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21.
Teresa Moreno Fulvio Amato Xavier Querol Andrés Alastuey Josep Elvira Wes Gibbons 《Environmental Geology》2009,57(2):411-420
Given the relevance of desert aerosols to environmental issues such as dust storms, climate change and human health effects,
we provide a demonstration of how the bedrock geology of an arid area influences the mineralogy and geochemistry of even the
finest particulate matter (i.e., the inhalable fraction <10 μm in size: PM10). PM10 samples extracted from desert sediments at geologically contrasting off-road sites in central and southeastern Australia
(granitic, high grade metamorphic, quartzitic sandstone) were analyzed using X-ray diffraction (XRD), scanning electron microscopy
(SEM), inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry
(ICP-MS). The “granitic” PM10 are highly alkali feldspathic and illitic, with a wide range of accessory minerals including rutile (TiO2), monazite [(Ce, La, Nd, Th, Y) PO4], xenotime (YPO4), apatite [Ca5(PO4)3 (F, OH, Cl)], hematite (Fe3O4), zircon (ZrSiO4) and thorite (ThSiO4). This mineralogy is reflected in the geochemistry which shows notable enrichments in rare earth elements (REE) and most
high field strength elements (both held in the accessory minerals), and higher than normal levels of low (<2.0) ionic potential
elements (Na, K, Li, Cs, Rb: held in alkali feldspar and illite). The “metamorphic” resuspended PM10 define a mineralogy clearly influenced by local exposures of pelitic and calc-silicate schists (sillimanite, muscovite, calcite,
Ca-amphibole), a dominance of monazite over other REE-bearing phases, and a geochemistry distinguished by enrichments in alkaline
earth metals (Ca, Mg, Ba, Sr) and depletion in heavy REE. The “quartzite” PM10, derived from rocks already recycled by Precambrian erosion and sedimentary transport, show a sedimentologically mature mineralogy
of mostly quartz and kaolinite, detrital accessory ilmenite, rutile, monazite and hematite, and the strongest geochemical
depletion (especially K, Rb, Cs, Na, Ca, Mg, Ba). 相似文献
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Brian R. Jicha Garret L. Hart Clark M. Johnson Wes Hildreth Brian L. Beard Steven B. Shirey John W. Valley 《Contributions to Mineralogy and Petrology》2009,157(2):189-207
Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and
its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources,
a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns
for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade
arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators
of slab-fluids such as (Sr/P)
n
and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle
source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification
is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most
Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and
isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass
of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization
in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher
than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. δ
18O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding
involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope
compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile
arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust. 相似文献
23.
Plinian/ignimbrite activity stopped briefly and abruptly 16 and 45 h after commencement of the 1912 Novarupta eruption defining three episodes of explosive volcanism before finally giving way after 60 h to effusion of lava domes. We focus here on the processes leading to the termination of the second and third of these three episodes. Early erupted pumice from both episodes show a very similar range in bulk vesicularity, but the modal values markedly decrease and the vesicularity range widens toward the end of Episode III. Clasts erupted at the end of each episode represent textural extremes; at the end of Episode II, clasts have very thin glass walls and a predominance of large bubbles, whereas at the end of Episode III, clasts have thick interstices and more small bubbles. Quantitatively, all clasts have very similar vesicle size distributions which show a division in the bubble population at 30 μm vesicle diameter and cumulative number densities ranging from 107–109 cm–3. Patterns seen in histograms of volume fraction and the trends in the vesicle size data can be explained by coalescence signatures superimposed on an interval of prolonged nucleation and free growth of bubbles. Compared to experimental data for bubble growth in silicic melts, the high 1912 number densities suggest homogeneous nucleation was a significant if not dominant mechanism of bubble nucleation in the dacitic magma. The most distinct clast populations occurred toward the end of Plinian activity preceding effusive dome growth. Distributions skewed toward small sizes, thick walls, and teardrop vesicle shapes are indicative of bubble wall collapse marking maturation of the melt and onset of processes of outgassing. The data suggest that the superficially similar pauses in the 1912 eruption which marked the ends of episodes II and III had very different causes. Through Episode III, the trend in vesicle size data reflects a progressive shift in the degassing process from rapid magma ascent and coupled gas exsolution to slower ascent with partial open-system outgassing as a precursor to effusive dome growth. No such trend is visible in the Episode II clast assemblages; we suggest that external changes involving failure of the conduit/vent walls are more likely to have effected the break in explosive activity at 45 h. 相似文献
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Crustal contributions to arc magmatism in the Andes of Central Chile 总被引：52，自引：4，他引：52
Fifteen andesite-dacite stratovolcanoes on the volcanic front of a single segment of the Andean arc show along-arc changes in isotopic and elemental ratios that demonstrate large crustal contributions to magma genesis. All 15 centers lie 90 km above the Benioff zone and 280±20 km from the trench axis. Rate and geometry of subduction and composition and age of subducted sediments and seafloor are nearly constant along the segment. Nonetheless, from S to N along the volcanic front (at 57.5% SiO2) K2O rises from 1.1 to 2.4 wt %, Ba from 300 to 600 ppm, and Ce from 25 to 50 ppm, whereas FeO*/MgO declines from >2.5 to 1.4. Ce/Yb and Hf/Lu triple northward, in part reflecting suppression of HREE enrichment by deep-crustal garnet. Rb, Cs, Th, and U contents all rise markedly from S to N, but Rb/Cs values double northward — opposite to prediction were the regional alkali enrichment controlled by sediment subduction. K/Rb drops steeply and scatters greatly within many (biotite-free) andesitic suites. Wide diversity in Zr/Hf, Zr/Rb, Ba/Ta, and Ba/La within and among neighboring suites (which lack zircon and alkali feldspar) largely reflects local variability of intracrustal (not slab or mantle) contributions. Pb-isotope data define a limited range that straddles the Stacey-Kramers line, is bracketed by values of local basement rocks, in part plots above the field of Nazca plate sediment, and shows no indication of a steep (mantle+sedimentary) Pb mixing trend. 87Sr/86Sr values rise northward from 0.7036 to 0.7057, and 143Nd/144Nd values drop from 0.5129 to 0.5125. A northward climb in basal elevation of volcanic-front edifices from 1350 m to 4500 m elevation coincides with a Bougueranomaly gradient from –95 to –295 mgal, interpreted to indicate thickening of the crust from 30–35 km to 50–60 km. Complementary to the thickening crust, the mantle wedge beneath the front thins northward from about 60 km to 30–40 km (as slab depth is constant). The thick northern crust contains an abundance of Paleozoic and Triassic rocks, whereas the proportion of younger arc-intrusive basement increases southward. Primitive basalts are unknown anywhere along the arc. Base-level isotopic and chemical values for each volcano are established by blending of subcrustal and deep-crustal magmas in zones of melting, assimilation, storage and homogenization (MASH) at the mantle-crust transition. Scavenging of mid-to upper-crustal silicic-alkalic melts and intracrustal AFC (prominent at the largest center) can subsequently modify ascending magmas, but the base-level geochemical signature at each center reflects the depth of its MASH zone and the age, composition, and proportional contribution of the lowermost crust. 相似文献
26.
A.N. Halliday A.E. Fallick J. Hutchinson W. Hildreth 《Earth and Planetary Science Letters》1984,68(3):379-391
The Bishop Tuff represents a single eruption of chemically zoned rhyolitic magma. Six whole rock samples spanning the compositional and temperature range yield initial87Sr/86Sr of 0.7060–0.7092 andδ18O of 5.9–10.3‰. Six constituent sanidines yield smaller ranges of initial87Sr/86Sr of 0.7061–0.7069 andδ18O of 6.7–7.9. In contrast143Nd/144Nd ratios for the six whole rocks and two constituent magnetites exhibit negligible variation with a mean of0.51258 ± 1. These data are used to show that the phenocrysts were precipitated from an already chemically zoned liquid, that the zoning process involved negligible assimilation of, or exchange with, country rocks and that the extreme Sr and O isotopic disequilibria are probably the result of post-eruptive interaction with meteoric water. The parent magma had?Nd = ?0.9, ?Sr = +23 andδ18O = 7‰ and was formed from mantle-derived magmas and/or melts of lower crustal rocks isotopically similar to parts of the Sierra Nevada Batholith. 相似文献
27.
Rory Quinn Wes Forsythe Colin Breen Martin Dean Mark Lawrence Steve Liscoe 《Geoarchaeology》2002,17(5):441-451
Rathlin Island, off the north coast of Ireland, has a history of settlement and seafaring from the Late Mesolithic period to the present day. The maritime Sites and Monuments Record (SMR) for Rathlin indicates many wrecking incidents. In 1999, a reconnaissance side‐scan sonar survey confirmed the presence of 46 targets of possible archaeological potential around Rathlin Island. Thirteen of these anomalies were positively identified as shipwrecks. Of the remaining 33 targets, nine were dived on in order to ground‐truth the geophysical data. A successful and rapid methodology of ground‐truthing side‐scan sonar data for archaeological purposes was developed. The results confirmed the presence of a Danforth Anchor at one site, while the remaining anomalies were identified as geological features. The results from the side‐scan survey and diver‐truthing exercise enhanced the existing maritime SMR. © 2002 Wiley Periodicals, Inc. 相似文献
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Volcán Quizapu,Chilean Andes 总被引：1，自引：0，他引：1
Quizapu is a flank vent of the basalt-to-rhyodacite Holocene stratocone, Cerro Azul, and lies at the focus of a complex Quaternary volcanic field on the Andean volcanic front. The Quizapu vent originated in 1846 when 5 km3 of hornblende-dacite magma erupted effusively with little accompanying tephra. Between 1907 and 1932, phreatic and strombolian activity reamed out a deep crater, from which 4 km3 of dacite magma identical to that of 1846 fed the great plinian event of 10–11 April 1932. Although a total of >9 km3 of magma was thus released in 86 years, there is no discernible subsidence. As the pre-plinian crater was lined by massive lavas, 1932 enlargement was limited and the total plinian deposit contains only 0.4 wt % lithics. Areas of 5-cm and 1-cm isopachs for compacted 1932 fallout are about half of those estimated in the 1930's, yielding a revised ejecta volume of 9.5 km3. A strong inflection near the 10-cm isopach (downwind 110 km) on a plot of log Thickness vs Area1/2 reflects slow settling of fine plinian ash — not of coignimbrite ash, as the volume of pyroclastic flows was trivial (<0.01 km3). About 17 vol.% of the fallout lies beyond the 1-cm isopach, and 82 wt% of the ejecta are finer than 1 mm. A least 18 hours of steady plinian activity produced an exceptionally uniform fall deposit. Observed column height (27–30 km) and average mass eruption rate (1.5x108 kg/s) compare well with values for height and peak intensity calculated from published eruption models. The progressive aeolian fractionation of downwind ash (for which Quizapu is widely cited) is complicated by the large compositional range of 1932 juvenile pumice (52–70% SiO2). The eruption began with andesitic scoria and ended with basaltic scoria, but >95% of the ejecta are dacitic pumice (67–68% SiO2); minor andesitic scoria and frothier rhyodacite pumice (70% SiO2) accompanied the dominant dacite. Phenocrysts (pl>hbopx>mt>ilmcpx) are similar in both abundance and composition in the 1846 (effusive) and 1932 (plinian) dacites. Despite the contrast in mode of eruption, bulk compositions are also indistinguishable. The only difference so far identified is a lower range of D values for 1846 hornblende, consistent with pre-eruptive degassing of the effusive batch. 相似文献
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