Intercumulus Liquid Research Articles

Trace Element and Platinum Group Element Distributions and the Genesis of the Merensky Reef, Western Bushveld Complex, South Africa

The Merensky Reef of the Bushveld Complex is one of the world’s largest resources of platinum group elements (PGE); however, mechanisms for its formation remain poorly understood, and many contradictory theories have been proposed. We present precise compositional data [major elements, trace elements, and platinum group elements (PGE)] for 370 samples from four borehole core sections of the Merensky Reef in one area of the western Bushveld Complex. Trace element patterns (incompatible elements and rare earth elements) exhibit systematic variations, including small-scale cyclic changes indicative of the presence of cumulus crystals and intercumulus liquid derived from different magmas. Ratios of highly incompatible elements for the different sections are intermediate to those of the proposed parental magmas (Critical Zone and Main Zone types) that gave rise to the Bushveld Complex. Mingling, but not complete mixing of different magmas is suggested to have occurred during the formation of the Merensky Reef. The trace element patterns are indicative of transient associations between distinct magma layers. The porosity of the cumulates is shown to affect significantly the distribution of sulphides and PGE. A genetic link is made between the thickness of the Merensky pyroxenite, the total PGE and sulphide content, petrological and textural features, and the trace element signatures in the sections studied. The rare earth elements reveal the important role of plagioclase in the formation of the Merensky pyroxenite, and the distribution of sulphide.

Perspectives on basaltic magma crystallization and differentiation: Lava-lake blocks erupted at Mauna Loa volcano summit, Hawaii

Explosive eruptions at Mauna Loa summit ejected coarse-grained blocks (free of lava coatings) from Moku'aweoweo caldera. Most are gabbronorites and gabbros that have 0–26 vol.% olivine and 1–29 vol.% oikocrystic orthopyroxene. Some blocks are ferrogabbros and diorites with micrographic matrices, and diorite veins (≤ 2 cm) cross-cut some gabbronorites and gabbros. One block is an open-textured dunite. The MgO of the gabbronorites and gabbros ranges ∼ 7–21 wt.%. Those with MgO > 10 wt.% have some incompatible-element abundances (Zr, Y, REE; positive Eu anomalies) lower than those in Mauna Loa lavas of comparable MgO; gabbros (MgO < 10 wt.%) generally overlap lava compositions. Olivines range Fo 83–58, clinopyroxenes have Mg#s ∼ 83–62, and orthopyroxene Mg#s are 84–63 — all evolved beyond the mineral-Mg#s of Mauna Loa lavas. Plagioclase is An 75–50. Ferrogabbro and diorite blocks have ∼ 3–5 wt.% MgO (TiO 2 3.2–5.4%; K 2O 0.8–1.3%; La 16–27 ppm), and a diorite vein is the most evolved (SiO 2 59%, K 2O 1.5%, La 38 ppm). They have clinopyroxene Mg#s 67–46, and plagioclase An 57–40. The open-textured dunite has olivine ∼ Fo 83.5. Seven isotope ratios are 87Sr/ 86Sr 0.70394–0.70374 and 143Nd/ 144Nd 0.51293–0.51286, and identify the suite as belonging to the Mauna Loa system. Gabbronorites and gabbros originated in solidification zones of Moku'aweoweo lava lakes where they acquired orthocumulate textures and incompatible-element depletions. These features suggest deeper and slower cooling lakes than the lava lake paradigm, Kilauea Iki, which is basalt and picrite. Clinopyroxene geobarometry suggests crystallization at < 1 kbar P. Highly evolved mineral Mg#s, < 75, are largely explained by cumulus phases exposed to evolving intercumulus liquids causing compositional ‘shifts.’ Ferrogabbro and diorite represent segregation veins from differentiated intercumulus liquids filter pressed into rigid zones of cooling lakes. Clinopyroxene geobarometry suggests < 300 bar P. Open-textured dunite represents olivine-melt mush, precursor to vertical olivine-rich bodies (as in Kilauea Iki). Its Fo 83.5 identifies the most primitive lake magma as ∼ 8.3 wt.% MgO. Mass balancing and MELTS show that such a magma could have yielded both ferrogabbro and diorite by ≥ 50% fractional crystallization, but under different fO 2: < FMQ (250 bar) led to diorite, and FMQ (250 bar) yielded ferrogabbro. These segregation veins, documented as similar to those of Kilauea, testify to appreciable volumes of ‘rhyolitic’ liquid forming in oceanic environments. Namely, SiO 2-rich veins are intrinsic to all shields that reached caldera stage to accommodate various-sized cooling, differentiating lava lakes.

Trapped intercumulus liquid in the Main Zone of the eastern Bushveld Complex, South Africa

Lateral variations in the amount of trapped intercumulus liquid in the Main Zone of the eastern Bushveld Complex are constrained by new Sr-isotopic, whole-rock and mineralogical data from three profiles that are separated laterally by ca.100 km and represent thicknesses of 551–1,127 m of Main Zone gabbronorites below the Pyroxenite Marker. An analysis of the An-contents (100×Ca/(Ca+Na)) of plagioclase cores within the Thornhill (north), Roossenekal and Stoffberg (south) profiles show similar systematic, up-section variations from An66 to An59. In contrast, both the An-contents of bulk plagioclase separates and the Mg-numbers (100×Mg/(Mg+FeT)) of orthopyroxenes show pronounced lateral variations from Thornhill (An67-61; Mg#67-61), through Roossenekal (An64-58; Mg#64-55) to Stoffberg (An59-55; Mg#59-50). These mineralogical variations are interpreted to be the result of reaction between cumulus minerals and an increasing amount of trapped liquid from north to south. Modelling of the trapped liquid shift of orthopyroxene compositions suggests that the amount of trapped liquid in the cumulates increased from near 0% at Thornhill, through 10–30% at Roossenekal to 30–45% at Stoffberg. A two- to eightfold southward increase in whole-rock concentrations of P, Ti, Y, Zr and Ba is consistent with the trapped liquid model. However, the 14-fold increase in Rb from Thornhill to Stoffberg is too great to be accounted for by trapped liquid alone, but can be explained by local assimilation of partial melts of the country rock. Constant initial 87Sr/86Sr isotopic ratios of Main Zone plagioclase separates (0.7081–0.7085) in all the three profiles do not preclude assimilation of adjacent basaltic to rhyolitic country rock with initial ratios between 0.6924–0.7096. The southward increase in the amount of trapped liquid is ascribed to an increased cooling rate by enhanced heat loss and partial assimilation of country rock xenoliths in the distal cumulate sequence at Stoffberg.

Modeling of in-situ crystallization processes in the Permian mafic layered intrusion of Mont Collon (Dent Blanche nappe, western Alps)

The Mont Collon mafic complex is one of the best preserved examples of the Early Permian magmatism in the Central Alps, related to the intra-continental collapse of the Variscan belt. It mostly consists (> 95 vol.%) of ol + hy-normative plagioclase-wehrlites, olivine- and cpx-gabbros with cumulitic structures, crosscut by acid dikes. Pegmatitic gabbros, troctolites and anorthosites outcrop locally. A well-preserved cumulative sequence is exposed in the Dents de Bertol area (center of intrusion). P– T calculations indicate that this layered magma chamber emplaced at mid-crustal levels at about 0.5 GPa and 1100 °C. The Mont Collon cumulitic rocks record little magmatic differentiation, as illustrated by the restricted range of clinopyroxene mg-number (Mg# cpx = 83–89). Whole-rock incompatible trace-element contents (e.g. Nb, Zr, Ba) vary largely and without correlation with major-element composition. These features are characteristic of an in-situ crystallization process with variable amounts of interstitial liquid L trapped between the cumulus mineral phases. LA-ICPMS measurements show that trace-element distribution in the latter is homogeneous, pointing to subsolidus re-equilibration between crystals and interstitial melts. A quantitative modeling based on Langmuir's in-situ crystallization equation successfully duplicated the REE concentrations in cumulitic minerals of all rock facies of the intrusion. The calculated amounts of interstitial liquid L vary between 0 and 35% for degrees of differentiation F of 0 to 20%, relative to the least evolved facies of the intrusion. L values are well correlated with the modal proportions of interstitial amphibole and whole-rock incompatible trace-element concentrations (e.g. Zr, Nb) of the tested samples. However, the in-situ crystallization model reaches its limitations with rock containing high modal content of REE-bearing minerals (i.e. zircon), such as pegmatitic gabbros. Dikes of anorthositic composition, locally crosscutting the layered lithologies, evidence that the Mont Collon rocks evolved in open system with mixing of intercumulus liquids of different origins and possibly contrasting compositions. The proposed model is not able to resolve these complex open systems, but migrating liquids could be partly responsible for the observed dispersion of points in some correlation diagrams. Absence of significant differentiation with recurrent lithologies in the cumulitic pile of Dents de Bertol points to an efficiently convective magma chamber, with possible periodic replenishment.

Kiglapait mineralogy IV: The augite series

Based on twenty-five new chemical analyses, augites from the Upper Zone of the Kiglapait layered intrusion, Labrador, span the range from En 7 3 to En 0 . disposed about a straight line sloping slightly upward from WO 4 0 to Wo 4 2 in the pyroxene quadrilateral. They contain little tetrahedral Al, which decreases with Fe enrichment as tetrahedral ferric Fe 3 + increases. Little or no jadeite or Ca-Tschermaks component is present. Titanium rises only slightly with Fe, and the acmite component is scattered near mole fraction 0.03. Excess cations relative to 6 oxygen atoms occur chiefly by solid solution toward olivine, the only co-crystallizing mafic silicate phase. Values of n β increase linearly over the range 1.670 at fictive En 1 0 0 to 1.7448 at En 0 . The position of the 150 powder X-ray diffraction peak shifts regularly from 5.0372 to 4.1727 degrees away from the 220 peak of silicon metal over the same composition range. Exsolution of orthopyroxene near (100) is succeeded up-stratigraphy by that of pigeonite near (001), and the switch occurs at about the same augite composition as the switch from external orthopyroxene to pigeonite in the Skaergaard intrusion. Exsolution of ilmenite in or near (100) and magnetite-ulvospinel near the a and c directions in (010) produces the black color of these augites in hand specimen. Subsolidus augite-olivine tie lines and exchange relations in Fe-Mg allow the distinction between equilibrium pairs and those affected by exchange with external Fe-Ti oxide minerals or with intercumulus liquid in Upper Border Zone lithologies. Liquids producing augite and olivine with plagioclase follow a Ca-depletion trend until crystallization of abundant olivine and titanomagnetite drives them on a Ca-enrichment trend, to a point near Mg-free augite. Activities of silica consistent with augite-olivine pairs range from <0.6 to 0.98. The Kiglapait augite series exemplifies the crystallization of a magma occupying the narrow regime between silica saturation and undersaturation, in extreme fractionation from troctolite to Mg-free syenite.

A crystal size-distribution study of the Kiglapait layered mafic intrusion, Labrador, Canada: evidence for textural coarsening

Crystal size distributions (CSD) of plagioclase, olivine and clinopyroxene from the Kiglapait intrusion, Labrador were measured for a series of 13 samples spanning most of the exposed intrusion. All CSDs lack small crystals and are slightly curved to the right, concave up. All CSDs of a mineral sweep out a fan, as expected where there is relatively little variation in phase proportion. CSD dispersion can be replicated using the communicating neighbours model of textural coarsening (Ostwald ripening). Most of the textural variation in the intrusion is the result of variable degrees of coarsening. It is proposed that coarsening occurred in the outer part of the crystallisation boundary layer where release of latent heat and changes in the intercumulus liquid composition resulted in a reduction in undercooling, suppression of nucleation and coarsening. Particularly coarse-grained areas reflect focusing of circulating magmatic fluids during coarsening. The effects of crystal settling were not observed in the CSDs, even within single well-developed layers. They were probably concealed by the pervasive textural coarsening. CSDs do not suggest that mechanical compaction has played an important role in the formation of texture, although it is very likely to have occurred. However, samples from the lower part of the intrusion show textural evidence of pressure-solution compaction.

Cumulatic Diorites Related to Post-Collisional, Brasiliano/Pan-African Mafic Magmatism in the Vila Nova Belt, Southern Brazil

The Capivaras Diorite, in the Vila Nova region, NW of the Sul-rio-grandense Shield, is composed of six NE- to NNE-oriented rock bodies of late-tectonic emplacement relative to the D3 deformation phase which forms subvertical high-strain zones in basement gneiss sequences. Within these intrusive bodies, a shape foliation is present, generally parallel to contacts and displaying a local solid-state deformational component. The internal structure of the Capivaras Diorite main intrusion is marked by a zone of intense flow and mingling, characterized by strong shape foliation and layers of variable texture and composition, which result from cumulative processes, heterogeneous flow and interaction of coeval, compositionally contrasting magmas. The central part of this intrusion is texturally homogeneous and slightly foliated, even though cumulative processes have remained important during its formation. Along the contact with basement gneisses, fine-grained diorites are found, which are considered to be compositionally close to the parental magma of the Capivaras Diorite. This magma has a mildly alkaline affinity and shows moderate to high contents of Zr, Ti and P. Highly-fractionated REE patterns, low Nb contents, as well as high contents of K, Sr, Ba, and Rb, are suggestive of its provenance from mantle sources which have been previously affected by subduction processes, such as those of mature magmatic arcs or post-collisional settings. Magmatic evolution was controlled by cumulative processes and gave origin to pyroxene orthocumulates, plagioclase-pyroxene orthocumulates, pyroxene adcumulates, and more rarely plagioclase adcumulates. The cumulative origin of these rocks is indicated by field, textural and geochemical features, which are distinct from those of crystallized liquids. The compositional diversity of cumulates has led to the generation of compositionally different melts. The early-formed pyroxene cumulates have caused Ca, Al, Na, Ba, and Sr enrichment in the magmatic liquid, leading to plagioclase crystallization and accumulation. Coarse-grained mafelsic cumulates were formed during the late stages of magmatic crystallization, due to volatile enrichment of the intercumulus liquid. Considering geological relations, as well as tectonic and compositional features of the Capivaras Diorite, it is interpreted as part of Neoproterozoic magmatism related to the post-collisional stage of Brasiliano/Pan-African Orogenic Cycle in southern Brazil.

The Giant Mottled Anorthosite: a transitional sequence at the top of the Upper Critical Zone of the Bushveld Complex

Research Article| March 01, 2002 The Giant Mottled Anorthosite: a transitional sequence at the top of the Upper Critical Zone of the Bushveld Complex Andrew A. Mitchell; Andrew A. Mitchell Department of Geology, University of Durban-Westville, Private Bag X54001, Durban, 4000 Search for other works by this author on: GSW Google Scholar Rogenie Manthree Rogenie Manthree Department of Geology, University of Durban-Westville, Private Bag X54001, Durban, 4000 Search for other works by this author on: GSW Google Scholar Author and Article Information Andrew A. Mitchell Department of Geology, University of Durban-Westville, Private Bag X54001, Durban, 4000 Rogenie Manthree Department of Geology, University of Durban-Westville, Private Bag X54001, Durban, 4000 Publisher: Geological Society of South Africa First Online: 07 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2002 Geological Society of South Africa South African Journal of Geology (2002) 105 (1): 15–24. https://doi.org/10.2113/1050015 Article history First Online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Andrew A. Mitchell, Rogenie Manthree; The Giant Mottled Anorthosite: a transitional sequence at the top of the Upper Critical Zone of the Bushveld Complex. South African Journal of Geology 2002;; 105 (1): 15–24. doi: https://doi.org/10.2113/1050015 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Previous studies have placed the transition between the Upper Critical and Main Zones of the Bushveld Complex somewhere within or above the distinctive Giant Mottled Anorthosite, but have lacked the resolution to precisely pinpoint the level at which Main Zone geochemical signatures predominate over Critical Zone signatures. Using a sample spacing of 5m or less, we demonstrate that the uppermost unit of the Upper Critical Zone (an anorthosite with large intercumulus pyroxene mottles, designated unit HW5 at Impala Platinum Mines), whilst petrographically part of the Giant Mottled Anorthosite, has some chemical attributes that link it to the Main Zone. We attribute the hybrid geochemical signature of unit HW5 to partial melting of the anorthosite, due to an influx of Main Zone magma above it, followed by hybridization of the remelted intercumulus material with evolved intercumulus liquids from the overlying Main Zone. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Spinels and Mg Ilmenites from the Noril'sk 1 and TalnakhIntrusions and Other Mafic Rocks of the Siberian Flood Basalt Province

Spinels from the mineralized Noril’sk 1 and Talnakh intrusions show a broad and distinctive compositional spectrum, from normal chromite to Ti-enriched aluminous magnetite. In the Talnakh intrusion, in particular, the range of spinel compositions extends across a compositional space where spinels are rare in other layered intrusions. Spinel compositions vary widely on a scale of individual thin sections and are influenced by the nature of the enclosing silicate phase. Spinels enclosed in olivines are less Ti enriched than those outside olivine grains. Compositional variance in the spinels is attributed to variable but generally advanced reaction between cumulus chromite grains and solidifying trapped intercumulus liquid. The advanced nature of the process is attributed to the open nature of the intrusions. Continuous, prolonged flow-through of hot magma enabled cumulates within the intrusions to remain above their solidus temperature for longer than would be normal for sills of comparable thickness. The barren Chibacheka River intrusion shows some of the same features but differs from Talnakh in that spinel-olivine pairs record equilibration temperatures higher than those at Talnakh. Temperatures recorded by spinel-olivine pairs at Talnakh are anomalously low for such a small intrusion, again indicating prolonged slow cooling consistent with a flow-through model for the sill. Spinel compositions support a hypothesized comagmatic relationship between the Chibacheka River intrusion and the Ti-, Cr-rich Gudchichinsky picritic basalts. Spinels in the Noril’sk and Talnakh intrusions imply a parent magma with a lower Cr/Al ratio than the Gudchichinsky picritic basalts. Spinels from the Siberian intrusions show a similar range in compositions to the feeder intrusions of the Karoo flood basalt province. Jinchuan spinels share the features of high Ti contents and anomalously low equilibration temperatures with the Noril’sk-Talnakh spinels. Mg ilmenites are a feature of the Siberian and Jinchuan intrusions. They are attributed to crystallization of ilmenite from evolved trapped intercumulus liquid buffered by surrounding Mg-rich cumulus phases. Their existence implies moderately but not unusually Ti-rich parent magmas, consistent with compositions of the Siberian Traps lavas.

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.

Chrome spinels from the Jinchuan Ni-Cu sulfide deposit, Gansu Province, People's Republic of China

Chrome spinels occur in olivine-rich cumulates in the Jinchuan intrusion, host to one of the largest known accumulations of magmatic Ni-Cu in the world. The chemistry and mode of occurrence of these spinels is compared with those in two other similar but unmineralized mafic-ultramafic intrusions in the same belt.The Jinchuan intrusion contains Mg-poor chromites with widely varying TiO 2 contents, showing many features that are typical of chromites in normal tholeiitic layered intrusions. Wide compositional variance occurs on the scale of single thin sections, and is attributed to reaction between cumulus chromite and trapped intercumulus liquid. This process operated extensively, even on grains that were armored by cumulus olivine crystals. Different reaction paths correlate with the abundance of sulfides in the rock.The unmineralized intrusions show similar reaction trends, but are offset from the Jinchuan trends. Yejili spinels are generally higher in Al and Ga, while Zangbutai chromites are systematically higher in Fe (super 3+) . Jinchuan spinel-olivine pairs record equilibration temperatures well below the solidus, and are slightly but significantly lower than blocking temperatures recorded at Yejili.The Jinchuan ore zone contains low Cr, high Fe (super 3+) chromites that are unusual and distinctive. A plausible model for these unusual compositions is that these grains originated as normal aluminous spinels crystallizing from the parent silicate melt, and subsequently became enlarged and modified during crystallization of the sulfide melt. Ti-enriched chromites enclosed within sulfides record a prehistory of extensive reaction with trapped liquid.Sulfide-associated chromites from Jinchuan are strongly nickel depleted relative to expected Ni values for their Fe (super 3+) content when compared with the barren intrusions. This feature, combined with the distinctive chemistry of the sulfide-related spinels, may have applications in the use of heavy resistate minerals during exploration.Distinctively high Ti chromites are also a feature of intrusions associated with the Karoo flood basalts, suggesting that Jinchuan may have a similar affinity. However, TiO 2 enrichment in chromite can be a product of interaction with differentiated trapped liquid, as indicated by similar trends in lava lakes, layered intrusions, and at Sudbury, and therefore cannot be taken to prove unusually Ti-rich parent magmas.

Re-evaluation of intercumulus liquid composition and oxidation state for the Shergotty meteorite

The Shergotty Martian meteorite contains pigeonite and augite with homogeneous magnesian cores, thought to be cumulus crystals. Using elemental maps of thin sections, we estimate the abundance of cumulus pyroxene cores at 13 vol%. This estimate is roughly half that determined from prior melting experiments (28%), and coincides with the observation of a similar discrepancy between the proportions of cumulus pyroxenes estimated from modal observations and melting experiments in the Zagami shergottite. An intercumulus liquid composition calculated using this new cumulus pyroxene estimate is virtually identical to that calculated for Zagami. Our results suggest the possibility that melting experiments may not have been carried out under conditions appropriate for crystallization of the shergottites. We consider two possible explanations: crystallization of pyroxene cores at depth or under different redox conditions. Prior arguments for polybaric crystallization of shergottites, based on pyroxene intergrowths and kaersutite inclusions, are inconclusive. Using the MELTS program, we have explored crystallization at higher pressure. A greater proportion of pyroxene crystallizes at higher pressures, rather than a smaller amount as necessary in this model. Even in calculations at 1 bar, pigeonite crystallizes before augite, and the temperature interval between the appearance of pigeonite and augite expands with pressure. Both pigeonite and augite appear to be in equilibrium with the same liquid composition and thus are thought to have co-crystallized in Shergotty, so it seems likely that MELTS may not correctly predict pyroxene crystallization behavior in this Fe-rich magma. Fe,Ti-oxide compositions in Shergotty were previously used as evidence that the magma crystallized at redox conditions near QFM. However, Ghosal et al. (1998) recently suggested that the oxides record subsolidus, reducing conditions of QFM-3 at 602°C, and our analyses support that conclusion, if a new solution model for oxides is used. They also proposed that Fe 3+ in Shergotty pyroxenes indicates magmatic crystallization at approximately QFM-3, and our estimates of Fe 3+ calculated from pyroxene stoichiometry in microprobe analyses match their reported values. However, we doubt that stoichiometric calculations provide accurate Fe 3+ determinations. Other indicators suggest that magmatic conditions were oxidizing. The FeO∗ content of Shergotty maskelynite is greater and the negative Eu anomaly in pyroxene cores is shallower than in another basaltic shergottite, QUE94201, which is thought to have crystallized under reducing conditions near QFM-4. Also, the presence of silica, fayalite, and magnetite in mesostasis indicates oxiding conditions, at least at a late magmatic stage. Thus we argue that Shergotty crystallized under redox conditions near QFM.

Geochemical modeling of acid–basic magma interaction in the Sardinia–Corsica Batholith: the case study of Sarrabus, southeastern Sardinia, Italy

The Hercynian intrusive rocks outcropping in the Sarrabus area, southeastern Sardinia, Italy, consist of microgranular mafic enclave (MME)-bearing granites (>95%) associated with coeval stratified gabbroic complexes and basic septa (BS) (<5%). The coexistence of partially molten magmas of contrasting composition offers the opportunity to study interaction processes in intrusive environments and the geochemical characteristics of hybrid products. In this paper we present a petrogenetic model for the differentiation of basic magmas injected into coeval, partially molten, granite magmas, on the basis of field, petrographic and geochemical data. Samples from the stratified basic complex of Capo Carbonara (CCB) and BS were grouped into outer and inner facies based upon the sampling sites close to, or far away (a few meters) the host granite contact. MME were grouped according to their composition: gabbro-diorites and tonalites. The inner facies of the basic complex and BS represent cumulus of amphibole+clinopyroxene+plagioclase with trapped intercumulus liquid. The calculated chemical composition of the parental magma is well within those calculated for the Hercynian basic magmas of the Sardinia–Corsica Batholith. The geochemical features of the MME and the outer facies of the basic complex and BS establish an origin by mixing/mingling mechanisms controlled by fractional crystallisation and contamination (CFC) of the parental magma of the inner gabbroic facies. Additional refinements of the model, addressed in this paper, establish the occurrence of a filter pressing process operating during the CFC evolution of the basic magma injected into the acid magma.

Chromatographic separation of the platinum-group elements, gold, base metals and sulfur during degassing of a compacting and solidifying igneous crystal pile

The major platinum-group elements (PGE) concentrations in layered intrusions are typically associated with zones in which the sulfide abundance begins to increase. In a number of layered intrusions, there is also a distinct stratigraphic separation in the peak concentrations of the PGE from those of the base metals, gold and sulfur through these zones. These stratigraphic “offsets” are characterized by a lower, typically S-poor, Pt- and Pd-enriched zone overlain by a zone enriched in the base metals, S and Au. The separations amount to a few decimeters to several tens of meters. In some instances, the high Pt and Pd concentrations are associated with trivial amounts of sulfide. Theoretical considerations suggest that these offsets can be modeled as chromatographic peaks that develop during an infiltration/reaction process. Using Pd as a typical PGE and Cu as a typical base metal, a numeric model is developed that illustrates how metal separations can develop in a vapor-refining zone as fluid evolved during solidification of a cumulus pile leaches sulfide and redeposits it higher in the crystal pile. The solidification/degassing ore-element transport is coupled with a compaction model for the crystal pile. Solidification resulting from conductive cooling through the base of the compacting column leads to an increasing volatile concentration in the intercumulus liquid until it reaches fluid saturation. Separation and upward migration of this fluid lead to an upward-migrating zone of increasingly higher bulk water contents as water degassed from underlying cumulates enriches overlying, fluid-undersaturated interstitial liquids. Sulfide is resorbed from the degassing regions and is reprecipitated in these vapor-undersaturated interstitial liquids, producing a zone of relatively high modal sulfide that also migrates upward with time. Owing to its strong preference for sulfide, Pd is not significantly mobile until all sulfide is resorbed. The result is a zone of increasing PGE enrichment that follows the sulfide resorption front as solidification/degassing continues. In detail, the highest Pd concentrations occur stratigraphically below the peak in S and base metals. The high Pd/S ratio mimics values conventionally interpreted as the result of high (silicate liquid)/(sulfide liquid) mass ratios (“R” values). However, in this case, the high Pd/S ratio is the result of a chromatographic/reaction front enrichment and not a magmatic sulfide-saturation event.

Chromite in Komatiites, 1. Magmatic Controls on Crystallization and Composition

Chromite is a widespread accessory mineral in komatiites, ranging Zhou & Kerrich, 1992). This is the first in a series of papers which addresses the occurrence of chromite in from skeletal through euhedral and equant to lobate and rarely poikilitic habits depending on cooling regime and the composition komatiites and its chemical variability. This paper looks at the factors controlling the crystallization of chromite of the coexisting olivine. Chromite is least abundant in highly magnesian chrome-undersaturated lavas, and most abundant in from komatiites, and at the textural and compositional variation within chromites which have apparently restrongly differentiated layered cumulate bodies. Abundances are typically lower than the expected cotectic proportions. Chromite tained some or all of their igneous characteristics. Textural and compositional features of chromites are compositions reflect variations in lava composition and oxygen fugacity; reaction with trapped intercumulus liquid; and sub-solidus interrelated in complex and interesting ways, and show systematic relationships to host rock chemistry and crysFe–Mg exchange with olivine. Primary chromites in thick dunitic channels and sheet flows have very low Fe contents, whereas tallization regime. Future papers in the series will deal with superimposed metamorphic effects. comparable thin flow chromites contain higher Fe. This is attributed to an initial reduced state of the magma, thin flow lavas A large body of new data on chromite compositions, including high-precision wavelength-dispersive analyses being subsequently oxidized because of post-eruption processes. Extensive reaction of chromite with trapped liquid causes decreasing of minor and trace elements Ti, V, Mn, Zn, Ni and Co, is presented from the Norseman–Wiluna Greenstone Belt Mg/(Mg+ Fe), and enrichment in Fe, TiO2, V2O5, MnO, CoO and ZnO. Nickel trends depend on sulphide content. These in Western Australia, and synthesized with a compilation of literature data taken mainly from Roeder (1994) and trends are best developed in rocks combining presence of trapped intercumulus liquid with prolonged cooling histories. The deficiency various unpublished sources. Whole-rock geochemical data are similarly compiled from the literature and some of chromite in channelized environments is partly a consequence of more primitive lavas being found in these environments, but is unpublished sources, and include a large body of data collected over the last 15 years at CSIRO. Details of mainly due to the difficulty in nucleating chromite crystals under low degrees of supercooling. data sources are listed in Appendix A, and details of electron microprobe analytical techniques in Appendix B.

Chromite in Komatiites, 1. Magmatic Controls on Crystallization and Composition

Chromite is a widespread accessory mineral in komatiites, ranging from skeletal through euhedral and equant to lobate and rarely poikilitic habits depending on cooling regime and the composition of the coexisting olivine. Chromite is least abundant in highly magnesian chrome-undersaturated lavas, and most abundant in strongly differentiated layered cumulate bodies. Abundances are typically lower than the expected cotectic proportions. Chromite compositions reflect variations in lava composition and oxygen fugacity; reaction with trapped intercumulus liquid; and sub-solidus Fe–Mg exchange with olivine. Primary chromites in thick dunitic channels and sheet flows have very low Fe3+ contents, whereas comparable thin flow chromites contain higher Fe3+. This is attributed to an initial reduced state of the magma, thin flow lavas being subsequently oxidized because of post-eruption processes. Extensive reaction of chromite with trapped liquid causes decreasing Mg/(Mg + Fe2+), and enrichment in Fe3+, TiO2, V2O5, MnO, CoO and ZnO. Nickel trends depend on sulphide content. These trends are best developed in rocks combining presence of trapped intercumulus liquid with prolonged cooling histories. The deficiency of chromite in channelized environments is partly a consequence of more primitive lavas being found in these environments, but is mainly due to the difficulty in nucleating chromite crystals under low degrees of supercooling.

The Cumulate Paradigm Affirmed

McBirney and Hunter's conclusion that the cumulate paradigm or theory is an impediment to progress is unjustified because they did not demonstrate that the original cumulate terminology cannot be adapted; neither did they question the fundamental cumulate assumption of bottom upward solidification for many large mafic intrusions. Because the decoupling of mineral, modal and whole rock chemical compositions may reflect either the failure of cumulate theory or may be an artifact of sampling of large mafic intrusions on scales smaller than the scale of the heterogeneities produced by the irregular distribution and migration of intercumulus liquids, we conclude that a compelling case to abandon cumulate theory has not been made. We suggest that the determination of whether cumulus and intercumulus phases were in equilibrium with each other, and whether disequilibrium textures might yield a complete record of the crystallization history of the cumulus plagioclase, are likely to be among the approaches that wil...

Chrome–spinels in gabbro–wehrlite intrusions of the Pechenga area, Kola Peninsula, Russia: emphasis on alteration features

Chrome–spinels are common accessory minerals of the Pechenga ultramafic rocks in NW Russia, varying in metamorphic grade from sub-greenschist to amphibolite facies. Magmatic chrome–spinels can be grouped into two main types. Spinel-1, which crystallized first, is represented by Al,Ti-chromite. It occurs mainly as inclusions in olivine and, rather less commonly, enclosed within interstitial minerals. Spinel-2 crystallized after spinel-1 and has composition varying from Ti-rich chromite to Ti-rich chrome-magnetite. Spinel-2 occurs in the interstitial spaces between olivine crystals and enclosed within intercumulus silicates. Zoned crystals, with spinel-1 in the core, overgrown by spinel-2 are also common. Cr-poor titanomagnetite occurring in pyroxenites and gabbro apparently represents further evolution of spinel-2 phases. Metamorphic chrome–spinels are represented by ferritchromit and Cr-magnetite which are the dominant chrome–spinel phases in the amphibolite facies rocks. The continuous trend of the magmatic chrome–spinels composition is indicative of their intensive post-cumulus alteration by reaction with evolving intercumulus liquid. This reaction led to depletion of the earlier crystallized chrome–spinels in Mg, Cr, and Al and their enrichment in Ti and Fe 3+. Oxidation–exsolution of ilmenite from Ti-rich chrome–spinels is another common process of post-crystallization alterations. Serpentinization of olivine led to formation of magnetite, which overgrew and cut chrome–spinel grains. Low-temperature hydrothermal processes, in particular talc–carbonate alterations, could cause a local loss of Ti and further redistribution of Mg, Mn and Zn. Prograde metamorphic reactions led to formation of ferritchromit and magnetite replacing primary chrome–spinels. Another common process, affecting Ti-rich chrome–spinels, is a prograde metamorphic recrystallization of exsolved ilmenite, which changes from submicron lamellae through a regular grid of coarse ilmenite crystals and further to their irregular blebs and rims. Metamorphism of epidote–amphibolite and amphibolite facies has led to the liberation of most of the elements. Metamorphically liberated Cr was captured by antigorite.

Experimental study of a Kiglapait marginal rock and implications for trace element partitioning in layered intrusions

Experiments on a chilled marginal rock from the Kiglapait Intrusion, Labrador, have been performed in order to constrain the partitioning behaviour of major and trace elements during formation of Kiglapait Lower Zone (LZ) cumulates. At 0.1 MPa and f o 2 close to FMQ, the liquidus is bracketed between 1225 and 1212°C and the only liquidus phases are plagioclase (An 73−74) and olivine,(Fo 82–85) In contrast LZ troctolites contain Fo 67±5 olivine and An 63±4 plagioclase. Plagioclase-melt partition coefficients ( D's) for Li, Ti, Rb, Sr, Y, Zr, Ba, La and Eu, determined by ion-microprobe analysis of run products, are similar to those for other basaltic systems, but differ systematically from apparent D's calculated from bulk analyses of LZ plagioclases and modelled liquid compositions obtained by summation. The discrepancies in D's and phase compositions are inconsistent with diffusion-controlled boundary layer growth. Crystallisation of trapped intercumulus liquid is quantitatively plausible; however, (a) the volume of liquid required to reconcile experimental and model D's (20±1%) is larger than that estimated from observed proportions of intercumulus phases (11 ± 2%), and (b) this mechanism alone cannot reduce X An in LZ plagioclases due to very slow Al ⇔ Si interdiffusion. The discrepancy between experiment and model is best resolved if the parental melt to LZ cumulates was more evolved (higher Fe/Mg, Na/Ca, REE and alkalis, lower Sr) than the summed bulk composition, in accord with a recent revision of the Kiglapait Fe/Mg based on olivine compositions

Petrology and geochemistry of a calcic and ferrous granitoid pluton: the Mitsuhashi Granite in the Ryoke Belt, southwest Japan.

The Mitsuhashi Granite pluton, 8×8 km in extent, was emplaced within the Ryoke metamorphic rocks, yielding a radiometric age of c.80 Ma. The main rock types are coarse-grained hornblende-biotite tonalite and low-K2O granodiorite, with subordinate quartz diorite which bears cummingtonite or garnet. At the margin of the pluton, wallrock assimilation produced garnet-biotite granite and granodiorite. Geochemically the rocks are unusually calcic (alkali-lime index=65), peraluminous and metaluminous, and low-potassic, and also have characteristically high Fe/Mg ratios. They have high (av. 280 ppm) Zr and low (<50 ppm) Rb concentrations.Geobarometric calibrations and mineral parageneses indicate that crystallization took place deeper than 15 km (∼0.42 GPa), under a reduced condition. Trace element variations and REE patterns suggest mainly biotite and some plagioclase fractionation. Modal and geochemical variations are modelled principally by varying the ratios of cumulus plagioclase and intercumulus liquid. The potential protoliths of this pluton are mafic rocks of continental lower crust beneath the Ryoke Belt.