Alvarez-Valero, A. M.Sumino, H.Burgess, R.Nunez-Guerrero, E....
13页
查看更多>>摘要：Noble gas isotopes, although present in trace amounts, are generally more reliable and less ambiguous recorders of their source than the major volatile species. In volcanic settings in particular, this advantage derives from their chemical inertness, as noble gas isotopic and elemental fractionations are strongly coupled to their source and modified only by physical processes during magma ascent and eruption. The Neogene volcano El Hoyazo (Betic Cordillera, SE Spain) is a highly favourable natural laboratory to study the links between partial crustal melting processes occurring at depth and the eruptive products at the surface, because partially melted crustal xenoliths are preserved in silicic lavas. Comparing the noble gas isotopic compositions of xenoliths and lavas has the potential to yield new insights into volatile behaviour during melting processes at inaccessible depths in the crust. At El Hoyazo, noble gases trapped in lava glasses, and the fluid/melt inclusions within xeno- and phenocrysts, provide novel information on: (i) their response to the crustal melting process including mechanisms such as magma mixing (and crustal assimilation) of two endmembers: i.e. the extracted felsic melt from the country metapelitic crust, and the basic-intermediate magma from the underplating in the region. The results reveal significant modification of magmatic noble gases by the interaction with the partially melted crust; (ii) noble gas variations during degassing and magma ascent, showing higher atmospheric influence in the lava samples from shallower depths than in the deeper lavas and minerals; and (iii) higher magmatic influence in crystals of garnet from deeper lava than in both shallower crystals of amphibole, and garnet crystals within the crustal xenoliths. In addition, we find that noble gases in melt inclusions are also likely accumulating in their shrinkage bubbles, and not only remaining dissolved in the melt.
Le Moigne, Y.Vigouroux, N.Russell, J. K.Williams-Jones, G....
18页
查看更多>>摘要：The Northern Cordilleran Volcanic Province (NCVP) is the most active volcanic belt in Canada, yet there are few detailed petrologic studies of the individual volcanoes. Tseax volcano is the southernmost volcanic centre of the NCVP, the second youngest (similar to 1700 CE) eruption in Canada, and led to one of Canada's worst natural disasters killing up to similar to 2000 Nisga' a First Nation people. We present a conceptual model for the origins, evolution and pre-eruptive storage of the Tseax magma which erupted in similar to 0.5 km(3) of volcanic material, mainly in the form of valley-filling lava flows. All Tseax products are alkali Fe- Ti- rich basanite-to-tephrite having trace element distributions similar to the other mafic NCVP lavas. Phenocrysts consist of plagioclase, olivine and titanomagnetite, often forming glomemcrysts suggesting co-crystallisation of these 3 mineral phases. Clinopymxene is never observed as a phenocryst phase. The lavas have low Mg-#' s implying they are fractionated relative to a mantle-derived 'parent' magma. Trace elements indicate the magma derives from melting of a fertile mantle source (i.e., (Nb/K)(n) - 1), most likely the upper asthenosphere. There is no evidence in the Tseax volcanic rocks for magma mixing or lithospheric contamination during ascent. The phenocryst assemblage suggests rapid ascent of a low viscosity magma to <5.5 km where titanomagnetite becomes the first phase on the liquidus (similar to 1133 degrees C). There the magma stalled for a very short period of time under PT conditions where clinopymxene did not reach saturation. Based on the size of the plagioclase phenocrysts, the magma stalled for less than a year and cooled down to 1094-1087 degrees C prior to eruption.
Fan, RongQian, GujieLi, YubiaoShort, Michael D....
11页
查看更多>>摘要：Acid rock drainage (ARD) is generated when mineral sulfides, primarily pyrite (FeS2), are exposed to weathering. Pyrite oxidation and dissolution may also result in the precipitation of secondary sulfate minerals, commonly found in weathered mine wastes, that release 'stored acidity' on subsequent dissolution. To better understand the release of this stored acidity, a kinetic leach column experiment using a synthetic pyritic (5 wt%) mine waste was conducted for a period of 524 weeks. The first 260 weeks watering/flushing was carried out using Milli-Q water during which the leachate pH decreased from 6.0 to 2.0, pyrite passivation layers present initially were removed and secondary sulfate minerals formed. The rate at which acidity was generated from secondary sulfate mineral dissolution was significantly greater than from pyrite oxidation after the calcite-saturated silicate water treatment during the period from 260 to 440 weeks. This suggests that in weathered pyritic wastes, both the amount of acidity and the rate at which it is released may be dominated by acid sulfate minerals rather than residual pyrite. Importantly, it was found that passivation strategies appropriate for reducing pyrite oxidation rates were not effective for reducing the rate of acid release from secondary mineralisation. Significant amounts of lime-saturated water (with high alkalinity) were required to dissolve the secondary sulfate minerals containing the stored acidity before circum-neutral conditions necessary for pyrite passivation could be achieved. It follows therefore that it is necessary to quantify the amount of stored acidity in secondary minerals so that appropriate remediation and acid management strategies can be designed and implemented to achieve long-term sustainable ARD control and mine closure.
查看更多>>摘要：The sulfur cycle at convergent margins remains poorly constrained yet is fundamentally important for understanding the redox state of Earth's reservoirs and the formation of ore deposits. In this study we investigate the sulfur isotope composition of high temperature volcanic gases emitted from the Nicaraguan (average of +4.8 +/- 1.3 parts per thousand) and Costa Rican (average of +2.3 +/- 1.3 parts per thousand) arc segments contributing to emissions from the Southern Central American Volcanic Arc (SCAVA; average of +3.8 +/- 1.7 parts per thousand). Along-arc variations in geochemical tracers at SCAVA are widely accepted to reflect variations in subduction parameters and deep fluid sources and correlations between these parameters and gas S isotope compositions are observed. These correlations suggest that gas emissions are sourced from a mixture of mantle S with delta S-34 similar to 0 parts per thousand and isotopically heavy slab-derived sulfur with delta S-34 >= similar to +8 parts per thousand. We employ Monte Carlo mass balance modeling to constrain S inputs to the subduction zone and relative contributions from mantle and slab to arc sulfur emissions. The models indicate that bulk subduction input in Nicaragua has a S isotope composition of +1.4 +/- 0.5 parts per thousand compared to -0.2 +/- 0.4 parts per thousand in Costa Rica, requiring preferential release of isotopically heavy oxidized S from the slab to explain the relatively high delta S-34 observed in arc outputs. We show that the flux of S from the slab is sufficient to oxidize the entire mantle wedge within the lifetime of the arc, indicating that S is a primary oxidizing agent in subduction zones. Furthermore, the preferential removal of heavy S from the slab requires retention of isotopically light S in the residual slab. Subduction-scale fractionation of S isotopes is fundamentally important in explaining why Earth's bulk surface reservoirs are isotopically positive.
查看更多>>摘要：The knowledge of the sources of base cations in stream water is a prerequisite to assess potential effects of changing environmental conditions such as changing rainfall, weathering or groundwater flows on cation export with stream water. This study use stable Mg isotopes to identify potential sources in the well-studied catchment of Krycklan located on gneissic bedrock covered by quaternary sediments in Sweden. Samples were collected from open filed rain, throughfall, stream, soil, rock and litterfall. The delta Mg-26 values of these samples was determined and the contributions of different sources to Mg fluxes in the stream were determined from the variation of the Mg isotope and Sr / Mg ratios. The results show an overall variation of 1.10 parts per thousand between all samples. In addition, Magnesium isotope ratios varied little in the streamwater and in soil solution, except during snowmelt periods during which a large portion of the annual runoff occurs. Magnesium in the streamwater is explained as a mixture of three pools (open field rain, soil solution and groundwater) with the latter two influenced by catchment processes. Outside the snow-melt period, Mg in streamwater mainly derived from the groundwater, assumed to be mineral weathering signature in this catchment, with a contribution ranging from 12 to 63% to Mg fluxes. Open field rain dominates Mg fluxes in streamwater during spring flood (0 to 78%) and may contribute significantly during larger summer and autumn rainfall events. Soil solution input to streamwater range from 16 to 59% of Mg fluxes in streamwater. Our results demonstrate that delta Mg-26 values together with Mg concentrations and Sr/Mg ratios can be used to constrain the Mg sources of stream water and quantify weathering release rates.
查看更多>>摘要：Pyroxenite veining is widely preserved in peridotite massifs, and used to derive information on the origin and evolution of upper mantle domains. These lithospheric mantle sections can be isolated from the convecting mantle for >1 Ga or more, suffering a long history of melting and/or melt-rock reaction processes, which modify their original chemical and isotopic compositions. Here, we show the effect of ancient process of melt-rock reaction in the chemistry of garnet pyroxenites from Lanzo North Massif, an iconic lithospheric mantle section exhumed during the opening of the Jurassic Alpine Tethys. Selected pyroxenites are more than 10 cm thick, and embedded within peridotites that have textures and chemical compositions indicative of a complex history of interaction with migrating melts. Whole rock and clinopymxene Nd-Hf isotopes of the pyroxenites consistently indicate that the first melt-rock reaction event occurred at similar to 400 Ma, likely in combination with exhumation from the garnet to the spinel-facies mantle conditions. Two samples still retain textural relicts and chemical evidence of precursor garnet and have high epsilon(Nd) (similar to 12) for comparatively low EHf (similar to 10), when recalculated at 400 Ma, which suggest that they were less affected by this ancient percolation process. The chemical evidence of such a long history of melt-rock reactions was preserved from 400 Ma until present. Finally, two pyroxenites located within plagioclase peridotites show evidence for an event of re-equilibration at plagioclase-facies conditions, likely triggered by infiltration of melt in the host rock. These samples reveal the coexistence of two internal Sm-Nd isochrones at 152 +/- 30 Ma and 149 +/- 13 Ma, thereby providing temporal constraints to the event of melt impregnation of the host peridotites as consequence of the opening of the Ligurian Tethys ocean.
查看更多>>摘要：Phyllosilicates may trap hydrogen (H-2) in the crust, but they may also produce it through various processes, including oxidative dehydrogenation. The dehydrogenation temperature depends on the type and composition of the phyllosilicates considered, but it may be as low as 300 degrees C. Here, we document the release of H-2 and CO during thermal treatment of chloritite (300 degrees C) and talc (500 degrees C) from the Trimouns deposits (Eastern Pyrenees, France). Thermal release of gases coupled to stable isotope analysis has been used to recover and characterize H-2 and CO, the two detected gases. Hydrogen content may be as high as 7 ppm with delta D-H2 values ranging from -258 parts per thousand to - 224 parts per thousand for sub-pure chloritite and - 140%0 for pure talc. CO content ranges between 3 ppm and 35.3 ppm with very homogeneous delta C-13(CO) values between -27.6 parts per thousand and - 25.7 parts per thousand. This study supports the idea that H-2 was produced during experiments by dehydrogenation. The origin of CO remains enigmatic, but its carbon isotope composition suggests a link to the few amounts of graphite documented in chloritite and talc from the deposit. This work also reports extensive hydrogen isotope fractionation between H-2 produced by dehydrogenation and both talc and chloritite. Dehydrogenation of phyllosilicates is a potential source term of H-2 in numerous magmatic-hydrothermal settings and must thus be accounted for in the budget of the H-2 geochemical cycle.
查看更多>>摘要：Carbonate weathering regulates the short-term carbon (C) cycle and global climate due to its fast response to hydrological processes. The carbonate weathering flux needs to be well constrained to better understand the climate change at short time scale. Riverine magnesium (Mg) isotopes are sensitive to primary mineral dissolution and so have great potential to trace carbonate weathering. Global large rivers draining continental crust dominate weathering flux to the oceans, but how riverine Mg isotopes respond to carbonate weathering remains unclear. The Yangtze River drainage basin (YRDB) was selected to test the robustness of riverine Mg isotopes (delta Mg-26) in tracing continental carbonate weathering because it spans a wide range in lithology, geomorphology and climate. The riverine delta Mg-26 values within the YRDB show a decreasing trend from the headwater to the mainstream ranging from -1.36 parts per thousand to -0.59 parts per thousand The dissolved delta Mg-26 have strong negative correlations with carbonate weathering rate and intensity within the YRDB, indicating a sensitive response of riverine delta Mg-26 to the carbonate weathering flux. In a compilation of Mg fluxes and delta Mg-26 in the world's largest rivers, there is similar dominance of carbonate weathering on riverine Mg fluxes and isotopes. Therefore, we propose that riverine delta Mg-26 in large rivers are a robust tracer of carbonate weathering intensity. Intensifying carbonate weathering under global warming tends to increase riverine Mg and C fluxes to the oceans and thus the atmospheric CO2 sink at the millennial time scale.
Simpson, S. L.Longstaffe, F. J.Osinski, G. R.Caudill, C. M....
17页
查看更多>>摘要：The peak-ring of the 66 Ma, similar to 180 km Chicxulub impact structure in the northern Yucatan peninsula and southern Gulf of Mexico was sampled during the International Ocean Discovery Program and International Continental Scientific Drilling Program (IODP-ICDP) Expedition 364 at Site M0077 (21.45 degrees N, 89.95 degrees W). Secondary clay minerals are pervasive throughout the upper peak-ring lithologies as a product of ubiquitous altered glass present throughout the impact melt and melt-bearing breccia sequence. Here we present the first detailed study of the clay mineralogy (microprobe, pXRD, spectral reflectance from 350 to 2500 nm) and isotope geochemistry (delta H-2 and delta O-18) of the <0.2 mu m size-fraction from upper peak-ring lithologies. The clay mineralogy is dominated by smectitic clay minerals, whose composition varies with stratigraphic position. Trioctahedral Mg-Fe smectite (var. saponite) is most common in Units or Subunits 2A, 2C, 3 and 4, while a section of Subunit 2B contains a more dioctahedral, Al-rich smectite. Higher porosity regions of the lower to mid, dioctahedral smectite-dominated intervals have higher delta O-18 (+14.2 to +18.6 parts per thousand) whereas intervals dominated by trioctahedral smectite have lower delta O-18 (+10.4 to +14.1 parts per thousand). The range of smectite delta H-2 (-105 to -87 parts per thousand), in comparison to that of oxygen isotopes, is proportionally much less variable and unrelated to smectite mineralogy. When combined, the oxygen and hydrogen isotope compositions of the smectitic clay minerals suggest low temperature (similar to 20 to 50 degrees C) formation from meteoric water-dominated fluids. The lower end of this temperature range is below current ambient conditions, which conceivably could suggest smectite formation before much of the overlying sedimentary rocks were deposited (similar to 56 Ma?). Calculated temperatures are generally lower than those associated with impact-generated hydrothermal alteration. Calculated delta O-18 and delta H-2 of meteoric water-dominated fluids associated with low-temperature formation of these clay minerals are lower than known for modern meteoric water in the Yucatan region. The simplest explanation for the source of these ancient fluids is meteoric water-dominated Gulf Coast brines. A more remote possibility is orogenically-driven, long-distance transport of groundwater from highlands to the east via an artesian aquifer formed in part by fractured Mesozoic rocks extending laterally beneath the impact structure.
NSTL
Elsevier