) recycling of internal metamorphic fluids during isothermal decompression and (c) shallow interaction with meteoric water during exhumation. No interaction with fluids derived from the surrounding metapelites during subduction was identified.

Description: Abstract In situ analysis of a garnet porphyroblast from a granulite facies gneiss from Sør Rondane Mountains, East Antarctica, reveals discontinuous step‐wise zoning in phosphorus and large δ18O variations from the phosphorus‐rich core to the phosphorus‐poor rim. The gradually decreasing profile of oxygen isotope from the core (δ18O = ~15‰) to the rim (δ18O = ~11‰) suggests that the 18O/16O zoning was originally step‐wise, and modified by diffusion after the garnet rim formation at ~800°C and 0.8 GPa. Fitting of the 18O/16O data to the diffusion equation constrains a duration of the high‐T event (~800°C) to c. 0.5–40 Ma after the garnet rim formation. The low δ18O value of the garnet rim, together with the previously reported low δ18O values in metacarbonates, indicates regional infiltration, probably along a detachment fault, of low δ18O fluid/melt possibly derived from meta‐mafic to ultramafic rocks.

Description: Most monazite reference materials (RMs) for in situ U-Pb geochronology are rich in Th, however, many hydrothermal ore deposits contain monazite that is low in trace element contents including Th, U and Pb. Because of potential problems with matrix effects and lack of appropriate matrix-matched RMs, such variations can bias dating of hydrothermal deposits. In this paper, we describe a polycrystalline low-U and low-Th Diamantina monazite from the Espinhaço Range, SE Brazil. It has a U-Pb ID-TIMS weighted mean 207 Pb*/ 235 U ratio of 0.62913 ± 0.00079, 206 Pb*/ 238 U of 0.079861 ± 0.000088 and 207 Pb*/ 206 Pb* of 0.057130 ± 0.000031, yielding a weighted mean 206 Pb*/ 238 U date of 495.26 ± 0.54 Ma (95% c.l.). In situ dates acquired with different methods (LA-(Q, SF, MC)-ICP-MS and SIMS) are within uncertainty of the ID-TIMS data. U-Pb LA-(Q, MC)-ICP-MS runs, using Diamantina as a primary RM, reproduced the ages of other established RMs within less than 1% deviation. The LA-MC-ICP-MS analyses yielded homogeneous Sm-Nd isotopic compositions ( 143 Nd/ 144 Nd = 0.511427 ± 23, 2 s ; 147 Sm/ 144 Nd = 0.1177 ± 13, 2 s ) and εNd (495 Ma) of -18.7 ± 0.5 (2 s ). SIMS oxygen isotope determinations showed measurement reproducibility better than ± 0.3‰ (2 s ) confirming Diamantina's relative homogeneity at test portion masses below 1 ng. This article is protected by copyright. All rights reserved.