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1.
Exploration by the NASA rover Opportunity has revealed sulfate- and hematite-rich sedimentary rocks exposed in craters and other surface features of Meridiani Planum, Mars. Modern, Holocene, and Plio-Pleistocene deposits of the Río Tinto, southwestern Spain, provide at least a partial environmental analog to Meridiani Planum rocks, facilitating our understanding of Meridiani mineral precipitation and diagenesis, while informing considerations of martian astrobiology. Oxidation, thought to be biologically mediated, of pyritic ore bodies by groundwaters in the source area of the Río Tinto generates headwaters enriched in sulfuric acid and ferric iron. Seasonal evaporation of river water drives precipitation of hydronium jarosite and schwertmannite, while (Mg,Al,Fe3+)-copiapite, coquimbite, gypsum, and other sulfate minerals precipitate nearby as efflorescences where locally variable source waters are brought to the surface by capillary action. During the wet season, hydrolysis of sulfate salts results in the precipitation of nanophase goethite. Holocene and Plio-Pleistocene terraces show increasing goethite crystallinity and then replacement of goethite with hematite through time. Hematite in Meridiani spherules also formed during diagenesis, although whether these replaced precursor goethite or precipitated directly from groundwaters is not known. The retention of jarosite and other soluble sulfate salts suggests that water limited the diagenesis of Meridiani rocks.Diverse prokaryotic and eukaryotic microorganisms inhabit acidic and seasonally dry Río Tinto environments. Organic matter does not persist in Río Tinto sediments, but biosignatures imparted to sedimentary rocks as macroscopic textures of coated microbial streamers, surface blisters formed by biogenic gas, and microfossils preserved as casts and molds in iron oxides help to shape strategies for astrobiological investigation of Meridiani outcrops.  相似文献   

2.
Sedimentary rocks exposed in the Meridiani Planum region of Mars record aqueous and eolian deposition in ancient dune and interdune playa-like environments that were arid, acidic, and oxidizing. On Earth, microbial populations have repeatedly adapted to low pH and both episodic and chronic water limitation, suggesting that, to a first approximation, the Meridiani plain may have been habitable during at least part of the interval when deposition and early diagenesis took place. On the other hand, the environmental conditions inferred for Meridiani deposition would have posed a challenge for prebiotic chemical reactions thought to have played a role in the origin of life on Earth. Orbital observations suggest that the combination of sulfate minerals and hematite found in Meridiani rocks may be unusual on the martian surface; however, there is reason to believe that acidity, aridity, and oxidizing conditions were broadly distributed on ancient Mars. When these conditions were established and how much environmental heterogeneity existed on early Mars remain to be determined. Because sulfates and iron oxides can preserve detailed geochemical records of environmental history as well as chemical, textural and microfossil signatures of biological activity, Meridiani Planum is an attractive candidate for Mars sample return.  相似文献   

3.
New data returned from the Mars Exploration Rover (MER) mission have revealed abundant evaporites in the sedimentary record at Meridiani Planum. A working hypothesis for Meridiani evaporite formation involves the evaporation of fluids derived from the weathering of martian basalt and subsequent diagenesis. On Earth, evaporite formation in exclusively basaltic settings is rare. However, models of the evaporation of fluids derived from experimentally weathering synthetic martian basalt provide insight into possible formation mechanisms. The thermodynamic database assembled for this investigation includes both Fe2+ and Fe3+ in Pitzer's ion interaction equations to evaluate Fe redox disequilibrium at Meridiani Planum. Modeling results suggest that evaporation of acidic fluids derived from weathering olivine-bearing basalt should produce Mg, Ca, and Fe-sulfates such as jarosite and melanterite. Calculations that model diagenesis by fluid recharge predict the eventual breakdown of jarosite to goethite as well as the preservation of much of the initial soluble evaporite component at modeled porosity values appropriate for relevant depositional environments (< 0.30). While only one of several possible formation scenarios, this simple model is consistent with much of the chemical and mineralogical data obtained on Meridiani Planum outcrop.  相似文献   

4.
Impure reworked evaporitic sandstones, preserved on Meridiani Planum, Mars, are mixtures of roughly equal amounts of altered siliciclastic debris, of basaltic provenance (40 ± 10% by mass), and chemical constituents, dominated by evaporitic minerals (jarosite, Mg-, Ca-sulfates ± chlorides ± Fe-, Na-sulfates), hematite and possibly secondary silica (60 ± 10%). These chemical constituents and their relative abundances are not an equilibrium evaporite assemblage and to a substantial degree have been reworked by aeolian and subaqueous transport. Ultimately they formed by evaporation of acidic waters derived from interaction with olivine-bearing basalts and subsequent diagenetic alteration. The rocks experienced an extended diagenetic history, with at least two and up to four distinct episodes of cementation, including stratigraphically restricted zones of recrystallization and secondary porosity, non-randomly distributed, highly spherical millimeter-scale hematitic concretions, millimeter-scale crystal molds, interpreted to have resulted from dissolution of a highly soluble evaporite mineral, elongate to sheet-like vugs and evidence for minor synsedimentary deformation (convolute and contorted bedding, possible teepee structures or salt ridge features). Other features that may be diagenetic, but more likely are associated with relatively recent meteorite impact, are meter-scale fracture patterns, veins and polygonal fractures on rock surfaces that cut across bedding. Crystallization of minerals that originally filled the molds, early cement and sediment deformation occurred syndepositionally or during early diagenesis. All other diagenetic features are consistent with formation during later diagenesis in the phreatic (fluid saturated) zone or capillary fringe of a groundwater table under near isotropic hydrological conditions such as those expected during periodic groundwater recharge. Textural evidence suggests that rapidly formed hematitic concretions post-date the primary mineral now represented by crystal molds and early pore-filling cements but pre-date secondary moldic and vug porosity. The second generation of cements followed formation of secondary porosity. This paragenetic sequence is consistent with an extended history of syndepositional through post-depositional diagenesis in the presence of a slowly fluctuating, chemically evolving, but persistently high ionic strength groundwater system.  相似文献   

5.
Evidence for sand motion is found in repeated observations of sand dunes at three sites in the Martian tropics by the High Resolution Imaging Science Experiment on Mars Reconnaissance Orbiter. An eroding outcrop of layered sediments is identified as a possible source of the sand in Pasteur crater. Ancient layered sediments in Becquerel crater are actively being carved into flutes and yardangs by the blowing sands. Dunes in an un‐named crater in Meridiani near the Mars Exploration Rover Opportunity landing site advanced as much as 50 cm over an interval of one Martian year. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

6.
The temperature rise caused by frictional heating during seismic slip is able to indicate dynamic frictional properties of the seismic fault,which provides an approach to understand the dynamic process and energy budget of an earthquake.The residual indicators of frictional heating within the fault zone also can be taken as an evidence for seismic events.The vitrinite reflectance is a commonly-used geothermometer in the coal,oil and gas industries.It also has some potential applications in the studies of fault rock and fault mechanics.We studied vitrinite reflectance (VR) of fault rocks collected from surface outcrops of the Wenchuan earthquake fault zone in this paper.The measured data reveal that the VR of fault rocks are affected by fault motion,and there is a trend that the VR increases towards the fault core,which indicates the effects of frictional heating.The VR of fault rocks from the Bajiaomiao outcrop is much higher than those from the Shenxigou outcrop,which probably suggests the difference in fault activity at the two outcrops.Our study also suggests that systematic measurement of VR across the fault zone is helpful in identifying slip zones and determining their widths.From the VR measurement on an oriented specimen containing the slip surface of the Wenchuan earthquake from the Shenxigou outcrop,we observed anomalous high VR values in two black-colored slip zones of about 2mm in width near the slip surface.The numerical calculation shows that the maximum temperature rise on the fault plane near Shenxigou was probably less than 162℃ during the Wenchuan earthquake,which means the dynamic fault strength was quite low.These estimations are roughly in accord with the results from the high-velocity frictional experiments.  相似文献   

7.
The Campi Flegrei (Naples, Campanian Plain, southern Italy) geothermal system is hosted by Quaternary volcanic rocks erupted before, during and after the formation of the caldera that represents one of the major structural features in the Neapolitan area. The volcanic products rest on a Mesozoic carbonate basement, cropping out north, east and south of the area. Chemical (major, minor and trace elements) and stable isotope (C, H, O) analyses were conducted on drill-core samples recovered from geothermal wells MF-1, MF-5, SV-1 and SV-3, at depths of ˜ 1100 to 2900 m. The study was complemented by petrographic and SEM examination of thin sections. The water which feeds the system is both marine and meteoric in origin. Mineral zonation typical of a high-temperature geothermal system exists in all the geothermal wells; measured temperatures in wells are as high as ˜ 400 °C. The chemical composition of the waters suggests the existence of two reservoirs: a shallow reservoir (depth < 2000 m) fed by seawater that boiled at 320 °C and became progressively diluted by steam-heated local meteoric water during its ascent; and a deeper reservoir (depth > 2000 m) of hypersaline water. The drill-cores are mainly hydrothermally altered volcanics of trachy-latitic affinity, but some altered pelites and limestones are also present. Published Na, Mg and K concentrations of selected geothermal waters indicate that the hydrothermal fluids are in equilibrium with their host rocks, with respect to K-feldspar, albite, sericite and chlorite. The measured δ18O(SMOW) values of rocks range from +4.3 to + 16.5%. The measured δD(SMOW) values range from − 79 to − 46%. The calculated isotopic composition of the fluids at equilibrium with the samples vary from + 1 to + 8.3%. δ18O and from − 52 to + 1%. δD. The estimated isotopic composition of the waters at equilibrium with the studied samples confirmed the existence of two distinct fluid types circulating in the geothermal system. The shallower has a marine water signature, while the deeper water has a signature consistent both with magmatic and meteoric origins. In the latter case, the recharge of this aquifer likely occurs at the outcrop of the Mesozoic Limestones surrounding the Campanian Plain; after infiltration, the water percolates through evaporitic layers, becoming hypersaline and D-depleted.  相似文献   

8.
A geochemical study of thermal and cold springs, stream waters and gas emissions has been carried out in the Mt. Amiata geothermal region.The cold springs and stream waters do not seem to have received significant contribution from hot deep fluids. On the contrary, the thermal springs present complex and not clearly quantifiable interactions with the hot fluids of the main geothermal reservoir.The liquid-dominated systems in the Mt. Amiata area, like most of the high-enthalpy geothermal fields in the world, are characterized by saline, NaCl fluids. The nature of the reservoir rock (carbonatic and anhydritic), and its widespread occurrence in central Italy, favor a regional circulation of “Ca-sulfate” thermal waters, which discharge from its outcrop areas. Waters of this kind, which have been considered recharge waters of the known geothermal fields, dilute, disperse and react with the deep geothermal fluids in the Mt. Amiata area, preventing the use of the main chemical geothermometers for prospecting purposes. The temperatures obtained from the chemical geothermometers vary widely and are generally cooler than temperatures measured in producing wells.Other thermal anomalies in central Italy, apart from those already known, might be masked by the above-mentioned circulation. A better knowledge of deep-fluid chemistry could contribute to the calibration of specific geothermometers for waters from reservoirs in carbonatic rocks.  相似文献   

9.
The eroded Colle Fabbri volcano comprises intrusive and extrusive rocks which cover an area of about 10,000 m2 in the Umbria region, Central Italy. The outcrop is located at the SW boundary of the Umbria Valley Graben (Umbria Region) on an N140 normal fault inside the Intramountain Ultra-Alkaline Province of carbonatite and melilitite rocks of central–southern Italy. A field survey of the outcrop allowed a reconstruction of igneous activity events of this unusual small-scale volcano. It is younger than 0.7 m.y. and rests on Lower Pleistocene conglomerates with intercalated clay beds. A palaeosoil marks the base of the volcanic sequence. It follows several metres of extrusive breccias composed of fragments of thermometamorphosed clay and travertine. Key features of these breccias are mud shells on blocks, plastic mud lumps, slumps, and mechanical injection from one layer into another, as well as plastic and vesiculated, micro-brecciated matrix. The breccias are cemented by a variety of silicate, sulphate, and sulphate–carbonate minerals deposited by intense hydrothermal circulation. The breccias are related to phreatic explosions triggered by the eruption of a superficial hydrothermal system. Up to 1 m thick sill and a dykelet swarm intrude the breccias. In the southern part of the outcrop there is a plug of melilitolitic composition which intrudes the breccias and deforms them upward. A variety of contact breccias is also scattered around the sill and the plug. In some places, hyaloclastite formed when melt invaded water pockets contained in the encasing rocks under hydrostatic pressure conditions. A thermal aureole, which moves up to 10 m away from the contact, is characterised by cordierite–trydimite association, thus indicating high-temperature (>>1000 °C) contact phenomena.  相似文献   

10.
Geochemical studies on cold meteoric waters, post-1980 hot spring waters, fumarole emissions from the dacite dome, and volcanic rocks at Mount St. Helens (MSH) from 1985 to 1989 show that magmatic volatiles are involved in the formation of a new hydrothermal system. Hot spring waters are enriched in 18O by as much as 2 and display enrichments in D relative to cold waters. A well-defined isotopic trend is displayed by the isotopic composition of a>400°C fumarole condensate collected from the central crater in 1980 (-33 D, +6 18O), of condensate samples collected on the dome, and of cold meteoric and hot spring waters. The trend indicates that mixing occurs between local meteoric water and magmatic water degassing from the dacite dome. Between 30 and 70% magmatic water is present in the dome fumarole discharges and 10% magnatic water has been added to the waters of the hydrothermal system. Relations between Cl, SO4 and HCO3 indicate that the hot spring waters are immature volcanic waters formed by reaction of rocks with waters generated by absorption of acidic volcanic fluids. In addition, the B/Cl ratios of the spring waters are similar to the B/Cl ratios of the fumarole condensates (0.02), values of 13C in the HCO3 of the hot springs (-9.5 to-13.5) are similar to the magmatic value at MSH (-10.5), and the 3He/4He ratio, relative to air, in a hot spring water is 5.7, suggesting a magmatic origin for this component.managed by Martin Marietta Energy Systems, Inc., under contract DE-AC05-84OR21400 with the US Department of Energy  相似文献   

11.
湘西南兰蓉岩体为一加里东期小侵入体,由黑云母二长花岗岩和二云母二长花岗岩组成.（443.5±8.1）Ma的锆石SHRIMP U Pb年龄表明花岗岩形成于早志留世早期.主量元素组成表明岩体总体属钙碱性高钾钙碱性系列强过铝质花岗岩类.该侵入体Ba、（Ta＋Nb）、Sr、P、Ti强烈亏损,Rb、（Th＋U＋K）、（La＋Ce）、Nd、（Zr＋Hf＋Sm）、（Y＋Yb＋Lu）等相对富集;稀土元素含量较高、轻稀土富集明显、Eu显著亏损;Isr值为0.71299,εSr（t）值为120,εNd （t）值为 8.11和-8.89,t2DM为1.82和1.84Ga.C/MF-A/MF图解显示其源岩为泥质岩和砂屑岩.上述地球化学特征表明兰蓉岩体为陆壳碎屑岩石部分熔融形成的S型花岗岩.基于岩石成因、构造环境判别以及区域构造演化过程,推断兰蓉岩体的具体形成机制为：奥陶纪末志留纪初的北流运动（板内造山运动）导致地壳增厚、升温,尔后在挤压减弱、应力松弛的后碰撞减压构造环境下,中、上地壳酸性岩石发生部分熔融并向上侵位而形成兰蓉岩体.  相似文献   

12.
Outcrop exposures of sedimentary rocks at the Opportunity landing site (Meridiani Planum) form a set of genetically related strata defined here informally as the Burns formation. This formation can be subdivided into lower, middle, and upper units which, respectively, represent eolian dune, eolian sand sheet, and mixed eolian sand sheet and interdune facies associations. Collectively, these three units are at least 7 m thick and define a “wetting-upward” succession which records a progressive increase in the influence of groundwater and, ultimately, surface water in controlling primary depositional processes.The Burns lower unit is interpreted as a dry dune field (though grain composition indicates an evaporitic source), whose preserved record of large-scale cross-bedded sandstones indicates either superimposed bedforms of variable size or reactivation of lee-side slip faces by episodic (possibly seasonal) changes in wind direction. The boundary between the lower and middle units is a significant eolian deflation surface. This surface is interpreted to record eolian erosion down to the capillary fringe of the water table, where increased resistance to wind-induced erosion was promoted by increased sediment cohesiveness in the capillary fringe. The overlying Burns middle unit is characterized by fine-scale planar-laminated to low-angle-stratified sandstones. These sandstones accumulated during lateral migration of eolian impact ripples over the flat to gently undulating sand sheet surface. In terrestrial settings, sand sheets may form an intermediate environment between dune fields and interdune or playa surfaces. The contact between the middle and upper units of the Burns formation is interpreted as a diagenetic front, where recrystallization in the phreatic or capillary zones may have occurred. The upper unit of the Burns formation contains a mixture of sand sheet facies and interdune facies. Interdune facies include wavy bedding, irregular lamination with convolute bedding and possible small tepee or salt-ridge structures, and cm-scale festoon cross-lamination indicative of shallow subaqueous flows marked by current velocities of a few tens of cm/s. Most likely, these currents were gravity-driven, possibly unchannelized flows resulting from the flooding of interdune/playa surfaces. However, evidence for lacustrine sedimentation, including mudstones or in situ bottom-growth evaporites, has not been observed so far at Eagle and Endurance craters.Mineralogical and elemental data indicate that the eolian sandstones of the lower and middle units, as well as the subaqueous and eolian deposits of the Burns upper unit, were derived from an evaporitic source. This indirectly points to a temporally equivalent playa where lacustrine evaporites or ground-water-generated efflorescent crusts were deflated to provide a source of sand-sized particles that were entrained to form eolian dunes and sand sheets. This process is responsible for the development of sulfate eolianites at White Sands, New Mexico, and could have provided a prolific flux of sulfate sediment at Meridiani. Though evidence for surface water in the Burns formation is mostly limited to the upper unit, the associated sulfate eolianites provide strong evidence for the critical role of groundwater in controlling sediment production and stratigraphic architecture throughout the formation.  相似文献   

13.
Low18O/16O values have been found in rocks from the Captains Bay pluton and surrounding country rocks on Unalaska Island in the Aleutian Islands. The pluton is surrounded by a metamorphic aureole in which the country rocks have been domed, faulted and chaotically fractured. Samples with lower18O/16O values (δ18O = ?4.1 to +2.7‰, SMOW) commonly have interstitial micrographic intergrowths of quartz and turbid potassium feldspar. These facts suggest extensive interaction of circulating meteoric waters with the pluton. Unalaska Island must have been subaerially exposed at or shortly after the time of intrusion of the pluton. The pervasive low-grade metamorphism of the “early series” volcanic rocks of all of the Aleutian Islands might be a result of the interaction of intrusive bodies with meteoric waters rather than the consequence of ridge subduction.  相似文献   

14.
The paper reports the first research on karst solution processes and rates in New Zealand. The study area is an IHD representative basin in the northwest corner of the South Island in a mountain range consisting principally of Ordovician marble. The climate is sunny and warm (17°C) in summer and wet and cool (7°C) in winter. Average precipitation is 2,158 mm of which 525 mm evapotranspires, yielding a discharge of 51–75 l/s/km2 in the river basin studied. Almost half of the catchment of 45.1 km2 consists of karst which occurs mainly as a doline covered plateau at 600–900 m within which most drainage is subterranean. Water tracing is with fluorescein defined drainage patterns. Marble solution was established by estimating inputs, throughputs and outputs of water and dissolved calcium and magnesium in both autogenic and allogenic karst drainage systems. Particular attention was paid to estimating errors. Water samples for chemical analysis were taken irregularly for approximately one year, and a rating curve relating chemical load to discharge was established. The best estimate of solution loss from the basin yields a mean rate of 100 ±M24 m3/km2/a. Of this 80 per cent is derived from solution of marble by autogenic waters, mostly in the top 10–30 m of the marble outcrop. The remaining 20 per cent is accomplished by allogenic stream solution. Approximately 9.9 per cent of the dissolved calcium and magnesium load leaving the basin originates from non-karst rocks and 4.6 per cent is initially introduced by rainfall. River flows that are exceeded only 5 per cent of the time transport approximately 44 per cent of the annual dissolved load, while mean to low flows that occur for 75 per cent of the time transport 35 per cent of the annual solute load. This confirms the importance of low frequency-high magnitude events, but indicates also that in corrosion systems high frequency events of moderate to low magnitude can also accomplish significant work.  相似文献   

15.
The Arakapas fault belt, southern Troodos Massif, which is interpreted as a fossil oceanic fracture zone, contains a sedimentary and volcanic “fill” including basaltic lava breccias, volcaniclastic sandstones, intercalated Fe-rich mudstones, and minor Cu sulphide mineralisation. The volcaniclastic sedimentary rocks were produced by submarine weathering and gravity redeposition of basalt within the transform zone. In contrast, the Fe-mudstones predominantly originated as chemical precipitates from hydrothermal solutions generated by hydrothermal leaching of basalt by seawater. The rare earth elements, and some trace metals, including Co, Ni, Pb, V and Zr, are thought to have been incorporated from seawater into the ferruginous precipitates. Unusually high Fe/Mn ratios of the Fe-mudstones relative to most modern ocean-ridge sediments may reflect deposition in relatively oxygen-depleted waters on the floor of a major bathymetric depression created by the transform faulting; release of acidic thermal waters would have further suppressed Mn precipitation. Contrary to recent suggestions of metallogenesis along oceanic fracture zones, none of the major Cyprus cupriferous sulphide bodies lie along the fossil Arakapas transform zone.  相似文献   

16.
Talat  Ahmad  Kabita C.  Longjam  Baishali  Fouzdar  Mike J.  Bickle  Hazel J.  Chapman 《Island Arc》2009,18(1):155-174
The Sakoli Mobile Belt comprises bimodal volcanic rocks that include metabasalt, rhyolite, tuffs, and epiclastic rocks with metapelites, quartzite, arkose, conglomerate, and banded iron formation (BIF). Mafic volcanic rocks are tholeiitic to quartz‐tholeiitic with normative quartz and hypersthene. SiO2 shows a large compositional gap between the basic and acidic volcanics, depicting their bimodal nature. Both the volcanics have distinct geochemical trends but display some similarity in terms of enriched light rare earth element–large ion lithophile element characteristics with positive anomalies for U, Pb, and Th and distinct negative anomalies for Nb, P, and Ti. These characteristics are typical of continental rift volcanism. Both the volcanic rocks show strong negative Sr and Eu anomalies indicating fractionation of plagioclases and K‐feldspars, respectively. The high Fe/Mg ratios for the basic rocks indicate their evolved nature. Whole rock Sm–Nd isochrons for the acidic volcanic rocks indicate an age of crystallization for these volcanic rocks at about 1675 ± 180 Ma (initial 143Nd/144Nd = 0.51017 ± 0.00017, mean square weighted deviate [MSWD] = 1.6). The εNdt (t = 2000 Ma) varies between ?0.19 and +2.22 for the basic volcanic rock and between ?2.85 and ?4.29 for the acidic volcanic rocks. Depleted mantle model ages vary from 2000 to 2275 Ma for the basic and from 2426 to 2777 Ma for the acidic volcanic rocks, respectively. These model ages indicate that protoliths for the acidic volcanic rocks probably had a much longer crustal residence time. Predominantly basaltic magma erupted during the deposition of the Dhabetekri Formation and part of it pooled at crustal or shallower subcrustal levels that probably triggered partial melting to generate the acidic magma. The influence of basic magma on the genesis of acidic magma is indicated by the higher Ni and Cr abundance at the observed silica levels of the acidic magma. A subsequent pulse of basic magma, which became crustally contaminated, erupted as minor component along with the dominantly acidic volcanics during the deposition of the Bhiwapur Formation.  相似文献   

17.
This paper discusses patterns that are observable in the alteration of effusive rocks that were discharged by Baranskii Volcano (central Iturup Island) under the action of sulfate chloride as well as acidic and ultra-acidic water (in the Kipyashchaya Rechka thermal brook). We acquired data on changes in the chemical and mineralogic composition of the rocks, structural features, porosity, and petrophysical properties. The dynamics of leaching and the leaching phase in a flowing acidic (ultra-acidic) geothermal environment are described. We note that the mechanism that is responsible for hydrogen sulfate leaching of rocks at the ground surface may be largely analogous to the generation of secondary quartzites (mono-quartzites) in the zones of ascending acidic gas flows above small gabbro-diorite and diorite intrusions.  相似文献   

18.
Full chemical analyses, including some trace elements and both oxygen and strontium isotope abundance data are presented for samples collected from a traverse across the outcrop of the early Tertiary Loch Uisg Granophyre. Chemically, the body is rhyodacite with very uniform major and trace element composition. In contrast, depleted δ18O values vary widely from +1.5‰ in the south to ?3.7‰ in the north (a distance of about 21/2 km), a range comparable to that for the intrusive rocks of Mull as a whole. This indicates more extensive groundwater interaction (i.e. higher water/rock ratios and/or higher temperatures of isotope exchange) towards the focus of the central intrusive complex. There is some degree of correlation between δ18O and iron oxidation ratios but no other evidence that the primary igneous geochemistry of these rocks has been significantly modified by hydrothermal alteration after emplacement of the pluton. Initial87Sr/86Sr ratios range from 0.71350 ± 9 to 0.71624 ± 6 and correlate with both Rb content and Rb/Sr ratio, the latter correlation yielding a pseudo-isochron of 260 ± 54 Ma at the time of emplacement. These results confirm a major contribution from an old crustal source region, ruling out formation of the granophyre solely from a basic parent magma. However, Rb-Sr data are presented for the Moine schists exposed in Mull and Morvern which also appear to rule out their involvement in the petrogenesis of the granophyre, either as a source region for melting, or as a bulk contaminant for a mantle-derived magma. The only viable hypotheses are assimilation at depth of? Lewisian into a basaltic fractionation sequence or partial melting of a Proterozoic basement such as that involved in the production of Caledonian granites in the Scottish Highlands.  相似文献   

19.
The Northern Junggar Basin experienced extensive subduction and a complex tectono-magmatic evolution during the Late Paleozoic,resulting in a heterogeneous distribution of volcanic rocks in the Junggar Basin.In this study,the Carboniferous tectono-magmatic evolution of the northern Luliang arc was described by exploring the petrography and geochemistry of Carboniferous volcanic rocks collected from well Y-2 and outcrop WW' in the northern Luliang Uplift.The distribution,types,and formation ages of these volcanic rocks were characterized and the volcanic sequence in well Y-1 was divided into upper and lower parts according to vertical variations in selected geochemical data.Then the petrogenesis and tectonic settings of different volcanic rocks were evaluated and this was used to infer the tectonomagmatic evolution of the northern Luliang arc during the Carboniferous.The results indicate that:(1) Carboniferous high-K calc-alkali andesite-dacite associations are distributed in the west of the northern Luliang Uplift,and Lower Carboniferous calc-alkali basalt-dacite-rhyolite assemblages are preserved in its east.(2) The intermediateacid volcanic rocks in wells Y-1 and Y-2 were derived from calc-alkali basaltic magma through melting of the juvenile lower crust,and geochemical variations indicate increasing addition of slab melting in a subduction-related arc environment.The bimodal volcanic rocks from outcrop WW' were derived from lithospheric underplating of basaltic magma in an intra-arc extensional setting.(3) The closure of the eastern Keramaili Oceanic basin occurred before the Early Carboniferous,and the tectono-magmatic difference between the east and the west of the northern Luliang Uplift appeared before the Carboniferous period.  相似文献   

20.
The Active Crater at Rincón de la Vieja volcano, Costa Rica, reaches an elevation of 1750 m and contains a warm, hyper-acidic crater lake that probably formed soon after the eruption of the Rio Blanco tephra deposit approximately 3500 years before present. The Active Crater is buttressed by volcanic ridges and older craters on all sides except the north, which dips steeply toward the Caribbean coastal plains. Acidic, above-ambient-temperature streams are found along the Active Crater's north flank at elevations between 800 and 1000 m. A geochemical survey of thermal and non-thermal waters at Rincón de la Vieja was done in 1989 to determine whether hyper-acidic fluids are leaking from the Active Crater through the north flank, affecting the composition of north-flank streams.Results of the water-chemistry survey reveal that three distinct thermal waters are found on the flanks of Rincón de la Vieja volcano: acid chloride–sulfate (ACS), acid sulfate (AS), and neutral chloride (NC) waters. The most extreme ACS water was collected from the crater lake that fills the Active Crater. Chemical analyses of the lake water reveal a hyper-acidic (pH0) chloride–sulfate brine with elevated concentrations of calcium, magnesium, aluminum, iron, manganese, copper, zinc, fluorine, and boron. The composition of the brine reflects the combined effects of magmatic degassing from a shallow magma body beneath the Active Crater, dissolution of andesitic volcanic rock, and evaporative concentration of dissolved constituents at above-ambient temperatures. Similar cation and anion enrichments are found in the above-ambient-temperature streams draining the north flank of the Active Crater. The pH of north-flank thermal waters range from 3.6 to 4.1 and chloride:sulfate ratios (1.2–1.4) that are a factor of two greater than that of the lake brine (0.60). The waters have an ACS composition that is quite different from the AS and NC thermal waters that occur along the southern flank of Rincón de la Vieja.The distribution of thermal water types at Rincón de la Vieja strongly indicates that formation of the north-flank ACS waters is not due to mixing of shallow, steam-heated AS water with deep-seated NC water. More likely, hyper-acidic brines formed in the Active Crater area are migrating through permeable zones in the volcanic strata that make up the Active Crater's north flank. Dissolution and shallow subsurface alteration of north-flank volcanoclastic material by interaction with acidic lake brine, particularly in the more permeable tephra units, could weaken the already oversteepened north flank of the Active Crater. Sector collapse of the Active Crater, with or without a volcanic eruption, represents a potential threat to human lives, property, and ecosystems at Rincón de la Vieja volcano.  相似文献   

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