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21.
Melting of Amphibole-bearing Wehrlites: an Experimental Study on the Origin of Ultra-calcic Nepheline-normative Melts 总被引：2，自引：0，他引：2
MEDARD ETIENNE; SCHMIDT MAX W.; SCHIANO PIERRE; OTTOLINI LUISA 《Journal of Petrology》2006,47(3):481-504
Olivine + clinopyroxene ± amphibole cumulates have beenwidely documented in island arc settings and may constitutea significant portion of the lowermost arc crust. Because ofthe low melting temperature of amphibole (1100°C), suchcumulates could melt during intrusion of primary mantle magmas.We have experimentally (piston-cylinder, 0·5–1·0GPa, 1200–1350°C, Pt–graphite capsules) investigatedthe melting behaviour of a model amphibole–olivine–clinopyroxenerock, to assess the possible role of such cumulates in islandarc magma genesis. Initial melts are controlled by pargasiticamphibole breakdown, are strongly nepheline-normative and areAl2O3-rich. With increasing melt fraction (T > 1190°Cat 1·0 GPa), the melts become ultra-calcic while remainingstrongly nepheline-normative, and are saturated with olivineand clinopyroxene. The experimental melts have strong compositionalsimilarities to natural nepheline-normative ultra-calcic meltinclusions and lavas exclusively found in arc settings. Theexperimentally derived phase relations show that such naturalmelt compositions originate by melting according to the reactionamphibole + clinopyroxene = melt + olivine in the arc crust.Pargasitic amphibole is the key phase in this process, as itlowers melting temperatures and imposes the nepheline-normativesignature. Ultra-calcic nepheline-normative melt inclusionsare tracers of magma–rock interaction (assimilative recycling)in the arc crust. KEY WORDS: experimental melting; subduction zone; ultra-calcic melts; wehrlite 相似文献
22.
Erping BI Torsten SCHMIDT 《地学前缘》2005,12(Z1):199-208
有机组分的土水分配系数(Kd)是描述有机组分在地下系统中吸附特征的重要参数。同时,它也是物质运移模拟和环境评价中的主要参数之一。影响Kd的因素可概括为三个方面:土壤性质、有机组分本身特征及水相的物理化学性质。一般而言,对于非极性和弱极性有机组分,土壤中的有机质含量(foc )是影响Kd的最主要因素。但是,对于极性有机组分(POCs), 特别是在土壤有机质含量较低的情况下,土壤中矿物的种类和含量、水化学组分特征(pH、离子力等)经常在吸附过程中起重要作用。实验室内测定Kd的方法包括批实验和柱实验方法。批实验法适用于研究Kd较高情况下的吸附。在Kd较低的情况下,如低有机质土壤对极性有机污染物的吸附,土柱色谱法(SCC)是更适宜的选择。另外,可用土柱色谱法快速了解各种因素对吸附过程的影响,并获取详细的吸附和解吸信息。应用土柱色谱法时应当注意非平衡吸附和可能的柱堵塞问题。很多文献中提到结合柱实验和已有的吸附数据来预测土壤有机碳标准化的分配系数Koc(=Kd/foc)。但是,如果没有考虑吸附中特定的作用过程(如矿物吸附),对极性有机组分Koc的预测将会产生很大的误差。在环境评价中,将从一种土壤测定的Koc 应用到不同性质的土壤中,可能会导致错误的认识。在进行室内实验时,应把标准土(如Eurosoi 相似文献
23.
Ultra-calcic Magmas Generated from Ca-depleted Mantle: an Experimental Study on the Origin of Ankaramites 总被引：3，自引：1，他引：3
Ultra-calcic ankaramitic magmas or melt inclusions are ubiquitousin arc, ocean-island and mid-ocean ridge settings. They areprimitive in character (XMg > 0·65) and have highCaO contents (>14 wt %) and CaO/Al2O3 (>1·1). Experimentson an ankaramite from Epi, Vanuatu arc, demonstrate that itsliquidus surface has only clinopyroxene at pressures of 15 and20 kbar, with XCO2 in the volatile component from 0 to 0·86.The parental Epi ankaramite is thus not an unfractionated magma.However, forcing the ankaramite experimentally into saturationwith olivine, orthopyroxene and spinel results in more magnesian,ultra-calcic melts with CaO/Al2O3 of 1·21–1·58.The experimental melts are not extremely Ca-rich but high inCaO/Al2O3 and in MgO (up to 18.5 wt %), and would evolve tohigh-CaO melts through olivine fractionation. Fractionationmodels show that the Epi parent magma can be derived from suchultra-calcic experimental melts through mainly olivine fractionation.We show that the experimental ultra-calcic melts could formthrough low-degree melting of somewhat refractory mantle. Thelatter would have been depleted by previous melt extraction,which increases the CaO/Al2O3 in the residue as long as someclinopyroxene remains residual. This finding corrects the commonassumption that ultra-calcic magmas must come from a Ca-richpyroxenite-type source. The temperatures necessary for the generationof ultra-calcic magmas are 相似文献
24.
Regional and local patterns of low-grade metamorphism in the North Shore Volcanic Group, Minnesota, USA 总被引：2，自引：0，他引：2
S. Th. SCHMIDT 《Journal of Metamorphic Geology》1993,11(3):401-414
Abstract The North Shore Volcanic Group in northern Minnesota is part of the Middle Proterozoic Keweenawan sequence, one of the largest plateau lava provinces in the world. The primary geochemistry of the basalts suggests that volcanism occurred in an intracontinental rift environment. The subaerial lava flows, mainly amygdaloidal olivine tholeiites and tholeiites, have undergone low-grade metamorphism from zeolite to lower greenschist facies. On the basis of alteration phases replacing the primary magmatic minerals, infilling amygdales and veins, and replacing secondary minerals, the following zones have been distinguished: (1) thomsonite-scolecite-smectite, (2) heulandite-stilbite-smectite, (3) laumontitechlorite-albite, (4) laumontite-chlorite-albite ± prehnite ± pumpellyite and (5) epidote-chlorite-albite ± actinolite zone. In addition to the overall zonation based on mineral parageneses, zonations in the composition of the Ab content of the newly formed albite replacing primary Ca-rich plagioclase and of the newly formed mafic phyllosilicates are observed within the sequence and within single flows. Mafic phyllosilicates in the upper part of the sequence (mainly smectites and mixed-layer smectite/chlorites) display high Si and Ca + Na + K contents, whereas in the lower part of the sequence the amounts of Si and Ca + Na + K are markedly lower (mainly chlorites and mixed-layer chlorite/smectites). Similar zonations are observed within the individual flows. The albite content of the newly formed plagioclase is highest, and the Si and Ca + Na + K content of the phyllosilicates lowest in the amygdaloidal flow top while the opposite is true for the massive flow interior. The above features suggest that the overall pattern is one of burial-type metamorphism associated with extension in the rift setting. In detail, the mineral assemblages are controlled not only by the stratigraphic position but also by the flow morphology controlling permeability whose effect on the assemblages is most pronounced in the stratigraphically upper parts. This suggests that at the first stages of alteration (lowest grade) the patterns of fluid flow were important effects in controlling the assemblages. At greater burial depth, assemblages are more homogeneous, perhaps representative of a more even and pervasive flow pattern. Using the observed assemblages at face value to define grade and/or facies, different conditions would be assigned within the different morphological flow portions. Thus at low-grade metamorphic conditions it is essential to integrate assemblages from different morphological flow portions in order to define satisfactorily the overall metamorphic conditions. 相似文献
25.
Six-year record of atmospheric carbon dioxide and methane at a high-altitude mountain site in Poland 总被引：2，自引：0，他引：2
26.
Magnitude and frequency of bedload transport was examined in the Lainbach River, Bavaria, using magnetically tagged particles. During the study, 18 small to moderate events and one major event occurred. About 1 per cent of the flow period exceeded the entrainment threshold and at least once every year about 50 per cent of the tagged particles were mobile. Themajor event which occurred during the study period resulted in the deposition of a thick layer of sediment in parts of the channel and passive burial of most of the recovered particles. The step–pool pattern, which characterized the study site prior to the large event, was obliterated. However, the channel recovered quickly and has returned to a new step–pool pattern. The event changed the boundary conditions by increasing the availability of loose sediment and creating higher river-bed slopes in reaches between breached check dams. As a result, movement of individual particles measured for events of both the same magnitude and duration, occurring before and after the large event, yielded different values. For events which occurred after the large event, the range and the mean distance of movement were about ten times higher. 相似文献
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CHEN Xuegang YU Mingzhen LOH Peisun Dieter GARBESCH?NBERG Mark SCHMIDT CHEN Chen Tung Arthur ZHENG Hao YE Ying 《《地质学报》英文版》2019,93(Z2):12-13
A group of low‐angle normal faults developed in banded gabbro of Moa Ophiolite, Cuba. The dark gabbro was cut into puddings by several normal faults, while light gabbro was just swelling in layer thickness. In Hongliuhe ophiolite at eastern segment of South Tien Shan Suture Zone in China, the extensional deformation concentrates on fine cumulus gabbro which is typically mylonitized. Abundant structural features were discovered in HLH ophiolite such as S‐C foliation, C’ foliation, extensional crenulation cleavage, small toughness normal fault, low‐angle normal faults and high‐angle normal faults. According to the above tectonic phenomenon from the ophiolite belts in Cuba and China, we will get the conclusion: the maximum principal compressive stress (b1) is vertical to cumulus bedding, and the maximum tensile stress (b3) is paralleling to cumulus bedding. Considering of the above evidence, the extensional tectonic event should developed at mid‐ocean ridge. Due to seafloor spreading, the maximum tensile stress is paralleling to cumulus layer, and extensional tectonic is kept in cumulus gabbro. In this way, normal faults developed in dark gabbro, while brittle‐ductile extensional developed in light gabbro. A large number of domes, folds paralleling to ocean ridge and detachment faults represented by low angle normal fault were discovered near ocean ridge in Indian Ocean and Atlantic Ocean. In this way, materials from deep oceanic lithosphere (e.g. gabbro, mantle peridotite) outcrop at the crust surface of ocean basin. The above evidences from China and Cuba are consistent with extensional tectonic and metamorphic core complex from slowly and super‐slowly spreading Indian Ocean and Atlantic Oceanic lithosphere based on ODP. Therefore, extensional deformation in the ophiolite belt is of significant meaning for clarifying the formation process and mechanism of ancient oceanic basin. 相似文献
30.
Timing and nature of fluid flow and alteration during Mesoproterozoic shear zone formation, Olary Domain, South Australia 总被引：5，自引：1，他引：5
The development of shear zones at mid‐crustal levels in the Proterozoic Willyama Supergroup was synchronous with widespread fluid flow resulting in albitization and calcsilicate alteration. Monazite dating of shear zone fabrics reveal that they formed at 1582 ± 22 Ma, at the end of the Olarian D3 deformational event and immediately prior to the emplacement of regional S‐type granites. Two stages of fluid flow are identified in the area: first an albitizing event which involved the addition of Na and loss of Si, K and Fe; and a second phase of calcsilicate alteration with additions of Ca, Fe, Mg and Si and removal of Na. Fluid fluxes calculated for albitization and calcsilicate alteration were 5.56 × 109 to 1.02 × 1010 mol m?2 and 2.57 × 108–5.20 × 109 mol m?2 respectively. These fluxes are consistent with estimates for fluid flow through mid‐crustal shear zones in other terranes. The fluids associated with shearing and alteration are calculated to have δ18O and δD values ranging between +8 and +11‰, and ?33 and ?42‰, respectively, and ?Nd values between ?2.24 and ?8.11. Our results indicate that fluids were derived from metamorphic dehydration of the Willyama Supergroup metasediments. Fluid generation occurred during prograde metamorphism of deeper crustal rocks at or near peak pressure conditions. Shear zones acted as conduits for major crustal fluid flow to shallow levels where peak metamorphic conditions had been attained earlier leading to the apparent ‘retrograde’ fluid‐flow event. Thus, the peak metamorphism conditions at upper and lower crustal levels were achieved at differing times, prior to regional granite formation, during the same orogenic cycle leading to the formation of retrograde mineral assemblages during shearing. 相似文献