Automated statistical matching of multiple tephra records exemplified using five long maar sequences younger than 75 ka,Auckland, New Zealand     

Rebecca M. Green  Mark S. Bebbington  Shane J. Cronin  Geoff Jones 《Quaternary Research》2014

Detailed tephrochronologies are built to underpin probabilistic volcanic hazard forecasting, and to understand the dynamics and history of diverse geomorphic, climatic, soil-forming and environmental processes. Complicating factors include highly variable tephra distribution over time; difficulty in correlating tephras from site to site based on physical and chemical properties; and uncertain age determinations. Multiple sites permit construction of more accurate composite tephra records, but correctly merging individual site records by recognizing common events and site-specific gaps is complex. We present an automated procedure for matching tephra sequences between multiple deposition sites using stochastic local optimization techniques. If individual tephra age determinations are not significantly different between sites, they are matched and a more precise age is assigned. Known stratigraphy and mineralogical or geochemical compositions are used to constrain tephra matches. We apply this method to match tephra records from five long sediment cores (≤ 75 cal ka BP) in Auckland, New Zealand. Sediments at these sites preserve basaltic tephras from local eruptions of the Auckland Volcanic Field as well as distal rhyolitic and andesitic tephras from Okataina, Taupo, Egmont, Tongariro, and Tuhua (Mayor Island) volcanic centers. The new correlated record compiled is statistically more likely than previously published arrangements from this area.  相似文献   

A cascade of magmatic events during the assembly and eruption of a super-sized magma body     

Aidan.?S.?R.?Allan  Simon?J.?Barker  Marc-Alban?Millet  Daniel?J.?Morgan  Shane?M.?Rooyakkers  C.?Ian?Schipper  Colin?J.?N.?WilsonEmail authorView author&#;s OrcID profile 《Contributions to Mineralogy and Petrology》2017,172(7):49

We use comprehensive geochemical and petrological records from whole-rock samples, crystals, matrix glasses and melt inclusions to derive an integrated picture of the generation, accumulation and evacuation of 530 km³ of crystal-poor rhyolite in the 25.4 ka Oruanui supereruption (New Zealand). New data from plagioclase, orthopyroxene, amphibole, quartz, Fe–Ti oxides, matrix glasses, and plagioclase- and quartz-hosted melt inclusions, in samples spanning different phases of the eruption, are integrated with existing data to build a history of the magma system prior to and during eruption. A thermally and compositionally zoned, parental crystal-rich (mush) body was developed during two periods of intensive crystallisation, 70 and 10–15 kyr before the eruption. The mush top was quartz-bearing and as shallow as ~3.5 km deep, and the roots quartz-free and extending to >10 km depth. Less than 600 year prior to the eruption, extraction of large volumes of ~840 °C low-silica rhyolite melt with some crystal cargo (between 1 and 10%), began from this mush to form a melt-dominant (eruptible) body that eventually extended from 3.5 to 6 km depth. Crystals from all levels of the mush were entrained into the eruptible magma, as seen in mineral zonation and amphibole model pressures. Rapid translation of crystals from the mush to the eruptible magma is reflected in textural and compositional diversity in crystal cores and melt inclusion compositions, versus uniformity in the outermost rims. Prior to eruption the assembled eruptible magma body was not thermally or compositionally zoned and at temperatures of ~790 °C, reflecting rapid cooling from the ~840 °C low-silica rhyolite feedstock magma. A subordinate but significant volume (3–5 km³) of contrasting tholeiitic and calc-alkaline mafic material was co-erupted with the dominant rhyolite. These mafic clasts host crystals with compositions which demonstrate that there was some limited pre-eruptive physical interaction of mafic magmas with the mush and melt-dominant body. However, the mafic magmas do not appear to have triggered the eruption or controlled magmatic temperatures in the erupted rhyolite. Integration of textural and compositional data from all available crystal types, across all dominant and subordinate magmatic components, allow the history of the Oruanui magma body to be reconstructed over a wide range of temporal scales using multiple techniques. This history spans the tens of millennia required to grow the parental magma system (U–Th disequilibrium dating in zircon), through the centuries and decades required to assemble the eruptible magma body (textural and diffusion modelling in orthopyroxene), to the months, days, hours and minutes over which individual phases of the eruption occurred, identified through field observations tied to diffusion modelling in magnetite, olivine, quartz and feldspar. Tectonic processes, rather than any inherent characteristics of the magmatic system, were a principal factor acting to drive the rapid accumulation of magma and control its release episodically during the eruption. This work highlights the richness of information that can be gained by integrating multiple lines of petrologic evidence into a holistic timeline of field-verifiable processes.  相似文献   

Exploitation of gold in a historic sewage sludge stockpile, Werribee, Australia: resource evaluation, chemical extraction and subsequent utilisation of sludge     

Shane J. Reeves  Ian R. Plimer  David Foster 《Journal of Geochemical Exploration》1999,65(2):397

Sewage sludges are dewatered end products of human sewage waste and are recognised repositories of organic pollutants and heavy metals. They may be considered targets for economic extraction of Au because of the documented Au content of sewage sludges worldwide which are of the order of some ore deposits currently mined for Au. They are also highly nutrient enriched (nitrogen and phosphorus) and therefore amenable to use as agricultural fertiliser or as covers for mine wastes. The sewage of Melbourne, Australia, a city with a current population of 3.3 million, was stockpiled in large, closed, lagoonal tanks from 1898 until 1980. In 1995 Echidna Mining, an Australian gold exploration company, acquired the exploration rights to the ground surrounding the historic sludge reserves and commenced a program of resource evaluation, utilising RNAA, INAA, GFAAS, ICP–MS and FLAAS to determine 31 elements, including Au, Ag, Sb, As, Cd, Hg, Zn, Cu, and Pb. The study was initiated to determine Au, Ag and other metal variations in both space and time and to investigate the economics of chemical extraction of the precious metals. A total of 149 samples from over 50 hand-auger drillholes to a depth up to 4 m have been analysed from the stockpiles, with Au assays yielding remarkably consistent results. Average grades of 0.77 g/t Au and 18.8 g/t Ag have been documented for a measured resource of 770,000 m³ (of an estimated 1.6–2.5 million m³ contained) at a density of 1.0 g/cm³ and an average moisture content of around 40%. Laboratory-based extractive metallurgy of the Werribee sludges has demonstrated that Au, Ag and Zn can be removed with relative ease by heap-leaching using modified conventional technology, albeit with prohibitive reagent consumption. The extraction of the precious metals also results in the variable removal of contaminant metals such as Cd, As, Sb, Hg and Cr which may render the sludges fit for sale as agricultural fertiliser, provided organic pollutants and pathogenic organisms are below governmental environmental protection limits, an area beyond the scope of this paper. Another potential avenue of the exploitation of sewage sludges is discussed: that of the utilisation of sludges to extract contaminant metals from waste water and contaminated mine waters, which we demonstrate on pure aqueous synthetic samples. This paper presents a study of the exploitation of an historic sludge resource for its contained Au and residue post-metal extraction.  相似文献   

The effects of disproportional load contributions on quantifying vegetated filter strip sediment trapping efficiencies     

Heather?E.?GallEmail authorView author&#;s OrcID profile  Daniel?Schultz  Tamie L.?Veith  Sarah C.?Goslee  Alfonso?Mejia  Ciaran J.?Harman  Cibin?Raj  Paul?H.?Patterson 《Stochastic Environmental Research and Risk Assessment (SERRA)》2018,32(8):2369-2380

Vegetated filter strips (VFSs) are a best management practice (BMP) commonly implemented adjacent to row-cropped fields to trap overland transport of sediment and other constituents present in agricultural runoff. Although they have been widely adopted, insufficient data exist to understand their short and long-term effectiveness. High inter-event variability in performance has been observed, yet the majority of studies report average removal efficiencies over observed or simulated events, ignoring the disproportional effects of loads into and out of VFSs over longer periods of time. We argue that due to positively correlated sediment concentration-discharge relationships, disproportional contribution of runoff events transporting sediment over the course of a year (i.e., temporal inequality), decreased performance with increasing flow rates, and effects of antecedent moisture condition, VFS removal efficiencies over annual time scales may be significantly lower than reported per-event averages. By applying a stochastic approach, we investigated the extent of disparity between reporting average efficiencies from each runoff event over the course of 1 year versus the total annual load reduction. Additionally, we examined the effects of soil texture, concentration-discharge relationship, and VFS slope in contributing to this disparity, with the goal of revealing potential errors that may be incurred by ignoring the effects of temporal inequality in quantifying VFS performance. Simulation results suggest that ignoring temporal inequality can lead to overestimation of annual performance by as little as < 2% and to as much as > 20%, with the greatest disparities observed for soils with high clay content.  相似文献