Trace elements and REE geochemistry of Siriwasan carbonatite, Chhota Udaipur, Gujarat
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Carbonatite
Pyroxene
Amphibole
Pyroxene
Anorthosite
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A wide variety of intergrowth microstructures have been observed with high-resolution transmission electron microscopy in eight different pyroxene specimens that have been partially or wholly altered to other minerals. The product phases ofthe alteration reactions include amphibole, clinojimthompsonite, chain-width disordered pyribole, and several sheet silicates. Textural considerations indicate that there are a number of different paths for pyroxene hydration reactions. These include the simple paths pyroxene --+ amphibole, pyroxene -+ clinojimthompsonite, and pyroxene -+ sheet silicate, as well as more complicated, stepwise paths, such as pyroxene -> amphibole -+ sheet silicate and pyroxene ---> clinojimthompsonite -+ sheet silicate. In some cases, multiple reaction paths are observed in the same specimen. The microstructures indicate that in addition to multiple paths for reaction, there may be multiple mechanisms by which a specific reaction may occur. For example, replacement of pyroxene by amphibole or other hydrous pyriboles can take place either by the nucleation and growth of narrow lamellae of the product mineral, or by a bulk replacement mechanism along a broad reaction front. Replacement of pyroxene by amphibole takes place in such a way that multiple nucleation events may result in several diferent types of out-of-phase boundaries in the product amphibole. The distinction between exsolution and alteration reaction as a mechanism for the formation of narrow amphibole lamellae in pyroxenes is chemical, rather than structural. The determination of which mechanism has operated must therefore be based on chemical and textural arguments. It is concluded that all cases of pyroxene replacement by amphibole that have been reported are at least consistent with an alteration mechanism, while the textures occurring in some specimens are clearly inconsistent with an exsolution mechanism. With presently available data it is not possible to identify the physical and chemical conditions that lead to specific types of reaction behavior. Failure to recognize the presence of finely intergrown hydrous pyriboles in pyroxene could lead to significant errors in the application of geochemical techniques relying on cation partitioning, such as geothermometric and geobarometric methods utilizing pyroxene chemistrv.
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Amphibole
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Abstract Amphibole in the lower parts of the Lilloise layered intrusion occurs interstitially and as a replacement of pyroxene; in the upper rocks it is a major cumulus phase. There is an overall trend of increasing Fe/(Fe + Mg) with height. Coupled substitutions which effect the variation in composition of the amphiboles are chiefly Na,K( A )+Al( T ) for □ A +Si T ) and Ti+Al( T ) for Fe 3+ ( C )+ Si( T ). There is considerable variation in composition both on the specimen scale and within individual grains. This variation, plus scatter found in plots of the coupled substitutions, is partly attributed to many of the amphiboles having replaced pyroxene and also to the effects of magmatic-hydrothermal fluids.
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Pyroxene
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Pigeonite
Pyroxene
Diopside
Norite
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Amphibole
Pyroxene
Diopside
Hornblende
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The estimated initial plagioclase compositions and plagioclase/liquid partitions for $$X_{An}$$ in the Kiglapait intrusion and Makaopuhi lava lake are the same: $$An_{67}$$ crystal, $$An_{57}$$ liquid. The Kiglapait residuum amounts to 0.01% by volume at $$An_{11.5}$$, whereas the Makaopuhi residuum amounts to 3% at $$An_{6.4}$$. The large difference in secular plagioclase variation can be ascribed only to augite, which is abundant in the lava lake and causes strong depletion of Ca. The effective secular fractionation for $$X_{An}$$ can be described as a function of CIPW-normative di/(di + fsp), here denoted $$F'_{di}$$, by $$D^{eff} = 1.81F'_{di}+ 1.04$$, where $$D^{eff}$$ represents the effective distribution coefficient $$X^{plag}_{An}/X^{lig}_{An}$$. This relation holds for systems ranging at least from olivine tholeiite to high alumina basalt in bulk composition. Plagioclase fractionation alone accounts for a bulk D of only about 1.17. Augite is the major cause of extreme An depletion. These relations can be used to estimate the stage of fractionation at several points in the Skaergaard intrusion, and they account for the small amount of plagioclase variation typically seen in augite-poor massif anorthosites.
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Abstract Experimental data from the system CaO-MgO-Na 2 O-Al 2 O 3 -SiO 2 and from the system CaO-MgO-FeO-Al 2 O 3 -SiO 2 are used to show the nature of the changes in composition, with temperature and with pressure, of liquids in equilibrium with augite, plagioclase, and olivine as MgO in the system CaO-MgO-Al 2 O 3 -SiO 2 is partially replaced by FeO and as Na 2 O replaces CaO. As differentiation of tholeiitic basalts proceeds, Fe/Mg increases, Na/Ca increases, and normative hypersthene increases and these effects alter the solubilities, and hence the proportions of augite, plagioclase, and olivine which co-precipitate. In particular olivine becomes more soluble (less is precipitated) as Fe/Mg increases; plagioclase becomes more soluble but augite less soluble as Na/(Na + Ca) increases; augite also becomes less soluble as normative hypersthene increases, and as pressure is reduced during ascent. Experimental data from remelted ocean tholeiites encounter the same equilibrium and are presented and compared with the data from synthetic systems. Some natural examples of daughter liquids related to parents by crystallization of augite, plagioclase, olivine, are combined with the experimental data and with some model calculations to demonstrate that only a very limited range of proportions of augite : plagioclase: olivine (approximately 30 : 50 : 15 to 20 : 60: 20) produces daughter liquids which lie on the major element variation of the tholeiitic basalt series. Other proportions lead to daughter liquids which are not basalts. In the light of these restrictions, several recent publications are reinterpreted as examples of fractionation of augite, plagioclase, and olivine, rather than examples of partial melting. The recognition of the small effect that massive (50 to 83%) crystallization has on basaltic chemistry by contrast with the large effect that small accumulations of phenocrysts can have is particularly emphasized in this reinterpretation. Comparisons and possible relationships are suggested between some low-K 2 O, low-TiO 2 tholeiites; some calcic-, aphyric-, dykes of some tholeiitic provinces; some chilled margins of some layered intrusions; some basalts from Iceland, and some basalts from the ocean ridges.
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Amphibole
Pyroxene
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