Cumulus Grains Research Articles

Poikilitic Textures, Heteradcumulates and Zoned Orthopyroxenes in the Ntaka Ultramafic Complex, Tanzania: Implications for Crystallization Mechanisms of Oikocrysts

The late Proterozoic Ntaka Ultramafic Complex is a body of dominantly pyroxenitic cumulate rocks containing cyclic alternations of olivine-orthopyroxene cumulates. Chemical zoning in the pyroxenes has been imaged at 25-40 μm resolution using desktop microbeam X-ray fluorescence mapping followed up with laser ablation-inductively coupled plasma mass spectrometry analysis for minor and trace elements on selected samples. Poikilitic and granular harzburgites are finely intermingled, in some cases on a centimetre scale in the same thin section. Poikilitic varieties display spectacular textures, ranging from isolated equant orthopyroxene oikocrysts within olivine-rich heteradcumulate harzburgites to rocks composed entirely of interlocking centimetre-sized anhedral orthopyroxene oikocrysts containing sharply bounded idiomorphic Cr-enriched cores. The poikilitic harzburgites are interlayered with cumulate pyroxenites in which orthopyroxene grains show a variety of zoning patterns: Cr-rich cores similar to those in the oikocrysts; sharply bounded oscillatory zoned cores; and reverse zoning with Cr-poor cores and Cr-enriched rims. A further variation is the presence of a mingled harzburgite lithology in which dunite or poikilitic harzburgite is invaded on a centimetre scale by diffuse vein networks or patches of coarse orthopyroxenite. This range of textures and lithologies attests to a more complex set of processes than implied by the standard cumulus theory model in which oikocrysts are considered to have crystallized from intercumulus liquid within a permeable crystal mush. A range of hypotheses is proposed, including infiltration metasomatism of original olivine cumulates by migrating orthopyroxene-saturated pore fluid; however, the textural relationships, whole-rock chemistry and Cr zoning within the grains can best be explained by a model in which the orthopyroxene oikocrysts form in part or whole as mechanically accumulated cumulus grains. The complexity of zoning patterns is attributed to stirring of entrained olivine and orthopyroxene crystals within a heterogeneous flowing crystal mush, where the transporting magma has a wide range of silica contents owing to poorly stirred incorporation of siliceous country-rock material. The Cr-rich orthopyroxenite component grew from Sienriched chromite-saturated magma. Mingled lithologies developed after accumulation as a result of percolation and infiltration metasomatism by Si-enriched liquid derived by melting of xenoliths within the crystal pile. The model may be more generally applicable: dunite-harzburgite cycles, common in many layered intrusions, may reflect variable degrees of contamination rather than cycles of fractional crystallization and replenishment.

Strontium isotope disequilibrium of plagioclase in the Upper Critical Zone of the Bushveld Complex: evidence for mixing of crystal slurries

We report in situ Sr isotope data for plagioclase of the Bushveld Complex. We found disequilibrium Sr isotopic compositions on several scales, (1) between cores and rims of plagioclase grains in the Merensky pyroxenite, the Bastard anorthosite, and the UG1 unit and its noritic footwall, (2) between cores of different plagioclase grains within thin sections of anorthosite and pyroxenite of the Merensky unit, the footwall anorthosite of the Merensky reef and the footwall norite of the UG1 chromitite. The data are consistent with a model of co-accumulation of cumulus plagioclase grains that had crystallized from different magmas, followed by late-stage overgrowth of the cumulus grains in a residual liquid derived from a different level of the compacting cumulate pile. We propose that the rocks formed through slumping of semi-consolidated crystal slurries at the top of the Critical Zone during subsidence of the center of the intrusion. Slumping led to sorting of crystals based on density differences, resulting in a layered interval of pyroxenites, norites and anorthosites.

Magmatic and metamorphic imprints in 2.9 Ga chromitites from the Sittampundi layered complex, Tamil Nadu, India

The ca. 2.9 Ga old Sittampundi layered complex (SLC) of South India belongs to a special group of deformed and metamorphosed Archaean rocks where chromitites are interlayered with anorthosite. Detailed field studies have revealed that the SLC comprises of a sequence of magmatic rocks consisting of clinopyroxenite, anorthosite and mafic rocks of tholeiitic composition. Minor BIF (Banded Iron Formations) intercalated with the mafic rocks are the only sedimentary rock within the SLC. Chromitite occurs as continuous to discontinuous bands (a few mm up to 6 m thick) within clinopyroxenite and highly calcic anorthosite (An 99–100) but never in the enclosing mafic rocks. Chromite occurs broadly in three textural types: (a) cumulus grains in chromitite layers; (b) disseminated grains in clinopyroxenite and anorthosite; and (c) small grains included in clinopyroxene and plagioclase. A suite of felsic magma intruded the SLC at ca. 2.51 Ga. Subsequently the entire rock sequence underwent polyphase deformation and metamorphism at ca. 2.50–2.45 Ga (granulite facies) and at 0.72–0.50 Ga (amphibolite facies). During the latter event, extensive alteration of clinopyroxene to amphibole and chlorite, of plagioclase to clinozoisite and of chromite to Cr-poor green spinel took place. Chromite compositions show large variations (Cr# = 26–60, Mg# = 4–45, Fe 3+# = 2–24) depending upon textural type and nature of the associated phases. The most conspicuous compositional variation took place when chromite was metasomatically replaced by green spinel (Cr# = 3–21; Mg# 45–61) during amphibolite facies metamorphism. On a Cr# vs. Mg# plot, pristine magmatic compositions of chromite define an array that suggest increasing Cr# with concomitant decrease in Mg#, presumably during magmatic fractionation. Highly calcic plagioclase in anorthosite and Fe–Al rich chromite in chromitite in the SLC are consistent with a hydrous parental magma of tholeiitic composition. This hydrous magma was generated in an oceanic arc environment and crystallized close to the surface of the earth. In terms of mineralogy and field relations, chromitite-bearing rocks of the SLC show remarkable similarity with the Archaean anorthosite-hosted chromitite deposits in layered magmatic complexes at Fiskenaesset (Greenland) and Messina (South Africa) but contrasts sharply with the chromitite deposits of Proterozoic and Phanerozoic age. A change in the style of arc magmatism appears to be the controlling factor for marked change in chromite compositions across the Archaean–Proterozoic boundary.

Chemical Evolution of Intercumulus Liquid, as Recorded in Plagioclase Overgrowth Rims from the Skaergaard Intrusion

The intercumulus liquid of a crystal mush fills pore spaces, and typically solidifies to form overgrowths on cumulus grains and poikilitic post-cumulus minerals. If the liquid is immobile, solidification produces zoned intercumulus minerals, as a result of progressive fractionation of the residual liquid. Convection within the mush results in buffering of the liquid composition, and thus limits mineral zonation. For fully solidified cumulates, fossil' changes in liquid composition or porosity are difficult to identify. However, detailed study of immobile minor components of plagioclase overgrowth rims can provide information about the progressive solidification of intercumulus material. Ti contents of plagioclase overgrowths, in samples from the lowermost parts of the Skaergaard Intrusion, show strong variations with anorthite content. With decreasing XAn, Ti concentrations first rise and then fall, consistent with changing TiO2 contents of the intercumulus liquid during solidification. TiO2 in plagioclase decreases sharply at An55, reflecting local saturation of Fe-Ti oxides. Ti in clinopyroxene oikocrysts also falls rimward, but zoning in faster diffusing species (Fe, Mg) is limited. Other than slight reverse zones that may occur on the plagioclase margins, XAn falls continuously during crystallization. The reverse zoning is interpreted as the result of compaction-driven dissolution and reprecipitation of plagioclase. The continual decrease in XAn is exploited, together with back-scattered electron images of the cumulates, to produce calibrated images showing regions of progressive crystallization. This allows the regions crystallizing at each stage of solidification to be visualized. These images show that the final remnants of interstitial melt were present in triangular pockets and as thin grain-boundary melt films. This approach can provide information about the progressive reduction of porosity during cumulate solidification.

Martian Dunite NWA 2737: Petrographic constraints on geological history, shock events, and olivine color

Meteorite Northwest Africa (NWA) 2737 is the second known chassignite, an olivine‐rich igneous rock with mineral compositions and isotopic ratios that suggest it formed on Mars. NWA 2737 consists of ∼85% vol. olivine (Mg#, molar Mg/(Mg + Fe), of 78.3 ± 0.4%), which is notable because it is black in hand sample and brown in thin section. Other minerals include chromite, pyroxenes (augite, pigeonite, orthopyroxene), and diaplectic glass of alkali‐feldspar composition. Aqueous alteration is minimal and appears only as slight dissolution of glass. NWA 2737 formed by accumulation of olivine and chromite from a basaltic magma; the other minerals represent magma trapped among the cumulus grains. Minerals are compositionally homogeneous, consistent with chemical equilibration in late and postigneous cooling. Two‐pyroxene thermometry gives equilibration temperatures ∼1150°C, implying a significant time spent at the basalt solidus. Olivine‐spinel‐pyroxene equilibria give ∼825°C (possibly the T of mesostasis crystallization) at an oxidation state of ∼QMF‐1. This oxidation state is consistent with low Fe3+ in olivine (determined by EMP, Mössbauer spectra, and synchrotron micro‐XANES spectroscopy) and with ∼10% of the iron in pyroxene being Fe3+. NWA 2737 experienced two shock events. The first shock, to stage S5–S6, affected the olivine by producing in it planar deformation features, intense mosaicism and lattice strain, and abundant droplets of iron‐nickel metal, 5–15 nm in diameter. At this stage the olivine became deeply colored, i.e., strongly absorbing at visible and near‐infrared (NIR) wavelengths. This shock event and its thermal pulse probably occurred at ∼170 Ma, the Ar‐Ar age of NWA 2737. The colored olivine is cut by ribbons of coarser, uncolored olivine with long axes along [100] and shorter axes on {021} planes: These are consistent with the easy slip law for olivine [100]{021}, which is activated at moderate strain rate at high temperature. Within these ribbons the olivine was coarsened and the iron metal globules coalesced to micron‐sized grains. The ribbons also are mosaicized and cut by planar fractures, which bespeak a second shock event, possibly that of ejection from Mars. The deeply colored olivine in NWA 2737 is unusual and represents a new “ground truth” type for remote sensing of Mars. Understanding the occurrence of the brown color in olivine in NWA 2737 places important constraints on interpretation of optical measurements.

Using apatite to dispel the “trapped liquid” concept and to understand the loss of interstitial liquid by compaction in mafic cumulates: an example from the Stillwater Complex, Montana

The size, paragenetic setting, distribution, and composition of apatite grains in 16 thin sections from one sample of an olivine gabbro from the Stillwater Complex, Montana were documented to determine whether they grew from trapped interstitial liquid or in an open system. The grains show extreme variability in cross-sectional area, ranging from 300,000 μm2. The apatite grains are not associated with quartz, Fe–Ti oxides, or evolved overgrowths on cumulus grains as expected for the crystallization of a trapped liquid, which suggests that they grew in an open system. The grains are very irregularly distributed, with a single cluster comprising almost 25% of the cumulative apatite cross-sectional area from all 16 slides. The spatial patterns of the apatite indicate that the interstitial liquid moved in tubular channels during the final stages of compaction. The trace-element composition of liquids calculated from apatite and silicate mineral compositions are similar, indicating that the apatite crystallized from liquids that were not dramatically enriched in incompatible elements. We conclude that the interstitial liquid never actually became trapped, that liquid was lost in an open system to essentially zero porosity, and that the model of cumulates as a combination of cumulus grains and trapped liquid should be discarded.

Formation of a martian pyroxenite: A comparative study of the nakhlite meteorites and Theo's Flow

Abstract— The unusual composition of the nakhlites, a group of pyroxenitic martian meteorites with young ages, presents an opportunity to learn about nonbasaltic magmatic activity on another planet. However, the limited number of these meteorites makes unraveling their history difficult. A promising terrestrial analog for the formation of the nakhlites is Theo's Flow in Ontario, Canada. This atypical, 120 m‐thick flow differentiated in place, forming distinct layered lithologies of peridotite, pyroxenite, and gabbro. Theo's pyroxenite and the nakhlites share strikingly similar petrographies, with concentrated euhedral to subhedral augite grains set in a plagioclase‐rich matrix. These two suites of rocks also share specific petrologic features, mineral and whole‐rock compositional features, and size and spatial distributions of cumulus grains. The numerous similarities suggest that the nakhlites formed by a similar mechanism in a surface lava flow or shallow intrusion. Their formation could have involved settling of crystals in a phenocryst‐laden flow or in situ nucleation and growth of pyroxenes in an ultramafic lava flow. The latter case is more likely and requires steady‐state nucleation and growth of clusters of pyroxene grains (and olivine in the nakhlites), circulating in a strongly convecting melt pool, followed by settling and continued growth in a thickening cumulate pile. Trapped pockets of intercumulus liquid in the pile gradually evolved, finally growing Fe‐enriched rims on cumulus grains. With sufficient evolution, the melt reached plagioclase supersaturation, causing rapid growth of plagioclase sprays and late‐stage mesostasis growth.

Evolution of the lunar crust: SIMS study of plagioclase from ferroan anorthosites

The lunar crust, down to a depth of ∼65 km, is composed of older (>4.5 Ga) ferroan anorthosites and younger (4.43-4.17 Ga) Mg-suite lithologies which include dunites, troctolites, and norites. The anorthosites are generally inferred to represent floating cumulates in a lunar magma ocean (possible depth 800 km, moon's radius ∼ 1,738 km). The cumulates that are inferred to be located near the base of the magma ocean are dominantly olivine and pyroxene. The last dregs of the magma ocean are enriched in incompatible elements and have been named KREEP (K, rare earth elements, P). KREEP, formed in this manner, is probably concentrated near the crust/mantle boundary at ∼70 km depth. We are attempting to characterize melts parental to ferroan anorthosites and Mg-suite norites by analyzing REEs (La, Ce, Nd, Sm, Eu, Dy, Er, Yb) and Ba, Sr, and Y in their cumulus plagioclase. If the cumulus grains have not been compromised by postcrystallization effects and if we know the relevant mineral/melt partition coefficients (Ds) we can invert the trace element data for plagioclase to parental melt compositions. Melts parental to ferroan anorthosites are estimated to contain REE at concentrations ten to fifty times chondrites. Melts parental to the earlier crystallizing anorthosites (lower REE) have virtually no Eu anomaly, while melts parental to later crystallizing anorthosites (higher REE) have small negative Eu anomalies. This is qualitatively consistent with the fractionation of Eu relative to other REE by crystallization of plagioclase with large positive Eu anomalies. Melts parental to the Mg-suite have much higher total REE and very large negative Eu anomalies. The characteristics of Mg-suite norite parental magmas may have been derived by the assimilation of KREEP (near the crust/mantle boundary) by Mg-rich basaltic melts formed deeper in the cumulate pile or near the contact between the lower cumulate horizons of the magma ocean and primitive, unprocessed lunar mantle (∼800 km).

An evaluation of models of apatite compositional variability using apatite from the Middle Banded series of the Stillwater Complex, Montana

Apatite is a relatively common accessory mineral in the olivine-bearing zones (OB-III and OB-IV) of the Middle Banded series of the Stillwater complex, occurring interstitial to cumulus grains, as monomineralic inclusions in cumulus grains, and in polymineralic clusters which may or may not be included in the cumulus grains. Eighty-nine of 185 samples examined were found to contain some apatite. The F-Cl-OH content of the apatite show distinct stratigraphic variations. XClAp (mole fraction Cl in apatite) of interstitial apatite increases upsection in both OB-III and OB-IV with average values ranging from 0.15 to 0.85 in OB-III and from 0.03 to 0.60 in OB-IV. XFAp varies inversely with XClApwith average values ranging from 0.00 to 0.70, while XOHAp remains relatively constant near 0.40 or decreases slightly with height. These variations are remarkable given that no appreciable stratigraphic variations in either the major or trace element compositions of any of the cumulus minerals are found in the 800 m of section that comprise OB-III and OB-IV. Within-sample variation of XClAp for samples containing Cl-rich apatite is substantially larger (up to 0.65 XClAp) than for samples with more F-rich apatite (XClAp varies by approximately 0.15). Although interstitial apatite is found throughout OB-III and OB-IV, apatite occurring as monomineralic inclusion in cumulus grains or in polymineralic clusters is almost exclusively found in samples with Cl-rich apatite. The data are best explained by a model involving the degassing of a Cl-rich volatile phase from the crystallizing interstitial liquid. The up-section migration of this fluid resulted in the crystallization of F-rich apatite in the lower portion and progressive Cl-enrichment in the apatite with height. The presence of hornblende-bearing dikes, veins and pegmatoids at the level of maximum Cl-enrichment is consistent with a fluid migration model.

Textures exhibited by feldspars in the Giant Mottled Anorthosite (GMA) of the bastard unit in the Upper Critical Zone, Western Bushveld Complex

The Bastard Unit, overlying the Merensky Unit, is the uppermost and most complete of the cyclic units in the Upper Critical Zone and its upper contact is taken as the boundary between the Critical and Main Zones (as recommended by SACS, 1980). The Unit is different from underlying units for three reasons: it does not have a well-defined top contact (with an overlying unit); it is much thicker than the Merensky and Footwall Units; and it has an unusually thick sequence of mottled anorthosite at its top. This distinctive composite anorthositic sequence is termed the Giant Mottled Anorthosite (GMA) which is predominantly made up of poikilitic anorthosite. The GMA can broadly be subdivided into three distinct parts; the Lower Giant Mottled Anorthosite (LGMA), Giant Mottled Middling (GMM) and the Upper Giant Mottled Anorthosite (UGMA). Petrographic examination of 82 samples taken in the GMA from 11 borehole profiles around the western limb of the Complex revealed that the LGMA and GMM are essentially adcumulates while the UGMA is orthocumulate in character. By its very nature the dominant phase throughout the GMA is cumulus plagioclase feldspar (≥ 85%) with the balance being made up of intercumulus orthopyroxene and clinopyroxene and minor biofite — all of which constitute the darker mottles. Intercumulus inverted pigeonite occurs in the upper part of the GMM, throughout the UGMA and for c. 30 m into the Main Zone. K-feldspar also joins the paragenesis within the UGMA and occurs both as intercumulus orthoclase and antiperthite. Cumulus plagioclase grains, showing varying degrees of complex oscillatory-zonal growth are common in the upper part of the GMA. These grains are interpreted as records of growth episodes during earlier influxes of magma which were responsible for the development of the underlying cyclic units, but escaped incorporation into the cumulate pile.

As, Bi, Hg, S, Sb, Sn and Te geochemistry of the J-M Reef, Stillwater Complex, Montana: constraints on the origin of PGE-enriched sulfides in layered intrusions

The J-M Reef is an interval of disseminated sulfides in the Lower Banded series of the Stillwater Complex that is enriched in the platinum group elements (PGE). Palladium and Pt occur in solid solution in base-metal sulfides and as discrete PGE minerals. PGE minerals include sulfides, tellurides, arsenides, antimonides, bismuthides, and alloys with Fe, Sn, Hg, and Au. Several subpopulations can be delineated based on whole-rock chemical analyses for As, Bi, Cu, Hg, Pd, Pt, S, Sb and Te for samples collected from and adjacent to the J-M Reef. In general, samples from within the reef have higher Pt/Cu, Pd/Cu, Pd/Pt, Te/Bi and S/(Te+Bi) than those collected adjacent to the reef. Vertical compositional profiles through the reef suggest that Pd/Cu and Pt/Cu decrease systematically upsection from mineralized to barren rock. The majority of samples with elevated As, Sb and Hg occur adjacent to the reef, not within it, or in sulfide-poor rocks Neither magma mixing nor fluid migration models readily explain why the minor quantities of sulfide minerals immediately adjacent to the sulfide-enriched layers that form the J-M Reef have different element ratios than the sulfide minerals that form the reef. If all the sulfides formed by exsolution during a magma mixing event and the modal proportion of sulfide now in the rocks are simply the result of mechanical processes that concentrated the sulfides into some layers and not others, then the composition of the sulfide would not be expected to be different. Models that rely upon ascending liquids or fluids are incompatible with the presence of sulfides that are not enriched in PGE immediately below or interlayered with the PGE-enriched sulfides layers. PGE-enriched postcumulus fluids should have reacted to the same extent with sulfides immediately outside the reef as within the reef. One explanation is that some of the sulfide minerals in the rocks outside the reef have a different origin than those that make up the reef. The sulfide minerals that form the reef may represent a cumulus sulfide phase that formed as the result of a magma-mixing event, achieved its high PGE contents at that time, and accumulated to form a layer. The rocks outside the reef may contain a large proportion of postcumulus sulfide minerals that formed as the last dregs of intercumulus liquids trapped in the interstitial spaces between the cumulus grains reached sulfur saturation and exsolved a sulfide liquid or precipitated a sulfide mineral. The PGE contents of these sulfides would be expected to be less than the cumulus sulfides that form the reef since they would have equilibrated with a much smaller volume of silicate liquid. Another explanation is that some of the sulfide droplets that formed as a result of the mixing event were trapped as inclusions in silicate minerals soon after they formed. This would reduce the amount of magma these sulfide droplets could equilibrate with and effectively reduce their PGE tenor.

The parental magma of the Nakhla achondrite: Ultrabasic volcanism on the shergottite parent body

The Nakhla meteorite is an augite-rich igneous cumulate rock; magma trapped among the cumulus grains is now represented by fine-grained mesostasis and post-cumulus overgrowths on cumulus grains. The composition of the intercumulus magma, calculated by mass balance, is ultrabasic (SiO2 < 45%) and enriched in incompatible elements (LaLu = 6 × CI). The cumulus augite and intercumulus magma were in chemical equilibrium, implying that the intercumulus magma is a sample of Nakhla's parental magma. Complex processes like infiltration metasomatism were therefore unimportant. Nakhla contains olivine crystals which are as large or larger than the cumulus augite crystals. The largest olivine crystals, probably xenocrysts, preserve core compositions too Fe-rich to have been in equilibrium with the intercumulus magma. Other smaller olivine grains may have been phenocrysts. The closest terrestrial analogs to Nakhla (and the other nakhlites) are pyroxene-rich cumulate layers in differentiated picrite and komatiite flows and shallow sills. Spinel or garnet was fractionated from the parental magmas of the nakhlites or their parental mantle, based on the superchondritic CaOAl2O3 ratio of Nakhla's intercumulus magma and its lack of an Eu anomaly. The three nakhlites and Chassigny are half of the known samples of the Shergottite Parent Body (SPB). One may tentatively infer that ultrabasic volcanism was much more important on the SPB than on the moon, Eucrite Parent Body, or present-day Earth.

Origin of chromite compositional variation in the Panton Sill, Western Australia

The compositional variation of chromite and associated olivine in chromite-rich and chromitepoor cumulus layers of the Panton Sill is described and a diffusion-controlled crystallization mechanism is proposed to explain this variation. By this mechanism, chromite initially precipitates with a fairly uniform composition, irrespective of the relative proportions of coprecipitating olivine and chromite, and is modified by continued growth during the postcumulus stage. The effect of postcumulus overgrowth of chromite, K d =(Mg/Fe2+)liquid/(Mg/Fe2+) chromite≃6, is to deplete the surrounding magma in chromium and decrease Fe2+ relative to Mg such that a chemical gradient exists between the overlying magma, through which the cumulus grains settled, and the magma in contact with settled chromite grains near the magma/crystal pile interface. Postcumulus equilibration of olivine and chromite with the surrounding magma results in higher Mg/(Mg + Fe2+) ratios of both olivine and chromite and higher Al content of chromite. The extent of this postcumulus modification is directly related to the proportion of chromite to olivine in a particular layer. This model can be extended to stratiform intrusions elsewhere in which chromite coprecipitates with olivine, orthopyroxene or plagioclase and displays similar compositional trends.

Some problems with the cumulus theory

There are a number of features in layered intrusions which do not appear to be consistent with the cumulus theory. The most important of these are: (1) Rhythmic layering in plagioclase-pyroxene cumulates from some intrusions (e.g. Jimberlana and Stillwater) may be inverted with the light plagioclase-rich cumulates at the base of the layer and heavy pyroxene-rich cumulates at the top. (2) Hydraulic sorting in rhythmically layered cumulates is often poor, suggesting that chemical and not mechanical processes control the distribution of minerals in this type of layered sequence. (3) The textures of cumulates from the steeply dipping marginal zone of Jimberlana, which cannot have formed by gravity settling, are indistinguishable from those found in the flatly dipping central layers of the intrusion. Similarly, it is difficult to explain the cumulate textures in the ‘overturned’ marginal layered series of Jimberlana. (4) Igneous layering, including cross-bedding, graded bedding and trough banding, has been described in layered sequences with near-vertical primary dips. (5) Calculations and experiments with a centrifuge furnace suggest that plagioclase cannot sink in Fe-rich tholeiitic liquids. (6) The settling of crystals as individual grains assumes that nucleation is homogeneous. This is unlikely. It is probable that heterogeneous and self-nucleation are the dominant nucleation mechanisms during the formation of cumulates. This gives rise to two possibilities. Firstly, that most grains nucleate against pre-existing settling crystals to form composite grains which gravitate to the floor of the chamber. Secondly, that the cumulus grains nucleate in situ at the temporary floor of the magma chamber.

The origin of heteradcumulate and adcumulate textures in the Jimberlana Norite

SUMMARYIn the Jimberlana Norite the heteradcumulate and adcumulate rocks are found in adjacent layers. The heteradcumulate occur immediately before the heterad minerals appear as cumulus grains in an adcumulate rock. It is considered that both heteradcumulate and adcumulate textures result from supercooling in the magma. Heteradcumulates develop when seeds of the heterad mineral are not available in the magma and adcumulates after they have formed.