What controls clumped isotopes? Stable isotopes of a molecule can clump together in several combinations, depending on their mass. Even for simple molecules such as O 2 , which can contain 16 O, 17 O, and 18 O in various combinations, clumped isotopes can potentially reveal the temperatures at which molecules form. Away from equilibrium, however, the pattern of clumped isotopes may reflect a complex array of processes. Using high-resolution gas-phase mass spectrometry, Yeung et al. found that biological factors influence the clumped isotope signature of oxygen produced during photosynthesis (see the Perspective by Passey). Similarly, Wang et al. showed that away from equilibrium, kinetic effects causing isotope clumping can lead to overestimation of the temperature at which microbially produced methane forms. Science , this issue p. 431; p. 428; see also p. 394

Tracing ancient hydrogeological fracture network age and compartmentalisation using noble gases, Geochimica et Cosmochimica Acta (2017), doi: https://doi. AbstractWe show that fluid volumes residing within the Precambrian crystalline basement account for ca 30 % of the total groundwater inventory of the Earth (> 30 million km 3 ). The residence times and scientific importance of this groundwater are only now receiving attention with ancient fracture fluids identified in Canada and South Africa showing: 1. microbial life which has existed in isolation for millions of years; 2. significant hydrogen and hydrocarbon production via waterrock reactions; and 3. preserving noble gas components from the early atmosphere. Noble gas (He, Ne, Ar, Kr, Xe) abundance and isotopic compositions provide the primary evidence for fluid mean residence time (MRT). Here we extend the noble gas data from the Kidd Creek Mine in Timmins Ontario Canada, a volcanogenic massive sulfide (VMS) deposit formed at 2.7 Ga, in which fracture fluids with MRTs of 1.1-1.7 Ga were identified at 2.4km depth (Holland et al., 2013); to fracture fluids at 2.9km depth. We compare here the Kidd Creek Mine study with noble gas compositions determined in fracture fluids taken from two mines (Mine 1 & Mine 2) at 1.7 and 1.4 km depth below surface in the Sudbury Basin formed by a meteorite impact at 1.849 Ga.The 2.9 km samples at Kidd Creek Mine show the highest radiogenic isotopic ratios observed to date in free fluids (e.g. 21 Ne/ 22 Ne = 0.6 and 40 Ar/ 36 Ar = 102,000) and have MRTs of 1.0 to 2.2 Ga. In contrast, resampled 2.4 km fluids indicated a less ancient MRT (0.2-0.6 Ga) compared with the previous study (1.1-1.7 Ga). This is consistent with a change in the age distribution of fluids feeding the fractures as they drain, with a decreasing proportion of the most ancient endmember fluids. 129 Xe/ 136 Xe ratios for these fluids confirm that boreholes at 2.4 km versus 2.9 km are sourced from hydrogeologically distinct systems. In contrast, results for the Sudbury mines 3 have MRTs of 0.2-0.6 and 0.2-0.9 Ga for Mines 1 and 2 respectively. While still old compared to almost all groundwaters reported in the literature to date, these younger residence times compared to Kidd Creek Mine are consistent with significant fracturing created by the impact event, facilitating more hydrogeologic connection and mixing of fluids in the basin. In all samples from both Kidd Creek Mine and Sudbury, a 124-128 Xe excess is identified over modern air values. This is attributed to an early atmospheric xenon component, previously identified at Kidd Creek Mine but which has to date not been observed in fluids with a residence time as recent as 0.2-0.6 Ga. The temporal and spatial sampling at Kidd Creek Mine is also used to verify our proposed conceptual model which provides key constraints regarding distribution, volumes and residence times of fracture fluids on the smaller, regional, scale.

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This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International licence Newcastle University ePrints-eprint.ncl.ac.uk Telling J,

A record of fluid flow has been documented within a Paleozoic carbonate platform sequence by U–Pb dating of calcite in veins and vugs from rock core sampled through a shallowly dipping sequence of sedimentary rocks beneath the Bruce nuclear site, Ontario, Canada. Secondary calcite from >650 m deep Ordovician carbonate rocks yields a Silurian age of 434 ± 5 Ma possibly related to infiltration of seawater from overlying evaporitic basins as well as hydrothermal solutions that infiltrated from below. In contrast, near-surface Devonian rocks mostly give vein infill ages over the range of 80–100 Ma with evidence for younger infill down to 50 Ma. Vein calcite samples previously dated from surface outcrops of Ordovician carbonate exposed up to 500 km to the east yielded similar U–Pb ages. Coincidence of near-surface vein calcite ages indicates widespread vein emplacement synchronous with a change in direction of motion of the North American plate as well as possible erosional unroofing following passage of the region over the Great Meteor hotspot approximately 125 Myr ago. Deeper carbonate formations have remained apparently impermeable to post-Paleozoic disturbance despite these perturbations.