Cumulate Pile Research Articles

Slow, dense replenishments of a basic magma chamber: the layered series of the Newark Island layered intrusion, Nain, Labrador

The Newark Island layered intrusion is a composite layered intrusion within the Nain anorthosite complex, Labrador. The intrusion comprises a lower layered series (LS) dominated by troctolites, olivine gabbros and oxide-rich cumulates and an upper hybrid series (HS) characterized by a wide range of mafic, granitic and hybrid cumulates and discontinuous layers of chilled mafic rocks (Wiebe 1988). The HS crystallized from a series of replenishments of both silicic and basic magmas. The LS crystallized from periodically replenished basic magmas. The LS has a lower zone that consists mainly of olivine-plagioclase cumulates and contains minor cryptic reversals in mineral compositions that resulted from replenishments of relatively primitive magma. An upper zone is dominated by olivine-plagioclaseaugite-ilmenite cumulates. Cumulus titanomagnetite and pyrrhotite occur within some oxide-rich cumulates, and the stratigraphically highest layers contain cumulus apatite. At intermediate levels in the sequence, cumulus inverted pigeonite occurs in place of olivine. Several prominent regressions in the stratigraphy of the upper zone are marked by fine-grained troctolitic layers with much higher Mg no. [100 MgO/(MgO+FeO)] and anorthite than underlying cumulates. These layers coarsen upward and grade back to oxide-bearing olivine gabbros within thicknesses ranging from 10 cm to 15 m. Dikes that cut the LS have major- and trace-element compositions that strongly suggest that they are feeders for the replenishments. In the lower zone when olivine and plagioclase were the only cumulus phases, replenishments were less dense than the resident magma and rose as plumes and mixed with it. Precipitation of cumulus oxides in the upper zone lowered the density of resident magma so that subsequent replenishments were more dense than resident magma. Replenishments that occurred after oxides began to precipitate had small injection velocities. These post-oxide injections flowed along the interface between resident magma and the cumulate pile and precipitated flow-banded, fine-grained troctolites.

A chemical model for generating the sources of mare basalts: Combined equilibrium and fractional crystallization of the lunar magmasphere

It is generally considered that mare basalts were generated by the melting of a cumulate mantle formed in an early Moon-wide magma ocean or magmasphere. However, the nature and chemistry of this cumulate mantle and the logistics of its origin have remained elusive. Extensive studies of terrestrial layered mafic intrusions over the past sixty years have emphasized the imperfection of fractional crystallization and attendant crystal-crystal and crystal-liquid separation in a convecting magma chamber. Crystal-liquid and crystal-crystal separations were similarly inefficient during evolution of the lunar magma ocean (LMO), allowing for the trapping of interstitial liquid and entrainment of a small proportion of less-dense plagioclase into the denser mafic cumulate mush. Indeed, petrography of lunar highlands samples demonstrates this for anorthosites (with 1–10% olivine). The residual liquid after 80–90% crystallization was very evolved (in fact KREEPy) and, even in small proportions (1–5%), would have a noticeable effect on the trace-element chemistry of melts generated from these cumulates. This trapped residual liquid would elevate total REE abundances in the cumulate pile, while synchronously deepening the already negative Eu anomaly. Essentially, this trapped liquid will make the cumulate more fertile for melting to generate both KREEP basalt and mare basalt magmas. Plagioclase entrained in the mafic cumulate pile adds an essential Al component to the high-Ti basalt source and will moderate the requisite negative Eu anomaly in the cumulate. Early in the evolution of the lunar mantle, when the LMO still was largely liquid, it is likely that vigorous convection was an important factor in crystallization. Such convection would allow crystals to remain suspended and in equilibrium with the LMO liquid for relatively long periods of time. This extended period of equilibrium crystallization would then have been followed by fractional crystallization once plagioclase became a liquidus phase and began to float to form the lunar highlands crust. Previous authors have proposed a three-component model for the evolution of high-Ti mare basalt source regions. This model includes KREEP, early (olivine-rich, high Mg#) cumulates, and late (ilmeniterich, low Mg#) cumulates in various proportions. However, we propose a model for high-Ti basalt parent magmas which is in accord with studies of terrestrial layered intrusions. This model for the high-Ti source includes trapped instantaneous residual liquid (TIRL; 1–3%) and entrainment of a small (2–5%) proportion of plagioclase into the late-stage cumulate pile in order to account for both the observed Al compositions and trace-element characteristics of high-Ti mare basalts. Melting of this relatively shallow, ilmenite- and clinopyroxene-bearing, late-stage cumulate can generate high-Ti mare basalt magmas. Furthermore, we are in agreement with other workers that only through a process of nonmodal melting will the high Ti values for the parent magmas be realized. Large-scale convective overturn of the cumulate pile and mixing of KREEP with early- and late-stage cumulates is not required. However, localized overturn of the upper tenth of the cumulate pile is likely and, in fact, required to achieve an appropriate major-element balance for the high-Ti mare basalt source region.

Petrogenesis of the nakhlite meteorites: Evidence from cumulate mineral zoning

A simple igneous petrogenesis for the meteorite Nakhla has previously been called into question because Mg Fe ratios in olivine indicate substantial disequilibrium between the predominant cumulus minerals (olivine and augite). Comparative analyses of simulated diffusive zoning and the observed cumulus mineral zoning for all three nakhlites (Nakhla, Governador Valadares, and Lafayette) show that their current compositions do not necessarily reflect parental magma compositions. Instead, diffusion has altered primary cumulus compositions to varying degrees, Nakhla being the least affected, and Lafayette being almost completely re-equilibrated. Only the cores of augite grains in Nakhla and Governador Valadares appear to have preserved their original compositions. Mineral zoning in each meteorite is strongly concentric around mesostasis areas, suggesting that reaction with intercumulus liquid has controlled the observed zoning. The presence of pigeonite and orthopyroxene overgrowths in Nakhla and Governador Valadares, and of poikilitic orthopyroxene enclosing olivine relicts in Lafayette, indicate substantial late-magmatic reactions. Two-pyroxene geothermometry for Lafayette indicates temperatures around 950°C, suggesting subsolidus equilibration as well. The nakhlites appear to be a series of relatively simple cumulate rocks which have undergone various amounts of late-magmatic and subsolidus diffusion, possibly reflecting their relative positions in a cooling cumulate pile.

Petrogenesis of the Merensky Reef in the Rustenburg section of the Bushveld Complex

A petrogenetic model for the Merensky Reef in the Rustenburg section of the Bushveld Complex has been developed based on detailed field and petrographic observations and electron microprobe data. The model maintains that the reef formed by reaction of hydrous melt and a partially molten cumulate assemblage. The model is devised to account for several key observations: (1) Although the dominant rock type in the Rusterburg sections is pegmatoidal feldspathic pyroxenite, there is a continous range of reef lithology from pyroxenite to pegmatoidal harzburgite and dunite, and small amounts of olivine are present in nearly all pegmatoids. (2) The pegmatoid is usually bounded above and below by chromitite seams and the basal chromitite separated from underlying norite by a centimeter-thick layer of anorthosite. The thicknesses of the two layers exhibit a well-defined, positive correlation. (3) Inclusion of pyroxenite identical to the hanging wall and of leuconorite identical to the footwall are present in the pegmatoid. The leuconorite inclusions are surrounded by thin anorthosite and chromitite layers in the same sequence as that at the base of the reef. (4) Chromite in seams adjacent to plagioclase-rich rocks is characterized by higher Mg/Mg+Fe and Al/R3 and lower Cr/R3 than that in seams adjacent to pyroxene-rich rocks. Similar variations in mineral compositions are observed across individual chromitite seams where the underlying and overlying rock types differ. The chromite compositional variations cannot be rationalized in terms of either fractional crystallization or reequilibration with surrounding silicates. It is proposed that the present reef was originally a melt-rich horizon in norite immediately overlain by relatively crystallized pyroxenite. Magmatic vapor generated by crystallization of intercumulus melt migrated upward through fractures in the cumulate pile below the protoreef. The melt-rich protoreef became hydrated because fractures were unable to propagate through it and because the melt itself was water-undersaturated. Hydration of the intercumulus melt was accompanied by melting, and the hydration/melting front migrated downward into the footwall and upward into the hanging wall. In the footwall melting resulted first in the dissolution of orthopyroxene and then of plagioclase. With continued hydration chromite was stabilized as melt alumina content increased. The regular variations in chromite compositions reflect the original gradients in melt composition at the hydration front. The stratigraphic sequence downward through the base of the reef or pegmatoid (melt)-chromitite-anorthosite-norite represents the sequence of stable mineral assemblages across the hydration/melting front. The sequence is shown to be consistent with knowledge gained from experiments on melting of hydrous mafic systems at crustal pressures. With cooling the hydrated mixture from partial melting of norite footwall and more mafic hanging wall crystallized in the sequence chromite-olivine-pyroxene-plagioclase, with peritectic loss of some olivine. Calculations of mass balance indicate that a significant proportion of the melt was lost from the melt-rich horizon. Variations in the development of the pegmatoid and associated lithologies and amount of modal olivine in the pegmatoids along the strike of the Merensky Reef resulted because the processes of hydration, melting and melt loss operated to varying extents.

Investigations of the Stillwater Complex: Part V. Apatites as indicators of evolving fluid composition

Variations in the F, Cl and OH contents of apatite are not constrained by crystal-chemical factors (in contrast to micas and amphiboles), and thus changes in the abundance of these components provide an indicator of halogen fugacity variations and insights into the degassing history of igneous rocks. Microprobe analysis of intercumulus apatites from the Stillwater Complex reveal that Cl-rich apatites, typically containing 6.0 wt % Cl, occur throughout the lower 1/3 of the complex excluding the Basal series. A change from Cl-rich to more F-rich apatite occurs within olivine-bearing zone I (OB I) of the Banded series, the host zone of the platiniferous J-M Reef. Although apatite compositions are somewhat variable above the J-M Reef, more F-rich apatites predominante and typically contain >1.2 wt % F and <3.0 wt % Cl. The most F-rich apatites occur in the uppermost exposed cumulates. Pristine apatites from coeval sills and dikes from below the complex and from the Basal series are similarly F-rich. In all apatites, the Cl and F contents are lower in rocks affected by later metamorphic fluids. Rare earth element (REE) concentrations in chlorapatites show a marked peak in the olivine-rich rocks of the J-M Reef, and contain up to 2 wt % Ce2O3 + La2O3. The trend of first increasing, then decreasing Cl/F ratios with stratigraphic height is modeled by a vapor-driven zone refining process occurring within the cumulate pile causing Cl-enrichment in the interstitial melt accompanied by degassing at the top of the magma chamber causing overall loss of Cl from the magma as crystallization proceeded. The abrupt change from Cl-rich to more F-rich apatites within OB I is interpreted as the result of a breakdown of the Cl-rich zone refining front and mixing with Cl-poor supernatant melt. Any high temperature fluids that exsolved and circulated through the lower 1/3 of the complex must have been enriched in Cl and could have transported REE and trace metals.

Segmentation at a fossil spreading axis: The plutonic sequence of the Wadi Haymiliyah area (Haylayn Block, Sumail Nappe, Oman)

Wadi Haymiliyah, in the central part of the Haylayn Block, offers a splendid continuous section across the crustal sequence of the Sumail Ophiolite. The plutonic sequence of this area presents several specific features of great interest for the understanding of the structure and evolution of this ophiolite. The petrological Moho and the whole plutonic sequence display a subvertical attitude, parallel to the mean dyke-complex orientation. High-temperature folding occurred in the mantle sequence and in the lower crust, whereas the upper crust remained unfolded and behaved as a rigid slab during this deformation, resulting in a tectonic decoupling between lower and upper crust. In the gabbro cumulate sequence, a remarkable large-scale curvature of the magmatic structures (layering and lamination planes), whose axis is totally discordant with the regional folds axes, is interpreted as a primary magmatic feature indicating the southern ending of a magma chamber. The lower crust, about 2 km thick, consists of a primary layered gabbroic sequence, consisting of rhythmic mineral-graded layers from several metres to several hundred metres thick. Syndepositional structures such as oblique stratification, microslumps, eroding mini-channels and small normal faults are widespread. The mineralogical composition, modal variations and cryptic evolution of the whole layered sequence are described, both at the megascale and on the simple cyclic scale. The data support a model of crystallization of the layered sequence by open-system fractionation at the floor and sides of a periodically replenished, steady-state magma chamber. A unique feature of this cumulate sequence is the ubiquitous presence of orthopyroxene throughout the whole cumulate pile. This is very unusual in the Sumail crust, with the exception of the Wadi Rajmi section in the Fizh block. Two huge plagioclase-free dunite-wehrlite laccoliths, about 300 m thick, and numerous ultramafic sills and mushroom-like bodies intrude the base of the layered sequence. These ultramafic intrusions exhibit in places an internal layering and dunitic differentiation products. They have sharp, sinuous and lobate contacts with the gabbros, which were still hot and probably not entirely consolidated at the moment of the intrusion. The mineral compositions of these intrusions are much more primitive than those of the adjacent gabbros. Higher in the plutonic section, the layered gabbros are overlain by 1 km of fine-grained, non-layered planar-laminated gabbronorites, characterized by the abundance of pleochroic cumulus hypersthene, the disappearance of olivine, and the development of Fe-Ti oxides and amphibole as post-cumulus phases. Numerous occurrences of concordant, coarse-grained layered gabbros occur in this unit, which is characterized also by a dense net of highly residual dioritic to tonalitic veins, resulting in a huge magmatic breccia in the uppermost few hundred metres where the gabbronorites are brecciated by the acidic rocks. Hydrothermal veins are also extremely abundant in this unit, which coincides with the core of the large-scale curvature. This unit, comprising unusually fractionated rocks, is interpreted as resulting from the evolution of the Haymiliyah magma chamber towards a closed-system fractionation, due to the cutting off of source supply. Our interpretation of both structural and petrological data may be summarized by the following major points: 1. (1) The Haymailiyah magma chamber was located at the southern tip of a spreading cell, and was cut off from the magma supply at the end of its evolution either vertically, when accretion slowed down, or horizontally, when it became isolated from the central part of the spreading segment. We suggest that orthopyroxene-bearing cumulates should be looked for all along the Sumail Nappe and used as markers of segmentation. 2. (2) Early, high-temperature compressional deformation took place in both the mantle and crust before the intraoceanic detachment of the ophiolite. The development of this compression regime activated filter-pressing processes, and in particular helped with the extraction of wehrlitic magmas from the upper mantle, and their ascent to the uppermost levels of the crust.

Discussion

Professor P. E. Brown writes: With respect to the suggested migration of liquid through the cumulus pile, has the author any feel for the thickness of the cumulate pile at the time the picritic magma was introduced and the depth in that pile at which the introduction took place? Ms R. Renner replies: at present the authors have no direct estimation of the thickness of the cumulus pile. Slump structures in the allivalites are tens of metres thick, suggesting that such thicknesses of unconsolidated cumulate might be typical.

Distribution of Orthocumulate Textures in the Jimberlana Intrusion

The P and U contents and the texture of cumulates have been used to document variations in residual porosity in the Jimberlana Intrusion. The results show that rocks with low residual porosity (adcumulate textures) occur where heat loss through the floor of the chamber was low or where the viscosity of the melt from which they crystallized was low. Low residual porosity also correlates with the appearance of plagio-clase as a cumulus phase and with zones of well-developed rhythmic layering. It is suggested that the initial porosity of cumulates is controlled by the balance between nucleation and crystal growth at the crystalliquid interface. Low initial porosity represents attainment of textural equilibrium and forms when adjacent crystals at the top of the cumulate pile continue to grow, expelling the intercumulus magma in the process. High initial porosities develop when new crystals nucleate at the top of the crystal pile before a high proportion of the intercumulus magma is expelled. Low viscosity in ...

Compaction of Igneous Cumulates

When initially deposited, mafic igneous cumulates consist of a solid matrix of mineral phases plus 50-60% interstitial liquid. Due to its greater density the matrix compacts, squeezing the interstitial liquid upward. A numerical fluid mechanical model of this process indicates that if the cumulate deposition rate is constant, compaction occurs in one of two distinct models. For rapid deposition, the system is permeability controlled with compaction occurring only at the base of the cumulate pile. For slowly deposited cumulates an additional compaction zone that is partially viscosity-controlled develops at the top of the pile. If the extent of compaction and thickness of this zone are known for a real system, the model can be used to estimate the permeability and effective viscosity of the compacting cumulates. Such calculations yield a permeability of $$1.5 \times 10^{-3}a^{2}f^{3}/(1 - f)^{2}$$ for olivine cumulates in the Muskox Intrusion. Reliable viscosity estimates await an empirical law relating ma...

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 (&lt; 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.

The distribution and main geologic features of discordant bodies of iron-rich ultramafic pegmatite in the Bushveld Complex

Transgressive bodies of iron-rich ultramafic pegmatite, composed predominantly of varying proportions of hortonolite, clinopyroxene, ilmenite, and Ti magnetite are abundantly distributed throughout the layered mafic sequence of the Bushveld Complex. New information from field and underground studies, together with interpretations of detailed magnetic surveys, is presented along with an overview of the available literature. Some whole-rock and mineralogical-chemical data are also included.Within the Bushveld Complex a number of distinct but possibly genetically related groups of postcumulus ultramafic rock are recognized. These include, besides the main group of iron-rich ultramafic pegmatite, bodies of magnesian dunite and the well-known platiniferous ultramafic (or dunitic) pipes. Transgressive, often pipelike bodies composed almost entirely of Fe-Ti oxides occur in the main and upper zones of the complex which, although part of the iron-rich ultramafic pegmatite suite, are described separately. Composite bodies in which massive Fe-Ti oxides occur as a core within a peripheral shell of silicate-rich pegmatite are also recognized. The major occurrences of iron-rich ultramafic pegmatite are documented and depicted in a series of maps. Their distribution is subject to two major stratigraphic controls. Iron-rich ultramafic pegmatite occurs largely above the ultramafic cumulates in the lower and lower critical zones. It is abundant in the upper critical and main zones where plagioclase is a major cumulus phase but is rarely observed above the highest cumulus Ti magnetite layer in the upper zone. It is also evident that these rocks become increasingly iron-rich toward the top of the layered sequence, so that in the upper zone, Fe-Ti oxide pegmatite is the dominant variety. Many bodies, particularly the large ones, are structurally controlled and occur in disturbed areas characterized by faulting and post-Bushveld dikes.Stratigraphic magnetic anomalies in the complex may be related to anorthositic layers containing small specks of iron-rich ferromagnesian silicates and Fe-Ti oxides. This iron-rich material is considered to represent the accumulated and trapped, late-stage, residual and intercumulus fluid resulting from fractional crystallization within a cyclic unit. Lateral migration and channeling of these pegmatitic fluids into areas of structural weakness resulted in the formation of large, pipelike bodies of iron-rich ultramafic pegmatite. These bodies may have resulted from metasomatic replacement of preexisting cumulates or they may have crystallized directly from the pegmatitic fluids. The authors believe that the pegmatitic fluids have been derived from within the crystallizing cumulate pile and it is concluded that the formation of iron-rich ultramafic pegmatite is an integral feature of the layered sequence.

The geochemistry of vanadiferous magnetite in the bushveld complex: Implications for crystallization mechanisms in layered complexes

Detailed trace-element analyses of pure magnetite from four continuous borehole intersections through the main magnetitite layer from the upper zone of the Rustenburg Layered Suite of the Bushveld Complex are presented. One section has been analysed at one centimetre intervals. Rapid depletion of Cr occurs over short, vertical sequences near the base of the layer, which is due to bottom-crystallization and the resulting chemical depletion of a thin layer of liquid. Sudden increases in Cr content of magnetite are attributed to convection cells which bring undepleted magma into the zone or crystallization. We suggest that these cells individually have lateral extents no greater than hundreds of metres, but collectively may be traced at a specific stratigraphic horizon for several tens of km. This lateral traceability of the effect of convection cells at approximately uniform stratigraphic height demonstrates the long-held implicit assumption that time-planes are in general parallel to the layering, and does not support the hypothesis that layers in the Bushveld Complex grew laterally. The activity of these cells is highly variable, with long periods of quiescence interspersed with periods of rapid, small-scale overturn. Most convection cells do not impinge upon the floor, and the abruptness of the resulting chemical reversal is largely a function of the thickness of the layer of depleted liquid trapped between the cumulates and the sole of the convection cell. Occasionally, these cells do touch the cumulate pile and may even cause erosion. This material may be redeposited elsewhere in the magnetitite layer either as mineralogically distinct fragments if erosion penetrated below the layer or, in the present instance, as a chemically chaotic pile of magnetite. The abruptness of the chemical reversals severely restricts the extent to which post-cumulus redistribution of elements or re-equilibration with percolating trapped liquid (infiltration metasomatism) may have occurred. The appearance of disseminated plagioclase in magnetite layers in variable proportions and in a non-systematic manner in the four profiles is attributed to fluctuations in pressure.

An origin of small-scale fluctuations in orthopyroxene composition in the lower and critical zones of the Bushveld Complex, South Africa

Numerous, small and apparently haphazard reversals in Mg/Fe ratio of orthopyroxene grains have been reported by E.N. Cameron in the lower and lower critical zones of the Bushveld Complex. A hypothesis for their origin is presented by extending the suggestion of Cameron that modal variation in layered complexes may be attributed to pressure fluctuation. An increase in pressure will cause the magma to be supercooled; thus speeding up crystallization rates. This results in a greater proportion of interstitial liquid being trapped in the cumulate pile. On crystallization, this liquid adds relatively Fe-rich orthopyroxene to the cumulus of Mg-rich phase. Homogenization produces minerals whose compositions are displaced from the cumulus composition towards the low-temperature end-member by an amount proportional to the percent of interstitial liquid. Decreases in pressure produce the opposite effect, and may explain the origin of pure adcumulate rocks. Quantitative data are presented from the lower zone-critical zone contact and from the Duluth Complex to demonstrate this effect.

Kiglapait Geochemistry III: Potassium and Rubidium

The regular geometry and completeness of the Kiglapait intrusion permit its bulk composition to be obtained by summation, and the composition of successive liquids to be obtained by subtraction. The summations for K and Rb give 1806 and 1.08 ppm, yielding R frsol| K/ Rb= 1670 for the intrusion, taken as equal to the parent magma. R increases slightly from this initial value to 2000 at the end of crystallization where MgO approaches zero in the rocks. K and Rb are therefore closely coherent and their distribution coefficients can differ only by a small amount in the Kiglapait system. Apparent feldspar/liquid distribution coefficients (D F/L) can be estimated from detailed plots of feldspar and liquid compositions against F L . The Kiglapait data imply that these coefficients are linear 1:1 functions of plagioclase composition within experimental error, having values given by D K F/ L = 1.42− X An D Rb F/ L = 1.13− X An with minimum values of 0.75 and 0.49, respectively. The ratio R F R L lies in the range of 1.53± 0.03 for the plagioclase composition range X An = 0.34 to 0.67 showing that high-R rocks such as anorthosite crystallized from high- R liquids. The apparent feldspar distribution coefficients are much closer to 1.0 than common literature values. They can be reduced by assuming that the cumulate pile was continuously recharged by the circulating magma until an advanced stage of differentiation was reached, and assuming that alkalies were exchanged to the feldspars from the magma. When such an ‘aquifer recharge’ model is calibrated using olivine-liquid equilibria as a time marker for the liquid, the inferred minimum equilibrium values of the distribution coefficients are D K F L = 0.42, D Rb F L = 0.25 at the base of the intrusion. Their variation is given by D K F L = 1.66−1.88X An , D Rb F L = 1.17−1.41X An , The equilibrium values are considered to be appropriate for deducing liquid compositions in plutonic bodies where alkali exchange can be shown or inferred to have been inhibited, such as in small intrusions. The apparent values are considered to be appropriate, even though they may be artificial, for large intrusions similar to the Kiglapait. The bulk K and Rb concentrations in the Kiglapait intrusion are consistent with a plagioclase-rich abyssal tholeiite magma. Clinopyroxene and olivine fractionation in the mantle may contribute to the production of such high- Rmagmas.

Fine-scale layering in igneous intrusions: a possible mechanism for a non-depositional origin

SummaryExamination of layered rocks from the Currywongaun-Doughruagh Intrusion, Connemara, Eire, has suggested a possible connexion between adcumulate crystallization and the formation of thin, yet very persistent igneous layering. It is thought that as a result of adcumulate crystallization proceeding under undisturbed conditions near the top of a pile of cumulus crystals, a zone of liquid, supersaturated in those components not required for adcumulate growth, could be generated in the magma just above the cumulate pile. Crystallization from such a supersaturated zone would result in the cumulate phases, inter-directly beneath. The rhythmic deposition is lacking, could by this mechanism build up a sequence of cumulate layers interbedded with fine-scale layers of a non-depositional origin.