Cumulus Olivine Research Articles

Flood basalt-related Fe–Ti oxide deposits in the Emeishan large igneous province, SW China

In the Panzhihua–Xichang region (Sichuan Province, SW China), there are a number of world-class magmatic Fe–Ti oxide deposits. They are hosted as conformable masses in lower parts of layered mafic–ultramafic intrusions that are part of the end-Guadalupian (∼ 260 Ma) Emeishan large igneous province. The ore-bearing Panzhihua, Hongge, Baima, Taihe and Xinjie intrusions are spatially and temporally associated with flood basalts and granitoids in the province. New mineralogical data for Fe–Ti oxide ores in the Hongge and Baima intrusions, combined with an overview of previously published data, provide a better understanding on the formation of these enigmatic deposits. Geochemical and Sr–Nd isotopic data confirm a genetic relation between the intrusions and high-Ti Emeishan flood basalts. Parental magma compositions, estimated using the most Mg-rich cumulus olivine in the intrusions, are characterized by slight to moderate degrees of fractionation (MgO = 7.2–11 wt.%). Occurrences of rare Cr-bearing titanomagnetite (Cr 2O 3 = 1.2–10.7 wt.%) in the Panzhihua, Hongge and Xinjie intrusions are consistent with early crystallization of Fe–Ti oxides. Oxide-silicate equilibria and low V concentration (< 4800 ppm) in magnetite are consistent with relatively high oxygen fugacity. Flood basalt-related origin, early Fe–Ti oxide crystallization and relatively high oxygen fugacity, are characteristic of Fe–Ti oxide deposits in the Panxi region but are fundamentally different from those hosted in highly-differentiated, upper parts of large layered intrusions (e.g. the Bushveld Complex). Similar Fe–Ti oxide deposits have not been documented in other well-studied large igneous provinces to date, but a number of Fe–Ti oxide ore occurrences worldwide are potentially similar to this type of flood basalt-related oxide deposits.

A conduit-related genesis of the Lengshuiqing intrusive assemblage (Sichuan, SW China)

Lengshuiqing is part of the late Proterozoic igneous province from the western margin of the Yangtze craton. The Lengshuiqing area comprises five ultramafic–mafic intrusions, emplaced in the late Proterozoic Yanbian Group. The intrusions from Lengshuiqing contain cumulate ultramafic zones (peridotite + olivine pyroxenite), with cumulus olivine and Cr-spinel, and intercumulus pyroxenes, hornblende, phlogopite and plagioclase. Ni–Cu ore (pyrrhotite + pentlandite + chalcopyrite) is hosted in the ultramafic zones. Olivine-free diorite–quartz diorite ± gabbro and granite zones commonly occur above the ultramafic rocks. The genesis of the intrusions (conduit-related accumulation or differentiation from stagnant magma) was investigated. The amount of sulphides in the intrusions from Lengshuiqing is one order of magnitude bigger than the sulphides that can be dissolved by a volume of mafic magma similar with the volume of the intrusions. Most intrusions from Lengshuiqing have bulk composition (peridotite ± diorite ± granite) more magnesian (MgO = 21–22%; Mg# > 78) than the deduced composition of their parental magma (MgO = 9–11%; Mg# = 64–67). This indicates the accumulation of sulphide and mafic silicates from a volume of magma much bigger than the volume of the intrusions, which can be explained by the fractionation from magma ascending through the intrusions to shallower depths. A continuous supply and vent of magma is consistent with the lack of chilled margins, the melting of the wall rocks and the generation of high-temperature mineral assemblages (K-feldspar, diopside, and sillimanite) in the Yanbian Group. The intrusions from Lengshuiqing are seen as microchambers on conduits draining olivine-, Cr-spinel-, and sulphide-bearing mafic magma from a larger staging chamber.

Mineral compositional constraints on petrogenesis and oxide ore genesis of the late Permian Panzhihua layered gabbroic intrusion, SW China

A systematic study of mineral compositions has been carried out on stratigraphic sections through the Panzhihua intrusion, a layered gabbroic sill of late Permian age that hosts significant magmatic Fe–Ti–V oxide deposits in the Emeishan large igneous province, SW China. The intrusion, which is ∼ 19 km long and ∼ 2 km thick, is divided into a Marginal zone (MGZ), Lower zone (LZ), Middle zone (sub-zones MZa and MZb) and Upper zone (UZ) from the base upwards. Compositions of the most primitive cumulus olivine (Fo 81) and plagioclase (An 73) found in the intrusion suggest that the parental magma was similar to the Mg-rich end-member of the coeval high-Ti basalts in the region. Two reversals in terms of mineral compositions are found in the intrusion, one in the lower MZa and the other at the transition between the MZb and the UZ. The first reversal is marked by a gradual increase in the An content of plagioclase (∼ An 60 to ∼ An 70) over a ∼ 100 m vertical interval. The other is characterized by abrupt changes in the compositions of olivine (∼ Fo 28 to ∼ Fo 70), clinopyroxene (Mg# = ∼ 51 to 75) and plagioclase (∼ An 24 to ∼ An 51) over a < 20 m vertical interval. These suggest that the Panzhihua intrusion opened at least twice for recharge of more primitive magmas. Strong depletion of incompatible trace elements, such as Zr, in the cumulate sequence without corresponding enrichment in any border series is suggested to be evidence for significant loss of residual liquid at a late-stage during solidification of the intrusion. Compared to other well-studied layered intrusions, the Panzhihua intrusion contains Fe–Ti oxides crystallized at a high temperature, consistent with evidence obtained in earlier studies. Early appearance of liquidus Fe–Ti oxides in the Panzhihua magma is most likely a combined effect of a rather high degree of initial oxidation and elevated volatile fugacity.

The roles of melt infiltration and cumulate assimilation in the formation of anorthosite and a Cr-spinel seam in the Rum Eastern Layered Intrusion, NW Scotland

Thin (~ 2 mm) Cr-spinel seams are present at the bases of several of the coupled peridotite–troctolite units that comprise the Rum Eastern Layered Intrusion. In some cases, ‘subsidiary’ Cr-spinel seams have also developed at between 6 and 12 cm below the unit boundaries. The subsidiary seams are thinner than the normal seams (approx. 1 mm), discontinuous, and occur within and at the base of a thin (~ 10 cm) layer of anorthosite, which is sandwiched between peridotite and troctolite. The troctolite contains cumulus olivine, but in the Cr-spinel seam and anorthosite, olivine is intercumulus only. Cr-spinel is scarce in the troctolite, but common in the anorthosite, where it has a different composition (more Fe-and Cr-rich) and crystal size distribution (CSD) profile to the Cr-spinel in the subsidiary seam, suggesting that it represents a different crystal population. A model involving downward infiltration of hot picrite and resulting troctolitic cumulate assimilation is developed here to explain the subsidiary seams. This is based on petrographic observation, quantitative textural measurement and mineral chemical analyses. It is suggested that as the picritic magma was emplaced, downward percolation of this melt occurred into a troctolite mush. The anorthosite represents a layer of almost completely melted troctolite, formed by growth of high-anorthite zones from the contaminated picrite onto residual cumulus plagioclase. Assimilation of large amounts of the troctolite cumulate mush forced the contaminated picrite onto the olivine-spinel cotectic, leading to Cr-spinel crystallisation. Subsequently, the Cr-spinel crystals in the anorthosite have reacted with intercumulus melt over a wider temperature interval and have gained a more Fe 3+-rich composition than the subsidiary seam Mg- and Al-rich Cr-spinels. It is suggested that the separation of a small fraction of immiscible sulphide liquid in the Cr-spinel seams is the result of locally changing SiO 2 and oxygen fugacity at the crystal–melt interface, during the peritectic reaction of some of the Cr-spinel to produce olivine and plagioclase.

Abundant Fe–Ti oxide inclusions in olivine from the Panzhihua and Hongge layered intrusions, SW China: evidence for early saturation of Fe–Ti oxides in ferrobasaltic magma

Abundant Fe–Ti oxide inclusions in cumulus olivine (Fo77–81) from the Panzhihua and Hongge intrusions, Emeishan large igneous province, SW China, document the first evidence for early crystallization of Fe–Ti oxides in ferrobasaltic systems in nature. The intrusions also contain significant stratiform Fe–Ti–V oxide ores. The oxide inclusions are sub-rounded or irregular, range from ∼5 to 50 μm in diameter, and are dominated by either titanomagnetite or ilmenite. The fact that the inclusions are either titanomagnetite- or ilmenite-dominant suggests that they are trapped crystals, instead of immiscible oxide melt, formed during growth of the host olivine. The absence of other silicate phases in the inclusion-bearing olivine is difficult to reconcile with a possible xenocrystic origin of the oxide inclusions. These oxide inclusions are thus interpreted to be cumulus minerals crystallized together and trapped in olivine from the same parental magma. In addition to Fe–Ti oxides, some inclusions contain amphibole + biotite ± fluorapatite that might have formed by reaction of trapped hydrous liquid with the host olivine. Numerical modeling of high-Ti Emeishan basalts using the MELTS program successfully simulates early crystallization of olivine (∼Fo81) and Fe–Ti spinel in the presence of a moderate amount of H2O (∼1.5 wt%) under pressure and fO2 conditions generally pertinent to the Panzhihua and Hongge intrusions. The modal mineralogy of the oxide inclusions is in good agreement with the bulk compositions of the ore, as inferred from whole-rock data, in a given intrusion. This is consistent with the interpretation that the stratiform oxide ores in the intrusions formed by accumulation of Fe–Ti oxide crystals that appeared on the liquidus with olivine and clinopyroxene.

Petrogenesis of olivine-phyric shergottite Yamato 980459, revisited

Primitive magmas provide critical information on mantle sources, but most Martian meteorites crystallized from fractionated melts. An olivine-phyric shergottite, Yamato 980459 (Y-980459), has been interpreted to represent a primary melt, because its olivine megacrysts have magnesian cores (Fo84–86) that appear to be in equilibrium with the Y-980459 whole-rock composition based on Fe–Mg partitioning. However, crystal size distribution (CSD) plots for Y-980459 olivines show a size gap, suggesting a cumulus origin for some megacrysts. Because melting experiments using the Y-980459 whole-rock composition have been used to infer the thermal structure and volatile contents of the Martian mantle, the interpretation that this rock is primitive should be scrutinized. We report major, minor and trace element compositions of Y-980459 olivines and compare them with results from melting experiments (both hydrous and anhydrous) and thermodynamic calculations. Cores of the olivine megacrysts have major and minor element contents identical to those of the most magnesian olivines from the experiments, but they differ slightly from those of thermodynamic calculations. This is probably because the Y-980459 whole-rock composition lies near the limit of the range of liquids used to calibrate these models. The megacryst cores (Fo80–85) exhibit minor and trace element (Mn–Ni–Co–Cr–V) characteristics distinct from other olivines (megacryst rims and groundmass olivines, Fo < 80), implying that the megacryst cores crystallized under more reduced conditions (∼IW + 1). Y-980459 contains pyroxenes with orthopyroxene cores mantled by pigeonite and augite. We also found some reversely zoned pyroxenes that have augite cores (low-Mg#) mantled by orthopyroxenes (high-Mg#), although they are uncommon. These reversely zoned pyroxenes are interpreted to have grown initially as atoll-like crystals with later crystallization filling in the hollow centers, implying disequilibrium crystallization at a moderate cooling rate (3–7 °C/h). The calculated REE pattern of a melt in equilibrium with normally zoned pyroxene is parallel to those of glass and the Y-980459 whole-rock as well as other depleted olivine-phyric shergottites, suggesting that Y-980459 was derived from a depleted mantle reservoir. Considering the CSD patterns of Y-980459 olivines, we propose that the olivine megacrysts are cumulus crystals which probably formed in a feeder conduit by continuous melt replenishment, and the parent melt composition was indistinguishable from the Y-980459 whole-rock with 0–2 wt% of H 2O and 0–5 wt% of CO 2. The final magma pulse entrained these cumulus olivines and then crystallized groundmass olivines and pyroxenes. Although Y-980459 contains small amounts of cumulus olivine (<∼6 vol%), we conclude that the Y-980459 whole-rock composition closely approximates a Martian primary melt composition.

Alkalic parental magmas for chassignites?

Abstract Detailed analysis of cumulate and melt inclusion assemblages in the chassignites provide important constraints on the nature of the melt trapped as inclusions in cumulus olivine (and, by extension, parental magma compositions), the pressures of crystallization, and magmatic volatile contents. These mineral assemblages show strong similarities to the experimental fractionation assemblages that produce the sodic silica‐saturated alkalic lavas on Earth (e.g., Ascension Island, Azores, the Nandewar volcano of Australia). The experimental assemblages were produced from silica‐saturated hawaiite at pressures above 4.3 kbar with dissolved water contents above 0.5 wt%. Such pressures are consistent with Ti:Al ratios of the melt‐inclusion pyroxenes in the Chassigny meteorite. Pyroxene compositions suggest early high crystallization temperatures and thus relatively low initial water and F contents. Feldspars indicate that melt evolution proceeded to rhyolite compositions both within the interstices of the cumulate olivine and within the melt inclusions, even though rhyolitic glass is only found within olivine‐hosted polyphase melt inclusions. The observed rhyolite glass is compositionally similar to the alkali‐rich rhyolite of Ascension Island which is produced experimentally by crystallization of hawaiite. It is proposed that the melt trapped in cumulus olivine of the Chassigny dunite was similar to a terrestrial silica‐saturated hawaiite, while that trapped in olivine of the Northwest Africa (NWA) 2727 dunite was less evolved, perhaps mildly alkalic basalt. Melts similar to terrestrial intra‐plate tholeiite could be parental to the cumulus minerals and evolve upon crystallization at pressures above 4.3 kbar and water contents above ˜0.4 wt% to mildly alkalic basalt, silica‐saturated hawaiite, and alkali‐rich rhyolite. The melt inclusion assemblages are inconsistent with either crystallization of a low‐Al, high‐Fe basalt, or low‐pressure crystallization of a terrestrial‐like tholeiite.

Magmatic lineations inferred from anisotropy of magnetic susceptibility fabrics in Units 8, 9, and 10 of the Rum Eastern Layered Series, NW Scotland

The Eastern Layered Series of the Rum Layered Suite, NW Scotland, comprises a sequence of sixteen (30–150 m thick) cyclic units. The upper troctolite–olivine gabbro parts of each of these units exhibit small-scale modal layering and a pervasive, layer-parallel mineral lamination that is often associated with ‘soft-sediment’ deformation structures. A sporadic, macroscopic magmatic lineation measurable on mineral lamination surfaces is also observed in places. Anisotropy of magnetic susceptibility (AMS) fabrics were studied in three of these cyclic units, (8, 9, and 10) in the northern part of the Eastern Layered Series. Magnetic fabrics measured in the troctolites and gabbros yield one dominant trend in which magnetic foliations parallel magmatic layering and magnetic lineations trend NW–SE and plunge gently. Magnetic fabrics measured for two detailed traverses through Unit 10 on the northern side of Hallival also yield one dominant trend, similar to that measured elsewhere in the Eastern Layered Series. However, toward the centre of Unit 10 in each traverse, magnetic lineations sometimes plunge approximately downdip (SW) on the magnetic foliation planes. The implications of these results are discussed with reference to previous textural and fabric observations on Rum. A model is suggested in which weak linear arrangements of cumulus olivine and plagioclase crystals are developed due to slumping and soft-sediment deformation of unconsolidated crystal mushes during central sagging of the Rum Layered Suite.

Anomalous Reef-Type Platinum-Group Element Mineralisation in the Wiluna Domain, Agnew-Wiluna Greenstone Belt, Western Australia

The ultramafic horizons of the Wiluna domain (Agnew-Wiluna Greenstone Belt, Western Australia) form two main tabular and sheet-like fractionated units, which generally comprise a 5-10m-thick basal pyroxenite layer, a 50-100m-thick meso-to-orthocumulate-textured peridotite horizon, which is overlain by a core of wehrlite, ranging 10-50m in thickness, grading upwards into oikocrystic pyroxenites (30-50m-thick) and upper gabbroic margins (30-50m-thick). Primary textures are exceptionally well preserved: pegmatoidal and dendritic (harrisitic) olivine domains are common in all lithologies, whereas spinifex-textured horizons and flowtop breccias are absent. The fractionation sequence and the geochemical profile of the two ultramafic units are almost indistinguishable, suggesting that the belts may have been structurally duplicated. The ultramafic units display a steady trend towards increasing Fe content up stratigraphy, above a narrow basal zone showing a reversed trend. Calculated Ni contents of cumulus olivines indicate that the magma remained sulfur-undersaturated up to about the middle of the unit, where fractionation of a small proportion of sulfide (R>1000) occurred and depleted the upper fractionated levels in Ni. The presence of various anomalous PGE-enriched horizons (SPt-Pd= 200-300 ppb) in the fractionated pyroxenite layer directly overlying the basal ortho-mesocumulate-textured unit indicates the occurrence of localised weak reef-style PGE mineralisation. Reef-style PGE mineralisation at Wiluna is more similar in stratigraphic setting, style and composition to PGE-rich disseminated Fe-Cu sulfide mineralisation zones within thick differentiated intrusions (e.g. Platinova reef in the Skaergaard intrusion) rather than to other zones in komatiite-hosted systems.

Cyclicity in the Main and Upper Zones of the Bushveld Complex, South Africa: Crystallization from a Zoned Magma Sheet

The major element composition of plagioclase, pyroxene, olivine, and magnetite, and whole-rock Sr/Sr data are presented for the uppermost 2.1 km of the layered mafic rocks (upper Main Zone and Upper Zone) at Bierkraal in the western Bushveld Complex. Initial Sr/Sr ratios are near-constant (0.7073 ± 0.0001) for 24 samples and imply crystallization from a homogeneous magma sheet without major magma recharge or assimilation. The 2125 m thick section investigated in drill core comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite) layers and changes up-section from gabbronorite (An72 plagioclase; Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayalite ferrodiorite (An43; Mg# 5 clinopyroxene; Fo1 olivine). The overall fractionation trend is, however, interrupted by reversals characterized by higher An% of plagioclase, higher Mg# of pyroxene and olivine, and higher V2O5 of magnetite. In the upper half of the succession there is also the intermittent presence of cumulus olivine and apatite. These reversals in normal fractionation trends define the bases of at least nine major cycles. We have calculated a plausible composition for the magma from which this entire succession formed. Forward fractional crystallization modeling of this composition predicts an initial increase in total iron, near-constant SiO2 and an increasing density of the residual magma before magnetite crystallizes. After magnetite begins to crystallize the residual magma shows a nearconstant total iron, an increase in SiO2 and decrease in density. We explain the observed cyclicity by bottom crystallization. Initially magma stratification developed during crystallization of the basal gabbronorites. Once magnetite began to crystallize, periodic density inversion led to mixing with the overlying magma layer, producing mineralogical breaks between fractionation cycles. The magnetitite and nelsonite layers mainly occur within fractionation cycles, not at their bases. In at least two cases, crystallization of thick magnetitite layers may have lowered the density of the basal layer of melt dramatically, and triggered the proposed density inversion, resulting in close, but not perfect, coincidence of mineralogical breaks and packages of magnetitite layers.

Northwest Africa 1950: Mineralogy and comparison with Antarctic lherzolitic shergottites

— NWA 1950 is a new lherzolitic shergottite recently recovered from Morocco and is the first sample of this group found outside Antarctica. Major constituent phases of NWA 1950 are olivine, pyroxenes, and plagioclase glass (“maskelynite”) and the rock shows a two distinct textures: poikilitic and non-poikilitic typical of lherzolitic shergottites. In poikilitic areas, several-millimeter-sized pyroxene oikocrysts enclose cumulus olivine and chromite. In contrast, pyroxenes are much smaller in non-poikilitic areas, and olivine and plagioclase glass are more abundant. Olivine in non-poikilitic areas is more Fe-rich (Fa29–31) and shows a narrower distribution than that in poikilitic areas (Fa23–29). Pyroxenes in non-poikilitic areas are also more Fe-rich than those in poikilitic areas that show continuous chemical zoning suggesting fractional crystallization under a closed system. These observations indicate that pyroxene in non-poikilitic areas crystallized from evolved interstitial melts and olivine was re-equilibrated with such melts. NWA 1950 shows similar mineralogy and petrology to previously known lherzolitic shergottites (ALH 77005, LEW 88516, Y-793605 and GRV 99027) that are considered to have originated from the same igneous body on Mars. Olivine composition of NWA 1950 is intermediate between those of ALH 77005-GRV 99027 and those of LEW 88516-Y-793605, but is rather similar to ALH 77005 and GRV 99027. The subtle difference of mineral chemistry (especially, olivine composition) can be explained by different degrees of re-equilibration compared to other lherzolitic shergottites, perhaps due to different location in the same igneous body. Thus, NWA 1950 experienced a high degree of re-equilibration, similar to ALH 77005 and GRV 99027.

Petrogenesis of the new lherzolitic shergottite Grove Mountains 99027: Constraints of petrography, mineral chemistry, and rare earth elements

We report petrography, mineral chemistry, and microdistribution of rare earth elements (REE) in a new lherzolitic shergottite, Grove Mountains (GRV) 99027. The textural relationship and REE patterns of minerals suggest precipitation of cumulus olivine and chromite, followed by equilibrium crystallization of a closed system with a bulk composition of the inferred intercumulus melt. Subsolidus equilibrium temperatures of pyroxenes and olivine range from 1100 to 1210 °C, based on a two-pyroxene thermometry and Ca partitioning between augite and olivine. Oxygen fugacity of the parent magma is 1.5-2.5 (av. 2.0 ± 0.4) log units below the quartz-fayalite-magnetite (QFM) buffer at 960-1360 °C, according to the olivine-orthopyroxene-chromite barometer. The ilmenite-chromite barometer and thermometer show much wider ranges of oxygen fugacity (1.0-7.0 log unit below QFM) and temperature (1130-480 °C), suggesting subsolidus equilibration of the oxides at low temperatures, probably due to deep burial of GRV 99027 on Mars. The low oxygen fugacity and LREE depletion of the parent magma of GRV 99027 suggest low contamination by martian crust.Characteristics of GRV 99027 demonstrate similarity of lherzolitic shergottites, suggesting a high possibility of launch pairing or a homogeneous upper mantle of Mars if they were ejected by individual impact events. However, GRV 99027 probably experienced severe post-shock thermal metamorphism in comparison with other lherzolitic shergottites, based on the re-crystallization of maskelynite, the homogeneity of minerals, and the low subsolidus equilibrium temperatures between chromite and ilmenite.

Komatiites and nickel sulfide ores of the Black Swan area, Yilgarn Craton, Western Australia. 3: Komatiite geochemistry, and implications for ore forming processes

The Black Swan komatiite sequence is a package of dominantly olivine-rich cumulates with lesser volumes of spinifex textured rocks, interpreted as a section through an extensive komatiite lava flow field. The sequence hosts a number of nickel sulfide orebodies, including the Silver Swan massive shoot and the Cygnet and Black Swan disseminated orebodies. A large body of whole rock analyses on komatiitic rocks from the Black Swan area has been filtered for metasomatic effects. With the exception of mobile elements such as Ca and alkalis, most samples retain residual igneous geochemistry, and can be modelled predominantly by fractionation and accumulation of olivine. Whole rock MgO–FeO relationships imply a relatively restricted range of olivine compositions, more primitive than the olivine which would have been in equilibrium with the transporting komatiite lavas, and together with textural data indicate that much of the cumulus olivine in the sequence was transported. Flow top compositions show evidence for chromite saturation, but the cumulates are deficient in accumulated chromite. Chromite compositions are typical of those found in compound flow-facies komatiites, and are distinct from those in komatiitic dunite bodies. Incompatible trace element abundances show three superimposed influences: control by the relative proportion of olivine to liquid; a signature of crustal contamination and an overprint of metasomatic introduction of LREE, Zr and Th. This overprint is most evident in cumulates, and relatively insignificant in the spinifex rocks. Platinum and palladium behaved as incompatible elements and are negatively correlated with MgO. They show no evidence for wholesale depletion due to sulfide extraction, which was evidently restricted to specific lava tubes or pathways. The lack of correspondence between PGE depletion and contamination by siliceous material implies that contamination alone is insufficient to generate S-saturation and ore formation in the absence of sulfide in the assimilant. Contamination signatures in spinifex-textured rocks may be a guide to Ni-sulfide mineralisation, but are not entirely reliable in the absence of other evidence. The widespread vesicularity of the sequence may be attributable to assimilated water rather than to primary mantle-derived volatiles, and cannot be taken as evidence for primary volatile-rich magmas. The characteristic signature of the Black Swan Succession is the presence of highly localised disseminated sulfide within a sequence showing more widespread evidence for crustal contamination and interaction with its immediate substrate. This has important implications for the applicability of trace element geochemistry in exploration for komatiite-hosted nickel deposits.

Platinum group element geochemistry of komatiites from the Alexo and Pyke Hill areas, Ontario, Canada

Thirty-three whole-rock drill core samples and thirteen olivine, chromite, and sulfide separates from three differentiated komatiite lava flows at Alexo and Pyke Hill, Canada, were analyzed for PGEs using the Carius tube digestion ID-ICP-MS technique. The emplaced lavas are Al-undepleted komatiites with ∼27% MgO derived by ∼50% partial melting of LILE-depleted Archean mantle. Major and minor element variations during and after emplacement were controlled by 30 to 50% fractionation of olivine Fo93–94. The emplaced lavas are characterized by (Pd/Ir)N = 4.0 to 4.6, (Os/Ir)N = 1.07, and Os abundances of ∼2.3 ppb. Variations in PGE abundances within individual flows indicate that Os and Ir were compatible (bulk DOs,Ir = 2.4–7.1) and that Pt and Pd were incompatible (bulk DPt,Pd < 0.2) during lava differentiation, whereas bulk DRu was close to unity. Analyses of cumulus olivine separates indicate that PGEs were incompatible in olivine (DPGEsOl-Liq = 0.04–0.7). The bulk fractionation trends cannot be accounted for by fractionation of olivine alone, and require an unidentified Os-Ir-rich phase. The composition of the mantle source (Os = 3.9 ppb, Ir = 3.6 ppb, Ru = 5.4 ppb, Pt and Pd = 5.7 ppb) was constrained empirically for Ru, Pt, and Pd; the Os/Ir ratio was taken to be identical to that in the emplaced melt, and the Ru/Ir ratio was taken to be chondritic, so that the absolute IPGE abundances of the source were determined by Ru. This is the first estimate of the PGE composition of a mantle source derived from analyses of erupted lavas. The suprachondritic Pd/Ir and Os/Ir of the inferred Abitibi komatiite mantle source are similar to those in off-craton spinel lherzolites, orogenic massif lherzolites, and enstatite chondrites, and are considered to be an intrinsic mantle feature. Bulk partition coefficients for use in komatiite melting models derived from the source and emplaced melt compositions are: DOs,Ir = 2.3, DRu = 1.0, DPt,Pd = 0.07. Ruthenium abundances are good indicators of absolute IPGE abundances in the mantle sources of komatiite melts with 26 to 29% MgO, as Ru fractionates very little during both high degrees of partial melting and lava differentiation.

The geochemical structure of the Insizwa lobe of the Mount Ayliff complex with implications for the emplacement and evolution of the complex and its Ni-sulphide potential

Detailed petrographic, modal and geochemical studies on a number of deep boreholes (exceeding 1.2km in some instances) along the southeastern margin of the Insizwa lobe of the Mount Ayliff Complex reveal the existence of a geochemical stratigraphy in the mafic intrusive rocks. Using combined geochemical and petrographic criteria new definitions are presented for the Basal Zone and Central Zone of Scholz (1936). Within the Basal Zone a coherent sequence of ultramafic rocks (plagioclase lherzolites), previously referred to as picrites, up to 350m thick, occur and are distinguished from associated olivine-rich gabbronorites by a new set of criteria which involves bulk rock composition (MgO>30%) and petrography (cumulus olivine and spinel only). This definition can be consistently applied in all borehole cores. The Basal Zone gabbronorites have variable olivine concentrations and can be distinguished from similar rocks in the overlying central zone by Sr/Zr, Sr/Al 2 O 3 and P/Zr and by overall higher incompatible element contents. The latter implies that Basal Zone rocks are orthocumulates and have retained a higher proportion of their residual liquid than the Central Zone gabbronorites which are mesocumulates. Within the Central Zone the gabbronorite sequence can be subdivided (from the top down) on the basis of geochemistry into a Top Gabbronorite, Cu-Poor Gabbronorite, and Upper Gabbronorite. These subdivisions can be recognized in all borehole cores. In borehole INS96-02, one of the deepest boreholes, the plagioclase lherzolite layer in the Basal Zone is ‘split’ by a sequence of gabbronorites showing chilled contacts and variable effects of thermal metamorphism. Specifically within this zone are two units of gabbronorite which are thermally metamorphosed by Basal Zone gabbronorites and which have chemical characteristics of the Central Zone. This is evidence that the Insizwa lobe was emplaced from the top down with intrusion of magmas which crystallized Central Zone gabbronorites early, followed by emplacement of Basal Zone magmas. The composition of olivine (%Fo, Ni/MgO) mimics whole-rock Mg-number and Ni/MgO variation through the sequence. Specifically olivines from the Upper Gabbronorite and the Cu-poor Gabbronorite are depleted in Ni relative to MgO, whereas olivine from the other units has ‘normal’ Ni/Mg ratios. The thick Taylor’s Koppie dyke to the E of Insizwa is built of olivine gabbronorites with variable olivine contents and scattered blebs of sulphide and is a potential feeder to the Insizwa sequence. It has whole-rock geochemical signatures and Ni-depleted olivines which are very similar to the Upper Gabbronorite of the Central Zone. The wide compositional range exhibited by contact margin samples at various levels in the Basal Zone indicates that this zone was built from doleritic magma variably charged with olivine phenocrysts and which had a wide range of incompatible element ratios and Sr-isotopic compositions. The Insizwa lobe was an open magmatic system formed by multiple intrusive events of compositionally variable magmas.

Compositional variations of olivine from the Jinchuan Ni–Cu sulfide deposit, western China: implications for ore genesis

The Jinchuan Ni–Cu sulfide deposit is hosted by an elongated, olivine-rich ultramafic body that is divided by subvertical strike-slip faults into three segments (central, eastern, and western). The central segment is characterized by concentric enrichments of cumulus olivine crystals and interstitial sulfides (pyrrhotite–pentlandite–chalcopyrite intergrowth), whereas the eastern and western segments are characterized by an increase of sulfides toward the lower contacts. In all segments sulfides are concentrated at the expense of intercumulus silicates. Olivine re-crystallization is found to be associated with actinolite alteration in some samples. The compositional variations of primary olivine from the sulfide-poor samples can be explained by a small degree of olivine crystallization ( 50 modal%) and sulfide (averaging ~5 wt%) in the rocks are consistent with the interpretation that the Jinchuan deposit was formed by olivine- and sulfide-laden magma successively ascending through a conduit to a higher, now-eroded, level. Sulfide enrichment toward the center in the central segment and toward the lower contact in the eastern and western segments may have, in part, resulted from flow differentiation and gravitational settling during magma ascent, respectively.

Hydromagmatic amphibole in komatiitic, tholeiitic and ferropicritic units, Abitibi greenstone belt, Ontario and Qu�bec: evidence for Archaean wet basic and ultrabasic melts

¶Detailed petrographic, electron microprobe and ion probe studies of Archaean hydromagmatic amphiboles from the Abitibi greenstone belt, Canada, yield new insights into the origin of Al-undepleted komatiitic and Al-depleted tholeiitic and ferropicritic melts. The amphiboles are present in peridotite layers and basal chill zones of thick differentiated basic and ultrabasic sills and flows, and are titanian pargasite–hastingsite in composition. They can be grouped into two petrographic types: (1) amphibole in the groundmass; and (2) amphibole-bearing melt inclusions. The groundmass amphiboles are oikocrysts, rims and interstitial grains, present in minor to major amounts. The oikocrysts host cumulus olivines (Fo83–84) that are rounded in shape, embayed, and smaller in size. The amphibole-bearing melt inclusions are hosted in cumulus olivines (Fo83–84 in komatiitic rocks and Fo79 in tholeiitic rocks), spherical to ovoid in shape, 50–500 µm in size, and dominated modally by amphibole. The melt inclusions also contain euhedral chromite and aluminous spinel and micrometric clinopyroxene and glass, and sub-micrometric iron–nickel sulphide, chloro-apatite and ilmenite. In-situ ion probe analyses indicate the amphibole is: (1) enriched in Nb, LREE and Zr and depleted in Sr and HREE relative to primitive mantle; (2) contains up to 1–3 wt% H2O; and (3) overall displays δD values from 50‰ to −140‰, including many values in the accepted magmatic range of −60‰ to −90‰.

An occurrence of igneous orthorhombic amphibole, Eriksberg gabbro, southern Sweden

We document the first reported igneous occurrence of the orthoamphibole, gedrite, from a troctolitic cumulate in the Eriksberg gabbro, Sweden. The gedrite is sodic and coexists in textural equilibrium with plagioclase, orthopyroxene, Ca-amphibole, and Na-rich phlogopite. The gedrite crystallized from evolved interstitial liquid that was buffered by cumulus olivine. Geothermometry indicates that the assemblage equilibrated at around 900 °C and 4-6 kbar. Gedrite shows a convex- downward REE-profile, with a positive Eu anomaly. The rare earth element concentrations are ~0.2 to 0.8 times chondritic abundances and much lower than in coexisting Ca-amphibole. Laser ablation ICP-MS analyses show that the gedrite has much lower concentrations of Zr, Nb, Th, and U, and lower concentrations of Sr, Ba, Rb, and Pb relative to co-crystallizing Ca-amphibole. However, nickel and cobalt partition preferentially into gedrite. Comparison of Zr and Nb concentrations in early crystallized Ca-amphibole with gedrite and associated Ca-amphibole reveals that Xmg of amphibole is not the best criteria to use for estimates of (DAmpf/LNb/Zr ) for cumulates where olivine is buffering the Mg/(Mg + Fe).

Formation of hybrid cumulates: melatroctolites in Intrusion 4 of the Honningsvåg Intrusive Suite, northern Norway

New mapping shows that the 1160-m-thick Layered Series of the mafic to ultramafic Intrusion 4 of the Honningsvåg Intrusive Suite, North Norwegian Caledonides, comprises approximately 25% feldspathic peridotite, 55% melatroctolite (olivine-rich troctolite), 5% troctolite, <2% olivine melagabbro (olivine-rich olivine gabbro) and 13–15% olivine gabbro. The cumulate sequence can be divided into eight macrocyclic units, the base of each unit resulting from magma replenishment. Compositions of olivine and plagioclase obtained from ca. 1700 electron microprobe point analyses, display complex and limited variations (Fo 87–74 and An 80–67) relative to stratigraphic height. The origin of olivine-rich cumulates, such as melatroctolites (52–72% olivine) and olivine melagabbros (29–37% olivine), in large quantities, is difficult to explain by either simple fractional crystallisation or conventional layer-forming processes. A new genetic model is proposed involving suspension of pre-existing cumulus crystals during magma recharge in a periodically replenished magma chamber, their partial resorption and simultaneous crystallisation of liquidus olivine. Cumulates formed in this way represent mixtures of crystals that crystallised from compositionally different magmas and are, therefore, termed hybrid cumulates. Apart from their distinctly non-cotectic bulk modal proportions, the hybrid nature of the olivine-rich cumulates is evidenced by partially resorbed plagioclases enclosed within cumulus olivines. The occurrence of resorbed plagioclase inclusions in olivine gabbros and large variations in modal plagioclase content suggest that these may also be hybrid cumulates.

Spinel-bearing symplectites in Palaeoproterozoic ultramafic rocks from two different geological settings in Finland: Thermobarometric and tectonic implications

Spinel-bearing symplectites and reaction rims after olivine and plagioclase are common in metamorphic mafic rocks. Two such occurrences in Palaeoproterozoic rocks in Finland are described and their conditions of metamorphism calculated. Both occur in olivine cumulates between cumulus olivine and intercumulus plagioclase, and were formed at ca. 1.9 Ga during the Svecofennian orogeny. They, nevertheless, represent quite different tectonic environments, as is reflected in their mineralogy. The first is from a Svecofennian tholeiitic intrusion at Saarijärvi in SE Finland. This intrusion is assigned to juvenile island arc setting with a MORB-type substrate, and the symplectite was formed under low-P/high-T granulite-facies metamorphism during the accretion of the island arc to a continental margin following emplacement and crystallization of the intrusion. The reaction rim consists of orthopyroxene against olivine and pargasite-spinel symplectite against the plagioclase side. The calculated PT-conditions are 900°C and 5-6 kbar. The other rock unit is located at the margin of the Lapland granulite belt, which constitutes a Palaeoproterozoic mobile collision zone between two Archaean blocks or microcontinents. The origin of the intrusion and its primary age is not clear, but metamorphism took place under relatively high pressures, either at the crust-mantle boundary during 1.9 Ga metamorphism of the granulite belt or slightly afterwards in a subduction zone beneath the overthrust granulite belt and Inari-Kola craton. This symplectite consists of orthopyroxene in the olivine side and pargasite-spinel and clinopyroxene-spinel symplectite on the plagioclase side. Calculated PT conditions are 670°C and 11–12 kbar.