Cumulate Phases Research Articles

Lower crustal control in the iron isotope systematics of plutonic xenoliths from Adak Island, Central Aleutians, with implications for arc magma geochemistry

We present bulk-rock and mineral Fe isotope data of ultramafic to mafic xenoliths and basaltic to andesitic lavas from Adagdak Volcano (Adak Island, Central Aleutians) to study the effects of early differentiation on the Fe isotopic evolution of island arc basalts and their crystallization products. The Fe isotope composition of ultramafic cumulate xenoliths increases from dunite (δ56Fe = –0.09 to –0.02 ‰) to clinopyroxenite (δ56Fe = +0.06 to +0.09 ‰), consistent with higher modal proportions of clinopyroxene (δ56Fe = –0.05 to +0.11 ‰) relative to olivine (δ56Fe = –0.10 to +0.06 ‰) in the latter. Mid-crustal cumulate amphibole gabbro and hornblendite cumulates also record heavier Fe isotope compositions (δ56Fe = +0.04 to +0.08 ‰) due to the abundance of isotopically heavy amphibole (δ56Fe = +0.07 to +0.09 ‰) and magnetite (δ56Fe = +0.11 to +0.13 ‰) in these rocks. High inter-mineral fractionations observed in spinel-olivine and spinel-clinopyroxene pairs (Δ56Fespl-ol = +0.12 to +0.28 and Δ56Fespl-cpx = +0.06 to +0.19) suggest that spinel is not recording equilibrium crystallization conditions for the ultramafic assemblages, likely due to subsolidus Fe-Mg exchange. Our data also include Fe isotope measurements of one mantle dunite (δ56Fe = +0.03 ± 0.05 ‰). Five Adagdak lavas, spanning from basalts to andesites, yield a narrow range of δ56Fe between +0.03 and +0.06 ‰. Our results highlight the potential of amphibole in driving the Fe isotope depletion trends observed in many erupted arc lavas, as amphibole hosts 28–99 % of the FeOT budget in the amphibole gabbro and hornblendite cumulates. This is also supported by single-crystal synchrotron Mössbauer spectroscopy of two amphibole grains, the first from an amphibole gabbro and the second from a hornblendite, which yield Fe3+/ΣFe ratios of 0.55 ± 0.06 and 0.58 ± 0.02, respectively. Water content and hydrogen isotope compositions determined by secondary-ion mass spectrometry from the same amphibole grains indicate partial dehydrogenation. Using Rayleigh fractionation modeling to account for oxidation during post-crystallization dehydrogenation, we calculate magmatic Fe3+/ΣFe ratios of 0.41 ± 0.04 for the amphibole gabbro and 0.30 ± 0.05 for the hornblendite. These data are then used to estimate an appropriate Fe force constant for Adagdak amphibole and quantitatively evaluate the effects of amphibole fractionation. Through a fractional crystallization model, we show how arc melts may experience periods of increasing δ56Fe during olivine-dominated fractionation, followed by decreasing δ56Fe once magnetite and amphibole saturate as cumulate phases. Notably, this dichotomy between fractionation of isotopically light versus heavy cumulate assemblages and its effects on the Fe isotope evolution of arc magmas is not captured by the Adagdak lava record, highlighting the utility of cumulates in chronicling the early isotopic evolution of magmatic systems.

Geochemical diversity of continental arc basaltic mushy reservoirs driven by reactive melt infiltration

The reactive melt flow emerges as an important factor for diversification of basaltic magmatic reservoirs, but whether and how it influences continental arc basaltic mushes are enigmatic. Here, we used mineral and whole-rock geochemistry to examine the petrogenesis of a suit of mafic and intermediate plutons in western Yangtze Block, which were emplaced at continental arc crust and primarily had plagioclase and clinopyroxene as early cumulate mineral phases. We found the crystal mushes were infiltrated by externally-derived reactive melt with high δ18O and fertile crustal signatures, resulting in the changes of mineral phases (e.g., clinopyroxene transformed to hornblende) and bulk-rock geochemistry (including isotopes). Then, the reacted granitic melt was prone to either be extracted from or stall in the crystal mushes, generating quartz-poor (mafic) or quartz-rich (intermediate) plutons, respectively. This study supports the reactive melt infiltration may serve as an important engine for compositional diversity of basaltic mush system within continental arc settings.

Petrogenesis of oxide-apatite mineralization associated with proterozoic anorthosite massifs at Lac Mirepoix, Quebec, Canada: A multi-injection model for Fe-Ti-P mineralization in the Central Grenville Province

The Lac Mirepoix Fe-Ti-P mineralization presents several mineralized lenses that outcrop near the border of the 1080 (±2) Ma Vanel and the 1016 (±2) Ma Mattawa Anorthosite massifs, in the Central Grenville Province, Quebec, Canada. The mineralization is (hemo)-ilmenite-dominated, accompanied by magnetite and apatite. It is subdivided in three different zones due to the appearance of different cumulate phases: zone I comprises mainly massive oxide (>70 % hemo-ilmenite ± magnetite) layers hosted in anorthosite. Towards the center of the mineralization (zone II), massive oxide layers are less common whereas apatite-bearing cumulates appear, forming massive nelsonite (50–70 % magnetite ± ilmenite and 25–30 % apatite) and oxide apatite norite (OAN, 15–25 % hemo-ilmenite ± magnetite and 8–20 % apatite). Finally, zone III is marked by the alternance of OAN layers (10–25 m), richer in magnetite, in addition to (hemo)-ilmenite and apatite, and an absence of massive oxide and nelsonite. In-situ trace element analysis of plagioclase, apatite and oxides by laser ablation ICP-MS, reveals cryptic variations related to magma differentiation and multiple injections of ferrodiorite parental magmas of similar composition. In-situ U-Pb dating of zircon from the OAN mineralization itself indicates two different crystallization ages between zone III (1048 ± 8 Ma) and zone I (964 ± 9 Ma), favouring a model of multiple injections of similar Fe-Ti-P-rich melts of similar composition over a period of 80 Mys rather than in-situ crystallization of a single intrusion, which is supported by trace-element geochemistry. The Lac Mirepoix mineralization records the following fractional crystallization sequence of a high-Ti-P magma, residual after the anorthosite formation: first, massive oxides of hemo-ilmenite crystallized (high Ti/Fe) by oxide settling, with primitive compositions, similar to the world-class Lac Tio deposit. The oxide-apatite mineralization (nelsonite and OAN) crystallized from the residual liquid (lower Ti/Fe, evolved compositions) in which magnetite and apatite were more abundant, similar to nearby mineralization in the area (e.g. Lac à l’Orignal Fe-Ti-P deposit). The observed decrease in Ti content of the evolving melt is supported by the liquid line of descent of several ferrodiorite dykes within the host-rocks and mineralization. Finally, we propose a model of multiple injections of residual Fe-Ti-P-rich liquids drained from or filter-pressed within diapirs of plagioclase-rich mushes that were emplaced over 80 Mys apart along the same crustal detachment zone.

Developing an Integrated Petrogenetic Model for Understanding REE Deportment of the Ampasibitika Intrusion and Associated Ion Adsorption Deposits

Abstract Alkaline–peralkaline igneous systems are promising sources of rare earth elements (REEs). Preservation bias has resulted in a gap in the geological record for alkaline–peralkaline magmatic systems, with the hypabyssal plumbing system linking magma chambers to extrusive volcanic rocks poorly represented. Large plutonic varieties of these systems are often proposed to have fed (now eroded) volcanoes, and current peralkaline volcanic systems obscure the plutonic system at depth. The alkaline to peralkaline Ampasibitika Intrusion in Madagascar is a rare example where the magmatic–volcanic interface between a deeper level magma reservoir and its genetically related caldera volcano is exposed. This c. 24 Ma sub-volcanic intrusive system comprises silica-undersaturated to silica-oversaturated units, of peralkaline to metaluminous and peraluminous characters, with varying styles of REE mineralisation, including supergene ion adsorption-style REE occurrences in the overlying weather profiles. There are two main intrusive suites: (1) the concentric Marginal Dyke Swarm (MDS) formed of quartz–microsyenite and peralkaline granite dykes (PGDs), and (2) the Ampasibitika Ring Dyke (ARD) comprising alkali feldspar syenites and subordinate nepheline syenites, trachytes and phonolites. We present new field observations and geochemical data to indicate that the MDS was emplaced as a series of low-viscosity, volatile-rich melt batches, which coalesced in the magma reservoir roof zone and intruded prior to caldera collapse, whereas the ARD was emplaced into the ring fault as a heterogeneous mix of variably evolved syenitic crystal mushes and phonolitic to trachytic-melt batches. As such, we suggest the MDS represents the residual melt fraction of the magma reservoir, whereas the ARD contains portions of the fractionating, silica-neutral to silica-undersaturated syenite, cumulate assemblage. In this revised framework, we assess the major and trace element geochemistry of amphibole- and clinopyroxene-group minerals to gain insight into the magmatic evolution of the Ampasibitika Intrusion and partitioning of REE between early cumulate and residual melt phases. Ultimately, the most REE-enriched units, the PGDs of the MDS, are identified as the product of the most volatile-rich, highly evolved melts from the roof zone of the magma reservoir. However, although REE enriched, the mineralogy does not always enable efficient release of REE for ion adsorption-style mineralisation; instead, lower REE-content protoliths with REE-host phases more amenable to decomposition release a greater proportion of REE.

The kabr El-Bonaya peridotites, Southeastern Sinai, Egypt: petrology, geochemistry, and metamorphism of Neoproterozoic arc ultramafic cumulates

Two small, isolated ultramafic masses in the northeastern part of the Wadi Kid area, southeast Sinai, are composed of variably serpentinized harzburgite and lherzolite with minor talc-anthophyllite rock. The primary phases are dominantly olivine, orthopyroxene and Cr-spinel; clinopyroxene, amphibole, and phlogopite are also found in lherzolite samples. The whole-rock Mg# of harzburgite samples (89–91) is higher than that of lherzolite (average 82). The harzburgite samples contain olivine with higher Mg and Ni contents, orthopyroxene with higher Mg#, and Cr-spinel with higher Cr content than do the lherzolite samples. The REE patterns of clinopyroxene and amphibole in lherzolite are most consistent with a cumulate origin. Although several compositional characteristics of the harzburgites resemble those of residual mantle, in detail the Cr₂O₃ and Al₂O₃ contents of fresh Cr-spinel in harzburgite are different from those found in mantle samples or in any of the Neoproterozoic ophiolitic peridotites throughout the Arabian-Nubian Shield. Thus, all the ultramafic rocks at Kabr El-Bonaya are best explained as ultramafic cumulates, with harzburgite consisting of early-formed cumulate phases and lherzolite containing later-formed cumulate phases with higher REE abundances, primary hydrous minerals, evolved primary silicates, and high TiO₂ (0.77 wt.%) and Al₂O₃ (18 wt.%) contents in Cr-spinel. The trace-element characteristics of the rocks indicate a subduction-related parental magma: whole-rock chondrite-normalized REE patterns are LREE-enriched; calculated <i>f</i>O₂ values are elevated (+2.47 to +3.39 log units above the fayalite-magnetite-quartz buffer); and computed N-MORB-normalized trace element patterns for melts in equilibrium with clinopyroxene and amphibole have negative Nb-Ta anomalies and enrichment in large-ion lithophile elements. The low Al₂O₃/SiO₂ ratios (0.007–0.040) of harzburgite samples and the low TiO₂ contents and high Cr# of their Cr-spinel indicate derivation from a mantle source that experienced high-degree partial melting. From these characteristics, we infer a boninitic parental melt for the harzburgite. We offer an illustrative quantitative fractionation model that can explain the successive derivation of harzburgite and lherzolite cumulates along a single equilibrium, polybaric cooling path. We conclude that the Kabr El-Bonaya ultramafic cumulates represent the exposed roots of a Neoproterozoic island arc that was caught in the collision between East and West Gondwana.

An experimental investigation of F, Cl and H2O mineral-melt partitioning in a reduced, model lunar system

It is believed that the Moon formed following collision of a large planetesimal with the early Earth. Over the ∼4 Gyr since this event the Moon has been considerably less processed by geological activity than the Earth, and may provide a better record of processes and conditions in the early Earth-Moon system. There have been many studies of magmatic volatiles such as H, F, Cl, S and C in lunar materials. However, our ability to interpret variable volatile contents in the lunar sample suite is dependent on our understanding of volatile behaviour in lunar systems. This is currently constrained by limited experimental data. Here, we present the first experimental mineral-melt partitioning coefficients for F, Cl and H2O in a model lunar system under appropriately reduced conditions (log fO2 to IW-2.1, i.e. oxygen fugacity down to 2.1 log units below the Fe-FeO buffer). Data are consistent with structural incorporation of F, Cl and OH− in silicate melt, olivine and pyroxene under conditions of the lunar mantle. Oxygen fugacity has a limited effect on H2O speciation, and partitioning of H2O, F and Cl is instead largely dependent on mineral chemistry and melt structure. Partition coefficients are broadly consistent with a mantle source region for lunar volcanic products that is significantly depleted in F, Cl and H2O, and depleted in Cl relative to F and H2O, compared to the terrestrial mantle. Partitioning data are also used to model volatile redistribution during lunar magma ocean (LMO) crystallisation. The volatile content of lunar mantle cumulates is dependent upon proportion of trapped liquid during LMO solidification. However, differences in mineral-melt partitioning during LMO solidification can result in significant enrichment on F relative to Cl, and F relative to H2O, in cumulate phases relative to original LMO composition. As such, Cl depletion in lunar volcanic products may in part be a result of LMO solidification.

Architecture of a Super-sized Magma Chamber and Remobilization of its Basal Cumulate (Peach Spring Tuff, USA)

Abstract Using a combination of petrological and geochemical approaches, we investigate processes prior to and during eruption of the Miocene supereruption of the Peach Spring Tuff (PST; Arizona–California–Nevada), including those leading to assembly and destruction of its reservoir(s). We compare the dominant high-silica rhyolite outflow of the PST with the sparsely exposed but distinctive crystal-rich trachyte capping unit, which matches intracaldera trachyte in composition, texture, and phenocryst content. The details of the diverse glass chemistry in fiamme and pumice in the capping unit, coupled with glass compositions in the rhyolite outflow and phase chemistry in general, illuminate critical aspects of chamber geometry, conditions, and processes at the onset of the supereruption. Our results are consistent with a relatively simple single-chamber reservoir for the PST where the crystal-poor, high-silica rhyolite portion directly overlies a mushy, cumulate base. Rhyolite-MELTS phase-equilibria and amphibole geobarometers indicate that the high-silica rhyolite was extracted from its cumulate mush at a depth of ∼9·5–11 km (∼260–300 MPa) and subsequently stored and crystallized at ∼7·0–8·5 km (190–230 MPa). Three types of glass are distinguishable in PST pumice: trachyte (Trg; ∼68 wt% SiO2), low-silica rhyolite (LSRg; ∼72), and high-silica rhyolite (HSRg; ∼76·5). As many as three discrete, complexly mingled glasses are present in single trachyte fiamme. Trace element concentration profiles in sanidine and plagioclase phenocrysts from both the trachyte and HSR support growth from multiple distinct melts (Trg, LSRg, and HSRg). Glasses in trachyte fiamme have zircon saturation temperatures ≥100 °C higher than HSR glasses (850–920 vs ∼770 °C) and compositions indicating dissolution of cumulate phases: very high Zr and Zr/Hf (zircon), REE (chevkinite and titanite), Ba and Sr (feldspars), and P (apatite). Dominant processes of crystal accumulation in the formation of a mushy base, followed by efficient melt extraction, led to the formation of the voluminous high-silica rhyolite melt-rich body overlying a residual cumulate of trachytic composition. This was followed by heating, partial dissolution, and remobilization of the basal cumulate. This history is reflected in the contrasts that are evident in the PST (elemental compositions of pumice, phenocrysts, and glasses; crystal-fraction; temperatures). Reheating was presumably a result of injection of hot mafic magma, but isotopic uniformity of trachyte and rhyolite indicates minimal chemical interaction with this magma. Variability in dissolution textures in phenocrysts in the trachyte, revealed by resorbed and embayed shapes, and the large range of glass trace element concentrations, together with variable temperatures recorded in glasses by zircon and apatite saturation thermometry, suggest that heat transfer from the hotter rejuvenating magma was unevenly distributed. The late-stage heating event probably contributed to the onset of eruption, providing the thermal energy necessary to reduce the crystal fraction within the cumulate below the mechanical lock point. We estimate ∼50 % of the original cumulate phenocrysts dissolved before eruption, using Rhyolite-MELTS and trace element modeling. Sharp contacts with micron-scale compositional gradients between contrasting glass types in individual trachyte fiamme suggest that juxtaposition of contrasting magmas from different parts of the reservoir occurred during eruption.

Rapid orthopyroxene growth induced by silica assimilation: constraints from sector-zoned orthopyroxene, olivine oxygen isotopes and trace element variations in the Huangshanxi Ni–Cu deposit, Northwest China

The Permian Huangshanxi Ni–Cu-hosted intrusion, located in the southern Central Asian Orogenic Belt in NW China, is dominated by cumulus olivine and orthopyroxene. Various types of compositional zonings in cumulate phases were studied using high-resolution synchrotron X-Ray fluorescence imaging, electron microprobe, laser ablation inductively coupled plasma mass spectrometry and secondary ion mass spectrometry, to shed light on the relationship between the orthopyroxene-rich cumulate and sulfide accumulation. Normal, oscillatory, and sector zonings of Cr in orthopyroxene from both lherzolites and olivine websterites were noted, likely due to the different cut orientation of orthopyroxene crystals composed of sector-zoned cores and reverse-zoned rims. Significant Fe–Mg–Al–Si variations found in the exterior regions of the grains do not correspond to the Cr zoning. However, Cr content is positively correlated with Ti–Al contents in interior regions of orthopyroxene, reflecting coupled charge substitution during the development of sector zoning, and indicating diffusion-modified Fe–Mg–Al–Si variation in the exterior regions. Primary Cr-sector zoning in the core is likely the result of rapid in situ growth in a boundary layer at the cumulus stage, probably induced by Si addition from wall rocks and crystallization under supersaturated conditions. In addition, some olivine shows Mg–Fe–Mn–Ca–Cr variations in the rim but not in the core, and both core and rim of olivine show a uniform O isotope composition, illustrating that they have not been modified by post-cumulus processes. Given the distinct difference in O isotopic signatures between mantle-derived melts and crustal materials, olivine O isotope composition could be used to decipher the relative degree of Si addition at crystallization. The Fo value, O isotopic signature and trace element content in the cores of olivine grains from different depths show systematic variations, dividing the sulfide-bearing cumulate into two intervals; the lower and upper lherzolite zones with harzburgitic rocks at the contact. In the lower zone, the olivine δ18O values increase upward from ~ 6 to ~ 7‰ and the oxygen fugacity decreases from QFM to QFM-2, suggesting increasing contamination by more reduced crustal materials. Over the same interval, the modal proportion of orthopyroxene and modal orthopyroxene/olivine ratio increase with increasing depth with little variation in whole-rock Mg#, clearly illustrating that the non-cotectic orthopyroxene proportions resulted from Si addition from the wall rock. In the upper zone, the decrease in δ18O values, and increase in olivine nucleation density, Fo value and oxygen fugacity suggest there was a pulse of sulfide- and olivine-charged magma into the magma chamber, that mixed with resident magma to form a hybrid bottom layer in contact with the early-crystallized orthopyroxene-rich cumulate pile. The sulfide content decreases up-section in the lower zone, and then increases again towards the top of the cumulate sequence. This, combined with the compositional variations with depth, suggests that sulfides in the Huangshanxi Ni–Cu deposit settled downward during at least two sulfide-loaded magma pulses. The fast-growing orthopyroxene-rich cumulate may have acted as a permeability barrier, preventing sulfide percolation into the lower cumulate formed during the first magma pulse. Overall, sector zoning in orthopyroxene and compositional variations in olivine suggest that Si and reduced material addition during contamination is of great importance in forming magmatic Ni–Cu deposits hosted by olivine- and orthopyroxene-dominated intrusions.

Sulfide melt inclusions associated with magmatic Ni-Cu-platinum-group element (PGE) mineralization in the Caribou Lake Gabbro, Blatchford Lake intrusive suite, Northwest Territories, Canada

Magmatic Fe-Ni-Cu-Co sulfide melt inclusions are preserved within cumulate and intercumulate phases (apatite, plagioclase, olivine, ilmenite) in the mineralized Caribou Lake Gabbro, a marginal subalkaline-alkaline mafic-ultramafic rock assemblage that forms part of the multiphase Blatchford Lake intrusive suite, Northwest Territories, Canada. Three distinct styles of sulfide mineralization occur in the Caribou Lake Gabbro: (i) disseminated sulfides, (ii) semi-massive to massive sulfides, and (iii) trace sulfide “platinum-group element (PGE)-enriched” mineralization. Within all mineralization styles, the major mineral assemblage consists of pyrrhotite + chalcopyrite + pentlandite + magnetite + ilmenite. Trace metal phases identified in the disseminated and semi-massive to massive mineralization comprise sphalerite, galena, glaucodot-gersdorffite solid solution [(Co,Fe)AsS-NiAsS], molybdenite, tellurobismuthite (Bi2Te3), altaite (PbTe), electrum, nickeline, melonite (NiTe2), empressite (AgTe), and unidentified Pb-Se-Te, Bi-Te, and Pb-Te phases. No platinum-group minerals (PGM) were observed in any mineralization styles. Within coarse-grained olivine gabbro sulfide melt inclusions of primary origin occur in olivine and ilmenite, and secondary inclusions occur in plagioclase. Secondary sulfide melt inclusions also occur in cumulus apatite in pegmatitic gabbro. Sulfide melt inclusions consist predominantly of pyrrhotite, with lesser amounts and variable proportions of chalcopyrite, and cobaltian pentlandite (∼31–38 wt% Co), and magnetite. In a minority of inclusions, sphalerite, magnetite, and glaucodot-gersdorffite were also observed however, no PGM were observed in the sulfide melt inclusions.The metal tenors of sulfide melt inclusions and sulfide mineralization are comparable with respect to Cu concentration, with sulfide melt inclusions containing between 0.06 and 2.7 wt%, and mineralization containing between 0.54 and 2.9 wt% (recalculated to 100% sulfide). Cobalt concentrations are also similar in sulfide melt inclusions (950–4400 ppm) and mineralization (1350–3500 ppm). In contrast to Cu and Co, Ni is enriched in mineralization (0.94–2.1 wt%) compared to sulfide melt inclusions (0.13–0.59 wt%). The difference in Ni concentration suggests that the inclusions trapped a residual sulfide melt that had been compositionally modified by olivine crystallization and associated “trapped liquid shift effect” prior to inclusion entrapment. Notably, PGE concentrations in sulfide melt inclusions are very low (i.e., below limits of detection of LA-ICPMS; < ∼4 ppm), consistent with the observation of low PGE grades in the associated mineralization styles, and 1–2 orders of magnitude lower than in sulfide melt inclusions reported in economically mineralized layered mafic-ultramafic intrusions.Using Cu and Pd bulk rock compositions for various mineralization styles, together with estimates of starting magma composition (S-undepleted tholeiite, and local, fine-grained, fresh gabbro), R-factors were estimated numerically. S-undepleted tholeiite (from published data) is too Cu- and Pd-rich to be a suitable starting magma composition to source disseminated and semi-massive to massive sulfide, and would require an initial (earlier) stage of metal depletion. Using the local gabbro as a starting magma composition estimate, the metal tenors of disseminated, and semi-massive to massive sulfide mineralization are estimated to have resulted from an R-factor of <100. On the other hand, trace sulfide “PGE-enriched” mineralization could have formed from a S-undepleted tholeiite at an R-factor of at least 1000. It does not appear likely that all mineralization styles in the Caribou Lake Gabbro formed from the same starting silicate melt composition.Low metal tenors of sulfide mineralization within the Caribou Lake Gabbro are likely the result of a magmatic system with a low R-factor, supported by low Ni in olivine and modelling of Cu and Pd systematics. In this case, sulfide melt inclusions predict the sub-economic PGE tenor of the sulfide mineralization as being a primary feature rather than the result of post-solidus downgrading by later metamorphic or hydrothermal fluids. More broadly, sulfide melt inclusions may serve as a proxy for sulfide metal tenor at different stages in the history of a magmatic system, providing a potentially valuable tool for exploration where post-entrapment changes may have modified bulk metal tenors, or where sulfide mineralization has not yet been located.

Petrogenesis of the Dalongkai ultramafic-mafic intrusion and its tectonic implication for the Paleotethyan evolution along the Ailaoshan tectonic zone (SW China)

Layered ultramafic-mafic intrusions are usually formed in an arc/back-arc or intra-plate tectonic environment, or genetically related to a mantle plume. In this paper, we report on an ultramafic-mafic intrusion, the Dalongkai intrusion in the Ailaoshan tectonic zone (SW China), whose occurrence is closely associated with arc/back-arc magmatic rocks. The Dalongkai intrusion is composed of plagioclase-lherzolite, hornblende-peridotite, lherzolite and wehrlite at the bottom, cumulate plagioclase-pyroxenite at the middle part, changing to fine-grained gabbro towards the upper part of the intrusion, forming layering structure. Zircons from the plagioclase-pyroxenites and gabbros yielded U-Pb ages of 272.1±1.7Ma and 266.4±5.8Ma, respectively. The plagioclase-pyroxenites show cumulate textures, and are characterized by high MgO (25.0–28.0wt.%; mg#=80.6–82.3), Cr (1606–2089ppm) and Ni (893–1203ppm) contents, interpreted as early cumulate phases. By contrast, the gabbros have relatively lower mg# values (56.3–62.7), and Cr (157–218ppm) and Ni (73–114ppm) concentrations, and may represent frozen liquids. The plagioclase-pyroxenites and gabbros share similar chondrite-normalized REE patterns and primitive mantle-normalized trace element profiles which are analogous to those of typical back-arc basin basalts. The εNd(t) values for both rock types range from +2.20 to +4.22. These geochemical and isotopic signatures suggest that the Dalongkai ultramafic-mafic rocks originated from a MORB-like mantle source metasomatized by subduction-related, sediment-derived fluids. Our data, together with other geological evidence, indicate that the emplacement of the Dalongkai ultramafic-mafic intrusion most likely occurred in a back-arc extensional setting associated with subduction of the Ailaoshan Paleotethyan branch ocean during the Middle Permian, thus ruling out the previously speculated linkage to the Emeishan mantle plume, or to an intra-continental rift.

Kyanite/corundum eclogites from the Kaapvaal Craton: subducted troctolites and layered gabbros from the Mid- to Early Archean

An oceanic crustal origin is the commonly accepted paradigm for mantle-derived eclogites. However, the significance of the aluminous members of the eclogite suite, containing kyanite and corundum, has long been underrated and their role neglected in genetic models of cratonic evolution. Here, we present a geochemical and petrological study of a suite of kyanite- and corundum-bearing eclogites from the Bellsbank kimberlite, S. Africa, which originate from depths between 150 and 200 km. Although clearly of high-pressure provenance, these rocks had a low-pressure cumulative origin with plagioclase and olivine as major cumulate phases. This is shown by the very pronounced positive Eu anomalies, low REE abundances, and δ 18O values lower than the Earth’s mantle. Many chemical features are identical to modern-day troctolitic cumulates including a light REE depletion akin to MORB, but there are also distinguishing features in that the eclogites are richer in Na, Fe, and Ni. Two of the eclogites have a minimum age of ~3.2 Ga, defined by the extremely unradiogenic 87Sr/86Sr (0.7007) in clinopyroxene. Phase equilibria indicate that the parent melts were formed by partial melting below an Archean volcanic center that generated (alkali-)picritic to high-alumina tholeiitic melts from a mantle whose oxygen fugacity was lower than today. Fractional crystallization produced troctolites with immiscible sulfide melt droplets within the mafic crust. Instability of the mafic crust led to deep subduction and re-equilibration at 4–6 GPa. Phase relationships plus the presence of a sample with appreciable modal corundum but no Eu anomaly suggest that kyanite- and corundum-bearing eclogites may also originate as plagioclase-free, higher pressure cumulates of highly aluminous clinopyroxene, spinel, and olivine. This is consistent with the crystallizing phase assemblage from an olivine tholeiitic to picritic magma deeper in the Archean oceanic crust or uppermost mantle. We postulate that the magmatic and subduction processes driving modern plate tectonics already existed in the Meso- to Early Archean.

Mineral Chemistry and Petrogenesis of the Puesto La Peña Undersaturated Alkaline Potassic Complex, Mendoza, Argentina

The late Early Miocene Puesto La Pena alkaline potassic complex, Mendoza, Argentina, is a multiphase intrusion with a large variety of rock types (including pyroxenite, malignite, borolanite, syenite, mafic to felsic dikes, and volcanic necks). This study focuses on the mineralogy of the various lithologies and the mineral chemical data in order to define a petrogenetic model that explains the fractionating phases of the complex. Geothermobarometric data and characteristics of and variations in mineral composition are in agreement with a parental magma that began the crystallization process at around 35–40 km depth with initial temperatures of around 1350 °C. After cooling to about 900 °C, a fractional crystallization process was dominant in a crustal environment at moderate to shallow depths, the ascent of magma being related to crustal extension. Crustal contamination seems to have been limited. In the proposed model, pyroxenite is a cumulate phase of the original intrusion, being comagmatic with the malignite-syenite. The late-stage evolution of the complex involves evolution towards the calcite liquidus field, which explains the presence of clusters of calcite-dolomite crystals and carbonatitic dikelets associated with late ouachititic dikes. The mineral composition reflects an environment of crystallization at moderate to shallow depths, low pressure, low silica, and high temperature, which is consistent with the model for the complex, considered to represent the root zone of an alkaline caldera-style volcano emplaced in an extensional back-arc setting.

Mantle-derived sources of syenites from the A-type igneous suites — New approach to the provenance of alkaline silicic magmas

Granite is generally dominant in A-type igneous suites but these frequently include also alkali feldspar- and peralkaline- syenite and quartz syenite. Such syenites can provide essential information about magma sources and mode of generation of A-type silicic magma. This paper addresses the petrogenesis of syenites based on comparisons between the Mongolian–Transbaikalian Belt, Russia (MTB), and the northern Arabian–Nubian Shield (ANS) as exposed in the Sinai Peninsula, Egypt and adjacent areas of southern Israel. The syenitic rocks from MTB and ANS are characterized by high alkali content (Na2O+K2O=10.5 to 12.5wt.%) and are assigned to alkaline metaluminous and peralkaline granitoids. Peralkaline syenites are generally richer in Na and contain slightly less K and Ba than are metaluminous granitoids. REE abundances are similar in all types of syenites. The Eu/Eu* ratios range commonly from 0.35 to 0.65, although higher values, up to 1.15, attributed to presence of accumulated Afs and minor Pl, also occur in some plutons. The geochemical and Sr–Nd isotope characteristics of associated syenite, granite and monzogabbro from five igneous suites (~80 samples) suggest that the main rock types, silicic and mafic, are cogenetic in each suite. Syenite magmas were produced from mantle-derived source with little, if any silicic crustal component. The generation of abundant A-type granite and syenite magmas in the young juvenile crust (ANS) argues that old continental crust is not required for generation of highly alkaline silicic magmas, as commonly advocated. The most probable source of both syenite and granite was mantle-derived K-rich shoshonitic monzogabbro. The bimodal character of the A-type suites suggests that partial melting of monzogabbro, rather than fractional crystallization of basic magma, accompanied with enrichment of a cumulate phase in the mafic units, was the dominant mode of granitoid magma formation. Granite magmas were produced in the lower crust at ~20% of melting, whereas syenite–quartz syenite magmas could be generated by melting of ~25–30%.

Columbia Hills — An exhumed layered igneous intrusion on Mars?

The rover Spirit has now performed a series of whole-rock chemical analyses on rock outcrops and boulders along its traverse across the Columbia Hills of Gusev crater. The wide-spread consensus that the Columbia Hills represent a mafic volcanic pyroclastic sequence has problems, however, explaining the compositional variation observed within the Columbia Hills analyses. The positive trend in Fe–Mg space exhibited by the rocks of the Columbia Hills contrasts markedly with volcanic trends controlled by crystal fractionation and no plausible crystal fractionation scheme can explain the co-variation of Fe and Mg, if the analyzed rocks are assumed to represent volcanic compositions defining a liquid line of descent. Positive arrays in Fe–Mg space are, however, common features in whole-rock data collected across cyclically-layered igneous intrusions. The repeated alteration of feldspar-rich versus mafic-rich cumulates in such intrusions results in whole-rock compositions that scatter along mixing lines between feldspar, near the origin, and the mafic cumulate phases, reflecting the magmatic sorting of cumulus crystals as opposed to crystal–liquid equilibria. There is a striking correspondence between the Fe–Mg trends observed in the Columbia Hills data and the cumulate stratigraphy of a number of terrestrial layered intrusions. This raises the possibility that stratification seen at many localities along Spirit's traverse may reflect magmatic sedimentation in a large layered igneous intrusion. According to this interpretation, the most magnesium rocks are harzburgite to olivine norite cumulates without cumulus magnetite. The majority of the rocks of the Columbia Hills, however, appear to be olivine norites to gabbronorites with cumulus magnetite, which are characterized by variable proportions of feldspar versus mafic silicates plus magnetite. Furthermore, phosphorus-rich rocks, which have previously been interpreted to be alkaline basalts, are better explained as leucocratic norites with cumulus apatite. The sequence of crystallization of the magma responsible for the Columbia Hills is therefore interpreted to be: olivine, followed by orthopyroxene and plagioclase, then magnetite, and finally apatite. Ca-rich clinopyroxene appears to have been a late post-cumulus phase. If this interpretation is correct, then the Columbia Hills likely represent a Noachian-age layered intrusion that was exhumed by rebound following the meteorite impact that formed Gusev crater in the Hesperian.

Remanent and induced magnetic anomalies over a layered intrusion: Effects from crystal fractionation and magma recharge

The Bjerkreim-Sokndal (BKS) norite – quartz mangerite layered intrusion is part of the early Neoproterozoic Rogaland Anorthosite Province intruded into the Fennoscandian shield in south Norway at ~ 930 Ma. The BKS is exposed over an area of 230 km 2 with a thickness of ~ 7000 m and is of economic interest for ilmenite, magnetite and apatite deposits. From the point of view of magnetic minerals, in the course of fractional crystallization and magma evolution, the ilmenite becomes less Fe 3+-rich reflected by a change from ilmenite with hematite exsolution to nearly pure ilmenite. Magnetite starts to crystallize relatively late in the intrusive history, but its crystallization is interrupted by influxes of more primitive magma. The variations in aeromagnetic and ground-magnetic anomalies measured over the BKS can be explained in terms of the measured magnetic properties of NRM, susceptibility, and hysteresis presented here, and in terms of mineralogy. Early layers in the intrusion contain hemo-ilmenite. As the magma evolved and magnetite started to crystallize, this caused a distinct change over the layering from remanence-controlled negative anomalies to induced positive anomalies. When new, more primitive magma was injected into the system, hemo-ilmenite returned as the major oxide and the resulting magnetic anomalies are again negative. The most dramatic change in the magnetic signature is in the upper part of the intrusion in MCU IVe, where magnetite became a well established cumulate phase as indicated by susceptibility, but its induced magnetization is overcome by large NRMs associated either with hemo-ilmenite, or with hemo-ilmenite and magnetite exsolved from pyroxenes. The average natural remanent magnetizations change from ~ 3 A/m in MCU IVd, to 15 A/m in MCU IVe, and back to 2 A/m in the overlying MCU IVf, producing a strong negative remanent anomaly that has been followed along strike for at least 20 km by ground-magnetic measurements. The highly varied magnetic properties of this intrusion, caused by varied magmatic crystallization of combinations of opaque minerals, illustrate some of the possibilities to be considered in evaluating crustal magnetic anomalies.

Experimental effects of pressure and fluorine on apatite saturation in mafic magmas, with reference to layered intrusions and massif anorthosites

Apatite is a cumulate phase in the upper parts of some mafic layered intrusions and anorthositic complexes. We investigated the effect of pressure and fluorine on apatite saturation in mafic magmas to better understand under which conditions this mineral crystallizes. Apatite saturation gives information about the formation of silicate rocks, and is of interest in explaining the formation of apatite–oxide-rich rocks (e.g. nelsonites comprising approximately, one-third apatite and two-third Fe–Ti oxide). Two models of formation are proposed for this rock type: crystal fractionation followed by accumulation of apatite and Fe–Ti oxides and liquid immiscibility. New experiments carried out with mafic compositions at 500 MPa confirm that the most important variables on phosphate saturation are SiO2 and CaO. Fluorine addition leads to apatite saturation at lower SiO2 and higher CaO concentrations. Comparison of our results with those of previous experimental studies on liquid–liquid immiscibility at upper-to-mid-crustal conditions allows us to investigate the relative importance of apatite saturation versus liquid–liquid immiscibility in the petrogenesis of nelsonites and similar rocks. The liquid line of descent of three natural examples studied (the Sept-Iles intrusive suite, the anorthositic Complex of the Lac-St-Jean and the Skaergaard layered intrusion) do not cross the liquid–liquid immiscibility field before they reach apatite saturation. Thus, the apatite–oxide-rich rock associated with these three intrusive suites are best explained by crystal fractionation followed by accumulation of apatite and Fe–Ti oxides.

Experimental constraints on the parental liquid of the Chassigny meteorite: A possible link between the Chassigny meteorite and a Martian Gusev basalt

Most magmas proposed as parental to the Martian SNC meteorites are high in iron and low in alumina. Yet, experiments at low pressures on such liquids have not produced the cumulate or melt-inclusion assemblages seen in the chassignite meteorites. Therefore, elevated pressure experiments under anhydrous and hydrous (water-undersaturated) conditions were conducted on a high-Fe, low-Al liquid proposed to be parental to the Chassigny meteorite. These experiments failed to produce the most magnesian cumulate phases, as well as the olivine hosted kaersutite-bearing melt-inclusion assemblage, of the chassignites. These results suggest that the parental liquid to the chassignite meteorites is both more magnesium and aluminum-rich than the previously considered composition (A∗; Johnson et al., 1991). The proposed composition is similar to the Martian Adirondack class Gusev basalt Humphrey and suggests a link between the Chassigny meteorite and rocks on the surface of Mars.

Lithospheric modification during crustal extension in the Main Ethiopian Rift

Quaternary lavas erupted in zones of tectonomagmatic extension within the Main Ethiopian Rift (MER) preserve details of lithospheric structure in the East African Rift System. Despite observed source heterogeneity, basalts, trachybasalts, and basaltic trachyandesites erupted in the Wonjii Fault Belt (WFB) and the Silti‐Debre Zeyit Fault Zone (SDFZ) form coherent fractionation paths dominated by variable removal of observed phenocryst phases. Crustal assimilation is not widespread, though it is observed at the southern end of the WFB where both fault belts merge; farther north, assimilation of cumulate phases related to fractional crystallization of previous magmas is identified. Shallow fractionation conditions (∼1 kbar) within the WFB do not change from north to south. In contrast, lavas erupted within the contemporaneous SDFZ fractionate at various crustal depths. These results indicate a better developed magmatic system beneath the WFB where magmas rose quickly before undergoing more significant fractionation at near surface levels and a less developed system beneath the SDFZ. The distribution of magmatism and extant geophysical data indicate thinned crust and a single rift‐centered zone of magmatic activity northeast of 8°30′N, consistent with a transitional lithosphere between continental and oceanic settings. Southwest of 8°30′N, thicker crust and rift‐marginal axes of extension suggest lithosphere with continental affinities. The WFB is propagating southward in response to extension within the Red Sea Rift; the northward propagating SDFZ is related to rifting within the East African Rift System. This region records the unification of two rift systems, requiring care in interpreting the MER as simply transitional between continental and oceanic environments.

Multiple events in the Neo-Tethyan oceanic upper mantle: Evidence from Ru–Os–Ir alloys in the Luobusa and Dongqiao ophiolitic podiform chromitites, Tibet

In situ analysis of major elements, platinum-group elements and Os isotope composition data for 170 grains of Ru–Os–Ir alloys from podiform chromitites in the Luobusa and Dongqiao ophiolites, Tibet, provide new constraints on the sources of the alloy grains, the Os-isotope composition of the Neo-Tethyan oceanic upper mantle, and the timing of melt–rock reactions in the ultramafic peridotites of the host ophiolites. Most alloy grains are osmiridium and iridosmine. Based on elemental and isotopic compositions, two populations can be identified. Group I, found in both ophiolite bodies, has fractionated PGE patterns typical of alloys and homogeneous 187Os/ 188Os 0.12645 ± 4 (2s; n = 145). Group II alloys from the Dongqiao ophiolites show smooth PGE patterns and 187Os/ 188Os ranges from 0.12003 to 0.12194 (yielding Re-depletion ages ≥ 1.1 Ga). Both the PGE patterns and the Os isotopic compositions of Group II resemble those of sulfides residual after melting. Group I alloys represent an early cumulate phase during the crystallization of magmas and are interpreted as derived from an asthenospheric melt. In contrast, Group II alloys are inferred to derive from the breakdown of residual sulfides from the mantle section of the ophiolite, which may represent relics of a sub-continental lithospheric mantle. The coexistence of the two groups of alloys in Dongqiao samples reflects interaction between asthenosphere-derived melts and old depleted lithospheric mantle and suggests that the chromitites originated through melt–rock reactions. The data on the Group I alloys support the hypothesis that the Re–Os evolution of the convective oceanic mantle resembles that of enstatite chondrites.

Magmatic processes in developing oceanic crust revealed in a cumulate xenolith collected at the East Pacific Rise, 9°50′N

The petrology and geochemistry of a xenolith, a fragment of a melt‐bearing cumulate, within a recently erupted mid‐ocean ridge (MOR) lava flow provide information on petrogenetic processes occurring within the newly forming oceanic crust beneath the northern East Pacific Rise (NEPR). The xenolith reveals important petrologic information about MOR magmatic systems concerning (1) melt distribution in a crystal‐dominated mush; (2) melt‐crystal reactions within the mush; (3) the chemistry of melts that have contributed to the cumulate lithology; and (4) the chemistry of axial melts that enter the axial magma system. The xenolith was enclosed within a moderately primitive, normal mid‐ocean ridge basalt (NMORB) erupted in 1991 within the neovolcanic zone of the NEPR, at approximately 9°50′N. The sample is a matrix‐dominated, cumulate olivine anorthosite, composed of anorthite (An94‐90) and bytownite (An89‐70), intergranular olivine (Fo86±0.3), minor sulfide and spinel, and intergranular glass. Marginal corrosion of plagioclase, and possibly olivine, and internal remelting of plagioclase indicate syntexis. It is surmised that the pore volume was eviscerated several times with moderately primitive basaltic melts and reduced by intergranular crystallization of forsteritic olivine. The presence of anorthite as a cumulate phase in the xenolith and the observation of anorthite xenocrysts in NMORB lavas, and as a cumulate phase in ophiolite gabbros, indicate that Ca‐rich melts that are not a part of the NMORB lineage play an important role in the construction of the oceanic crust.