Scheelite skarn granitoids: An evaluation of the roles of magmatic source and process

The compositions, mineralogies, textures, and isotopic characteristics of granitoids associated with scheelite skarns indicate these plutonic rocks cannot be uniquely described in terms of source materials, although most show distinguishing features of I-type granites^*. Scheelite skarn granitoids exhibit variable evidence for crustal contamination (primarily in their Sr isotopic ratios), but there is no correlation between degree of contamination (as measured by compositiona, mineralogical, and isotopic data) and size or abundance of associated scheelite skarns. Scheelite and Cu skarn-associated granitoids are generally similar, which implies similar sources for these granitic rocks. Textural and bulk compositional data, however, suggest that scheelite skarn granitoids are different from Cu skarn granitoids by virtue of greater degree of differentiation and by crystallization in a comparatively deep plutonic environment. Consideration of relevant phase equilibria indicates that magmatic water does not exsolve until very late in the crystallization of a scheelite skarn granitoid. Through this means, tungsten is concentrated in exsolved magmatic fluids by a combination of large degree of fractional cystallization and magmatic equilibration with a very Cl-rich exsolved aqueous phase. In consequence, the search for scheelite skarns should be based not on the search for plutons with appropriate chemical compositions (major, trace, or isotopic), but rather for plutons displaying mineralogical, textural, and general geologic features which point to crystallization in a highly fractionated, relatively deep environment. Large-scale tectonic features which might give rise to crystallization in this environment include crustal overthickening, which could be caused by collisional (accretionary) events.     

Tectonic evolution of Northeastern Siberia and adjacent regions

Previous models for the tectonic evolution of northeastern Siberia have proposed the existence of a Kolyma plate composed of the Kolyma and Omolon massifs of presumed Precambrian age. Lithologic similarities between the Siberian platform and the Cherskiy Mountains and the presence of oceanic and island arc type deposits in the Kolyma-Indigirka interfluve suggest that no such plate exists. The eastern margin of the Siberian plate is suggested to lie along a line between the Ulakhan Sis Range, the Alazeya uplift and the Arga Tas Range; the Cherskiy Mountains and the Verkhoyansk fold belt are parts of the Siberian plate. The Paleozoic deposits of the Omolon massif are unlike those found in the Cherskiys or Siberia. Paleomagnetic data from the Omolon massif are discordant from data from Siberia. It is suggested that the Omolon massif represents a microplate which accreted onto Siberia in the Jurassic. Ophiolites in central Chukotka are of the same emplacement age as in the western Brooks Range and may have been emplaced at the initiation of the rotation of Arctic Alaska. Geometric and limited stratigraphic data suggest that the East Siberian Sea may be floored by oceanic crust left by an incomplete closure between Arctic Alaska, Siberia and Omolon. The tectonic position of the Prikolymsk massif remains ambiguous.     

A summary of sulfur dioxide emission rate measuremnts from Guatemalan volcanoes

Measurements of the sulfur dioxide (SO₂) emission rate from three Guatemalan volcanoes provide data which are consistent with theoretical and laboratory studies of eruptive and shallow magma chamber processes. In particular, unerupted magma makes a major contribution to the measured SO₂ emission rates at Santiaguito, a continuously erupting dacitic volcanic dome. Varying shallow magma convection rates can explain the variations in SO₂ emission rates at Santiaguito. At Fuego, a basaltic volcano currently in repose, SO₂ emission rate measurements are consistent with a high level magma body that is crystallizing and releasing volatiles. At Pacaya, a continuously erupting basaltic volcano, recent SO₂ emission rate measurements support laboratory simulation studies of strombolian eruptions; these studies indicate that the majority of gas escapes during eruptions and little gas escapes between eruptions.Average SO₂ emission rates over the last 20 years for Santiaguito, Fuego and Pacaya are 80, 160 and 260 Mg/d, respectively. On a global scale, these three volcanoes account for 1% of the annual global volcanic output of SO₂. Santiaguito and Pacaya, together, emit 6% of the total annual SO₂ emitted by continuously erupting volcanoes.Even though SO₂ measurements at these volcanoes have been made infrequently and by different investigators, the collective data help to establish a useful baseline by which to judge future changes. A more complete record of SO₂ emission rates from these volcanoes could lead to a better understanding of their eruption mechanisms and reduce the impact of their future eruptions on Guatemalan society.     

Evidence for discrete subpopulations of sea perch (Helicolenus ercoides) across four fjords in Fiordland,New Zealand

In coastal populations of invertebrates and fishes, the distribution of discrete subpopulations is influenced by adult and larval dispersal, as well as by the effects of habitat heterogeneity on site fidelity or connectivity. Here, we examine evidence for spatial structure of sea perch, Helicolenus percoides, populations among four fjords in the Fiordland region of southwestern New Zealand. We examine patterns in adult morphology, length-at-age, δ¹³C and δ¹⁵N of muscle tissue, and trace elemental composition of whole otoliths as proxies for population isolation among the four inner fjord regions. A multivariate analysis of morphometrics reveals significant differences among populations from each of the four sites, suggesting existence of four distinct subpopulations. These patterns are consistent with observed differences in δ¹³C and δ¹⁵N, and length-at-age estimates among the four subpopulations. Differences in whole otolith concentrations of Sr, Ba, Mg and Li, and high classification scores based on the whole otolith elemental fingerprint are also consistent with significant subdivision among areas. Patterns across all four markers are consistent with discrete subpopulation structure of adult sea perch among the four study sites. These data indicate that the newly implemented network of marine protected areas in Fiordland is likely to contain discrete populations of sea perch.     

新疆东准噶尔石炭纪火山机构类型与时限

东准噶尔卡拉麦里大气田的主要储层是石炭纪火山岩,石炭纪古火山机构的样式与组合特征,与火山岩储层展布关系密切。野外调查表明,隐爆角砾岩是东准噶尔石炭纪古火山机构最常见的标志之一。东准噶尔石炭纪古火山机构类型主要有中心式、裂隙式和裂隙-中心式等样式。中心式火山机构以大石头层状火山、柯克巴斯套锥状火山最为典型;巴塔玛依内山附近沿断裂分布的带状火山岩具有裂隙式喷发的特征;东黑山火山具有火山口串珠状排列特征,是裂隙-中心式喷发的产物。大石头地区原缪林托凯陶山组火山岩的锆石SHRIMP U-Pb年龄为345.6±7.1 Ma,而已知巴塔玛依内山组火山岩年龄是350.0±6.3Ma,因此东准噶尔火山活动的主要时期是早石炭世。     

Variation of Mo isotopes from molybdenite in high-temperature hydrothermal ore deposits

Measurable molybdenum isotope fractionation in molybdenites from different ore deposits through time provides insights into ore genesis and a new technique to identify open-system behavior of Re–Os in molybdenites. Molybdenite samples from six porphyry copper deposits, one epithermal polymetallic vein deposit, four skarns, and three Fe-oxide Cu–Au deposits were analyzed. The δ⁹⁷Mo‰ (where ) for all samples varied from 1.34 ± 0.09‰ to −0.26 ± 0.04‰. This is the largest molybdenum isotopic variation in molybdenite from high-temperature ore deposits recorded to date. δ⁹⁷Mo‰ of molybdenite varies as a function of the deposit type and the rhenium and osmium concentrations of the samples. Isotope values for Mo also vary within the individual deposits. In general, molybdenites from porphyry copper deposits have the lightest values averaging 0.07 ± 0.23‰ (1σ). Molybdenites from the other deposit types average 0.49 ± 0.26‰ (1σ). The variations could be related to the fractionation of Mo into different mineral phases during the ore-forming processes. A comparison of the Mo isotope ratios and the Re–Os ages obtained from the same aliquot may possess a geochronological evaluation tool. Samples that yielded robust ages have different Mo isotopic compositions in comparison to samples that yielded geologically unreasonable ages. Another observed relationship between the Re–Os and Mo isotope data reveals a weak correspondence between Re concentration and Mo isotope composition. Molybdenites with higher concentrations of Re correspond to lighter Mo isotope values.     

An example of fingerprint detection of greenhouse climate change

As an example of the technique of fingerprint detection of greenhouse climate change, a multivariate signal or fingerprint of the enhanced greenhouse effect is defined using the zonal mean atmospheric temperature change as a function of height and latitude between equilibrium climate model simulations with control and doubled CO₂ concentrations. This signal is compared with observed atmospheric temperature variations over the period 1963 to 1988 from radiosonde-based global analyses. There is a significant increase of this greenhouse signal in the observational data over this period.These results must be treated with caution. Upper air data are available for a short period only, possibly too short to be able to resolve any real greenhouse climate change. The greenhouse fingerprint used in this study may not be unique to the enhanced greenhouse effect and may be due to other forcing mechanisms. However, it is shown that the patterns of atmospheric temperature change associated with uniform global increases of sea surface temperature, with El NinoSouthern Oscillation events and with decreases of stratospheric ozone concentrations individually are different from the greenhouse fingerprint used here.     

Colonisation of South African kelp-bed canopies by the alien mussel Mytilus galloprovincialis: extent and implications of a novel bioinvasion

The Mediterranean mussel Mytilus galloprovincialis is the most significant invasive alien marine species in South Africa and, although not normally found subtidally, has recently been observed colonising heads and stipes of the kelp species Ecklonia maxima in False Bay. We quantified this invasion and explored its ecological implications. Transects laid across kelp beds revealed that 10.34% of kelp individuals surveyed bore canopy mussels, with kelp heads (10.07%) being far more commonly infected than stipes (0.27%). Twenty kelp individuals with infected heads and 20 with infected stipes were separately collected for more-detailed examination. Wet mass of mussels on these kelp heads ranged from 2.5 to 2 462 g (median 86.4 g, interquartile range 14.8–353.8 g) and that of mussels on the stipes from 7.6 to 3 492 g (median 595.5 g, interquartile range 194.0–955.0 g). Mussel clumps consisted mostly of individuals <40 mm in length. Mussel clumps supported a rich biota of 80 invertebrate and 13 algal species. Larger clumps supported more epibiotic species, and those on stipes more species than those of comparable mass on kelp heads. The mussels and their associated epibiotic species negatively affected kelp buoyancy, but rarely enough to overcome natural buoyancy. Some kelp individuals that had been toppled by the weight of mussels and their epibiotic species, however, were encountered in situ. Implications of this invasion include large increases in animal biomass and species richness in the kelp canopy, plus reductions in kelp buoyancy and increased hydrodynamic drag on infected kelps, increasing their probability of being uprooted. Uprooted kelp individuals can raft long distances, potentially transporting both native and alien species to distant sites.     

A 3100-year history of argon isotopic and compositional variation at El Chichón volcano

El Chichón volcano has produced at least 11 eruptive events in the past 8000 years, all of which display a relatively constant trachyandesitic composition. The constancy of the eruptive products attests to the tapping of a long-lived magma chamber and suggests a system held in steady state between the influx of basaltic magma and differentiation of resident magma. We have sampled eruptive products from eight of these events, subjecting their dominant phenocryst phases (plagioclase and hornblende) to argon isotopic analysis in order to investigate the dynamics of a steady state magmatic system. Plagioclase from the older eruptions, 1500 yr BP, 1600 yr BP, 2000 yr BP and 3100 yr BP, display variable enrichment of ⁴⁰Ar (excess argon), whereas hornblende from the same eruptions shows little or no enrichment. In contrast, both plagioclase and hornblende phenocrysts from the younger eruptions in 1982, 550 yr BP, 900 yr BP and 1250 yr BP have argon isotopic ratios near-atmospheric values. Isochron analysis of these mineral phases rules out xenocrystic contamination as the source of excess argon, while mafic enclaves and isotopic and compositional zoning evidence frequent recharge events, suggesting the source of this argon is most likely the same as the source of this basaltic magma; correlation with Cl points to melt/vapor inclusions as the primary host for Ar within the phenocrysts. Argon isotopic variations point to a disturbance of the system between 1500 yr BP and 1250 yr BP, while compositional evidence for a major mafic input is present in the 900 yr BP eruption, indicating a complex relationship between recharge and isotopic signature of eruptive products. The amount of excess argon within a plagioclase phenocryst are a function of variations in melt and vapor inclusion abundance, time elapsed between melt/vapor inclusion entrapment and eruption, variations in Ar abundance in melt (itself a function of vapor exsolution timing) and time variations in Ar isotopic composition of the melt; subdued behavior of hornblende is due to slower diffusion and minimal inclusions.