Smith Profiles of trace element abundance measured by LA-ICP-MS along traverses through zoned orthopyroxene oikocrysts along traverse lines in sample 8363 indicated in the Tornado three-element false colour map (Cr red, Fe green, Ca blue). Figure 4: Thin section photo with crossed-polarizers, of 64569,4, showing poikilitic texture with included clasts. E. Electron microprobe analyses of orthopyroxene from selected samples. A plausible scenario is one where an olivine–chromite framework grows in situ at a sloping sidewall; the crystallization site is subsequently swamped by contaminated magma with a high silica activity, causing a rapid burst of orthopyroxene growth from supersaturated magma and resulting peritectic dissolution of olivine and chromite, as above. The entire assemblage is more consistent with crystallization at the liquidus from compositionally variable magma. 14 d) traverses an oscillatory zoned cumulus orthopyroxene and shows similar compatible element profiles to 8363 ( Fig. The GeoHistory Facility at Curtin University was supported by an AGOS AuScope EIF grant. Brown J. G. (c) Similar to (b) showing Cr (red) and Ti (blue) only; the Ti enrichment in Ca-amphibole but lack of strong zonation associated with Cr-rich and Cr-poor zones should be noted. A cumulus origin for oikocrysts, either by mechanical deposition or by in situ growth from supersaturated magma, may be a widespread phenomenon, and potentially accounts for a number of puzzling features in layered ultramafic cumulates such as those of the Stillwater Complex and Great Dyke. (, Campbell C. A. (, Viola The grain-scale peritectic relationship between olivine and orthopyroxene is present at a larger scale in the form of mingled pyroxenite–harzburgite lithologies ( Fig. The texture is most easily observed in petrographic thin sections. Henderson W. D. (, Donaldson C. M. Poikilitic Anorthositic Gabbro . The paper was improved by an initial review by Bélinda Godel. However, the microprobe analyses were collected with a 50 µm defocused beam and so record the average of the spinel lamellae and the re-equilibrated matrix; hence the analyses of the exsolved Cr-rich zones are anomalously Fe-enriched compared with the original igneous composition, owing to preferential incorporation of Fe over Mg in the lamellae prior to the diffusive homogenization of the host. Mogessie (1973) suggest that poikilitic texture is a . In geology|lang=en terms the difference between porphyritic and poikilitic is that porphyritic is (geology) containing large crystals in a fine mass of material while poikilitic is (geology) describing the texture of an igneous rock that has small crystals of one mineral scattered among larger crystals of another. 17 ), particularly within samples, although the slopes vary somewhat between samples and between cores and rims. J. C. (a, b) Schematic illustrations to explain the origin of orthopyroxenite–dunite mingling textures by channelized reactive fluid flow and replacement of olivine by orthopyroxene. Texture of a rock is the appearance of the rock and how one feels touching it. M. A. Reverse zoning from Cr-poor cores to Cr-rich rims, of comparable magnitude to that seen in the normally zoned grains, is observed in some orthopyroxene-rich cumulates ( Fig. 16 (oscillatory zoned cumulus grain in 8386, and reverse zoned grain in 8321). Models for the postcumulus origin of oikocryst-rich poikilitic harzburgite as a result of migration of trapped melt within a layered sequence. This model is essentially similar to that proposed by Leuthold et al. 14 ). (b) False-colour three-element map: Cr in red channel; Ca in green; Fe in blue; values scaled to maximum. (, Irvine The trends for Ni are typically nearly constant with variable Cr, suggesting that postcumulus diffusion has evened out original within-grain zoning. Representative profiles of trace element abundance measured by LA-ICP-MS along traverses through zoned orthopyroxene oikocrysts along indicated traverse lines in samples 8386 (a) and 8321 (b). S. An inverse situation gave rise to the reverse zoned cumulus orthopyroxenes: initial crystallization from a chromite-undersaturated, silica-rich batch of flowing magma, followed by mixing into a more primitive olivine–chromite-saturated composition. C. L. , 2013 ), subvolcanic ultramafic sills ( Lesher, 2007 ) and thick komatiitic cumulate bodies ( Barnes, 2006). A. There is a tendency of Ti to be less enriched than other more incompatible elements, resulting in a trend towards higher Ti/REE, Ti/Y, etc. II. J. Hallworth There are several possible reasons for this: crystals and liquid may well become decoupled during processes of accumulation and intercumulus liquid migration; and the likely contaminants, relatively REE-poor quartz-rich gneiss and sulfidic mafic metasediments, have similar REE patterns to the olivine-rich cumulates. are supported by the Science Leader program of the CSIRO Office of the Chief Executive. S. This process may have been enhanced by the presence of partially molten siliceous country-rock xenoliths within the cumulates. L. L. R. Titanium and the REE show exactly opposite patterns; surprisingly, the incompatible element profiles are similar to those in the normally zoned grain in 8386. Most tellingly, there is no evidence in any of the samples studied of an evolution towards more LREE-enriched pyroxene rims. , 2007 ; Leuthold et al. Poikilitic olivine-chromite cumulate in XPL. De Geronimo However, this simple interpretation is inadequate to explain the oikocryst-rich olivine pyroxenite ‘oikocrystite’ lithology, in which the rock is composed almost entirely of orthopyroxene with a few per cent of relict olivine chadacrysts and has a whole-rock composition indistinguishable from that expected for an orthopyroxene meso-adcumulate. This accounts for the spectrum observed between mixed dunite–orthopyroxenite and mottled poikilitic harzburgite illustrated in Fig. S. Gurer The complex oscillatory zoning and sector zoning within orthopyroxene core, which also contains olivine and chromite inclusions, should be noted. Model for development of the Sleeping Giant cyclic layering and pyroxene zoning patterns by pyroxene and olivine growth within chaotically flowing, incompletely mixed, variably contaminated magma in a continuously recharged conduit. Austin Gollan M. The detailed profile in sample 8386 ( Fig. Maatta In all cases, including the oscillatory and sector zoned grains, the Cr-rich grain cores are idiomorphic, associated with the presence of chromite inclusions and usually also with olivine inclusions (chadacrysts in the case of the oikocrysts). A cumulus rather than a replacement origin for the Cr-poor rims as well as the Cr-poor cores of the Ntaka oikocrysts is preferred for a number of reasons, some of which have already been mentioned. In this case we could interpret the textures as the result of centimetre-scale reaction of olivine-rich lithologies with intercumulus liquids migrating through anastamosing high-permeability pathways through the partially consolidated crystal mush. Detailed grain-scale trace element mapping, particularly of the slow diffusing and relatively abundant trace elements Cr and Ti, is now easily achieved at hand-sample scale, using desktop microbeam XRF techniques; these open up a promising range of new opportunities in igneous petrology, particularly when combined with LA-ICP-MS. Close. G. M. J. H. Microbeam XRF element maps showing zoning patterns in orthopyroxenes in poikilitic harzburgite sample 8402, NAD12-326, 377 m. (a–c) Tornado XFM images corresponding to those in Fig. (ii) Poikilitic texture: Smaller grains of one mineral are completely enclosed in large, optically continuous grains of another mineral without common orientation. Fig. M. Biotite and hornblende frequently enclose feldspar ophitically; less commonly iron oxides and sphene do so. There are nine main types of igneous rock textures: Phaneritic, vesicular, aphanitic, porphyritic, poikilitic, glassy, pyroclastic, equigranular, and spinifex. I. R. We thank IMX Resources for their financial support and for access to drill core and exploration data. GAC–MAC–AGU Joint Assembly, Montreal, QC. In these cases no crystallographic relation exists between the two minerals (enclosing and enclosed). Li Poikilitic texture refers to igneous rocks where large later-formed less perfect crystals ('oikocrysts') surround smaller early-formed idiomorphic crystals ('chadacrysts') of other minerals. Inter-element correlations among trace elements in the zoned oikocrysts are plotted in Fig. The process is closely akin to that considered to have formed replacement peridotite and gabbro bodies in the Rum intrusion ( Bédard et al. R. Muhongo Investigations in the Stillwater Complex: part II. Poikilitic texture describes the occurrence of one mineral that is irregularly scattered as diversely oriented crystals within much larger host crystals of another mineral. This is true of many gabbros, aplites and granites. G. N. de Jonge Hoinkes S. J. Distinct patterns are seen in the reverse zoned grain in sample 8321 ( Fig. Define poikilitic. 15 ) crosses two adjacent grains with euhedral Cr-rich cores, both of which are clearly evident in strongly correlated profiles of the compatible elements Cr, V, Zn and Co. Manganese correlates broadly with these elements but lacks the ‘dip’ in the Cr-poor zone between the two cores. Three distinct zoning patterns are observed, within closely interlayered rock types. We note that the Rum examples are somewhat different from the Ntaka examples: the Cr-rich cores at Rum appear to be resorbed in contrast to the sharp idiomorphic cores at Ntaka; the rims at Rum are consistently enriched in incompatible REE; the oikocrysts are clinopyroxene having a peritectic reaction relationship with olivine. 9 ). Search: "Poikilitic texture" Suggested Topics within your search. S. J. Including the optional sand fractions will refine the calculation. Crystal slurries accumulating on the steeply dipping marginal portions of the sills became unstable and formed gravity currents ( Maier et al. It is attributed to the effect of high-temperature Cr-spinel exsolution in the originally Cr-rich zones, followed by diffusive redistribution of the divalent ions. Added 21 days ago|11/21/2020 11:59:36 AM J. • A >500 m Ø impact crater is required to excavate poikilitic impact melt from the center of a 100 m thick melt … Archived. Median, 25th and 75th percentiles on REE for cores and rims in the different samples are shown in Fig. 15 (normally zoned oikocryst, sample 8363) and Fig. The Cr-poor rims on the zoned oikocrysts, and on the oscillatory zoned cumulus grains, lack these inclusions, and also tend to lack olivine inclusions. 6 ). Geology and ore petrology of the Akanvaara and Koitelainen mafic layered intrusions and the Keivitsa–Satovaaa layered complex, northern Finland. 18 . Society of Economic Geologists, Special Publications, Growth and collapse of a deeply eroded orogen; insights from structural, geophysical, and geochronological constraints on the Pan-African evolution of NE Mozambique, The geology of the Great ‘Dyke’, Zimbabwe; the ultramafic rocks, Origin of ultra-nickeliferous olivine in the Kevitsa Ni–Cu–PGE-mineralized intrusion, northern Finland, Cogenetic Dykes the Key to Identifying Diverse Magma Batches in the Assembly of Granitic Plutons, Fo and Ni Relations in Olivine Differentiate between Crystallization and Diffusion Trends, Evidence for Multi-stage Melt Transport in the Lower Ocean Crust: the Atlantis Bank Gabbroic Massif (IODP Hole U1473A, SW Indian Ridge), Generation of Calc-Alkaline Magmas During Crystallization at High Oxygen Fugacity: An Experimental and Petrologic Study of Tephras from Buldir Volcano, Western Aleutian Arc, Alaska, USA, Periodic mixing of magmas recorded by oscillatory zoning of the clinopyroxene macrocrysts from an ultrapotassic lamprophyre dyke, Receive exclusive offers and updates from Oxford Academic. M. D. Sobolev (, Jackson (, Paterson save hide report. Barnes In the case of the Ntaka Hill harzburgites, the two textural types can be found within the same thin section, consistent with mechanical deposition of a mixed cumulus assemblage from a heterogeneous body of magma. S. J. The site of crystallization could have been at the floor, wall or roof. D. Li Sample 8321 is a granular olivine orthopyroxenite to harzburgite with reverse Cr zoning ( Fig. Chromium concentrations measured by microbeam XRF and defocused beam electron microprobe are thought to reflect their igneous values, which range from 5000 to 7500 ppm in the cores, and zone from 2000 to less than 1000 ppm in the overgrowths. S. J. A further point of significance here is the within-sample correlation observed between Cr and Al from cores to rims of orthopyroxene grains, and particularly the observation that this correlation applies within poikilitic and non-poikilitic grains. 1973). S. Bingen C. Poikilitic. , 2015 ). The Cr content of the magma would have been buffered at chromite saturation during this process, resulting in a near-constant Cr content of the growing orthopyroxene. Salpas Thomas 22 ). Poikilitic definition is - of, relating to, or consisting of a structural pattern in igneous rocks in which a crystal of one mineral encloses smaller unoriented grains of another mineral so that a lustrous mottling effect is produced. Poikilitic Texture. The bulk of the data for all samples except the reverse zoned pyroxenite 8321 have a combination of LREE enrichment (high La/Sm) and heavy-over-middle REE (HREE/MREE) enrichment (low Gd/Yb); that is, U-shaped REE patterns developed to varying degrees ( Fig. R. E. T. 8 d) in which olivine became unstable, reacting with the local magma during flow to form suspended orthopyroxene oikocrysts. (, Barnes For the language, see Pontic Greek#Romeyka. • 100 m thick impact melt sheets form in 50 km Ø craters on the Moon [16]. Patterns for cores shown in colour with rims in black. Spaggiari 6 . Poikilitic olivine-chromite cumulate in XPL. M. L. Hauzenberger A number of critical observations require explanation. M. B. M. Intergranular texture - a texture in which the angular interstices between plagioclase grains are occupied by grains of ferromagnesium minerals such as olivine, pyroxene, or … S. A. Borley 12 ), correlating exactly with the brown-coloured idiomorphic cores. The model outlined here gives us three alternative hypotheses for the Cr-poor overgrowths on the oikocrysts and oscillatory zoned grains: accumulated liquidus phase overgrowths, in situ trapped liquid overgrowths, or essentially replacive overgrowths as illustrated in Fig. Distinct slopes of Zn and Co versus Cr are observed in the samples, with the highest values observed in oikocryst cores, particularly in sample 8402. R. Reverse zoning from Cr-poor cores to relatively Cr-rich rims is observed in cumulus orthopyroxene grains in a small proportion of granular olivine orthopyroxenites ( Fig. (, Oxford University Press is a department of the University of Oxford. C. G. 18 ). MacRae (e) Semi-quantitative element profiles for Cr and Ti along traverse line X–Y; values calibrated against microprobe data. Postcumulus overgrowth or modification by percolating intercumulus liquid is a relatively minor process, with the exception of dispersed melt percolation channels giving rise to the metasomatic replacement products that we consider below. H. I. S. J. Each kind of texture has a variety of different characteristics that make them unique. These cores evolve towards relatively flat patterns in the rims (although the rims of the normally zoned pyroxenes in 8386 have widely variable REE abundances). (, Meurer Where the flux is higher, intercumulus liquid migration may self-organize into discrete channels on a scale larger than the grain size. I. S. Element mapping using a Bruker Tornado TM XRF micro-mapper reveals some striking zoning patterns in orthopyroxene, primarily defined by the Cr content ( Figs 11–14 ). LREE over MREE) in rims over cores that would be expected if the rims had grown from evolving intercumulus liquid. MacDougall K. 19 . D. Renner Poikilitic texture refers to igneous rocks where large component crystals contain smaller crystals of other minerals within them. P. A. de Azavedo White dashed outlines indicate margins of optically continuous grains. In-house report for Goldstream Mining NL & Warthog Resources Limited, 62 pp. Kiddie The Cr-rich portions of the zoned grains, both normal and reverse zoned, show a strong association with the presence of chromite inclusions. On thinglink.com, edit images, videos and 360 photos in one place. Microbeam XRF element maps showing zoning patterns in orthopyroxenes, sample 8363, NAD12-326, 282 m. ‘Oikocrystite’ poikilitic olivine orthopyroxenite. Considering variation with Ti, the normally zoned oikocrysts from harzburgites have higher Ti and higher ratios of Ti to REE, Y and Zr. C. Luolavirta Poikilitic and granular textured harzburgites are intimately intermingled, in some cases on a centimetre scale in the same rock ( Fig. Knowing that HREE are significantly more compatible in pyroxenes than are the LREE, advanced fractionation of evolving trapped liquid within these cumulates should give rise to strong LREE enrichment, which should be manifest in the compositions of the grain rims, if these rims were indeed the result of equilibration of cumulus cores with intercumulus liquid. Explore content created by others. Relationship of Cr and Ti along traverse line X–Y ; values calibrated against microprobe data akin. 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