In crosspolarized light fine oscillatory zoning is visible, in which the composition varies back and forth between more and less anorthiterich compositions. Department of the geophysical sciences the university of chicago, chicago, illinois 60637 abstract millimetersized plagioclase phenocrysts were explosively erupted in highalumina. Dec 12, 2006 the compositional zoning in the stage 2 plagioclase is highly complex compared to the simple zoning patterns observed in stage 1 plagioclase. Patchy zoning of corrosion, is commonly rather uniform in the plagioclase of a given magmatic body. Rotate stage place one pair of the twin in extinction position, measure angle of rotation to extinction position of other twin pair,dividethe total angle by 2 to determine extinction angle. Citeseerx document details isaac councill, lee giles, pradeep teregowda.
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 patchy texture is either an exsolution feature of a. Only two gabbronorites were examined in this study. Phase equilibria of dikes associated with proterozoic. Two types of fractures are present within the mantle. Textures and compositions of minerals can be used to infer the physiochemical conditions present within magmatic systems.
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 more sodic plagioclase. Water, co2, cl, and f in melt inclusions in phenocrysts from. Zoning in plagioclase is equivocal in some samples and systematic in others. Compositional zoning in plagioclase plg mineral phase plays a relevant role in petrogenetic studies due to the wide possible compositional variation and high sensitivity to changes of chemical and physical conditions. Learn vocabulary, terms, and more with flashcards, games, and other study tools. Plagioclase phenocrysts in site 648 basalts range in size from zoning patterns even within individual thin sections. Magma evolution and opensystem processes at shiveluch. These gabbronorites contain up to 8% modal orthopyroxene and appear not to contain olivine.
Mineral zoning, ptxm phase relations, and metamorphic. Please redirect your searches to the new ads modern form or the classic form. Backscattered electron bsederived zoning patterns of plagioclase phenocrysts are used to identify magma processes at bezymianny volcano, kamchatka, based on the 20002007 sequence of. Textural evidence indicates that patchy zoning is the result of a twostage replacement process involving partial resorption of early plagioclase crystals followed. In sar1, most plagioclase is an 46 and zoning is systematic to an 40 in the reaction zones and back to an 43 at the contact with pyroxene. The oscillatory zoning is believed to have formed in response to recurrent supersaturation of the melt in anorthite adjacent to the individual crystals.
Deep roots for midoceanridge volcanoes revealed by. In sections cut parallel to 010 the included plagioclase typically forms irregular. Given that plagioclase is an abundant phase in many magmatic systems, understanding the link between texture and process is vital. May 21, 2019 c plagioclase exhibiting type 2 patchy zoning characterised by low anorthite amoeboid patches that show a degree of crystallographic alignment along cleavage planes. Plagioclase textures and zoning patterns in the miocene dowdy. Other groundmass minerals include orthopyroxene, clinopyroxene, and ilmenite. Reverse and oscillatory zoning in plagioclase felspars. 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. Amphibole in an intergrowths with pyroxenes was found on the boundary between the patchy core and the concentrically zoned periphery of one of the phenocrysts of plagioclase with an 70. Two common examples of this type of zoning are the hourglass sand inclusion in gypsum crystals, and chiastolite those andalusite crystals with the xshaped cross sections. Hornblendes are commonly unzoned, but some show simple, multiple or patchy zoning.
The stage 2 plagioclase exhibits a distinct patchy zoning that represents a pattern of secondary compositional variability see below that has been superimposed upon the original stage 2 plagioclase zoning. The crystal cargo is complex, with both individual plagioclase and glomerocrysts showing large variations in crystal habit, zoning and resorption. The boundary between the core and rim may be euhedral or somewhat irregular and embayed. Given that plagioclase is an abundant phase in many magmatic systems. Plagioclase exhibits complex zoning such as oscillatory oz and patchy zoning pz in b and c. Plagioclase can be unzoned c or show patchy zoning a or reverse and normal zoning b. Unzoned, inclusionpoor cores, up to 2 mm thick have round edges and compositions between an 97 and an 88. The plagioclase in some igneous bodies characteristically shows two or more distinct stages of corrosion and patchy zoning see figs.
The crystals have unzoned, patchy zoned and oscillatoryzoned parts. Backscatteredelectron images of plagioclase ac and olivine df. Etna is the perfect site to investigate interactions between basaltic melts with different volatiles content and the effect of degassing on. Trace element and isotopic exchange during acidbasic. Minerals belonging to a continuous solid solution such as. This grain appears quite homogeneous in plane light, without concentric zones of inclusions that are commonly seen elsewhere. Plagioclase xenocrysts and mafic magmatic enclaves in some. Changes in composition during growth produce a zonation resembling tree rings which is interpreted in a somewhat similar manner although the periods of growth resulting in zones are not necessarily annual. Probable relations between plagioclase zoning and magma dynamics, fuego volcano, guatemala arpre,o t. The two types of plagioclase bearing ajata flows have very distinct plagioclase zoning types.
Plagioclase zoning as an indicator of magma processes at. One explanation for zoning in plagioclases formed from magmas can be implied from information known about the aban system. Recent work on oscillatory zoning in plagioclase springerlink. The significance of plagioclase textures in midocean. 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. Abstract many igneous plagioclases exhibit irregular zoning consisting of corroded cores embayed and surrounded in crystallographic continuity by more sodic plagioclase. Plagioclase zoning includes calcic cores with high sr concentration, and an extensive resorption zone with lower sr, patchy zoning and glass inclusions, surrounded by a narrow 10 m, highly calcic outer rim an 60 with low sr and very high fe contents, similar to the groundmass crystals. Phase equilibria of dikes associated with proterozoic anorthosite complexes mrnq. Rotate stage place one pair of the twin in extinction position, measure angle of rotation to extinction position of other twin pair,dividethe total angle by 2 to determine.
Complex phenocryst textures and zoning patterns in andesites and. Lets take gypsum for example to see how these interesting inclusions form. However, plagioclase grains throughout pca 91179 display patchy zoning as in gro 95533 n fig 1. Experimental petrology of the 19911995 unzen dacite. Trace element and isotopic exchange during acidbasic magma. In this work, major and some trace element abundances by electron microprobe emp and secondary ion mass spectrometry sims. Textural evidence indicates that patchy zoning is the result of a twostage. Abstract textures and compositional zoning of plagioclase reflect changes in the physical and chemical conditions of the host magma, thus representing an useful tool to investigate the processes occurring in the feeding system. Variable parent magmas and recharge regimes of the. This optical zoning is a reflection of chemical zoning in the mineral. Etna volcano has been used to explore the possible role that volcanotectonics exert on magma transfer dynamics. The compositional zoning in the stage 2 plagioclase is highly complex compared to the simple zoning patterns observed in stage 1 plagioclase. Probable relations between plagioclase zoning and magma.
In addition, the chemical zoning in the feldspars in 14303,353 is very patchy fig. 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. Magmatic processes in developing oceanic crust revealed in a. Fnlnr lamontdoherty geological observatory and department of geological sciences, columbia university. Plagioclase zone formation, especially those in the volcanic rocks is reflected in the.
Aban zoning of plagioclase is common, reflecting hybridization of magmas. Pdf the significance of plagioclase textures in mid. The euhedral plagioclase inclusions are interpreted to reflect disequilibrium during anatexis due to slow diffusion in plagioclase. Zoning crystal zoning is a texture developed in solidsolution minerals and characterized optically by changes in the colour or extinction angle of the mineral from the core to the rim. Plagioclase textures and zoning patterns in the miocene. Here, we present a database of textural and compositional data for 1800 plagioclase crystals in midocean ridge basalt from the gakkel ridge. Plagioclase zoning includes oscillatory and patchy zonation and sieve textures. Textural evidence indicates that patchy zoning is the result of a twostage replacement p partial resorption of early plagioclase crystals followed by crystallization. All plagioclase crystals are interpreted as inherited. Lastly, there is sector zoning, which can be caused in two ways. Oscillatory zoning in plagioclase crystals, 100x total magnification.
For example, a plagioclase can be zoned from a carich core to an narich rim. These patches have similar anorthite contents to the outer region of the crystal. 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. 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.
Pdf backscattered electron bsederived zoning patterns of plagioclase phenocrysts are used to identify magma processes at. Crystal residence times from trace element zoning in. The patchy zoning may have resulted from cellular growth of plagioclase. Chemical composition those in many rocks are zonedi. Inclusions of this sodic plagioclase within the corroded cores give the feldspar a mottled appearance and suggest the term patchy zoning for these textures. Magmatic processes in developing oceanic crust revealed in. In some places, the oscillatory zoning is replaced by patchy zoning or irregular, wavy zones e. Ilmenite is the most abundant oxide phase in pca 91179, often sharing boundaries with spinel. In crosspolarized light fine oscillatory zoning is visible, in which the composition varies back and. Sr isotope zoning in plagioclase from parinacota volcano. Additionally, the exercise is to give the student an. The lavas and pumices studied are hornblende plagioclase andesites with average preeruptive temperatures of 840c and fo2 of 1. Minerals free fulltext allanite geochemical response.
Patchy zoning 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. The significance of plagioclase textures in midocean ridge. The interpretation of plagioclase zoning has been the subject of much speculation among petrologists, phemister having recently contributed 1934 a valuable discussion of earlier work, together with certain new suggestions as to the probable causes of the reverse and oscillatory types. Pdf plagioclase zoning as an indicator of magma processes. Type 1 a, b and type 2 cg patchy zoning in plagioclase. Origin of patchy zoning in plagioclase from gabbroic rocks of. Inclusions of this sodic plagioclase within the corroded cores give the feldspar a mottled appearance and suggest the term patchy zoning. Crystal residence times from trace element zoning in plagioclase reveal changes in magma transfer dynamics at mt. Zoning the zoning is a compositional variations found in the crystals.
Chlorine is strongly partitioned into any fluid phase separating from magmas shinohara, 1994, and, if the. Garnet zoning directly adjacent to the inclusions demonstrates that the ca. The most common types of zoning are reverse and patchy. Plagioclase textures and compositional zoning in recent mt. Video produced as part of the open educational resource. Mineral zoning, ptxm phase relations, and metamorphic evolution of some adirondack granulites, new york frank s. When present, the kfeldspar is found only as an interstitial phase.
In et10, most plagioclase is an 50 and there are patches of an. The significance of plagioclase textures in midocean ridge basalt. The first type is represented by those that encompass the entire mantle and these are sometimes filled with a bsebright phase e. Volcanic rock series in island arcs and active continental. A sequence of oscillatory cores and thin, normally zoned, more sodic rims is. Monazite occurs both as small inclusions within the garnet zoning with th reaching concentra tions up to 25 wt% at the rims. Streck, 2008 when observed at backscattered electron images bse. Many igneous plagioclases exhibit irregular zoning consisting of corroded cores embayed and surrounded in crystallographic continuity by more sodic plagioclase.
Patchy zoning this sometimes occurs in plagioclase crystals where irregularly shaped patches of the crystal show different compositions as evidenced by going extinct at. Plagioclase and pyroxene tend to trap mi in zonal arrays, permitting some studies on the sequence of inclusion formation. The mechanism employed to explain this type of zoning is an increase in water pressure in the magma during the initial stages of crystallization. Textures and compositional zoning of plagioclase reflect changes in the physical and chemical conditions of the host magma, thus representing an useful tool to investigate the processes occurring in the feeding system. Growth of crystals from magma is one of the fundamental processes of geology. 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. Normal concentric, reverse, oscillatory, patchy, and sector zoning are all apparent, even in individual thin sections. Plagioclase zoning includes oscillatory and patchy. Plagioclase exhibits both internal and external resorption ir and er, respectively in b and c. Pdf plagioclase zoning as an indicator of magma processes at. Citeseerx magma evolution and opensystem processes at. Markussen department of earth and environmental science, rensselaer polytechnic institute, troy, ny 12180, usa detailed petrologic analysis of ten metaanorthosites and related introduction. Millimetersized plagioclase phenocrysts were explosively erupted in highalumina basaltic ash from fuego volcano, guatemala on october 14, 1974. Pdf the significance of plagioclase textures in midocean.
Mineral dissolution and reprecipitation mediated by an. Phenocryst zoning patterns are used to identify opensystem magmatic processes in the products of the 2001 eruption of shiveluch volcano, kamchatka. Orthopyroxene, along with plagioclase and clinopyroxene, is present as a primary igneous phase in a few gabbros without any apparent stratigraphic relations. Apr 24, 2018 the classical approach to fluidmediated mineral dissolution and reprecipitation assumes that minerals in contact with a fluid phase dissolve until the chemical potential of the solid phase equals. Plagioclase exhibits normal, reverse, oscillatory and patchy zoning patterns and sieve and sponge textures, with several populations of plagioclase crystals observed on the scale of a single thin section. Trace element zoning in plagioclase of selected alkaline lavas from the historic 16071892 ad and recent 198320 ad activity of mt.
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