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Skarn garnets in the Mineral King roof pendant of the south–central Sierra Nevada within Sequoia National Park, California, USA reveal variable fluid chemistry with a significant component of meteoric water during metasomatism in the Early Cretaceous Sierra Nevada Batholith. We focus on andradite garnet associated with Pb–Zn mineralization in the White Chief Mine. Laser fluorination oxygen isotope analyses of δ18O of garnet (δ18O(Grt)) from sites along the skarn show a large range of values (− 8.8 to + 4.6‰ VSMOW). Ion microprobe (SIMS) analyses elucidate that individual andradite crystals are strongly zoned in δ18O(Grt) (up to 7‰ of variation). Total rare-earth element concentrations (∑REE) across individual garnets show progressive depletion of skarn-forming fluids in these elements during garnet growth. These findings support a skarn model of earliest red high-δ18O grandite garnet consistent with a magmatic-dominated equilibrium fluid (δ18O(H2O) as high as ≈ + 8‰). Later, green andradite crystallized in equilibrium with a low-δ18O fluid indicating a significant influx of meteoric fluid (δ18O(H2O) ≈ − 6 to − 5‰), following a hiatus in garnet growth, associated with late-stage Pb–Zn mineralization. Latest orange overprint rims have higher δ18O values (δ18O(H2O) ≈ 0–2‰), and depleted total REEs, suggesting influx of high-δ18O, trace-element depleted fluid derived from regional metamorphism of the carbonate host. Remarkably, low δ18O(Grt) values in the White Chief canyon skarn require a significant proportion of meteoric fluid available during > 400 °C andradite-forming metasomatism. Fluid flow was channelized at the pluton–wallrock contact, evidenced by the narrow extent of skarn. [ABSTRACT FROM AUTHOR]

Radiogenic and stable isotopic studies of zircon are a powerful tool to investigate geologic processes because data can be placed in a temporal context using U-Pb ages. However, when zircon data lack information on the spatial distribution of the parent rock(s) (e.g., detrital data sets), interpreting changes in isotopic composition through time is not always straightforward. To evaluate and improve the utility of zircon isotopic data, we present a regional data set consisting of new zircon U-Pb, εHf(t), and δ18Ozrc data in 31 Triassic to early Miocene igneous rocks from a >1300-km-long transect in the southwestern U.S. Cordillera. This data set is combined and compared with a compilation of whole rock isotopic data from the same transect. Orogen-scale spatial and temporal isotopic trends are identified and interpreted, both in terms of the underlying mechanisms that generated the trends and the tectonic processes that have shaped this part of the Cordillera. Most Cordilleran magmatism originates in the upper mantle and zircon εHf(t) primarily reflects the isotopic composition of the mantle source region. East of ~114°W longitude in the southwestern U.S. Cordillera, the continental mantle lithosphere remained coupled to the crust until the late Miocene and zircon εHf(t) reflects the age of the lithosphere. Because the mantle lithosphere remained intact, zircon εHf(t) and δ18Ozrc of igneous rocks associated with low-angle to flat-slab subduction and crustal thickening during the Laramide orogeny are not significantly different from igneous rocks associated with Farallon slab rollback/foundering. Temporal isotopic trends identified in rocks east of ~114°W longitude are related to migration of magmatism into lithospheric terranes of a different age. West of ~114°E longitude, in regions like the Mojave Desert in southern California, USA, the continental mantle lithosphere is interpreted to have been partially removed and replaced by underplated Pelona--Orocopia--Rand schist and isotopically depleted asthenosphere or oceanic lithosphere during the Laramide orogeny. There is a temporal isotopic shift to more juvenile zircon εHf(t) and higher δ18Ozrc in igneous rocks west of ~114°W, which is used to estimate the position of the western edge of intact North American continental mantle lithosphere before and after the Laramide orogeny. The results suggest that regional (spatial) trends in zircon εHf(t) and δ18Ozrc data can be significantly larger than isotopic shifts at a specific location within a Cordilleran orogenic system. By accounting for regional spatial variations, temporal isotopic trends in zircon data can be more confidently interpreted in terms of tectonic and geodynamics processes. [ABSTRACT FROM AUTHOR]

No crustal rocks are known to have survived since the time of the intense meteor bombardment that affected Earth between its formation about 4,550 Myr ago and 4,030 Myr, the age of the oldest known components in the Acasta Gneiss of northwestern Canada. But evidence of an even older crust is provided by detrital zircons in metamorphosed sediments at Mt Narryer and Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, where grains as old as approximately 4,276 Myr have been found. Here we report, based on a detailed micro-analytical study of Jack Hills zircons, the discovery of a detrital zircon with an age as old as 4,404+/-8 Myr--about 130 million years older than any previously identified on Earth. We found that the zircon is zoned with respect to rare earth elements and oxygen isotope ratios (delta18O values from 7.4 to 5.0%), indicating that it formed from an evolving magmatic source. The evolved chemistry, high delta18O value and micro-inclusions of SiO2 are consistent with growth from a granitic melt with a delta18O value from 8.5 to 9.5%. Magmatic oxygen isotope ratios in this range point toward the involvement of supracrustal material that has undergone low-temperature interaction with a liquid hydrosphere. This zircon thus represents the earliest evidence for continental crust and oceans on the Earth.

Oxygen isotope ratios measured by ion microprobe in magnetite from granulite-facies marble of the Adirondack Mountains, New York, range from +2 to +11 per mil (standard mean ocean water) across a single grain that measures 3 millimeters by 5 millimeters. Low values are concentrated in irregular domains near the grain boundary but also occur in the grain's interior. In contrast, grains 1 millimeter in diameter that are from a second nearby sample show no significant heterogeneity, except within 10 micrometers of the grain boundary. These data, including large gradients of up to 9 per mil per 10 micrometers, provide important new constraints on the nature and origins of intragrain isotopic heterogeneity and on oxygen isotope thermometry. The differences between these magnetite grains result from contrasting mechanisms of isotope exchange with fluids after the peak of regional metamorphism. Volume diffusion of oxygen through the crystal structure of magnetite contributed to isotope exchange in the rims of small grains, but larger grains are crosscut by healed cracks that are not readily detected and that short-circuited diffusion.