Adcumulus Growth Research Articles

Convection in Aid of Adcumulus Growth

Convection in Aid of Adcumulus Growth

Cryptic variations within chromitites of the upper critical zone, northwestern Bushveld Complex

The close association of platinum-group elements with chromite in the upper critical zone calls for the appraisal of structures, textures, and composition of chromitites in the development of genetic models. Cryptic variations in successive chromitite layers of the UG-1 unit are consistent with fractionation of limited melt volumes. Individual chromitite bands within the composite UG-2 chromitite layer exhibit distinctive compositions, but all display long-range compositional trends, with height, superimposed on short-range homogeneity. Accessory chromites are substantially different from massive chromitites in composition; reaction with residual liquids and subsolidus reactions were significant. Recrystallization of chromitites was ubiquitous, but the importance of in situ growth is shown by relict textures. Long-range settling, adcumulus growth, or deposition from density currents are processes that find little support in the data presented.

Crystallization and compositional convection in a porous medium with application to layered igneous intrusions

Growing crystals can provide a local source of buoyancy by depleting melt of certain chemical components, and vigorous convective motions in the fluid can result. We report a quantitative experimental study, in which saturated aqueous solutions of CuSO4 and Na2SO4 in porous media of either glass balls or CuSO4•5H2O crystals were crystallized by cooling from below. The crystals growing in the pore space released light, chemically depleted solution which had the potential to convect out of the porous medium and be replaced by additional saturated solution from above. We explore the physical and chemical effects of compositional convection during the complete solidification of the interstitial solution by varying the permeability of the porous medium, the cooling rate, and the initial composition of the solution. This type of convection can form adcumulate rocks by enabling continuous circulation of melt from the main magma reservoir through the pores of a cumulus pile. Adcumulus growth can occur by this mechanism as long as the convective velocity in the porous medium is greater than the velocity of the solidification front. We scale the experimental results to magmatic conditions and infer that this process should occur commonly in layered igneous intrusions. When our porous media had a small permeability, compositional convection enabled the adcumulate growth of a completely solid contact between melt and cumulate. Trapping of dynamically unstable melt beneath this impermeable layer of crystals also occurred in these experiments and may be important geologically. Small, shallowly emplaced intrusions and the cumulate zones at the base of komatiites are probably cooled too rapidly for significant intercumulus convection to occur.

Postcumulus processes in layered intrusions

AbstractDuring the postcumulus stage of solidification in layered intrusions, fluid dynamic phenomena play an important role in developing the textural and chemical characteristics of the cumulate rocks. One mechanism of adcumulus growth involves crystallization at the top of the cumulate pile where crystals are in direct contact with the magma reservoir. Convection in the chamber can enable adcumulus growth to occur to form a completely solid contact between cumulate and magma. Another important process may involve compositional convection in which light differentiated melt released by intercumulus crystallization is continually replaced by denser melt from the overlying magma reservoir. This process favours adcumulus growth and can allow adcumulus growth within the pore space of the cumulate pile. Calculations indicate that this process could reduce residual porosities to a few percent in large layered intrusions, but could not form pure monomineralic rocks. Intercumulus melt may also be replaced by more primitive melt during episodes of magma chamber replenishment. Dense magma, emplaced over a cumulate pile containing lower density differentiated melt may sink several metres into the underlying pile in the form of fingers. Reactions between melt and matrix may lead to changes in mineral compositions, mineral textures and whole rock isotope compositions. Another important mechanism for forming adcumulate rocks is compaction, in which the imbalance of the hydrostatic and lithostatic pressures in the cumulate pile causes the crystalline matrix to deform and intercumulus melt to be expelled. For cumulate layers from 10 to 1000 metres in thickness, compaction can reduce porosities to very low values (< 1%) and form monomineralic rocks. The characteristic time-scale for such compaction is theoretically short compared to the time required to solidify a large layered intrusion. During compaction changes of mineral compositions and texture may occur as moving melts interact with the surrounding matrix. Both compaction and compositional convection can be interrupted by solidification in the pore spaces. Compositional convection will only occur if the Rayleigh number is larger than 40, if the residual melt becomes lower in density, and the convective velocity exceeds the solidification velocity (measured by the rate of crystal accumulation in the chamber). Orthocumulates are thus more likely to form in rapidly cooled intrusions where residual melt is frozen into the pore spaces before it can be expelled by compaction or replaced by convection.

Terminology of postcumulus processes and products in the Rhum layered intrusion

AbstractThe terminology of cumulates is discussed with particular reference to postcumulus processes as exemplified by the Rhum layered intrusion. The history of the terms orthocumulate, adcumulate, heteradcumulate and harrisitic cumulate is reviewed, and some modifications to the classification scheme proposed by Irvine (1982) are suggested. Possible mechanisms of adcumulus growth are considered and, on balance, the Rhum evidence confirms the view that postcumulus cementation occurred at a relatively early stage, while the intercumulus melt was in effective contact with the main body of magma.

Fluid dynamic and geochemical evolution of cyclic unit 10, Rhum, Eastern Layered Series

AbstractLithological, major element, trace element and Sr isotope data from cyclic unit 10 of the Rhum Eastern Layered Series are presented. The lower 65 metres of the unit are peridotite, subdivided on textural and geochemical grounds into a lower homogeneous portion approximately 50 metres thick and an upper heterogeneous portion approximately 15 metres thick. The uppermost 16.5 metres of the unit are allivalite. There are steep geochemical gradients across the peridotite-allivalite boundary in Ni content of olivine and whole-rock Sr isotope composition.Calculations are presented on the geochemical evolution of a Rhum picritic liquid undergoing olivine precipitation, both when the olivines remain suspended in the residual liquid as they precipitate, and when they are continuously fractionated. Quenched groundmass and olivine compositions from the Rhum dykes and the unit 10 peridotite olivines show good agreement with the suspension model but are inconsistent with the fractionation model. The Rhum chamber is thought to have been replenished with a picritic liquid from which olivine crystallized while held in suspension; however, replenishment by a highly olivine-phyric basalt is also possible. The peridotite probably accumulated rapidly as olivines were dumped out of suspension onto the chamber floor.The lower part of the peridotite is a poikilitic adcumulate; it is suggested that this formed by convective circulation of melt in the pores of the pile of cumulus olivines. In the latter stages of adcumulus growth, more Fe-rich and isotopically contaminated magma entered the top of the cumulus pile causing cumulus olivines to re-equilibrate and giving the intercumulus plagioclase a higher Sr isotope ratio than lower down. The olivines in the allivalite show steep stratigraphic gradients in major element composition but not in their Ni content. They also show substantial variation in major element composition laterally within the allivalite. It is suggested that these features are a consequence of postcumulus re-equilibration of olivine with migrating intercumulus magma.

Origin of strongly reversed rims on plagioclase in cumulates

Narrow reversed rims on plagioclase are ubiquitous in troctolites and olivine gabbros of the Kiglapait intrusion and may be a common feature of all such cumulates. The rims occur at plag/plag, plag/ol, and less strongly at plag/aug grain boundaries. They are optically obvious at ΔAn < 10 mol.% and can reach ΔAn = 32 mol.% or more. In parallel, K/Na drops sharply. Although ubiquitous from sample to sample, the reversed rims are only locally present at grain boundaries even for the same pair of crystals in contact; they are prominent in linear networks suggesting the last trace of intercumulus liquid. A subsolidus origin is ruled out by the absence of reactants at plag/plag and plag/ol boundaries and by the local rather than pervasive development of rims. The rims are required to grow from intercumulus liquid, in which the partition of An component between crystals and liquid increases with the trapped augite component of the liquid. Calculations from published experimental data show that ΔAn > 30 can easily be achieved by such a process. It is also probable that the trapped liquid is part of an An-rich boundary layer generated by solute rejection during adcumulus growth. The ability of the rims to sustain steep K/Na gradients despite a long subsolidus cooling history proves that the K Na exchange rate is vanishingly small over a geologic time scale in An-rich feldspar, suggesting that at low K content the potassium is site-bound to the tetrahedral Al/Si distribution. Reversed rims therefore provide important information on diffusion limits as well as on the late-stage solidification history of plagioclase-rich cumulates. Moreover, they demonstrate that plagioclase geothermometry cannot be divorced from effects of liquid composition and structure as monitored, for example, by augite content.

Petrology and comparative thermal and mechanical histories of clasts in breccia 62236

Lunar breccia 62236 contains large lithic fragments of troctolite, norite, and anorthosite. The mafic phases, olivine, inverted pigeonite, and augite, fill interstitial areas between larger plagioclases and appear to be cumulate phases with extensive adcumulus growth. Pyroxene compositional homogeneity indicates that cation exchange during cooling was limited to an area of about 1 mm. Primary augite and pigeonite both contain 30 μm‐wide lamellae of the other along ‘001’. Pigeonite inverted to orthopyroxene without retaining any crystallographic orientation and subsequently exsolved fine lamellae of augite on (100). Profiles across orthopyroxene‐augite interfaces obtained in the analytical transmission electron microscope show an increase of ∼5% wollastonite in augite within 0.5 μm of the interface, suggesting that short‐range cation exchange continued to temperatures below 500°C. The entire sample has undergone heterogeneous shock deformation. Shock melting of the troctolitic clast suggests pressures of 200–300 kb and well‐developed basal twinning in augite from the norite clasts suggests pressures of 50–300 kbars. The present evidence indicates that 62236 contains parts of a slowly cooled microlayered adcumulate that has been heterogeneously shocked several times and combined into the present breccia.

Adcumulus growth of anorthosite at the base of the lunar crust

Unzoned plagioclase crystals imply isothermal adcumulus solidification and cannot result from reactive equilibrium crystallization in dry magma. Extraction of latent heat upward through lunar crust results only in chilled or cotectic mesocumulate rocks, not anorthosite or adcumulates. Adcumulus growth of anorthosite requires exchange of refractory for incompatible components between the site of crystal growth and nearby fresh magma, and the extraction of calories from growing crystals to the flowing, supercooled magma. Supercooling can be acquired at remote sites like upwellings and then be carried by convective flow beneath crust which is accreting plagioclase by flotation or in situ nucleation and growth. If transport distances are larger than the scale of rockbergs, cotectic norites and troctolites may result. Impact‐induced upwellings tend to promote growth of buoyant rather than mafic crust, hence impacts are not solely destructive.

Noble metals in Thetford Mines ophiolites, Quebec, Canada; Part I, Distribution of gold, iridium, platinum, and palladium in the ultramafic and gabbroic rocks

The gold, iridium, palladium, and platinum contents of ophiolites from the Thetford mines area, Quebec, were determined by radiochemical neutron activation analysis. The main lithologies studied are the tectonite peridotites (harzburgites) of the lower structural unit and the ultramafic-mafic plutonic members of the overlying cumulates. Average noble metal contents of the main rock types are: Au (ppb) Ir (ppb) Pt (ppb) Pd (ppb)Tectonite peridotitesHarzburgite 1.5 3.2 10.0 3.8Dunite lenses in harzburgite 0.27 3.6 2.1 0.29Cumulate rocksOlivine-chromitite 0.28 30.0 1.5 0.33Dunite 1.3 2.4 28.0 21.0Pyroxenite 1.4 0.24 17.0 29.0Gabbro 1.7 0.017 4.5 3.0The harzburgites have chondritelike noble metal abundance patterns and relatively uniform noble metal contents. Both features are compatible with an origin for the harzburgite as a residue from extensive partial melting of mantle peridotite. The cumulates are highly variable in noble metal content. Iridium is enriched in early olivine-chromitite and progressively depleted in later rocks. Palladium increases from early to late ultramafic cumulates and decreases in gabbros, whereas Pt shows no systematic trend in the cumulates. Gold increases very slightly from early to late cumulates. The noble metal trends in cumulate rocks result mainly from fractional crystallization, but other processes including sulfur saturation of magma and adcumulus growth may enrich rocks in noble metals. Sulfur-rich late dunites and early pyroxenites probably have the best mineralization potential of the cumulate rocks.There is little preferential partition of noble metals into any ferromagnesian silicate, but the platinum-group elements, particularly Ir, are strongly associated with chromite. Acid-leaching experiments suggest that in harzburgites Ir is concentrated along chromite-silicate grain boundaries whereas in cumulus chromitites Ir occurs partly within chromite grains. It is suggested that Ir was originally incorporated into the chromite structure during a high-temperature magmatic stage, and in the case of the harzburgites, has diffused to grain boundaries in response to cooling and to recrystallization induced in part by tectonic stress.Serpentinization probably occurred in both oceanic and continental environments. Cumulates suffered intense serpentinization, but little noble metal mobility resulted. Harzburgites were serpentinized in both oceanic and continental environments and the latter process probably caused some mobilization of Au in conjunction with formation of asbestos veins.

Primary igneous load-cast deformation structures in the Fongen-Hyllingen layered basic intrusion, Trondheim Region, Norway

SummaryLoad-cast structures in the Caledonian, synorogenic, Fongen-Hyllingen layered basic complex closely resemble those deformations resulting from reverse density stratification in experimentally studied, water-saturated sediments. The mushroom-shaped structures are apparently restricted to a single horizon within an interlayered, metamorphosed, dunite–troctolite sequence. They generally have a symmetrical form, revealing a polygonal pattern in the plane of the layering. The density contrast between an olivine-rich crystal mush overlying a plagioclase-rich crystal mush, both very close to the igneous sediment–magma interface, is believed to have caused the deformation. Subsequent consolidation occurred by adcumulus growth.

Pallasite meteorites—mineralogy, petrology and geochemistry

Pallasites are highly differentiated meteorites and provide a unique sample from the deep interiors of solar system parent bodies. They contain evidence of the former existence of one or more residual melts. Olivine is a major phase. Its primary shape is rounded; the angular crystals in many pallasites are secondary. Tubular inclusions are widespread. They perhaps are the residence of CO 2, released during laboratory heating experiments. Phosphoran olivine, a new variety of olivine containing 4–5 wt% P 2O 5, occurs in a few pallasites. Its Fe/Mg ratio is apparently independent of the host olivine composition. Pyroxene (not previously described from pallasites) occurs in symplectic intergrowths in seven meteorites. Compositionally, it lies in the gap between pyroxenes in chondrites and most irons. There are two groups: Fs 11.6 ± 0.2 and Fs 16.7 ± 0.2 The pyroxene contains exceptionally low Ca (< 0.1–0.2 wt%) and there is an indication of an inverse relation between Fe and Ca. Modal analyses and density measurements were made on all available specimens and bulk compositions were calculated. The ‘average’ pallasite contains 65 vol. % olivine and 50.5 wt % total Fe. Many of the densities of pallasites cluster around that calculated for close-packed olivine. Pallasites are exotic cumulates. Their textures resemble terrestrial cumulates, as does the presence of olivine and chromite. The metal texture resembles a solidified intercumulus liquid. Those pallasites containing olivine in excess of close-packing were subjected to adcumulus growth, thereby also explaining the widespread mutual borders. There is abundant evidence of deformation. For olivines this includes their fragmental shape and kink banding. Troilite formed a eutectic-like melt with kamacite: pieces of spalled olivine and schreibersite were injected into and captured by this melt. Troilite polycrystallinity resulted from the deformation. This deformation occurred while the pallasites were still deeply buried, resulting in incipient spheroidization of olivine fragments, including the formation of elongate, rounded crystals. A later, lower temperature deformation disrupted plessite. Pallasites formed in multiple parent bodies by processes that recurred in several places within the solar system, as shown by the mineralogical and textural similarities between pallasites that differ in their isotopic and trace element compositions. Type IIIB irons still seem the most likely associated meteorites. Two new pallasites, Dora and Rawlinna, are described briefly.

Ultrabasic rocks of Rhum and Skye: the nature of the parent magma

Rocks from the peridotite dykes of southern Skye ( Gibb 1968 ) are virtually identical to those found in the large layered intrusions of Rhum and Skye, suggesting that they may have been formed in a similar manner. The rocks of the layered intrusions are generally regarded as having formed by the gravitative accumulation of the early precipitating phases from a basaltic magma, usually with subsequent extensive ad-cumulus growth. Such a mechanism is totally inapplicable to the dykes and it can be established that they were formed by flow differentiation of a suspension of olivine in an eucritic liquid which subsequently underwent near-equilibrium crystallization. The possibility therefore arises that the layered intrusions originated from an ultrabasic magma similar to that which formed the dykes. Consideration of this possibility suggests that the petrogenetic models proposed for the layered ultrabasic intrusions of Rhum and Sgurr Dubh (Skye) ( Brown 1956 , Wadsworth 1961 , Weedon 1965 , Wager &amp; Brown 1968) be slightly modified: in particular, that the parental magma was not a basaltic liquid but rather a suspension of olivine in a eucritic liquid. Such a modification is consistent with the features of the layered intrusions and appears to overcome several problems posed by the basaltic liquid hypothesis.

Mineralogic and Petrologic Study of Lunar Anorthosite Slide 15415,18

The anorthosite slide 15415,18 contains 98 percent subhedral plagioclase (97 mole percent anorthite), two pyroxenes: diopsidic augite (46 percent wollastonite, 39 percent enstatite, 16 percent ferrosilite) with subsidiary (100) lamellae and grains of hypersthene (2.5 percent wollastonite, 58 percent enstatite, 39.5 percent ferrosilite), and traces of ilmenite. The pyroxene occurs interstitial to, and as small grains enclosed within, plagioclase. The textures and compositions of the phases appear compatible with an origin by concentration and adcumulus growth of plagioclase from a gabbroic anorthosite (or hyperaluminous) magma in a "plutonic" environment.

A Note on Rhythmic Layering in Hornblende-rich Picrite and Dolerite Intrusions, near Rhiw, Caernarvonshire

AbstractBanding, recorded by previous workers in hornblende picrite and dolerite intrusions near Rhiw, is found to be rhythmic layering. At least three major units (200 to 250 feet in thickness) have been recognized, and intermediate and minor scale units also occur, predominantly within the basal major unit. The majority of the rocks display cumulate textures, and crystallization appears to have been completed by heteradcumulus or adcumulus growth. Thin coarser-grained sheets, rich in plagioclase, and slightly discordant to the layering, are considered to be contemporaneous segregation veins.