Tools for Comprehensive Assessment of Fluid-Mediated and Solid-State Alteration of Carbonates Used to Reconstruct Ancient Elevation and Environments

Carbonates are ubiquitous in the rock record and provide a broad array of stable isotope-based paleoclimatic proxies (i.e., δ18O, δ13C, ∆17O, ∆47, ∆48) that provide information on stratigraphy, carbon cycling, temperature, hydrology, and the altitude of ancient land surfaces. Thus, carbonates are an essential archive of environmental and topographic histories of continental terranes. However, carbonate minerals are highly susceptible to post-depositional alteration of primary isotopic values via fluid-mediated and solid-state reactions. We propose a hierarchical suite of techniques to comprehensively assess alteration in carbonates, from essential and readily accessible tools to novel, high-resolution techniques. This framework provides a means of identifying preserved textures in differentially altered samples that contain high-value environmental information. To illustrate this progressive approach, we present a case study of Tethyan nearshore carbonates from the Paleocene Tso Jianding Group (Tibet). We demonstrate the utility of each technique in identifying chemical and crystallographic indicators of post-depositional alteration at progressively finer spatial scales. For example, secondary ionization mass spectrometry (SIMS) oxygen isotope maps of micrite and bioclasts reveal significant isotopic heterogeneity due to grain-scale water-rock exchange in textures that were labeled “primary” by optical inspection at coarser spatial resolution. Optical and cathodoluminescence microscopy should be the minimum required assessment of carbonate samples used in stable isotope analyses, but supplemented when necessary by SIMS, PIC mapping, and other yet untapped technologies that may allow distinction of primary and altered fabrics at finer spatial resolutions.

Origin of Mesozoic Porphyritic Rocks and Regional Magmatic Evolution in the Zijinshan Ore Field of Fujian Province, China: Hf-O Isotope Characteristics of Magmatic Zircons

Mesozoic porphyritic rocks from the Zijinshan area, southwestern Fujian Province, China, are andesitic to rhyolitic in composition. The whole-rock SiO2 contents of these rocks are between 62.5% and 78.1%. Magmatic zircon from the Mesozoic porphyritic rocks was determined via secondary-ionization mass spectrometry (SIMS) for the U-Pb age and Hf and O isotopes. The zircon U-Pb ages could be mainly divided into three age groups: Group 1: ~138.8 Ma; Group 2: 109.2~107.4 Ma; and Group 3: 99.7~98.2 Ma. The εHf(t) and δ18O values of the porphyritic zircons showed that the porphyritic rocks in Group 2 were more affected by mantle-derived magma. Combined with previous research results, the medium-acidic magmatism in the southwestern Fujian Province can be divided into eight periods: Paleoproterozoic, Mesoproterozoic, Middle Neoproterozoic, Silurian to Lower Devonian, Permian to Triassic, Middle Jurassic to early Lower Cretaceous, late Lower Cretaceous, and late Lower Cretaceous to early Upper Cretaceous. The Paleoproterozoic crust was the predominant magmatic source for the subsequent Mesoproterozoic to Jurassic magmatism, but the only melts that were closely related to mineralization were derived from partial melting of the Mesoproterozoic crust and a more depleted upper mantle.

Photoluminescence Imaging of Whole Zircon Grains on a Petrographic Microscope—An Underused Aide for Geochronologic Studies

The refractory nature of zircon to temperature and pressure allows even a single zircon grain to preserve a rich history of magmatic, metamorphic, and hydrothermal processes. Isotopic dating of micro-domains exposed in cross-sections of zircon grains allows us to interrogate this history. Unfortunately, our ability to select the zircon grains in a heavy mineral concentrate that records the most geochronologic information is limited by our inability to predict internal zonation from observations of whole grains. Here we document the use of a petrographic microscope to observe and image the photoluminescence (PL) response of whole zircon grains excited under ultraviolet (UV) light, and the utility of this PL response in selecting grains for geochronology. While zircon fluorescence has long been known, there is limited documentation of its utility for and application to geochronologic studies. Our observations of zircon from an un-metamorphosed igneous rock, two meta-igneous rocks, and a placer deposit show that variations in the PL color are readily observable in real-time, both among grains in a population of zircons and within single grains. Analyses of cross-sections of the same grains demonstrate that the changes in PL correlate with zoning in backscattered electron (BSE) and cathodoluminescence (CL) images as well as with changes in U + Th concentration and spectroscopic proxies for radiation damage. In other words, the whole grain PL provides a low-resolution preview of the U + Th zoning expected in a cross-sectioned grain. We demonstrate the usefulness of this “preview” in identifying and selecting the subset of zircon grains in a heavy mineral separate that has metamorphic rims of sufficient width to date by secondary ionization mass spectrometry (SIMS). The data are also used to place preliminary constraints on the age and U + Th concentrations at which a yellow PL response is observed in natural samples. The PL response of zircon is well-known among spectroscopists, and these simple applications demonstrate several ways in which the response might be more effectively used by geochronologists.

Multimethod U–Pb baddeleyite dating: insights from the Spread Eagle Intrusive Complex and Cape St. Mary's sills, Newfoundland, Canada

Baddeleyite (ZrO2) is widely used in U–Pb geochronology but analysis and age interpretation are often difficult, especially for samples which have experienced post-intrusive alteration and/or metamorphism. Here, we combine high spatial resolution (secondary ionization mass spectrometry, SIMS) and high-precision (isotope dilution thermal ionization mass spectrometry, ID-TIMS) analyses of baddeleyite from the Spread Eagle Intrusive Complex (SEIC) and Cape St. Mary's sills (CSMS) from Newfoundland. Literature data and our own detailed microtextural analysis suggest that at least seven different types of baddeleyite–zircon intergrowths can be distinguished in nature. These include secondary baddeleyite inclusions in altered zircon, previously unreported from low-grade rocks, and likely the first discovery of xenocrystic zircon inclusions mantled by baddeleyite. 207Pb∕206Pb baddeleyite dates from SIMS and ID-TIMS mostly overlap within uncertainties. However, some SIMS sessions of grain mounts show reverse discordance, suggesting that bias in the U ∕ Pb relative sensitivity calibration affected 206Pb∕238U dates, possibly due to crystal orientation effects, and/or alteration of baddeleyite crystals, which is indicated by unusually high common-Pb contents. ID-TIMS data for SEIC and CSMS single baddeleyite crystals reveal normal discordance as linear arrays with decreasing 206Pb∕238U dates, indicating that their discordance is dominated by recent Pb loss due to fast pathway diffusion or volume diffusion. Hence, 207Pb∕206Pb dates are more reliable than 206Pb∕238U dates even for Phanerozoic baddeleyite. Negative lower intercepts of baddeleyite discordia trends for ID-TIMS dates for SEIC and CSMS and direct correlations between ID-TIMS 207Pb∕206Pb dates and the degree of discordance may indicate preferential 206Pb loss, possibly due to 222Rn mobilization. In such cases, the most reliable crystallization ages are concordia upper intercept dates or weighted means of the least discordant 207Pb∕206Pb dates. We regard the best estimates of the intrusion ages to be 498.7±4.5 Ma (2σ; ID-TIMS upper intercept date for one SEIC dike) and 439.4±0.8 Ma (ID-TIMS weighted mean 207Pb∕206Pb date for one sill of CSMS). This first radiometric age for the SEIC is consistent with stratigraphic constraints and indicates a magmatic episode prior to opening of the Rheic Ocean. Sample SL18 of the Freetown Layered Complex (FLC), Sierra Leone, was investigated as an additional reference. For SL18, we report a revised 201.07±0.64 Ma intrusion age, based on a weighted mean 207Pb∕206Pb date of previous and new baddeleyite ID-TIMS data, agreeing well with corresponding SIMS data. Increasing discordance with decreasing crystal size in SL18 indicates that Pb loss affected baddeleyite rims more strongly than cores. Our SL18 results validate that the SIMS in situ approach, previously used for Precambrian and Paleozoic samples, is also suitable for Mesozoic baddeleyite.

Stepwise chemical abrasion ID-TIMS-TEA of microfractured Hadean zircon

The Hadean Jack Hills zircons represent the oldest known terrestrial material, providing a unique and truly direct record of Hadean Earth history. This zircon population has been extensively studied via high spatial resolution, high throughput in situ isotopic and elemental analysis techniques such as secondary ionization mass spectrometry (SIMS), but not by comparatively destructive, high-temporal-precision (

Fingerprinting Paranesti Rubies through Oxygen Isotopes

In this study, the oxygen isotope (δ18O) composition of pink to red gem-quality rubies from Paranesti, Greece was investigated using in-situ secondary ionization mass spectrometry (SIMS) and laser-fluorination techniques. Paranesti rubies have a narrow range of δ18O values between ~0 and +1‰ and represent one of only a few cases worldwide where δ18O signatures can be used to distinguish them from other localities. SIMS analyses from this study and previous work by the authors suggests that the rubies formed under metamorphic/metasomatic conditions involving deeply penetrating meteoric waters along major crustal structures associated with the Nestos Shear Zone. SIMS analyses also revealed slight variations in δ18O composition for two outcrops located just ~500 m apart: PAR-1 with a mean value of 1.0‰ ± 0.42‰ and PAR-5 with a mean value of 0.14‰ ± 0.24‰. This work adds to the growing use of in-situ methods to determine the origin of gem-quality corundum and re-confirms its usefulness in geographic “fingerprinting”.