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. Twentytwo crystals from 1974 and thirtyseven crystals from 1999 ash were weighed, oriented, sectioned and optically characterized to delineate zoning differences. Also remember to distinguish between orthoclase, orthoklase, which is an individual species and oligoclase, ohligohklase, which is in the plagioclase. Compositions of the plagioclase phenocrysts and microlites of kvs and dvs lavas range from an70 to an59 and an 22 to an36, respectively. Plagioclase is a mixture of albite, or sodium aluminosilicate, and anorthite, or calcium aluminosilicate. The plagioclase grains are mostly subhedral and exhibit twining. Complex zoning in plagioclase feldspars from odp site 648. Zoning is relatively less common as compared to twining. Zoning in plagioclase is equivocal in some samples and systematic in others. 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. Magma storage constrains by compositional zoning of plagioclase. 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 igneous rocks. What happens is that lattice points in one crystal are shared as lattice points in another crystal adding apparent symmetry to the crystal pairs.
Extinction angles on multiple twins in plagioclase may be used to estimate feldspar composition 30 o carich. All plagioclase crystals are interpreted as inherited. Plagioclase can be unzoned c or show patchy zoning a or reverse and normal zoning b. It is euhedral, partly zoned with typical polysynthetic twinning. Minerals belonging to a continuous solid solution such as. This sometimes occurs in plagioclase crystals where irregularly shaped patches. This rollover demonstrates the ability to see twinning when the plagioclase mineral is under crossed polars. In sections cut parallel to 010 the included plagioclase typically forms irregular. In crosspolarized light fine oscillatory zoning is visible, in which the composition varies back and forth between more and less anorthiterich compositions. Two types of interpenetration twins occur in staurolite the 031 twins from a rightangled cross and the 231 twins form a cross at about 60 o. Plagioclase is the main felsic mineral of kvs and dvs. 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.
In the studied rocks, plagioclase crystals display a variety of textures such as finescale oscillatory zoning, resorption surfaces, sieved and patchy zones, and a wide variability of anorthite an and trace element te content. Plagioclase phenocrysts in the plagioclase phyric basalt from hole u1433b of iodp expedition 349 in the south china sea show complex zoning patterns e. In the plagioclase crystals from hole 648b, minor and trace element. Plagioclase thin section tectosilicates microckscopic. 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. Twinning is important to recognize, because when it occurs, it is often one of the most diagnostic features enabling identification of the mineral. Mineral zoning, p t x m phase relations, and metamorphic. Plagioclase zone formation, especially those in the volcanic rocks is reflected in the fact. Type 1 a, b and type 2 cg patchy zoning in plagioclase. Multilevel magma plumbing at agung and batur volcanoes. Zoning results from the mineral s inability to maintain chemical. Prince, tom petty, steve winwood, jeff lynne and others while my guitar gently weeps duration. Plagioclase minerals are extremely widespread and abundant. Bytownite ca %7090 na %3010 locality at bytown now the city of ottawa, canada.
Compositional zoning morphic clinopyroxene, minor metamorphic ordetails of the petrography and mineral zoning from three thopyroxene plus small amounts of plagioclase, biotite samples et10, as31 and sar1 are presented in figs and hornblende. The crystals have unzoned, patchy zoned and oscillatoryzoned parts. The oscillatory zoning is believed to have formed in response to recurrent supersaturation of the melt in anorthite adjacent to the individual crystals. Microtextural and mineral chemical analyses of andesite. Many igneous plagioclases exhibit irregular zoning consisting of corroded cores embayed and surrounded in crystallographic continuity by more sodic plagioclase.
Please redirect your searches to the new ads modern form or the classic form. An intergrowth of quartz and plagioclase that shows small wormlike bodies of quartz enclosed in plagioclase. Plagioclase, any member of the series of abundant feldspar minerals usually occurring as lightcolored, glassy, transparent to translucent, brittle crystals. Plagioclase zoning as an indicator of magma processes at. Zoning results from the minerals inability to maintain chemical equilibrium with a magma during rapid cooling. Minerals in thin section at university of minnesota twin. Reverse and oscillatory zoning in plagioclase felspars. Plagioclase zoning as an indicator of magma processes at bezymianny volcano, kamchatka article pdf available in contributions to mineralogy and petrology 1621. Patchy zoning is noticed locally within the plagioclase. Many plagioclase phenocrystals display patchy zones and oscilatory zoning. On sources of magma and enclaves can mineral february 1, 2007 45. Mineral chemistry and petrography of lava successions from. Pdf plagioclase zoning as an indicator of magma processes at. Select image and move cursor over image to view twinning at extinction, then rotated 45 o counterclockwise.
The zoning is a compositional variations found in the crystals. Both boxy cellular growth and resorbtion rims present. Textures and compositions of minerals can be used to infer the. In the experiments plagioclase of different compositions an96, an61, an54, an23, and an22 surrounded by glasses of six different compositions in the system diopsidealbiteanorthite was heated at. Plagioclase is a series of tectosilicate framework silicate minerals within the feldspar group. 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.
Of magma and enclaves can mineral february 1, 2007 45. A few grains also show reaction texture with the groundmass. Backscatteredelectron images of plagioclase ac and olivine df. Ophitic laths of plagioclase in a coarse grained matrix of pyroxene crystals, wherein the plagioclase is totally surrounded by pyroxene grains. This optical zoning is a reflection of chemical zoning in the mineral. Inclusions of this sodic plagioclase within the corroded cores give the feldspar a mottled appearance and suggest the term patchy zoning for these textures. United states zoning mineral framework silicate2, feldspar group. Complex phenocryst textures and zoning patterns in. Additionally, the exercise is to give the student an. 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.
This most commonly appears as bands of different colors, but can also be dark, oriented bands within the crystal. For example, a plagioclase can be zoned from a carich core to an narich rim. Details of the petrography and mineral zoning from three thopyroxene plus small amounts of plagioclase, biotite samples et10, as31 and sar1 are presented in figs and hornblende. In et10, most plagioclase is an 50 and there are patches of an. Origin of patchy zoning in plagioclase from gabbroic rocks of.
Plagioclase phenocrysts in site 648 basalts range in size from zoning patterns even within individual thin sections. Geological society of america special papers 101, abstract for 1966. Photomicrograph of plagioclase in anorthosite bv106 in crossed polarized light. Mineral textures and zoning as evidence for open system. Plagioclase zoning comprises a unique record of past changes in magmatic environment which can provide critical evidence on several fundamental problems of magmatic evolution. Probable relations between plagioclase zoning and magma. Millimetersized plagioclase phenocrysts were explosively erupted in highalumina basaltic ash from fuego volcano, guatemala on october 14, 1974. Complex phenocryst textures and zoning patterns in andesites and.
In crosspolarized light fine oscillatory zoning is visible, in which the composition varies back and. Anorthite ca %90100 na %100 from greek for not straight, because of its triclinic symmetry. Plagioclase also shows melt inclusions with occasional fresh core and altered rim. 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. Normal concentric, reverse, oscillatory, patchy, and sector zoning are all apparent, even in individual thin sections. Probable relations between plagioclase zoning and magma dynamics, fuego volcano, guatemala arpre,o t. Department of the geophysical sciences the university of chicago, chicago, illinois 60637 abstract millimetersized plagioclase phenocrysts were explosively erupted in highalumina. Inclusions of this sodic plagioclase within the cor roded cores give the feldspar a mottled appearance and suggest the term patchy zoning for these textures. An igneous rock is any crystalline or glassy rock that forms from cooling of a magma a magma consists mostly of liquid rock matter, but may contain crystals of various minerals, and may contain a gas phase that may be dissolved in the liquid or may be present as a separate gas phase magma can cool to form an igneous rock either on the surface of the earth. Monazite occurs both as small inclusions within the garnet zoning with th reaching concentra tions up to 25 wt% at the rims.
Mineral feature modal% size, morphology, distinguishing optical properties plagioclase 30 0. This paper is concerned with the significance of patchy zoning, a feature which, though widespread in igneous plagioclases, has been ignored by many petrologists. A sequence of oscillatory cores and thin, normally zoned, more sodic rims is. We compare plagioclase zoning relationships in the 1974 vulcanian and 1999 strombolian. The textures and kinetics of reaction between plagioclase and melts have been investigated experimentally, and origin of dusty plagioclase in andesites has been discussed. 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 interior of this crystal has patchy zoning rather than concentric, indicating. 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 plagioclases possess complex patchy and concentrically zoned patterns 1112 figure1. Zoning the zoning is a compositional variations found in the crystals.
Zoning results from the mineral s inability to maintain chemical equilibrium with a. Unzoned, inclusionpoor cores, up to 2 mm thick have round edges and compositions between an 97 and an 88. The plagioclase hosted in the megacrysts, as well as many phenocrysts in the groundmass, present a variety of textures, from patchy to direct or oscillatory zoning, and from skeletal to deeply resorbed shapes, and variable chemical composition. The significance of plagioclase textures in midocean ridge basalt. Evidence for crystal breakage, sieved rims and cores, patchy cores, plagioclase inclusions, olivine inclusions near rims. The plagioclase in some igneous bodies characteristically shows two or more distinct stages of corrosion and patchy zoning see figs. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or recordgroove effect.
Patchy zoning of corrosion, is commonly rather uniform in the plagioclase of a given magmatic body. Deciphering textural and chemical zoning of kfeldspar. There are several causes of these zones in crystals. Plagioclase series of minerals in the feldspar group, from greek for oblique and fracture, in reference to the oblique angle between its best cleavages. Textural evidence indicates that patchy zoning is the result of a twostage.
Sr isotope zoning in plagioclase from parinacota volcano. This grain appears quite homogeneous in plane light, without concentric zones of inclusions that are commonly seen elsewhere. Mineral zoning, ptxm phase relations, and metamorphic. Multilevel magma plumbing at agung and batur volcanoes increases risk of hazardous eruptions. The phenocrysts often display diffusionlimited growth textures e.136 384 838 597 64 1255 1077 425 741 448 444 1 1224 973 282 985 1351 1005 747 1296 845 49 754 37 941 1306 281 1251 476 1291 1233 248 1363 593 360 580 1091 1129 519 1023 44 1494 1495 1271