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1.
The relationship between the redox ratio Fe+2/(Fe+2+Fe+3) and the K2O/(K2O + Al2O3) ratio (K2O*) were experimentally investigated in silicate melts with 78 mol% SiO2 in the system SiO2-Al2O3-K2O-FeO-Fe2O3, in air at 1,400° C. Quenched glass compositions were analyzed by electron microprobe and wet chemical microtitration techniques. Minimum values of the redox ratio were obtained at K2O*0.5. The redox ratio in peralkaline melts (K2O*>0.5) increases slightly with K2O* whereas this ratio increases dramatically in peraluminous melts (K2O*<0.5) as K2O is replaced by Al2O3. These data indicate that all Fe+3 (and Al+3) occur as tetrahedral species charge balanced with K+ in peralkaline melts. In peraluminous melts, Fe+3 (and Al+3) probably occur as both tetrahedral species using Fe+2 as a charge-balancing cation and as network-modifying cations associated with non-bridging oxygen. 相似文献
2.
The role of phosphorus in rhyolitic liquids as determined from the homogeneous iron redox equilibrium 总被引：1，自引：1，他引：1
The redox ratio of iron is used as an indicator of solution properties of silicate liquids in the system (SiO–Al2O3–K2O–FeO–Fe2O3–P2O5). Glasses containing 80–85 mol% SiO2 with 1 mol% Fe2O3 and compositions covering a range of K2O/Al2O3 were synthesized at 1400°C in air (fixed fO2). Variations in the ratio FeO/FeO1.5 resulting from the addition of P2O5 are used to determine the solution behavior of phosphorus and its interactions with other cations in the silicate melt. In 80 mol% SiO2 peralkaline melts the redox ratio, expressed as FeO/FeO1.5, is unchanged relative to the reference curve with the addition of 3 mol% P2O5. Yet, the iron redox ratio in the 85 mol% SiO2 potassium aluminosilicate melts is decreased relative to phosphorus-free liquids even for small amounts of P2O5 (0.5 mol%). The redox ratio in peraluminous melts is decreased relative to phosphorus- free liquids at P2O5 concentrations of 3 mol%. In peraluminous liquids, complexing of both Fe+3–O–P+5 and Al+3–O–P+5 occur. The activity coefficient of Fe+3 is decreased because more ferric iron can be accommodated than in phosphorus-free liquids. In peralkaline melts, there is no evidence that P+5 is removing K+ from either Al+3 or Fe+3 species. In chargebalanced melts with 3 mol% Fe2O3 and very high P2O5 concentrations, phosphorus removes K+ from K–O–Fe+3 complexes resulting in a redox increase. P2O5 should be accommodated easily in peraluminous rhyolitic liquids and phosphate saturation may be suppressed relative to metaluminous rhyolites. In peralkaline melts, phosphate solubility may increase as a result of phosphorus complexing with alkalis. The complexing stoichiometry may be variable, however, and the relative influence of peralkalinity versus temperature on phosphate solubility in rhyolitic melts deserves greater attention. 相似文献
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4.
胶东焦家金矿床热液蚀变作用 总被引：12，自引：7，他引：5
胶东作为中国最重要的金矿集区,区内大型-超大型金矿床集中产出,已探明金矿资源量占全国近1/3。其中,破碎带蚀变岩型金矿床是最重要的金矿床类型,占胶东已探明金矿资源量的90%以上,焦家金矿床是著名的"焦家式"破碎带蚀变岩型金矿的命名地,内发育大规模的绢英岩化蚀变带(宽20~200m)和钾化蚀变带(50~300m),蚀变岩型金矿体主要发育在焦家断裂带下盘的绢英岩化蚀变带中。本文通过详细的野外地质观测,查清了焦家金矿床蚀变类型及矿物组合特征,系统采集了不同蚀变类型的岩石样品,进行了岩石元素地球化学分析,运用质量平衡方法讨论了热液蚀变过程中元素迁移规律,初步探讨了焦家金矿床热液蚀变机理。其中,钾化蚀变是成矿前蚀变,钾化花岗岩常以团块状或角砾状残留于黄铁绢英岩和绢英岩内;黄铁绢英岩化和绢英岩化蚀变受焦家断裂及其下盘的次级断裂控制,其规模大小受断裂的规模控制;其中焦家主断裂下盘的绢英岩化蚀变带规模最大,一般宽10~200m;而次级断裂控制的绢英岩化蚀变带规模相对较小,一般以0.1~1m宽的脉状发育在钾化花岗岩内,指示绢英岩化蚀变晚于钾化蚀变。相对于黑云母花岗岩,不同蚀变带岩石普遍表现出高K2O、低Al2O3、CaO和Na2O,而不同蚀变岩石Si、Fe、Mg等元素各表现出不同特征。钾化带岩石表现为K2O的富集,而绢英岩带和黄铁绢英岩带岩石表现为MgO、Fe2O3增加的趋势。黑云母花岗岩发生钾化蚀变过程中,SiO2、K2O表现为明显的带入,指示在钾长石化过程中,流体为富硅的碱性氧化流体。在钾化花岗岩→黄铁绢英岩过程中,Fe2O3表现为明显的带入,可能是由于黑云母等暗色矿物的分解造成的;此外,亲硫元素(Au、Ag、As、Pb、Zn)均表现为带入,特别是成矿元素Au表现为明显的带入。结合本区金的来源可能部分为玲珑黑云母花岗岩,本研究认为钾化过程中的富硅碱性氧化流体通过交代蚀变反应使金从围岩中释放、成为高价态离子活化进入成矿流体,即分散还原态的金(Au0)被活化为氧化态(Au+、Au3+)以AuH3SiO4形式随热液迁移。在绢英岩化过程中,热液中的SiO2等组分损失,引起热液中的AuH3SiO4稳定性降低,造成AuH3SiO4分解,Fe2+、Fe3+被消耗形成黄铁矿,导致金大量沉淀和聚集沉淀,此时完成了金由活化→迁移→沉淀富集成矿。 相似文献
5.
W. Seifert 《Mineralogy and Petrology》2005,84(3-4):129-146
Summary The mineral chemistry of a Variscan lamprophyre (kersantite) from the Frankenwald, Germany, has been investigated by electron
microprobe. This potassic, Si-saturated, mafic rock contains an assemblage of different generations of titanite and allanite-(Ce),
Th-rich zircon, and metamict REE–Ti–Zr–Th silicates. The primary ferroan-ceroan titanite contains unusually high contents
of REE2O3 (max. (ΣLa to Sm)+Y = 36.8 oxide wt.%), ZrO2 (max. 5.4 wt.%), and ThO2 (max. 3.1 wt.%). Its empirical formula averages to (Ca0.31 La0.17 Ce0.30 Pr0.03 Nd0.08 Sm0.01 Y0.01 Fe2+0.06 Th0.02 Mn0.01)Σ1.00 (Ti0.60 Fe2+0.22 Al0.06 Zr0.07 Mg0.04 Nb0.01)Σ1.00 O1.00(Si0.93 Al0.07)Σ1.00 O4. Element correlations reveal operation of the complex substitution Ca2++Ti4++Th4+ ⇔ REE3++Al3++Zr4+. In comparison to allanite-(Ce), ferroan-ceroan titanite preferentially incorporated the LREE and Th. This finding is inconsistent
with previous experimental studies and suggests that both minerals are not cogenetic. High Zr contents in titanite, usually
known only from Si-undersaturated alkaline rocks, and the predominance of Fe2+ suggest that the ferroan-ceroan titanite crystallized from an alkali-rich, low-fO2 residual melt. 相似文献
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The structural role and homogeneous redox equilibria of iron in peraluminous,metaluminous and peralkaline silicate melts 总被引：1，自引：3，他引：1
The compositional dependence of the redox ratio (FeO/FeO1.5) has been experimentally determined in K2O-Al2O3-SiO2-Fe2O3-FeO (KASFF) and K2O-CaO-Al2O3-SiO2-Fe2O3-FeO (KCASFF) silicate melts. Compositions were equilibrated at 1,450° C in air, with 78 mol % SiO2. KASFF melts have from 1 to 5 mol % Fe2O3 and include both peraluminous (K2O2O3) and peralkaline (K2O>Al2O3) compositions. KCASFF melts have 1 mol % Fe2O3 encompassing peraluminous, metaluminous (CaO+K2O>Al2O3) and peralkaline compositions. Peralkaline KASFF melts with 1 mol % Fe2O3 have low and constant values for the redox ratio, whereas in peraluminous melts the redox ratio increases with increasing (K2O/Al2O3). Increasing total iron concentration increases the redox ratio in peraluminous melts and slightly decreases the redox ratio in peralkaline melts. Substituting CaO for K2O at fixed total iron (1 mol %) increases the redox ratio in both peraluminous and metaluminous KCASFF melts; however, the redox ratio in peralkaline KCASFF melts is not affected by this exchange. These data indicate that Fe3+ is in four-fold coordination, with K+ or Ca2+ providing local charge balance. The tetrahedral ferric species is most stable in peralkaline melts and least stable in peraluminous melts, due to the competition between Al3+ and Fe3+ for charge balancing cations in the latter melt. Tetrahedral Fe3+ is also less stable when Ca2+ provides local charge balance. The data are consistent with a network modifying role for Fe2+ in the melt.The data are interpreted to reflect the effects of melt composition on the partitioning of K+ and Ca2+ and Fe3+ and Al3+ between various species in the melt. These relationships are discussed in terms of homogeneous equilibria between various iron-bearing and iron-free melt species. The results also reflect the effect of liquid composition on the exchange potentials Fe3+ Al–1 and Ca0.5K–1. The exchange potentials are relatively constant in peralkaline melts, but decrease in metaluminous and peraluminous melts as both (CaO+K2O)/(CaO+K2O+Al2O3) and K2O/CaO decrease. These qualitative observations imply that minerals exhibiting these exchanges will also be similarly affected as liquid composition changes.
Present address: Department of Geological Sciences, Virginia Tech, Blacksburg, VA 24061, USA 相似文献
8.
Si+4 Content of natural phengites 总被引：2，自引：0，他引：2
B. Velde 《Contributions to Mineralogy and Petrology》1967,14(3):250-258
The chemical compositions of white micas separated from adjacent rocks of glaucophane and greenschist facies are compared with respect to their Si+4 content. The micas are predominantly phengitic, i.e. between muscovite, K[Al2Si3AlO10(OH)2] and celadonite, K[(R+2R+3)Si4O10(OH)2] in composition. Constancy of Si content in micas coming from rocks of different bulk chemical composition but closely similar physical conditions indicates that the silica content of a potassic dioctahedral mica can be used to indicate the pressure and temperature conditions of its formation. This conclusion is in part based upon previous experimental data obtained for synthetic phengites. 相似文献
9.
用置换矢量概念解释了115个天然 Fe-Li 云母化学成分的变化。Fe-Li 云母是三八面体 Li-Fe-Al 云母,其基本置换是四锂云母置换。由于 Al-Li 白云母置换和白云母置换的影响,其化学组成变化的基本趋势呈明显的非线性,因而 Fe-Li 云母不是真正的二元系。作为 Fe-Li 云母,富铁黑云母和铁叶云母都是最富铁的成员,因而建议称 Fe-Li 云母为黑云母-锂云母系列。根据化学成分,晶胞参数和折光率的异常变化还提出了该系列自然分类的方案。 相似文献
10.
A. Mukhopadhyay A. Bhattacharya L. Mohanty 《Contributions to Mineralogy and Petrology》1992,110(2-3):346-354
Enthalpy of formation and third law entropy for andradite (Ca3Fe2Si3O12) and hedenbergite (CaFeSi2O6) at standard state have been retrieved from experimental P-T-f(O2) data on the stability relations of these phases. The data for hedenbergite were combined with the thermodynamic data for related phases (Berman 1988) to formulate the geobarometers based on the reactions CaFeSi2O6+2CaAl2Si2O8+2FeTiO3= Ca3Al2Si3O12+Fe3Al2Si3O12+2TiO2 (CPG-IR), 3CaFeSi2O6+3CaAl2Si2O8+3FeTiO3=Ca3Al2Si3O12 +2Fe3Al2Si3O12+3CaTiSiO5 (CPG-IS) and 3CaFeSi2O6+3CaAl2Si2O8=2Ca3Al2Si3O12+ Fe3Al2Si3O12+3SiO2 (CPG-O). The geobarometers involving rutile and sphene are proposed for the first time. In several granulite terranes, pressures have been estimated from the three geobarometers for basic granulites and charnockitic suite of rocks containing the above assemblages. In clinopyroxene-garnet-plagioclase-quartz-ilmenite-rutile assemblages, pressures obtained from CPG-IR are within ±500 bars of pressures calculated from the CPG-Q barometer. However, if rutile is absent, the pressures computed from the CPG-IR barometer are 300 to 1,200 bars higher than those calculated from the CPG-Q barometer. For maximum differences in pressures obtained from the two equilibria TiO2 activity [a(TiO2)] in the rocks is calculated to be 0.8. The sphene-bearing geobarometer (CPG-IS) also registers pressures comparable (<±500 bars) to those obtained from the CPG-Q barometer. The close agreement in pressure values obtained from the CPG-IR and the CPG-IS equilibria with those registered by the CPG-Q geobarometer in assemblages containing quartz implies that the rutile- and sphene-bearing geobarometers which do not involve quartz, can also be applied to estimate pressures from quartz-absent assemblages-assemblages which are not amenable to quantitative geobarometry using existing formulations. 相似文献
11.
Trace element variations in olivine phenocrysts from Ugandan potassic rocks as clues to the chemical characteristics of parental magmas 总被引：2，自引：0，他引：2
Stephen F. Foley Dorrit E. Jacob Hugh St. C. O’Neill 《Contributions to Mineralogy and Petrology》2011,162(1):1-20
Olivine phenocrysts in ugandite and leucite basanite from the western branch of the East African Rift have been analysed for
up to 34 trace elements by Laser-ICP-MS with detection limits as low as 1 ppb. A combination of point analyses with varying
ablation crater diameters and line scans allow the identification of subtle zonations from core to rim, as well as characterization
of the chemical effects of contamination along cracks. Trace element concentrations are remarkably uniform between large and
small phenocrysts; fractionated leucite basanites (Mg# 59) have higher D
Ca and D
Al, and less fractionated LREE/HREE than MgO-rich ugandites (Mg# 75–80). Minor zonation is seen in elements with cation charges
from 5+ to 2+ (P, Ti, Zr, Cr, Al, Sc, V, Cu, Mn, Ni) and show correlation between Ti and Al, but not P. Early phenocryst cores
with high Li or Ni, low Mn, or enrichments in many trace elements can be identified, whereas xenocrysts have exceptionally
low Na, Cr, Ti, V and Co. Partition coefficients for Ni are 31–35, less than in lamproites, with which they demonstrate an
approximately linear correlation with K2O content, K2O/Al2O3 and K2O/Na2O of the melt, but none with SiO2 content or Mg#. D-values for Cr, Mn and Co overlap with those of basalts, whereas those for Sc (0.011–0.018), Zn (0.44–0.49) and Ga (0.006–0.007)
are lower. D
V of various potassic rocks (0.015 in the Ugandan rocks) confirms the dependence on fO2 calibrated by the Fe3+/(Fe3++Fe2+) of spinels; the Ugandan potassic rocks crystallized at fO2 = FMQ to FMQ + 1. The ugandite olivines have some trace element characteristics reminiscent of those in metasomatized Kaapvaal
peridotites, but not ocean islands. Line scan analyses are contaminated in Al, Ca, Cu, Ga, Sr, Zr, Nb, La and Ce, elements
that are also concentrated in microcracks between subgrains, indicating smearing out during polishing, and demonstrating that
large spot analyses produce the best results. 相似文献
12.
Synthetic melilites on the join Ca2MgSi2O7 (åkermanite: Ak)-Ca2Fe3+AlSiO7 (ferrialuminium gehlenite: FAGeh) were studied using X-ray powder diffraction and 57Fe Mössbauer spectroscopic methods to determine the distribution of Fe3+ between two different tetrahedral sites (T1 and T2), and the relationship between ionic substitution and incommensurate (IC) structure. Melilites were synthesized from starting materials with compositions of Ak100, Ak80FAGeh20, Ak70FAGeh30 and Ak50FAGeh50 by sintering at 1,170–1,350 °C and 1 atm. The average chemical compositions and end-member components, Ak, FAGeh and Geh (Ca2Al2SiO7), of the synthetic melilites were Ca2.015Mg1.023Si1.981O7 (Ak100), Ca2.017Mg0.788Fe 0.187 3+ Al0.221Si1.791O7 (Ak78FAGeh19Geh3), Ca1.995Mg0.695Fe 0.258 3+ Al0.318Si1.723O7 (Ak69FAGeh25Geh6) and Ca1.982Mg0.495Fe 0.449 3+ Al0.519Si1.535O7 (Ak49FAGeh44Geh7), respectively. Rietveld refinements using X-ray powder diffraction data measured using CuK α -radiation at room temperature converged successfully with goodness-of-fits of 1.15–1.26. The refined Fe occupancies at the T1 and T2 sites and the Mg and Si contents determined by electron microprobe analysis gave the site populations of [0.788Mg + 0.082Fe3+ + 0.130Al]T1[0.104Fe3+ + 0.104Al + 1.792Si]T2 for Ak78FAGeh19Geh3, [0.695Mg + 0.127Fe3+ + 0.178Al]T1[0.132Fe3+ + 0.144Al + 1.724Si]T2 for Ak69FAGeh25Geh6 and [0.495Mg + 0.202Fe3+ + 0.303Al]T1[0.248Fe3+ + 0.216Al + 1.536Si]T2 for Ak49FAGeh44Geh7 (apfu: atoms per formula unit), respectively. The results indicate that Fe3+ is distributed at both the T1 and the T2 sites. The mean T1–O distance decreases with the substitution of Fe3+ + Al3+ for Mg2+ at the T1 site, whereas the mean T2–O distance increases with substitution of Fe3+ + Al3+ for Si4+ at the T2 site, causing decrease in the a dimension and increase in the c dimension. However, in spite of the successful Rietveld refinements for the X-ray powder diffraction data measured using CuK α-radiation at room temperature, each Bragg reflection measured using CuK α1-radiation at room temperature showed weak shoulders, which were not observed in those measured at 200 °C. The Mössbauer spectra of the melilites measured at room temperature consist of two doublets assigned to Fe3+ at the T1 site and two or three doublets to Fe3+ at the T2 site, implying the existence of multiple T1 and T2 sites with different site distortions. These facts can be interpreted in terms of the IC structure in all synthetic melilites at room temperature, respectively. The results of Mössbauer analysis indicate that the IC structure in melilite is caused by not only known multiple T1 site, but also multiple T2 site at room temperature. 相似文献
13.
Within the ‘glimmerite’ nodules occurring within kimberlite pipes we recognize the MARID suite consisting of varying proportions of mica, amphibole, rutile, ilmenite and diopside. Banding of some specimens is interpreted as cumulate layering. All specimens were deformed either before incorporation into the host kimberlite or during intrusion. Compared with minerals in peridotite xenoliths, the MARID ones are lower in Al2O3 and Cr2O3, but richer in total iron. The MARID micas, amphiboles, diopsides, ilmenites and probably rutiles contain substantial Fe2O3 indicative of oxidizing conditions. The amphibole is potassic richterite. Micas of the megacryst suite in kimberlite have less total iron and Fe2O3 than micas of the MARID suite. We suggest that the rocks of the MARID suite crystallized under oxidizing conditions from a magma, chemically similar to kimberlite, within the higher parts of the upper mantle: the presence of amphibole restricts the depth to less than ~ 100 km. A xenolith containing olivine and orthopyroxene as well as minerals similar to but not the same compositionally as MARID-types is interpreted as a metasomite, possibly representing wall-rock of a magma body from which MARID-suite rocks crystallized. 相似文献
14.
The saturation surface of pseudobrookite (Fe2TiO5) was determined for melts in the system SiO2-Al2O3-K2O-FeO-Fe2O3-TiO2 at 1400° C and 1 atm. The variation in concentrations of Fe2O3, TiO2 and Fe2TiO5 in liquids can be used to infer relative changes in activity coefficients of these components with changing K2O/(K2O+Al2O3) of the melts. Saturation concentrations of these components are low and relatively constant in the peraluminous melts and increase with increasing K2O/(K2O+Al2O3) in peralkaline liquids. The activity coefficients of Fe2O3 and TiO2 and Fe2TiO5, therefore, are higher in peraluminous liquids than in peralkaline liquids in this system. In addition, the iron redox ratio was measured as a function of K2O/(K2O+Al2O3) for liquids just below the saturation surface;
was fixed so all variations in redox ratio are entirely due to changes in melt composition. The redox ratio from unsaturated liquids was applied to saturated liquids where redox analysis of the glass is impossible. The homogeneous equilibrium experiments indicate that the activity coefficient of Fe2O3 relative to that of FeO is significantly greater in peraluminous melts than peralkaline melts. Both the heterogeneous and homogeneous equilibria suggest that in peralkaline liquids K+in excess of that required to charge balance tetrahedral Al+3 is used to stabilize both Fe+3 and Ti+4. Calculations show that ferric iron and titanium compete equally effectively for charge-balancing potassium but neither can outcompete aluminum. The observed changes in solution properties of Fe2O3 and TiO2 in the synthetic melts are used to explain variations in Fe-Ti oxide stabilities in natural peraluminous and peralkaline rhyolites and granites. Since the activity coefficients of both ferric iron and titanium are significantly higher in peraluminous liquids than in peralkaline liquids, Fe-Ti oxides should occur earlier in the crystallization sequence in peraluminous rhyolites than in peralkaline rhyolites. In addition, iron will be reduced in peraluminous granites and rhyolites relative to peralkaline ones under comparable P, T, and
. Finally, observed crystallization patterns for minerals containing highly charged cations other than ferric iron and titanium are evaluated in the context of this and other experimental studies. 相似文献
15.
The postperovskite phase transition of Fe and Al-bearing MgSiO3 bridgmanite, the most aboundant mineral in the Earth's lower mantle, is believed to be a key to understanding seismological observations in the D″ layer, e.g., the discontinuous changes in seismic wave velocities. Experimentally reported phase transition boundaries of Fe and Al-bearing bridgmanite are currently largely controversial and generally suggest wide two-phase coexistence domains. Theoretical simulations ignoring temperature effects cannot evaluate correctly two-phase coexistence domains under high-temperature. We show high-pressure and high-temperature phase transition boundaries for various compositions with geophysically relevant impurities of Fe2+SiO3, Fe3+Fe3+O3, Fe3+Al3+O3, and Al3+Al3+O3 derived from the ab initio finite-temperature free energies calculated combining the internally consistent LSDA + U method and a lattice dynamics approach. We found that at ～ 2500 K, incorporations accompanied by Fe3+ expand the two-phase coexistence domains distinctly, implying that D″ seismic discontinuities likely arise from the phase transition of Fe2+-bearing bridgmanite. 相似文献
16.
Nodular silcretes of the Cypress Hills Formation (upper Eocene to middle Miocene) of southern Saskatchewan, Canada 总被引：1，自引：0，他引：1
Siliceous nodules in the upper Eocene to middle Miocene Cypress Hills Formation in southern Saskatchewan formed on the distal portions of an extensive braidplain. The nodules are similar to silcretes described elsewhere in the literature but their morphology and occurrence suggests that they are of a rare variety. The silcretes are discrete equant to disc-shaped nodules, 1–3 cm thick, and up to 15 cm long. The silicretes form horizontal, discontinous layers parallel to bedding within an unweathered profile. Void spaces and fractures within the nodules are lined or filled with drusy quartz. The silcretes contain less than 0.07% TiO2. The TiO2/SiO2/Al2O3 and TiO2/SiO2/Fe2O3 ratios are similar to values obtained from nodules formed in an arid to semi-arid environment based on comparison with modern silcretes. This interpretation is confirmed by independent sedimentological and palaeontological climatic evidence for an arid to semi-arid climate throughout Oligocene time in the western Canadian plains. 相似文献
17.
Summary The results of microprobe analyses of clinopyroxenes from alkaline melasyenites and layered melagabbros, produced by intra-plate magmatism of Paleocene age at Punta delle Pietre Nere, are here given and discussed.The analysed pyroxenes range from diopsidic to acmite-rich compositions.The first crystallized pyroxenes (diopside) show AlVI contents suggesting shallow depths of crystallization. In addition pyroxenes from melasyenite and those from melagabbro display different Cr contents, Al/Ti and Mg/(Mg+Fe2++Fe3+) ratios confirming their crystallization from melts produced by different parental liquids.Diopsides and salites show an overall trend towards high Al, Ti and Fe3+, suggesting that the crystallization occurred under decreasing SiO2/Al2O3 ratios and under relatively highpH2O–pO2 conditions.Pyroxenes from the Pietre Nere melasyenite show a progressive variation towards acmite rich compositions at Mg/(Mg+Fe2++Fe3+) lower than 0.5; those from the layered melagabbro, instead, show a continuous enrichment in Ca Fe3+ AlSiO6. This different behaviour is due to the co-crystallization, with the latest pyroxenes, of phases with different K/Na and Si/Al ratios.
With 6 Figures 相似文献
Kristallisations-Tendenzen der Pyroxene aus Alkali-Subvulkaniten auf Punta delle Pietre Nere (Gargano, Süditalien)
Zusammenfassung Es werden die Ergebnisse der Mikrosonden-Untersuchungen von Klinopyroxenen aus Alkali-Melasyeniten und schichtigen Alkali-Melagabbros, die durch Intra-plate-Magmatismus paläozenen Alters auf Punta delle Pietre Nere entstanden sind, beschrieben und erörtert.Die untersuchten Pyroxene reichen von diopsidischen bis zu Akmit-reichen Zusammensetzungen.Die zuerst kristallisierten Pyroxene (Diopsid) zeigen AlVI-Gehalte, die auf geringe Tiefe des Kristallisationsvorganges hinweisen. Dazu zeigen die Pyroxene aus dem Melasyenit und aus dem Melagabbro unterschiedliche Cr-Gehalte; die Al/Ti- und Mg/(Mg+Fe2++Fe3+)-Verhältnisse bestätigen deren Kristallisation aus Schmelzen, die aus unterschiedlichen Ursprungsmagmen stammen.Die Diopside und Salite zeigen eine einheitliche Tendenz zu hohem Al-, Ti- und Fe3+-Gehalt; dies deutet darauf hin, daß die Kristallisation unter abnehmenden SiO2/Al2O3-Verhältnissen und unter relativ hohenpH2O–pO2-Bedingungen stattfand.Die Pyroxene aus dem Punta delle Pietre Nere-Melasyenit zeigen eine zunehmende Änderung zu Akmit-reichen Zusammensetzungen bei weniger als 0,5 Mg/(Mg+Fe2++Fe3+); die Pyroxene aus dem schichtig differenzierten Melagabbro zeigen dagegen eine allmähliche Zunahme von CaFe3+AlSiO6. Dieses unterschiedliche Verhalten rührt daher, daß Mineralphasen mit unterschiedlichen K/Na- und Si/Al-Verhältnissen zugleich mit den zuletzt gebildeten Pyroxenen kristallisierten.
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18.
首次在班公湖-怒江缝合带西段去申拉组中发现了泥质硅质岩,呈2个层位产出。为探讨泥质硅质岩的沉积环境、成因及与班-怒特提斯洋西段构造演化的关系,进行岩石学和地球化学分析,结果显示,第一层位泥质硅质岩Al_2O_3/(Al_2O_3+Fe_2O_3)、Ce/Ce~?、(La/Ce)_N、V/(Ni+V)、Ce/La、Ce_(anom)、Eu_(anom)平均值分别为0.60、0.80、1.24、0.72、1.84、-0.08、0.01,第二层位泥质硅质岩相应比值平均值分别为0.65、0.83、1.16、0.77、1.97、-0.07、0.02。结合泥质硅质岩的Fe_2O_3/TiO2-Al_2O_3/(Al_2O_3+Fe_2O_3)、(La/Ce)N-Al_2O_3/(Al_2O_3+Fe_2O_3)、Hf/3-Th-Ta关系图解,表明第一、二层位泥质硅质岩形成于活动大陆边缘,沉积时水-岩界面为水体分层不强烈的厌氧环境。U-Th、Zn-Ni-Co、La-Ce、La/Yb-REE关系图解和稀土元素特征指示了第一、二层位泥质硅质岩为热水成因,热水活动与玄武岩岩浆活动有关,第二层位泥质硅质岩沉积时热水活动更强烈。去申拉组泥质硅质岩的岩石学、地球化学特征表明,狮泉河地区班-怒特提斯洋至少在早白垩世仍具有一定规模的洋盆,其闭合时间应晚于约109Ma,进一步限定了洋盆的闭合时间。 相似文献
19.
Bruce Velde 《Contributions to Mineralogy and Petrology》1972,37(3):235-247
Phase relations for the bulk compositions of the celadonites between the MgAl, MgFe3+ and Fe2+Fe3+ types (celadonite = KR2+R3+ Si4O10 (OH)2) under magnetite-iron and nickel-nickel oxide solid-fluid buffers indicate the extent of solid solution possible in this potassic mica series at temperatures between 300° and 430° C at 2 Kb total pressure. Other possible combinations of Mg, Al, Fe ions in octahedrally coordinated sites did not produce single-phase mica products. The ferrous celadonite micas are stable only under oxygen fugacities where magnetite is the stable oxide—where both Fe2+ and Fe3+ can coexist. However the celadonite with the highest thermal stability at 2 Kb total pressure, nickel-nickel oxide buffer conditions is the KMgFe3+Si4O10(OH)2 phase which is stable up to 420°C, well into low grade metamorphic conditions. It is thus apparent that the presence of celadonite or glauconite mica will not be indicative of changing diagenetic conditions. 相似文献
20.
K.D. Litasov 《Russian Geology and Geophysics》2010,51(6):644-649
In this paper I present results of IR spectroscopic measurements of water solubility in Al-bearing periclase and ferropericlase (Mg# = 88) synthesized at 25 GPa and 1400–2000 °C. The IR spectra of their crystals show narrow absorption peaks at 3299, 3308, and 3474 cm?1. The calculated H2O contents are 11–25 ppm in periclase (Al2O3 = 0.9–1.2 wt.%) and 14–79 ppm in ferropericlase (Al2O3 = 0.9–2.9 wt.%). Ferropericlase contains more H2O and Al2O3 than periclase at 1800–2000 °C. I suggest that addition of Al2O3 does not influence the solubility of water in ferropericlase but can favor the additional incorporation of Fe2O3 into the structure. The incorporation of Fe3+ into ferropericlase increases water solubility as a result of iron reduction to Fe2+. It is shown that water has limited solubility in ferropericlase from mantle peridotite; therefore, ferropericlase cannot be considered an important hydrogen-bearing mineral in the lower mantle. 相似文献