Mineral zoning, p t x m phase relations, and metamorphic. The abrupt change to normally zoned rims in the zoning sequence is thought to reflect latestage saturation of the residual melt in volatiles. There are several causes of these zones in crystals and both their presence and absence can be interesting tools for learning about the mineral and how it grew. A kinetic mathematical model of crystal growth from the melt is used to describe quantitatively the phenomenon of oscillatory zoning in plagioc lase feldspar. Zoning the zoning is a compositional variations found in the crystals. Pdf plagioclase zoning as an indicator of magma processes at. Quartz is typically the most transparent mineral in rocks, because it is not very succeptible to alteration to finegrained minerals, and it has no cleavages. Mackenzie, university of manchester press, manchester.
Reverse and oscillatory zoning in plagioclase felspars. A phenocryst is an early forming, relatively large and usually conspicuous crystal distinctly larger than the grains of the rock groundmass of an igneous rock. Variable parent magmas and recharge regimes of the. Some phenocrysts may show more than one type in the different growth zones, indicating that several growthcontrolled phenomena may be at work during the crystallization of a single crystal. 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 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. Magmatic processes in developing oceanic crust revealed in. Poikilitic inclusions commonly present within the patches of sodic plagioclase are believed. 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. Probable relations between plagioclase zoning and magma. Type 1 a, b and type 2 cg patchy zoning in plagioclase. Complex zoning in plagioclase feldspars from odp site 648. These probably formed during faster crystal growth than the clear zones. 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.
The smaller unlabelled black crystals are crspinel. Open circles are outliers, which lie further than 1. Instead, plagioclase crystals that formed in different parts of a. Concentric zones are familiar to all as in phantom crystals or color zoning in watermelon tourmaline. Plagioclase phenocrysts in site 648 basalts range in size from zoning patterns even within individual thin sections. Oscillatory zoning this sometimes occurs in plagioclase grains wherein concentric zones around the grain show thin zones of different composition as. Zoning in crystal is the difference between the inner core of the crystal and its outer rim. Minerals belonging to a continuous solid solution such as. Plagioclase textures and zoning patterns in the miocene dowdy. Fe and mg concentrations increase overall from cores to rims. This sometimes occurs in plagioclase crystals where irregularly shaped patches of the crystal show different. Phenocryst zoning patterns are used to identify opensystem magmatic processes in the products of the 2001 eruption of shiveluch volcano, kamchatka. The significance of plagioclase textures in midocean. The significance of plagioclase textures in midocean ridge basalt.
Oscillatory zoning this sometimes occurs in plagioclase grains wherein concentric zones around the grain show thin zones of. Pdf plagioclase zoning as an indicator of magma processes. 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. Normal concentric, reverse, oscillatory, patchy, and sector zoning are all apparent, even in individual thin sections. Learn vocabulary, terms, and more with flashcards, games, and other study tools. This contrasts with the pyroxenes, which are usually unzoned. Magma interaction recorded in plagioclase zoning in granitoid. Plagioclase is a major constituent mineral in the earths crust, and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of. Magma interaction recorded in plagioclase zoning in. Oscillatory zoning in plagioclase phenocrysts, which may reflect rhythmic changes in temperature andor ph2o during crystal growth, shows both half high amplitudelow frequency and lahf low amplitudehigh frequency zoning.
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. The one on the right is known as watermelon for obvious reasons. A close scrutiny of the newer granites of the caledonian orogen in scotland lorence g. Plagioclase is a series of tectosilicate framework silicate minerals within the feldspar group. The significance of plagioclase textures in midocean ridge. All plagioclase crystals are interpreted as inherited. Phase equilibria of dikes associated with proterozoic. Abstract many igneous plagioclases exhibit irregular zoning consisting of corroded cores embayed and surrounded in crystallographic continuity by more sodic plagioclase. Probable relations between plagioclase zoning and magma dynamics, fuego volcano, guatemala arpre,o t. Mineral zoning, ptxm phase relations, and metamorphic evolution of some adirondack granulites, new york frank s. O and p refer to host olivine and plagioclase crystals. Millimetersized plagioclase phenocrysts were explosively erupted in highalumina basaftic ash from fuego volcano, guatemala on october 14, 1974.
Variable parent magmas and recharge regimes of the parinacota. Pdf the significance of plagioclase textures in mid. Zoning results from the minerals inability to maintain chemical equilibrium with a. In order to help resolve this conflict, we previously showed that ba zoning profiles for megacrysts from two. Complex zoning in plagioc lase feldspars from odp site 6481 peter s. Zircons are euhedral and prismatic and vary from 200400 m in length.
Complex phenocryst textures and zoning patterns in andesites and. Markussen department of earth and environmental science, rensselaer polytechnic institute, troy, ny 12180, usa detailed petrologic analysis of ten metaanorthosites and related introduction. Pdf backscattered electron bsederived zoning patterns of plagioclase phenocrysts are used to identify magma processes at. Plagioclase zoning in parinacota samples ranges from oscillatory zoning to rounded zone boundaries, especially at crystal corners, often crosscutting previous zones. Shown in the image are two faceted tourmalines cut from zoned crystals produced by growth. In fact, the formation of zoned crystals during growth and dissolution may be much more common than generally believed because growth zoning can be readily purged by diffusion in plutonic and metamorphic rocks during their long residence time at high temperature. Plagioclase textures and zoning patterns in the miocene. Patchy zoning shows a range in composition from 45% an in dark patches to 55% an in light patches and 510% change in feo content. Other groundmass minerals include orthopyroxene, clinopyroxene, and ilmenite. In crosspolarized light fine oscillatory zoning is visible, in which the composition varies back and forth between more and less anorthiterich compositions.
Plagioclase exhibits complex zoning such as oscillatory oz and patchy zoning pz. The crystal cargo is complex, with both individual plagioclase and glomerocrysts showing large variations in crystal habit, zoning and resorption. 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. Textural evidence indicates that patchy zoning is the result of a twostage replacement p partial resorption of early plagioclase crystals followed by crystallization. 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. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or recordgroove effect. In crosspolarized light you can see that the zones also have different birefringence, indicating. The oscillatory zoning is believed to have formed in response to recurrent supersaturation of the melt in anorthite adjacent to the individual crystals. Magma evolution and opensystem processes at shiveluch. The two types of plagioclasebearing ajata flows have very distinct plagioclase zoning types.
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. Magmatic processes in developing oceanic crust revealed in a. This describes explicitly the coupling between the interface kinetics and the diffusion of chemical species. Many igneous plagioclases exhibit irregular zoning consisting of corroded cores embayed and surrounded in crystallographic continuity by more sodic plagioclase. Deep roots for midoceanridge volcanoes revealed by. Unzoned, inclusionpoor cores, up to 2 mm thick have round edges and compositions between an 97 and an 88. If such rims are present on crystals they should be noted in the textural description. A new theory of oscillatory zoning in naturally grown plagioclase crystals is presented. Zircon is an abundant accessory phase occurring as 20150. Such rocks that have a distinct difference in the size of the crystals are called porphyries, and the adjective porphyritic is. Finally, crystal zoning describes faintly to very welldefined geometric arrangements of portions within individual crystals that differ significantly in composition or some other property from adjacent portions. Plagioclase crystals display irregular margins and appear to have crystallized after biotite. Abstract 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 the significance of plagioclase textures in midocean.
Patchy zoning is classified on an intensity scale of 15, a crystal with normal. Finegrained crspinel is present outside the seams, distributed around olivine crystals. Fowler oxawacarleton geoscience cente, depamnent of geology, university of ottawa, 140 ktuis pasteur, ottawa, ontario kin 6n5 abstract crystals characterized by oscillatory zoning show evidence for a quasicyclic alternation in the chemical composition of. Oscillatory zoning in plagioclase phenocrysts, which may reflect rhythmic changes in temperature andor ph2o during crystal growth, shows both half high amplitudelow frequency and lahf low amplitudehigh. Patchyzoned cores and regions are rich in inclusions of glass and gas, about 50. A boxandwhisker diagram of the srba ratio at cores and rims of both historic and recent plagioclase crystals. Plagioclase and pyroxene tend to trap mi in zonal arrays, permitting some studies on the sequence of inclusion formation. Plagioclase zone formation, especially those in the volcanic rocks is reflected in. The development of patchy textures at the core of plagioclase is still a matter of debate, mostly over whether it is a feature acquired during the growth or a consequence of dissolution cf. Life after death experience nde with steve gardipee, vietnam war story one of the best ndes duration.
These are therefore interpreted, and referred to in the following, as resorption surfaces. The petrology and geochemistry of a xenolith, a fragment of a melt. Sr isotope zoning in plagioclase from parinacota volcano. A sequence of oscillatory cores and thin, normally zoned, more sodic rims is. Four main types are present in plagioclase phyric oceanic basalts. Trace element zoning in plagioclase of selected alkaline lavas from the historic 16071892 ad and recent 198320 ad activity of mt. A kinetic mathematical model of crystal growth from the melt is used to describe quantitatively the phenomenon of oscillatory zoning in plagioclase feldspar. Etna volcano has been used to explore the possible role that volcanotectonics exert on magma transfer dynamics.
Plagioclase crystals an 3565 are characterized by numerous, more or less rounded, concentric resorption surfaces, commonly associated with concentric patchy zoning and large crystallized melt inclusions. However, patchy zoning occurs in almost all the plagioclase crystals. A homogeneous internal region, frequently the core. This grain appears quite homogeneous in plane light, without concentric zones of inclusions that are commonly seen elsewhere. Plagioclase oikocrysts are roundedelongate, and 15 mm in diameter, and display patchy optical zoning. Poikilitic inclusions commonly present within the patches of sodic plagioclase are believed to have crystal lized from the melt trapped in the cores after corrosion. Plagioclase feldspars are an important, if not the dominant, component of nearly every igneous rock, especially intermediate to mafic igneous rocks. For example, a plagioclase can be zoned from a carich core to an narich rim. We agree with andersons interpretation that the fine to mediumscale patchy zoning probably represents rapid crystal growth during sudden decompression. Filled circles indicate the mean values, solid lines in the grey boxes the medians. The lavas and pumices studied are hornblendeplagioclase andesites with average preeruptive temperatures of 840c and fo2 of 1. Rapid crystallization of preciousmetalmineralized layers. 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. Oscillatory zoning this sometimes occurs in plagioclase grains wherein concentric zones around the grain show thin zones of different composition as evidenced by extinction phenomena.
May 21, 2019 the crystal cargo is complex, with both individual plagioclase and glomerocrysts showing large variations in crystal habit, zoning and resorption. Its usually caused by a change in the composition of the crystal, showing a difference in the composition of the fluid the crystal crystallized from, i. The crystals of plagioclase are often particularly well developed in pegmatites, coarsely crystalline igneous rocks that form during late waterwet stages of a magma cooling. Minerals such as olivine or plagioclase which have a solid solution series are frequently zoned. Conflicting theories for the origin of kfeldspar megacrysts in granitic rocks are principally l the megacrysts crystallized from a melt or 2 they grew by replacement from a waterrich fluid phase under subsolidus conditions. Plagioclase crystals are usually complexly zoned and according to gill 1981, the complexlyzoned crystals in andesite usually include one or more of the following features. Unzoned, inclusionpoor cores, up to 2 mm thick have round edges and compositions between anet and an6s. Plagioclase crystals in magmatic rocks possess various textural types and zoning such as spongy, boxy and sieve texture, and oscillatory, patchy and normal zoning. Linking in situ crystallization and magma replenishment via. The crystals have unzoned, patchyzoned and oscillatoryzoned parts. Video produced as part of the open educational resource. Plagioclase feldspar grows in igneous environments and crystals which grow early in a cooling magma contain more calcium than those formed at the end of crystallization, which are enriched in sodium.
Oct 27, 20 zoning seen in plagioclase feldspar in 4x and 10x. Inclusions of this sodic plagioclase within the corroded cores give the feldspar a mottled appearance and suggest the term patchy zoning for these textures. Backscattered electron bsederived zoning patterns of plagioclase phenocrysts are used to identify magma processes at bezymianny volcano, kamchatka, based on the 20002007 sequence of. Feb 23, 2009 zoning in crystal is the difference between the inner core of the crystal and its outer rim.
Poikilitic inclusions commonly present within the patches of sodic plagioclase. Two common types of zoning are concentric zones and sector zones. Crystal outlines and zone morphologies range from euhedral. Plagioclase xenocrysts and mafic magmatic enclaves in some. These textures provide us with information concerning conditions of magmatic evolution and histories of magmatic systems. 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. Department of the geophysical sciences the university of chicago, chicago, illinois 60637 abstract millimetersized plagioclase phenocrysts were explosively erupted in highalumina. Patchy zones are a common and prominent feature of 1974 crystals but are rare and subtle in 1999 crystals. In the plagioclase crystals from hole 648b, minor and trace. The most common types of zoning are reverse and patchy. Oscillatory zoning in plagioclase crystals, 100x total magnification.
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