Rapid crystallization of preciousmetalmineralized layers. A kinetic mathematical model of crystal growth from the melt is used to describe quantitatively the phenomenon of oscillatory zoning in plagioc lase feldspar. Hornblendes are commonly unzoned, but some show simple, multiple or patchy zoning. Plagioclase group solid solution between albite anorthite. Unzoned, inclusionpoor cores, up to 2 mm thick have round edges and compositions between an 97 and an 88. Backscattered electron bsederived zoning patterns of plagioclase phenocrysts are used to identify magma processes at bezymianny volcano, kamchatka, based on the 20002007 sequence of eruptive products. The grain selected for sims analysis is typical of these fig. This paper is concerned with the significance of patchy zoning, a feature which, though widespread in igneous plagioclases, has been ignored by many petrologists. Etna volcano has been used to explore the possible role that volcanotectonics exert on magma transfer dynamics. The patchy zoning may have resulted from cellular growth of plagioclase. The patchyzoned regions are relatively sodic but overlap the compositions of the brightly banded regions.
The primary importance of plagioclase, however, derives from its role in rock formation. Plagioclase and pyroxene tend to trap mi in zonal arrays, permitting some studies on the sequence of inclusion formation. Phenocryst zoning patterns are used to identify opensystem magmatic processes in the products of the 2001 eruption of shiveluch volcano, kamchatka. The plagioclases possess complex patchy and concentrically zoned patterns 1112 figure1. A sequence of oscillatory cores and thin, normally zoned, more sodic rims is characteristic of many igneous plagioclases, especially those of quartz diorites and granodiorites.
The patchy zone includes large glass inclusions and, in some crystals, several internal discontinuities, interpreted as dissolution surfaces. In crosspolarized light fine oscillatory zoning is visible, in which the composition varies back and. Pdf plagioclase zoning as an indicator of magma processes at. The first one is the primary plagioclase, which in most cases shows patchy zoning, and therefore is considered to be crystallised from a melt vance, 1965. The lavas and pumices studied are hornblende plagioclase andesites with average preeruptive temperatures of. Plagioclase also shows melt inclusions with occasional fresh core and altered rim. Abstract oscillatory zoning in plagioclase is investigat ed at small scale. Patchy plagioclase consists of domains with contrast compositions. Zoning the zoning is a compositional variations found in the crystals. Grainrecycling zoning of plagioclase and metamorphic. Deep roots for midoceanridge volcanoes revealed by. Other alteration products include epidote and carbonates.
Plagioclase is used in the manufacture of glass and ceramics. Citeseerx processes and sources during latevariscan. Complex phenocryst textures and zoning patterns in andesites and. Backscatteredelectron images of plagioclase ac and olivine df. Plagioclase zoning comprises a unique record of past changes in magmatic environment which can provide critical evidence on several fundamental problems of magmatic evolution. Four main types are present in plagioclase phyric oceanic basalts. The significance of plagioclase textures in midocean. On the other hand, the observed patchy zoning suggests decompression starting under waterundersaturated conditions and ending under watersaturated conditions.
This sometimes occurs in plagioclase crystals where irregularly shaped patches of the crystal show different compositions as evidenced by going extinct at angles different from other zones in the crystal. Trace element zoning in plagioclase of selected alkaline lavas from the historic 16071892 ad and recent 198320 ad activity of mt. Under crossed polars, plagioclase will also display zoning continuous zoning as the stage is rotated, there will be a change in extinction due to a regular change. Complex zoning in plagioclase feldspars from odp site 648.
Minerals belonging to a continuous solid solution such as. Probable relations between plagioclase zoning and magma. Aban zoning of plagioclase is common, reflecting hybridization of magmas. The boundary between the core and rim may be euhedral or somewhat irregular and embayed. Plagioclase phenocrysts in site 648 basalts range in size from zoning patterns even within individual thin sections. Crystal residence times from trace element zoning in. Patchy zoning in plagioclase has been observed in gabbroic rocks in southwestern oregon. Variable parent magmas and recharge regimes of the. The grains con tain regions with lamellar zoning in th content and irregular patchy zoning. The smaller unlabelled black crystals are crspinel. Water, co2, cl, and f in melt inclusions in phenocrysts.
Oscillatory zoning in plagioclase feldspar science. Equation 59 can be used to estimate dra, using the lattice strain approach. Inclusions of this sodic plagioclase within the corroded cores give the feldspar a mottled appearance and suggest the term patchy zoning for these textures. The diversity of plagioclase zoning suggests that several batches of rhyolitic magmas with distinct histories might be involved. Plagioclase zone formation, especially those in the volcanic rocks is reflected in. Type 1 a, b and type 2 cg patchy zoning in plagioclase. Zoning is relatively less common as compared to twining.
Textural evidence indicates that patchy zoning is the result of a twostage replacement process involving partial resorption of early plagioclase crystals followed by crystallization of a. Plagioclase zoning includes oscillatory and patchy zonation and sieve textures. The sequence of development is visualized as follows. Normal concentric, reverse, oscillatory, patchy, and sector zoning are all apparent, even in individual thin sections. As a result of discontinuous growth of a particular grain, plagioclase can have discontinuous zoning or patchy zoning. The significance of plagioclase textures in midocean ridge basalt. Microtextural and mineral chemical analyses of andesite. The lavas and pumices studied are hornblende plagioclase andesites with average preeruptive temperatures of 840c and fo2 of 1. Metaluminous granitoids consist of mediumcoarsegrained. Molecular ion interferences in the mass spectra were minimized using an energy. The main minerals are the same as for the mme, but mafic phases are modally more prevalent, together with amphiboleclinopyroxene textures, and patchy zoning in plagioclase.
Interpretation of plagioclase zonation in calcic pelitic. Pdf the origin and petrogenesis of mount hasan small mt. Mineral zoning, p t x m phase relations, and metamorphic. Zoning patterns in plagioclase can also reflect the character of the matrix overgrown. Plagioclase can be unzoned c or show patchy zoning a or reverse and normal zoning b. Reverse and oscillatory zoning in plagioclase felspars.
Plagioclase zoning includes oscillatory and patchy. Citeseerx magma evolution and opensystem processes at. This grain appears quite homogeneous in plane light, without concentric zones of inclusions that are commonly seen elsewhere. The crystals have unzoned, patchy zoned and oscillatoryzoned parts. It is approximately hexagonal in outline, clear yellow. Highresolution quantitative imaging of plagioclase composition. Sr isotope zoning in plagioclase from parinacota volcano. This fractionation alters the range of plagioclase compositions available over a given temperature range, changing pt estimates obtained from assuming whole rock equilibrium. Trace element and isotopic exchange during acidbasic. Millimetersized plagioclase phenocrysts were explosively erupted in highalumina basaltic ash from fuego volcano, guatemala on october 14, 1974. Learn vocabulary, terms, and more with flashcards, games, and other study tools.
Plagioclase xenocrysts and mafic magmatic enclaves in some. When present, the kfeldspar is found only as an interstitial phase. Origin of patchy zoning in plagioclase from gabbroic rocks of. Plagioclase is characterized by complex zoning patterns, such as patchy zoning, asymmetrical zoning and strong resorption between anrich cores and more albitic rims. In most granites neogene plagioclase in contrast to relict plagioclase consists of two groups thought to be genetically different.
Magma storage constrains by compositional zoning of plagioclase. Patchy zoning is interpreted here in terms of decrease in confining pressure on waterdeficient magma during its rise in the crust. Please redirect your searches to the new ads modern form or the classic form. In this model, the functional dependence of crystal growth rate on both melt and crystal surface composition and the transport of material within the melt are explicitly considered.
The lavas and pumices studied\ud are hornblende plagioclase andesites with average preeruptive temperatures of 840c and fo2 of 1. These patches have similar anorthite contents to the outer region of the crystal. The importance of accurate knowledge concerning the causes of composition zoning in plagioclases lies in the possibility that such zoning may throw light on the magmatic history of the rock of which the zoned felspar is a component. Plagioclase phenocrysts from mafic magmatic enclaves and plagioclase crystals from host granitoids of some plutons of the central part of the sierra nevada batholith are complexly zoned and commonly divided into three neatly distinct parts. Patchy zoning is noticed locally within the plagioclase. Patchy zoning with little compositional range is present in many phenocrysts, either in cores or in thick zones, which are overgrown by thick, oscillatory zoned rims figs. Plagioclase zoning as an indicator of magma processes at. A few grains also show reaction texture with the groundmass. Groups of dully and brightly banded oscillatory zones correlate between grains and are respectively relatively calcic and sodic. So if we take a microprobe to a plagioclase crystal we should see a core with high calcium and a rim with high sodium. An example of this new type of zoning is presented from the lewisian gneiss, nw scotland, and a new pt path calculated for post deformation grain growth. Through the use of plagioclase, midocean ridges have been shown to be.
Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more properly known as the plagioclase feldspar series from the ancient greek for oblique fracture, in reference to its two cleavage angles. Variations in na not shown and e with coretorim variations in al of 015011 are patchy rather than. Magma evolution and opensystem processes at shiveluch. Oligoclase, the most common plagioclase, occurs in. Petrograpical fetures of each volcano are similar and they display disequilibrium textures indicative of mixingmingling processes, such as patchy zones and oscillatory zoning in plagioclase. Sometimes the zoning is quite pronounced and can be seen using a polarized light microscope. The plagioclase grains are mostly subhedral and exhibit twining. In metamorphic rocks, twinning, more often than not, is absent. Plagioclase is a series of tectosilicate framework silicate minerals within the feldspar group. The range of sr isotopic compositions recorded in plagioclase is 0706907091, greater than that of the wholerocks.
1268 1056 530 1582 1401 9 1546 678 255 469 387 181 246 410 357 803 454 1636 139 1298 1106 537 1033 246 216 1101 1206 1017 843 410 1205 1295