This contribution provides in-situ LA-ICP-MS U-Pb ages and trace element determinations of zircons from dacitic to rhyolitic lavas, ignimbrites and intrusions in the Southern Rocky Mountain Volcanic Field (SRMVF) in Colorado, USA. The data record a period of intense magmatic activity in the Oligocene-early Miocene (∼37-22 Ma) which gave rise to some of the largest explosive ignimbrites in the geological record (e.g. the Fish Canyon Tuff). Age data are drift corrected, but not corrected for radiation dosage or Th disequilibrium, in order to allow users to apply their own algorithms. Xenocrysts (much older crystals up to 2 Ga from the Proterozoic basement) are included in this record.
<p>The interpretation of paleoclimate records from speleothems remains a challenging task due to the individual characteristics of each specimen and cave system. Through recent advances in techniques like confocal microscopy and high-resolution geochemical analysis, fluorescent layers in speleothems have become a significant source of information to enhance paleo-seasonal reconstructions, improve age models and, consequently, constrain rates of past climate changes. In this framework, speleothem fluorescence originates from organic matter produced in the soil above the cave, from ancient organic compounds in the bedrock or from microbial processes within the karst. However, the mechanisms leading to the incorporation of fluorescent banding into calcite as well as the properties of transport, storage and decomposition of organic matter in natural karst systems are still under debate. We present results from a one-year monitoring study of fluorescence properties in drip water, sampled from May 2020 to May 2021 in a quasi-monthly resolution at 3-6 locations within the cave system La Vallina in Northwestern Spain. We have measured absorbance spectra and fluorescence exitation-emission matrices; and compare it to drip water geochemistry, fluorescence of active speleothems at the same site and vegetation type above the cave. Our results indicate high gradients of fluorescent properties in drip waters already on a small spatial scale. In the site where active speleothems show fluorescent banding, a humic-like fluorescent signal prevails in cave waters (AC peak, according to Coble nomenclature), while other sites are more likely to be influenced by microbial activity (B/M peak). Humic-like fluorescence is stronger in drip waters during the autumn season, probably due to the increased input by colloids. Yet, simple relationships between the fluorescence in drip water and colloid-associated trace elements like Cu and Y cannot be confirmed. Further, the difference in drip water fluorescence is small compared to the actual intra-seasonal difference retrieved by confocal microscopy in active stalagmites. Therefore, we find drip water composition unlikely to be solely responsible for seasonal enriched fluorescence incorporation in speleothems and favour conceptual models taking moisture-limitation and adsorption into account.</p>
Here, we map a series of stalagmites from Asturias, Spain, by laser ablation inductively-coupled-plasma mass spectrometry and confocal laser scanning (fluorescence) microscopy and discuss the origins of trace element and fluorescence variations. Seasonal banding is evident with both methods and may be attributed to lignins/humic acids based on fluorescence absorption and emission characteristics. Some lateral variations in fluorescence present as saw-tooth fluorescent "spires" and demonstrate disruptions of seasonal banding, corroborated by trace element variations (most prominently Mg and Na). Such features likely reflect the differential partitioning of trace elements by sectoral zoning as a result of low supersaturation and/or high organic matter load, combined with the effects of dissolved organic matter on the calcite growth surface and the association of each element with colloidal organic matter. The lateral variability of trace elements demonstrates the pitfalls of obtaining trace element information from one-dimensional transects without prior reconnaissance mapping. It is, however, possible that traditional drilling with ~1 mm holes homogenizes these features and provides reliable trace element estimates.
Abstract Speleothem fluorescence can provide insights into past vegetation dynamics and stalagmite chronology. However, its origin and especially the formation of fluorescent laminations in stalagmites are poorly understood. We conducted a year-long monthly monitoring of drip water fluorescence in La Vallina Cave (northern Iberian Peninsula) and compared the results to drip water chemistry and active speleothems from the same sites. Drip waters were analyzed using fluorescence spectroscopy and parallel factor analysis (PARAFAC). The resulting five-component model indicates contributions from vegetation, microbial activity, and bedrock. Intra-site fluorescence variability is mainly influenced by changes in overlying vegetation, water reservoir time, and respiration rates. Contrary to prevailing views, we find no systematic increase in drip water fluorescence during rainy conditions across drip sites and seasonal variations in drip water fluorescence are absent at a location where present-day speleothem layers form. Our findings challenge the notion of a higher abundance of humic-like fluorescence during the rainy season as the primary cause for layer formation and suggest additional controls on drip water fluorescence, such as bedrock interaction and microbial reprocessing. We also propose that growth rate may control the dilation of the fluorescence signal in stalagmites, indicating other potential mechanisms for fluorescent layer formation.
Some speleothems, in particular stalagmites, are laminated at the visible and microscopic scale, with the latter visible using fluorescence microscopy (e.g., confocal laser scanning microscopy). These laminations can be used to supplement speleothem chronologies, although this process is laborious and lateral variations in lamination geometry and quality necessitate a detailed look over an entire scan as opposed to a simple one-dimensional transect. In order to assist this process, we develop a classification-based machine learning algorithm using an open-source machine learning package. This algorithm is optimized for stalagmites growing at 20–100 μm yr−1 and outputs a 2-dimensional layer density map which may aid in quantitatively interpreting past variations in speleothem growth rate. This algorithm requires user supervision and interpretation, as image artefacts and magnification settings may complicate model output.
Speleothem fluorescence may elucidate past vegetation dynamics, while microscale fluorescent laminations can provide annually resolved chronology. However, the origin of speleothem fluorescence and the mechanism responsible for the formation of micrometer scale fluorescent lamination in stalagmites, are not well constrained by monitoring studies. Here, we present results from a year-long monthly drip water monitoring from seven locations in La Vallina cave (Northwestern Spain). Fluorescence was quantified by excitation-emission matrices (EEM) spectrofluorometry. Five distinct components were resolved by parallel factor analysis (PARAFAC) modeling, including previously described humic-like, and protein-like components as well as an additional component suggesting a contribution of indigenous fluorescence sourced from the bedrock. Variations in the overlying vegetation and in the water reservoir age contribute to differences in the fluorescent components among different drip sites. While some active stalagmites feature annual to sub-annual fluorescent laminae, the drip water does not support higher abundance of humic-like fluorescence during the rainy season as a primary cause for layer formation. The constancy in humic-like fluorescence, likely to arise due to abiotic interactions with the bedrock, highlights possible other mechanisms on fluorescent layer formation, such as growth rate control over the dilation of the fluorescence signal in the stalagmite.
The San Luis caldera complex in the Southern Rocky Mountain Volcanic Field (CO, USA) consists of three overlapping calderas that overlie the sources of three large-volume mid-Cenozoic ignimbrites: the Rat Creek Tuff (RCT; zoned dacite-rhyolite, 150 km3), the Cebolla Creek Tuff (mafic dacite, 250 km3) and the Nelson Mountain Tuff (NMT; zoned dacite-rhyolite, 500 km3), which are indistinguishable in age by 40Ar/39Ar dating. In this study, we argue for a shared magmatic history for the three units on the basis of mineral trace element compositions (plagioclase, sanidine, biotite, pyroxene, amphibole, titanite and zircon), as well as zircon U-Pb geochronology in the RCT and NMT. It is postulated that these latter two are cogenetic, having occupied an elongated magma reservoir that erupted in two stages, prior to and following the eruption of the Cebolla Creek Tuff. This necessitates large-scale lateral magma transport of the NMT magma, which is corroborated by the formation of the nearby Cochetopa caldera with a paucity of intracaldera eruptive products. The implications of lateral magma transport and evolution in integrated magma chambers are discussed in the context of calculating magma fluxes, which can be redefined as an area-normalized flux to avoid inconsistencies in flux estimations.
Abstract The integration of detrital zircon age and trace element analyses provides a powerful tool with which to reconstruct continental arc evolution. Detrital zircons from the Ross-Delamerian orogen along the Pacific-Gondwana margin in north Victoria Land in Antarctica yield a broad 700–500 Ma U-Pb age population that shows a prominent period of activity centered at ca. 630–550 Ma. This activity is well correlated with the highest zircon Th/U and U/Yb ratios, suggesting an increase in lithospheric contribution coincident with fluid input from oceanic slab subduction, respectively. A low Yb/Gd ratio over this same period also suggests crustal thickening. Determination of zircon parent rock types using trace element proxies reveals the presence of previously unrecognized distinct pulses of granitoid activity that occur over tens of millions of years. Lulls between granitoid flare-ups overlap with increases in mafic-carbonatite-alkaline magma production, suggesting an influx of mantle or lower crustal melts during syn-subduction extension. A concomitant increase in the number of metamorphic zircons (U/Th > 10) and 40Ar/39Ar white mica cooling ages found during these extensional episodes suggest that significant thermal perturbations of the crust coincided with orogenic cooling, which was possibly influenced by uplift and exhumation.
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