The Cornwall and Devon vein- and greisen-type copper and tin deposits of southwest England are spatially and genetically related to shallow-seated granitic intrusions. These late Variscan intrusions, ...collectively known as the Cornubian Batholith, extend over 200 km and form a continuous granitic spine from the Isles of Scilly Granite in the west to the Dartmoor Granite in the east. The granitic plutons of the Cornubian Batholith were intruded from ~ 295 to 270 Ma without a major hiatus. Twelve samples of cassiterite (SnO
2
) were obtained from tin deposits associated with seven different plutons within the Cornubian Batholith for in situ LA-ICPMS U–Pb dating. This study of cassiterite was undertaken to obtain the first results of direct dating of ore mineral to refine the geochronology of tin mineralization in this region. Of the cassiterite samples analyzed, the oldest ages were determined within the Kit Hill and Hingston–Gunnislake Granites in the central part of the Cornubian Batholith. The Hingston–Gunnislake cassiterite, from Drakewalls Mine, was the oldest sample dated at 291.8 ± 3.4 Ma. The next oldest dates, 290.5 ± 2.8 and 288.5 ± 2.9 Ma, were from two cassiterite samples extracted from the adjacent Kit Hill Consolidated Mines within the Kit Hill Granite. At the eastern end of the study area, two cassiterite samples within the Dartmoor Granite produced ages of 286.0 ± 1.8 and 284.1 ± 1.3 Ma. The youngest sample from this study, 275.4 ± 1.6 Ma, is from the Balleswidden Mine within the westernmost Land’s End Granite. The cassiterite dates do not reveal any readily observable relationship between ore ages and geographic relationship from west to east throughout the Cornubian Batholith. Incorporating the associated errors, the geochronology does indicate continuous mineralization within the granites for ~ 21 million years, from ca. 295 to 274 Ma. This span falls within the established period of granitic magmatism of ca. 295 to 270 Ma for the Cornubian Batholith and further confirms the reliability of in situ LA-ICPMS U–Pb dating of cassiterite.
This paper investigates applicability of cassiterite to dating ore deposits in a wide age range. We report in situ LA-ICPMS U-Pb and Pb-Pb dating results (n = 15) of cassiterite from six ore deposits ...in Russia ranging in age from ~1.85 Ga to 93 Ma. The two oldest deposits dated at ~1.83–1.86 Ga are rare metal Vishnyakovskoe located in the East Sayan pegmatite belt and tin deposits within the Tuyukan ore region in the Baikal folded region. Rare metal skarn deposits of Pitkäranta ore field in the Ladoga region, Fennoscandian Shield are dated at ~1.54 Ga. Cassiterite from the Mokhovoe porphyry tin deposit located in western Transbaikalia is 810 ± 20 Ma. The youngest cassiterite was dated from the deposits Valkumei (Russian North East, 108 ± 2 Ma) and Merek (Russian Far East, 93 ± 2 Ma). Three methods of age calculations, including 208Pb/206Pb-207Pb/206Pb inverse isochron age, Tera-Wasserburg Concordia lower intercept age, and 207Pb-corrected 206Pb*/238U age were used and the comparison of the results is discussed. In all cases, the dated cassiterite from the ore deposits agreed, within error, with the established period of magmatism of the associated granitic rock.
Cassiterite (SnO2), a main ore mineral in tin deposits, is suitable for U–Pb isotopic dating because of its relatively high U/Pb ratios and typically low common Pb. We report a LA-ICPMS analytical ...procedure for U–Pb dating of this mineral with no need for an independently dated matrix-matched cassiterite standard. LA-ICPMS U-Th-Pb data were acquired while using NIST 612 glass as a primary non-matrix-matched standard. Raw data are reduced using a combination of Iolite™ and other off-line data reduction methods. Cassiterite is extremely difficult to digest, so traditional approaches in LA-ICPMS U-Pb geochronology that utilize well-characterized matrix-matched reference materials (e.g., age values determined by ID-TIMS) cannot be easily implemented. We propose a new approach for in situ LA-ICPMS dating of cassiterite, which benefits from the unique chemistry of cassiterite with extremely low Th concentrations (Th/U ratio of 10−4 or lower) in some cassiterite samples. Accordingly, it is assumed that 208Pb measured in cassiterite is mostly of non-radiogenic origin—it was initially incorporated in cassiterite during mineral formation, and can be used as a proxy for common Pb. Using 208Pb as a common Pb proxy instead of 204Pb is preferred as 204Pb is much less abundant and is also compromised by 204Hg interference during the LA-ICPMS analyses.
Our procedure relies on 208Pb/206Pb vs 207Pb/206Pb (Pb-Pb) and Tera-Wasserburg 207Pb/206Pb vs 238U/206Pb (U-Pb) isochron dates that are calculated for a ~1.54 Ga low-Th cassiterite reference material with varying amounts of common Pb that we assume remained a closed U-Pb system. The difference between the NIST 612 glass normalized biased U-Pb date and the Pb-Pb age of the reference material is used to calculate a correction factor (F) for instrumental U-Pb fractionation. The correction factor (F) is then applied to measured U/Pb ratios and Tera-Wasserburg isochron dates are obtained for the unknown cassiterite analyzed in the same analytical session. This allows for U-Pb dating of cassiterite of any age with no need for an independently dated matrix-matched reference material, nor assumptions about the isotopic composition of common Pb.
Results for cassiterite from tin deposits in Bolivia, Brazil, China, Russia, Saudi Arabia, South Africa, Spain, and the United Kingdom, with ages ranging from ~20 Ma to ~2060 Ma, demonstrate the applicability of this approach across a broad range of geologic time. These ages are in good agreement with published geochronology of the host rocks associated with the tin deposits and with previously published U-Pb ages of some cassiterites from the same deposits. Thus, our in situ LA-ICPMS methodology verifies the use of cassiterite as a reliable U-Pb mineral-geochronometer with the advantages of fast and relatively low cost in situ analyses with moderate spatial resolution.
•Low Th in some cassiterite allows using 208Pb as a common Pb proxy.•208Pb/206Pb vs 207Pb/206Pb isochron used to determine the age of a cassiterite standard•Biased U-Pb ages corrected using the standard•Applicability of this approach demonstrated by LA-ICPMS cassiterite dating ranging in age from ~20 Ma to ~2060 Ma•Cassiterite is suitable for reliable U–Pb isotopic dating by LA-ICPMS.
•The maximum depositional age of the rift-related Ghanzi Group is refined to 1085.5 ± 4.5 Ma.•Copper-bearing marine siliciclastic rocks of the Ghanzi Group were deposited after ∼1050 to ...1060 Ma.•Source regions contained both juvenile mantle and crustal reservoirs.•Compilation of magmatic rock U-Pb, Lu-Hf, and Sm-Nd data from the Kalahari Craton.•The southwestern margin of the Kalahari Craton was the primary source for the rift basin infill.
New igneous and detrital zircon laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb geochronology and Lu-Hf isotopic data are presented for the Mesoproterozoic Kgwebe Formation and the unconformably overlying Ghanzi Group in northwestern Botswana. The Makgabana Hills porphyritic rhyolite flow from the Ghanzi area yielded a U-Pb concordia age of 1085.5 ± 4.5 Ma and provides a new maximum depositional age for the unconformably overlying Ghanzi Group. Detrital zircon (n = 448) from the Ghanzi Group yielded a 207Pb/206Pb age distribution with a dominant (70 to 90%) Mesoproterozoic population (∼1450 to ∼1050 Ma), a smaller (5 to 20%) Paleoproterozoic (∼2200 to ∼1700 Ma) population, and a few (n = 4) older (∼3000 Ma to ∼2450 Ma) grains. A maximum depositional age constraint of ∼1060 to ∼1050 Ma was obtained for middle and upper Ghanzi Group based on the weighted-mean 207Pb/206Pb age of the youngest clusters of overlapping zircon ages for each sample.
Initial hafnium ratios (εHfi) and corresponding crustal residence model ages (TCDM) for the Paleoproterozoic zircon populations indicate either fractionation from a chondritic uniform reservoir (CHUR) or mixing between juvenile mantle and older crustal components. Mesoproterozoic zircon with εHfi values between −20 and +15 and TCDM model ages between 3000 and 1200 Ma suggest that the source terrane(s) contained magmatic rocks including both juvenile material and substantially reworked Paleoproterozoic and possibly Archean crust.
Comparison with a compilation of published U-Pb, Lu-Hf, and Sm-Nd data from the Kalahari Craton suggests that the predominant Mesoproterozoic zircon population was derived from the Namaqua Sector, Rehoboth Basement Inlier, Kwando Complex, and Choma-Kalomo Block; some zircon may have had distal sources in adjacent Rodinia landmasses. Both Archean cratonic components and juvenile ∼1200 to ∼1000 Ma magmatic rocks of the Natal Sector and the Maud and Mozambique belts on the eastern margin of the craton are unlikely sources for the detrital zircon based on isotopic composition. Sediment transported from the western margin of the Kalahari Craton entered the northwest Botswana rift and mixed with sediments from the Rehoboth Basement Inlier and Paleo- to Mesoproterozoic terranes that bound the northwest Botswana rift.
U-Pb dating of cassiterite and zircon from the Yazov granite (Transbaikalia region, Eastern Siberia, Russia) and cassiterite from spatially associated tin mineralization in the Tuyukan ore district ...in the Tonod uplift was conducted using in situ laser ablation inductively coupled plasma mass spectrometry. These analyses allow comparison of isotopic systematics for both minerals, especially related to transport in granitic magma. These data are also useful for understanding possible genetic links between the granite and the tin mineralization. Most of the U-Pb zircon analyses define a
206
Pb/
238
U age of 719 ± 15 Ma for the granite; in addition, several zircon cores define an inheritance age of 1839 ± 21 Ma. U-Pb data for 10 nearly concordant analyses of disseminated cassiterite from the same samples yield a
206
Pb/
238
U age of 1838 ± 34 Ma. This is the first documented evidence of cassiterite inheritance in granitic magma. These data indicate the robust character of U-Pb isotope systematics in cassiterite, comparable to that in zircon. The presence of numerous inclusions of cassiterite in zircon from the Yazov granite (revealed by nanotomography) supports the interpretation of inherited cassiterite included during Neoproterozoic zircon crystallization. The data indicate that high tin concentrations in the Yazov granite are due to the incorporation of older cassiterite crystals from country rock, not coeval cassiterite crystallization. Cassiterite samples from two ore occurrences spatially associated with the Yazov granite yield Pb-Pb isochron ages of 1.86–1.82 Ga, indicating that tin mineralization occurred in the Paleoproterozoic, nearly 1 Ga before emplacement of the Yazov granite. Tin mineralization of the ore region is probably related to ~ 1.85 Ga Chuya-Kodar tin-bearing granitic rocks that host tin deposits. These results have broad implications for understanding how critical elements, such as tin, may become enriched in rare-metal granites and how they are related to regional to global geodynamic processes.
As a consequence of contemporary or longer term (since 15ka) climate warming, gas hydrates in some settings may presently be dissociating and releasing methane and other gases to the ocean–atmosphere ...system. A key challenge in assessing the impact of dissociating gas hydrates on global atmospheric methane is the lack of a technique able to distinguish between methane recently released from gas hydrates and methane emitted from leaky thermogenic reservoirs, shallow sediments (some newly thawed), coal beds, and other sources. Carbon and deuterium stable isotopic fractionation during methane formation provides a first-order constraint on the processes (microbial or thermogenic) of methane generation. However, because gas hydrate formation and dissociation do not cause significant isotopic fractionation, a stable isotope-based hydrate-source determination is not possible. Here, we investigate patterns of mass-dependent noble gas fractionation within the gas hydrate lattice to fingerprint methane released from gas hydrates. Starting with synthetic gas hydrate formed under laboratory conditions, we document complex noble gas fractionation patterns in the gases liberated during dissociation and explore the effects of aging and storage (e.g., in liquid nitrogen), as well as sampling and preservation procedures. The laboratory results confirm a unique noble gas fractionation pattern for gas hydrates, one that shows promise in evaluating modern natural gas seeps for a signature associated with gas hydrate dissociation.
► We demonstrate possible flaws in sampling and processing of natural hydrate samples. ► We demonstrated that noble gases fractionated in the synthetic hydrates. ► Data shows that He and Ne are retained in the synthetic hydrate samples. ► We show a unique trend in hydrate dissociation by noble gas fingerprinting.
The Abu Dabbab rare-metal granite in the Eastern Desert of Egypt is a highly-evolved alkali-feldspar granite with transitional magmatic-hydrothermal features. Extreme geochemical fractionation and ...the associated significant TaSn resource make the Abu Dabbab intrusion an important feature in the metallogenic evolution of the Arabian-Nubian Shield. UPb dating by laser ablation sector field (SF)-ICPMS analysis of igneous monazite yields a Concordia age of 644.7 ± 2.3 Ma, identical within uncertainty to a lower intercept Tera-Wasserburg isochron age of 644.2 ± 2.3 Ma obtained from hydrothermal cassiterite. Both ages place tight constraints on the timing of magmatic-hydrothermal processes in the Abu Dabbab granite which represents the oldest highly-evolved granite recognized so far in the Pan-African Arabian-Nubian Shield. Thus, the new ages also date the start of a period of late-orogenic metalliferous granite magmatism, when the basement of the Eastern Desert underwent a geodynamic transition from a compressive subduction-collision regime towards orogenic collapse in the late Cryogenian.
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•Combined UPb dating of monazite and cassiterite for rare-metal granite systems•Earliest rare-metal granite in the Panafrican of Arabian-Nubian shield at 644 Ma•Marker for the geodynamic transition from subduction-collision to orogenic collapse
Precipitation was collected between 1991 and 1997 at 41 locations within and adjacent to parts of the Great Basin lying in California, Oregon, Nevada, and Utah. These samples were analyzed for their ...deuterium (δD) and oxygen‐18 (δ18O) contents. Separate collections were made of summer and winter season precipitation at stations ranging in elevation from –65 m to 3246 m. The δD per mil values of stations that were closely spaced but at different elevations showed an average δD decrease of approximately 10‰/km rise in elevation. Data for all samples representing winter precipitation, when plotted on a δD versus δ18O plot, fall close to the Meteoric Water Line (δD = 8 δ18O + 10); samples representing summer precipitation define a line of slightly lower slope due to evaporation of the raindrops during their passage from cloud to ground. Comparison of our 1991–1997 δD data with those from the same three stations reported by an earlier study in the southeastern California shows seasonal differences ranging from 0 per mil to 19‰ (average: 15) and annual differences ranging from 0 to 13 per mil (average: 2), illustrating the degree of annual and seasonal variability in this region. When contoured, the δD values display gradients indicating a north to northwest decrease in deuterium, with values ranging from −60 to −125‰ in winter precipitation and from −40 to −110‰ in summer precipitation. These gradient trends can be explained by the predominance of air mass trajectories originating in the tropical Pacific, the Gulf of California, and (in summer) the Gulf of Mexico.
Secondary calcite, silica and minor amounts of fluorite deposited in fractures and cavities record the chemistry, temperatures, and timing of past fluid movement in the unsaturated zone at Yucca ...Mountain, Nevada, the proposed site of a high-level radioactive waste repository. The distribution and geochemistry of these deposits are consistent with low-temperature precipitation from meteoric waters that infiltrated at the surface and percolated down through the unsaturated zone. However, the discovery of fluid inclusions in calcite with homogenization temperatures (Th) up to ∼80°C was construed by some scientists as strong evidence for hydrothermal deposition. This paper reports the results of investigations to test the hypothesis of hydrothermal deposition and to determine the temperature and timing of secondary mineral deposition. Mineral precipitation temperatures in the unsaturated zone are estimated from calcite- and fluorite-hosted fluid inclusions and calcite δ18O values, and depositional timing is constrained by the 207Pb/235U ages of chalcedony or opal in the deposits. Fluid inclusion Th from 50 samples of calcite and four samples of fluorite range from ∼35 to ∼90°C. Calcite δ18O values range from ∼0 to ∼22‰ (SMOW) but most fall between 12 and 20‰. The highest Th and the lowest δ18O values are found in the older calcite. Calcite Th and δ18O values indicate that most calcite precipitated from water with δ18O values between −13 and −7‰, similar to modern meteoric waters.
Twenty-two 207Pb/235U ages of chalcedony or opal that generally postdate elevated depositional temperatures range from ∼9.5 to 1.9Ma. New and published 207Pb/235U and 230Th/Uages coupled with the Th values and estimates of temperature from calcite δ18O values indicate that maximum unsaturated zone temperatures probably predate ∼10Ma and that the unsaturated zone had cooled to near-present-day temperatures (24–26°C at a depth of 250m) by 2–4Ma. The evidence of elevated temperatures persisting in ash flow tuffs adjacent to parent calderas for as much as ∼8Ma is a new finding, but consistent with thermal modeling. Simulations using the HEAT code demonstrate that prolonged cooling of the unsaturated zone is consistent with magmatic heat inputs and deep-seated (sub-water table) hydrothermal activity generated by the large magma body ∼8km to the north that produced the 15–11Ma ash flows and ash falls that make up Yucca Mountain. The evidence discussed in this and preceding papers strongly supports unsaturated zone deposition of the secondary minerals from descending meteoric waters. Although depositional temperatures reflect conductive (and possibly vapor-phase convective) heating of the unsaturated zone related to regional magmatic sources until perhaps 6Ma, depositional conditions similar to the present-day unsaturated zone have prevailed for at least the past 2–4Ma.
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