The redox states of basic and silicic magmas: a reflection of their source regions?   总被引：22，自引：1，他引：21  

Ian S. E. Carmichael 《Contributions to Mineralogy and Petrology》1991,106(2):129-141

At present the best estimates of the oxygen fugacity of spinel-lherzolites that could be the source material of basic magmas is about five log units below the Ni–NiO buffer to one above it. However partially glassy basic lavas, ranging from MORBs to minettes, all with olivine on their liquidus, cover a wider range, and may have oxygen fugacities that extend to four log units above NNO. Surprisingly the range of oxygen fugacities observed in silicic lavas and ashflows with quartz phenocrysts is smaller, despite a crustal dominated evolution. The peralkaline silicic lava type pantellerite is the most reduced, equivalent to MORBs, whereas the large volume ashflows with phenocrysts of hornblende and/or sphene are the most oxidised. As the concentration of water in the basic lavas is correlated with increase in their redox state, it would seem that water could be the agent of this increase. That this is unlikely is seen in the behavior of silicic ashflows and lavas, particularly those of Yellowstone. Here the silicic magmas of the last 2Ma contain about 2 wt% FeO_(total), and typically phenocrysts of fayalite, quartz and Fe–Ti oxides. Despite extensive exchange of the ¹⁸O of the magma with meteoric water after caldera collapse (Hildreth et al. 1984), there is no displacement of the redox equilibria. Thus the thermal dissociation of molecular H₂O to H₂, and its subsequent diffusive loss to cause oxidation must have been minimal. This could only be so if the activity of water was small, as it would be if H₂O reacted with the silicate liquid to form OH groups (Stolper 1982). The conclusion is that silicic magmas with small amounts of iron and large amounts of water do not have their redox states reset, which in turn presumably reflect their generation. By analogy basic magmas with large amounts of iron and far less water are even less likely to have their redox equilibria disturbed, so that their oxygen fugacities will also reflect their source regions. The effect of pressure on the ferric-ferrous equilibrium in basic magmas can be calculated from experimental measurements of the partial molar volumes of FeO and Fe₂O₃, and their pressure derivatives V/P, in silicate liquids. The effect of pressure is found to be about the same on the liquid as it is for various solid oxygen buffers. Accordingly there should be mantle source regions covering the same range of oxygen fugacity as that found in basic lavas, but so far samples of spinel-lherzolite of equivalent oxygen fugacity to minettes or other potassic lavas have not been found. Whether or not the redox state of phlogopite-pyroxenites is equivalent to these potassic lavas cannot be established without experiment.  相似文献   

基于波动方程的广义屏叠前深度偏移   总被引：15，自引：7，他引：15       下载免费PDF全文

金胜汶  许士勇  吴如山 《地球物理学报》2002,45(5):684-690

地震波传播算子的计算效率和精度是制约三维叠前深度偏移的关键因素. 广义屏传播算子(GSP, Generalized Screen Propagator)是一种在双域中实现的广角单程波传播算子. 这一方法略去了在非均匀体之间发生的交混回响,但它可以正确处理包括聚焦、衍射、折射和干涉在内的各种多次前向散射现象. 通过背景速度下的相移和扰动速度下的陡倾角校正,广义屏算子能够适应地层速度的强烈横向变化. 这种算子可以直接应用于炮集叠前偏移,通过将广义屏算子作用于双平方根方程,还可以获得一种高效率、高精度的炮检距域叠前深度偏移方法,用于二维共炮检距道集和三维共方位角道集的深度域成像. 本文首先简述了炮检距域广义屏传播算子的理论,进而讨论了共照射角成像(CAI, Common Angle Imaging)条件,由此给出各个不同照射角(炮检距射线参数)下的成像结果,进而得到共照射角像集. 由于照射角和炮检距的对应关系,共照射角像集又为偏移速度分析和AVO(振幅随炮检距变化)分析等提供了有力工具.  相似文献   

Quaternary minettes and associated volcanic rocks of Mascota,western Mexico: a consequence of plate extension above a subduction modified mantle wedge   总被引：12，自引：0，他引：12  

I. S. E. Carmichael  R. A. Lange  James F. Luhr 《Contributions to Mineralogy and Petrology》1996,124(3-4):302-333

 Pleistocene-Holocene volcanism in the Jalisco block of western Mexico is confined to two conspicuous grabens, where potassic eruptives range from absarokites (48–52% SiO₂) and minettes (49–54% SiO₂) through basaltic andesites (53–57% SiO₂), the most voluminous type, to andesites and their lamprophyric equivalent spessartite (58–62% SiO₂); there are no contemporary rhyolitic rocks. This suite has high concentrations of Mg, Cr (<550 ppm) and Ni (<450 ppm) accompanied by large concentrations of K, P, Ba (<4000 ppm) and Sr (<5000 ppm) and elements such as LREE and Zr (<600 ppm). No combination of crystal fractionation and/or crustal contamination can reproduce the compositional range of these magmas, which nevertheless are believed to be genetically related because of their proximity in time and space. Hydrous minerals in the lamprophyres and the typical absence of plagioclase phenocrysts in both basaltic andesites and andesites reflect the relatively high concentrations of water in the magmas, which suppressed the crystallisation of feldspar. Experimental verification of the minimal amounts of water required to reproduce the phenocryst assemblages in selected rocks range from 3.5 to 6%. During ascent in a volcanic conduit, andesitic magma may lose water and consequently precipitate plagioclase, or it may ascend more rapidly, retaining more of its initial water, which stabilises phenocrysts of hornblende at the expense of plagioclase. Our estimates of water concentrations, which are consistent with the various low pressure phenocryst assemblages, will be minimal for the magmas in their source regions, and the process of magmatic dewatering on ascent may be typical in well established volcanic conduits. In accord with the compositions of phenocrystic olivine in the basaltic andesites and the minettes, the values of FeO and Fe₂O₃ of the bulk lavas and scoriae are demonstrably pristine. As a consequence, there are two characteristic features of the Mascota suite: the high range of relative oxygen fugacities (ΔNNO=1–5) and the high Mg# (MgO/MgO+FeO) that ranges from 0.70 to 0.91 (with only one andesite as low as 0.66). From the evidence of phlogopite phenocrysts, a partial melt involving phlogopite would have a higher Mg# than one from olivine (Fo₉₀) and pyroxene alone. As the Mascota series shows a correlation between K₂O and Mg#, we conclude that it was generated by partial fusion of the mantle wedge, with a variable contribution of phlogopite and apatite from veins throughout the lherzolitic assemblage. In conformity with an origin by varying increments of partial fusion of a phlogopite-bearing mantle, all incompatible elements vary linearly with Ti (or K) as if phlogopite (+apatite) in the source dominated their contribution to the partial melts. Fluids from dehydration of the subducting slab presumably deposit hydrous and other minerals in veins in the mantle wedge and also increase its redox state. As the Mascota volcanism occurs in grabens closer to the trench than the main andesite arc, it is concluded that the eruption of these small volumes of hydrous magmas require the tectonically favored ascent paths offered by the extensional grabens to reach the surface from their mantle sources. Received: 24 January 1995 / Accepted: 21 February 1996  相似文献   

In situ sampling reveals rapid uptake and depuration of polycyclic aromatic hydrocarbons by surf clams (Paphies subtriangulata) affected by the Rena oil spill     

PM Ross  RM Fairweather  DP Culliford  S Park  CA Pilditch  CN Battershill 《新西兰海洋与淡水研究杂志》2016,50(1):56-69

ABSTRACT

To investigate the uptake and depuration of polycyclic aromatic hydrocarbons associated with the Rena oil spill we sampled the surf clam Paphies subtriangulata at two open coast locations (6?km apart) just prior to oil coming ashore (7 October 2011), then at 1–3 week intervals for the next 4 months. Total polycyclic aromatic hydrocarbons (tPAH) increased at both sites from 1 to 96–124?µg?kg^?1 (wet weight) by 18 October before declining to low levels (<4?µg?kg^?1) by February 2012. Ongoing sampling throughout 2012–2014 included three additional sites to the north east (up to 30?km away) and a site 5?km to the south east revealing tPAH levels generally <10?µg?kg^?1 except in October 2013 where levels ranged between 39–45?µg?kg^?1 at all sites. A comparison of PAH component profiles with oil-contaminated beach sediment indicated that the high levels observed in surf clams between October–December 2011 were clearly associated with the Rena spill. However, the October 2013 peak had a PAH profile inconsistent with weathered Rena oil, suggesting an alternative source of contamination. Our results highlight the potential for P. subtriangulata as a PAH monitoring tool but recognise more study is needed to better quantify baseline levels and uptake and depuration dynamics.  相似文献   

Back to the future: reflections and directions of South African marine bioinvasion research     

ME Alexander  CA Simon  CL Griffiths  K Peters  S Sibanda  S Miza 《African Journal of Marine Science》2016,38(1):141-144

Biological invasions continue to increase around the world, with impacts on many coastal marine systems. Here we review the South African marine invasion literature which, despite the field being relatively new, has grown to have significant presence in both the local and international arenas. Of the 79 papers reviewed, 70% focused on the establishment and spread of alien species, with modes of transport and introduction largely overlooked. An emphasis was also apparent towards field studies, in particular survey work, with few experimental studies. The overwhelming majority of papers focused on a single species, the Mediterranean mussel Mytilus galloprovincialis, reflecting the scale of this invasion and the tractable nature of rocky shores as study systems. With the exception of this one species, the impacts of marine alien species have rarely been quantified. We suggest that future research extends the taxonomic coverage of present work and develops a better understanding of the mechanisms of introduction, establishment and spread of marine alien species. Through an experimental approach, the drivers of altered ecological patterns and processes resulting from invasions should be addressed, providing insight into associated impacts. This approach will maintain the local applicability and international relevance of South African marine invasion research.  相似文献   

The compressibility of silicate liquids containing Fe2O3 and the effect of composition,temperature, oxygen fugacity and pressure on their redox states     

Victor C. Kress  Ian S. E. Carmichael 《Contributions to Mineralogy and Petrology》1991,108(1-2):82-92

Ultrasonic longitudinal acoustic velocities in oxidized silicate liquids indicate that the pressure derivative of the partial-molar volume of Fe₂O₃ is the same in iron-rich alkali-, alkaline earth- and natural silicate melt compositions at 1 bar. The dV/dP for multicomponent silicate liquids can be expressed as a linear combination of partial-molar constants plus a positive excess term for Na₂O−Al₂O₃ mixing. Partial-molar properties for FeO and Fe₂O₃ components allow extension of the empirical expression of Sack et al. (1980) to permit the calculation of Fe-redox equilibrium in a natural silicate liquid as a function of composition, temperature, fo₂ and pressure; a more formal thermodynamic expression is presented in the Appendix. The predicted equilibrium fo₂ of natural silicate melts, of fixed oxygen content, closely parallels that defined by the metastable assemblage fayalite+magnetite+β-quartz (FMQ), in pressure-temperature space. A silicate melt initially equilibrated at 3 GPa and FMQ, will remain within approximately 0.5 log₁₀ units of FMQ during its closed-system ascent. Thus, for magmas closed to oxygen, iron-redox equilibrium in crystal-poor pristine glassy lavas represents an excellent probe of the relative oxidation state of their source regions.  相似文献   

Lamprophyric lavas in the Colima graben,SW Mexico   总被引：2，自引：3，他引：2  

J. F. Allan  I. S. E. Carmichael 《Contributions to Mineralogy and Petrology》1984,88(3):203-216

The Colima graben, located in SW Mexico, is one of three grabens which intersect about 50 km SSW of Guadalajara, forming a triple junction. The 90 km long, 20–60 km wide Colima graben represents a N-S rift of the E-W trending Mexican Volcanic Belt. Since the Early Pliocene, the Colima graben has served as a locus for the eruption of alkaline lavas, the most recent of which are basanites and minettes erupted from Late Pleistocene cinder cones (Luhr and Carmichael 1981). In this paper, we report on older alkaline lavas which crop out in the graben's walls. These rocks include phlogopite- and hornblende-bearing lamprophyres, a phlogopite-kalsilite-ankaratrite, and high-K andesites. These lavas crop out throughout the Colima graben area, and are intimately associated with calc-alkaline lavas in the field. Compared to these, the alkaline rocks are strikingly enriched in the incompatible elements, particularly Ba, Sr, P, and the LREE. Unlike the younger Late Pleistocene alkaline cinder cone lavas, most of the graben wall lamprophyres and the high-K andesites represent magmas that appear to have undergone significant evolution since their generation, including fractionation, crustal contamination, and possible magma mixing. Least-squares modeling indicates that the cinder cone minettes represent reasonable parental magmas for the graben lamprophyres. The occurrence of these alkaline lavas in an active calc-alkaline volcanic arc is unusual, and we suggest that they are a manifestation of the rifting processes which produced the Colima graben.  相似文献   

Experimental petrology of alkalic lavas: constraints on cotectics of multiple saturation in natural basic liquids     

Richard O. Sack  David Walker  Ian S. E. Carmichael 《Contributions to Mineralogy and Petrology》1987,96(1):1-23

Experimental study of natural alkalic lava compositions at low pressures (pO₂QFM) reveals that crystallization of primitive lavas often occurs in the sequence olivine, plagioclase, clinopyroxene, nepheline without obvious reaction relation. Pseudoternary liquidus projections of multiply saturated liquids coexisting with plagioclase (±olivine±clinopyroxene±nepheline) have been prepared to facilitate graphical analysis of the evolution of lava compositions during hypabyssal cooling. Use of (TiAl₂)(MgSi₂)_–1 and Fe³⁺ (Al)_–1 exchange components is a key aspect of the projection procedure which is succesful in reducing a wide range of compositions to a systematic graphical representation. These projections, and the experiments on which they are based, show that low pressure fractionation plays a significant role in the petrogenesis of many alkalic lava suites from both continental and oceanic settings. However, the role of polybaric fractionation is more evident in the major element chemistry of these lava suites than in many tholeiitic suites of comparable extent. For example, the lavas of Karisimbi, East Africa, show a range of compositions reflecting a polybaric petrogenesis from primitive picrites at 1360° C/18 kb and leading to advanced low pressure differentiates. Evolved leucite-bearing potassic members of this and other suites may be treated in a nepheline-diopside-kspar (+olivine+leucite) projection. Compositional curvature on the plagioclase+clinopyroxene+olivine+leucite cotectic offers a mechanism to explain resorption of plagioclase in alkalic groundmass assemblages and the incompatibility of albite and leucite. This projection is useful for evaluating the extent of assimilation of the alkalic portions of crustal granulites. Assimilation appears to have played some role in the advanced differentiates from Karisimbi.  相似文献   

P total,P_{H_2 O} and the occurrence of cummingtonite in volcanic rocks     

B. J. Wood  I. S. E. Carmichael 《Contributions to Mineralogy and Petrology》1973,40(2):149-158

The phenocryst assemblage of cummingtonite, orthopyroxene, quartz, titanomagnetite and ilmenite in rhyolites of New Zealand has been used to calculate P _total and . The values of P _total and depend strongly upon whether an ideal mixing, or an ordered, model is used for the solid-solutions, but in both cases P _total.The rhyolite magma contained over 9 per cent water (by weight) when the cummingtonite phenocrysts precipitated, and possibly as much as 12 per cent, so that it is surprising that one of these rhyolites is a coherent lava. The calculated values of P _total and are very sensitive to uncertainty in both the composition of the solid-solutions and temperature. Calculations show that >0.7–0.8 P _total for cummingtonite to precipitate in rhyolites, and that iron-rich olivine and cummingtonite could only exist in rhyolites over a small temperature range at a pressure near 5 kilobars. Hornblende phenocrysts co-existing with fayalitic olivine in rhyolites accordingly have a very low activity of Mg₇Si₈O₂₂(OH)₂.  相似文献   

Constraints on mantle source compositions imposed by phosphorus and the rare-earth elements     

A. E. Beswick  I. S. E. Carmichael 《Contributions to Mineralogy and Petrology》1978,67(3):317-330

There is a positive correlation between the concentration of P₂O₅ in basic magmas and the concentration of the REE and also between their light: heavy ratio, represented by both Ce/Yb and Nd/Sm. This suggests that a phosphate mineral, such as apatite or whitlockite, both of which can contain high concentrations of REE, is present in the magma source regions. Thermodynamic calculations indicate that fluorapatite is stable over the whole P-T range of magma generation, but at the greatest depths it may co-exist with whitlockite in the presence of fluorphlogopite. Using published REE crystal-liquid distribution coefficients, it is evident that for P₂O₅-rich basic magmas with 700–1000 times chondritic abundances of LREE, garnet need not have contributed significantly to their composition. The most convincing match of hypothetical liquid with actual basic magma is for the derivation of mid-ocean ridge (MOR) basalts from plagioclase- or spinel-lherzolite containing 3 times chondritic REE by ∼5% partial melting. The more P₂O₅-rich, and hence REE-rich, basic magmas are apparently derived from crystalline sources which are progressively impoverished in garnet and clinopyroxene, or in other words, the greater the REE concentration of basic magma, the more refractory is the mineral assemblage of the source. There is some evidence for a compositional dependence of radiogenic neodymium and lead in basic magma, and one way that this can be reconciled with mantle source-region evolution is to postulate that fusion is not always accompanied by isotopic equilibrium.  相似文献