Cumulus Crystals Research Articles

Trace Element Constraints on the Differentiation and Crystal Mush Solidification in the Skaergaard Intrusion, Greenland

New major and trace element analyses of plagioclase and clinopyroxene in gabbros from the Skaergaard intrusion are used to understand the mechanisms of crystallization in the main magma body and in the crystal mush. Crystal cores show a continuous chemostratigraphic evolution in the Layered Series, with compatible elements (e.g. Cr and Ni in clinopyroxene) being progressively depleted from the bottom up, whereas incompatible elements (e.g. Sr, Ba, REE in plagioclase and clinopyroxene) become progressively enriched. We performed numerical models and showed that these trends can be explained by a simple process of fractional crystallization, except for the upper 20% of the intrusion where plagioclase and clinopyroxene trace element compositions depart from fractional crystallization trends. At this stage of magma chamber solidification, fractional crystallization becomes less efficient and is replaced by a major proportion of in situ, equilibrium crystallization. Trace elements also show significant variations in interstitial overgrowths on plagioclase and clinopyroxene cumulus crystals. They result from crystallization of the interstitial liquid in the liquid + crystal mush. However, incompatible elements, and especially REE, show a degree of enrichment in plagioclase rims (> 20 ppm Ce) that strongly exceeds the highest concentrations observed in plagioclase cores (up to 4 ppm Ce) at the top of the Layered Series. Such a strong enrichment is difficult to reconcile with a simple process of fractional crystallization of the interstitial liquid in the crystal mush, but may be related to the development of silicate liquid immiscibility in the crystal mush or to delayed nucleation of apatite. After the crystallization of plagioclase and clinopyroxene overgrowths, diffusive re-equilibration during a period of 0.1-0.25 Myr significantly changed the original zoning profiles. Ce flux from clinopyroxene into the plagioclase lattice of adjacent crystals could also have contributed to the strong Ce enrichment observed in plagioclase rims.

Coronitic textures in the ferrogabbros of the Elet’ozero intrusive complex (Northern Karelia, Russia) as evidence for the existence of Fe-rich melt. 2. Origin of Fe-rich liquid

The study of coronitic textures in ferrogabbros and data on rhythmic layering of the Elet’ozero Massif supports the existence of a specific low-temperature Fe-rich liquid in nature. This liquid was formed during solidification of intrusion owing to the local multiple accumulation of Fe and Ti contents in a parental Fe-rich Fe–Ti basaltic melt. According to obtained data, this occurred on micro- and macroscale: 1) in the interglanular (intercumulus) space of the crystallization zone where intercumulus melt becomes rich in Fe and Ti owing to the crystallization of cumulus silicate minerals and is transformed into Fe-rich liquid, which concentrates residual components of an intergranular melt; 2) during formation of rhythmic layering when Fe-rich residual melt is accumulated before the upper part of the moving front of solidification; when Fe content reaches a certain limit, the melt is also transformed in a separate Fe-rich liquid, the interlayers of which form the upper (lowest temperature) members of rhythms. It was concluded that the emergence of a Fe-rich melt is related to its specific structure, which is formed when the Fe content reaches certain critical values in a liquid. Thus, this liquid is not a product of immiscible splitting of a melt, but represents a peculiar phenomenon. The preservation of primary textures and structures of the rocks is supposedly related to the lyophobic properties of surfaces, i.e., “repulsion” of nonwetting liquid by facets of cumulus crystals, especially plagioclase. Owing to this, the drops and even horizons of heavy Fe-rich liquid are retained in situ of their formation.

Petrology of a Neoproterozoic Alaskan-type complex from the Eastern Desert of Egypt: Implications for mantle heterogeneity

This paper details petrological and geochemical studies of an ultramafic–mafic intrusion in the Southern Eastern Desert of Egypt. The Dahanib complex shows a concentric zonation, from dunites at the core, through chromitites, clinopyroxene-rich dunites, wehrlites, harzburgites, gabbronorites and layered gabbros, to hornblende gabbros/diorites at the rim, similar to other Alaskan-type complexes. These lithologies typically feature cumulate textures and layering. Their pyroxenes (Mg#s, 0.54–0.94) evidence Fe, Mn and Na enrichment, but Al, Cr, Mg and Ti are depleted with differentiation. Their chromian spinels have a wide range of Cr# (0.31–0.61), along with high Ti and Fe, as a result of their origin through crystal accumulation and reaction with interstitial liquids. The clinopyroxenes (Cpxs) in peridotites and gabbroic rocks, which are high in REE concentration (2–100 times chondrite), are depleted in LREE relative to HREE and are similar to Cpx crystallized from asthenospheric melts. The mineral inclusions in spinel, the chemistry of Cpx in peridotites (rich in Al, Cr, Na, Ti and ΣREE=13.7), and the melts in equilibrium with Cpx suggest that the Neoproterozoic lithosphere were partially refertilized by trace asthenospheric melts. The early magmas were possibly enriched by Mg, Cr, Ni, Ti, V and Sr, while the evolved types were rich in Fe, Mn, Na, Li, Zr, Co and REE via crystal accumulation and the interaction with interstitial liquids. The Neoproterozoic sub-arc mantle in Egypt is chemically heterogeneous and generally low in Nb, Ta, Zr and K, due to the low solubility of HFSE in slab-derived fluids and no other external addition of these elements. The large variations in lithology and chemistry, as well as the occurrence of scattered chromitite clots in the Dahanib peridotites, are related to a continuous supply of primitive magmas and/or the reaction between interstitial liquids and early cumulus crystals during multistage fractional crystallization. The Dahanib Alaskan-type rocks were fractionally crystallized from the hydrous tholeiitic-basaltic melts associated with a continuous supply of primitive magmas at the mantle–crust boundary in a sub-arc setting. Their parental melts are a mixture of the sub-arc mantle-derived melts associated with trace asthenospheric melts from the mantle diapir. The changes in lithology type, mineral composition, and chemistry between the Dahanib intrusion and the nearest intrusions can perhaps be attributed to mantle heterogeneity by several mantle plumes and to slab-derived inputs. These two causes could explain the large variety of parental magmas for Alaskan-type intrusions. There is a genetic link between the origins of ophiolites and Alaskan-type complexes because both of them originated in the sub-arc setting and both exhibit extension characteristics and subduction-zone components.

Erratum: Triggering of major eruptions recorded by actively forming cumulates

Major overturn within a magma chamber can bring together felsic and mafic magmas, prompting de-volatilisation and acting as the driver for Plinian eruptions. Until now identification of mixing has been limited to analysis of lavas or individual crystals ejected during eruptions. We have recovered partially developed cumulate material (‘live’ cumulate mush) from pyroclastic deposits of major eruptions on Tenerife. These samples represent “frozen” clumps of diverse crystalline deposits from all levels in the developing reservoir, which are permeated with the final magma immediately before eruptions. Such events therefore record the complete disintegration of the magma chamber, leading to caldera collapse. Chemical variation across developing cumulus crystals records changes in melt composition. Apart from fluctuations reflecting periodic influxes of mafic melt, crystal edges consistently record the presence of more felsic magmas. The prevalence of this felsic liquid implies it was able to infiltrate the entire cumulate pile immediately before each eruption.

Triggering of major eruptions recorded by actively forming cumulates

Major overturn within a magma chamber can bring together felsic and mafic magmas, prompting de-volatilisation and acting as the driver for Plinian eruptions. Until now identification of mixing has been limited to analysis of lavas or individual crystals ejected during eruptions. We have recovered partially developed cumulate material (‘live’ cumulate mush) from pyroclastic deposits of major eruptions on Tenerife. These samples represent “frozen” clumps of diverse crystalline deposits from all levels in the developing reservoir, which are permeated with the final magma immediately before eruptions. Such events therefore record the complete disintegration of the magma chamber, leading to caldera collapse. Chemical variation across developing cumulus crystals records changes in melt composition. Apart from fluctuations reflecting periodic influxes of mafic melt, crystal edges consistently record the presence of more felsic magmas. The prevalence of this felsic liquid implies it was able to infiltrate the entire cumulate pile immediately before each eruption.

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.

Magmatic Fractionation by Compositional Convection in a Sheet-like Magma Body: Constraints from the Nosappumisaki Intrusion, Northern Japan

The mechanism of fractionation in a sheet-like magma body was investigated based on observations from the Nosappumisaki intrusion, northern Japan. This is one of a number of late Cretaceous shoshonite sills in the Nemuro peninsula and has a compound structure comprising a bottom cumulate zone containing � 50 vol. % cumulus crystals and a overlying middle zone free of such cumulus crystals. These zones are sandwiched between porphyritic rocks of upper and lower chilled and marginal zones. It is inferred that massive crystal settling or flotation from a magma initially laden with � 20 vol. % of crystals (primocrysts) in a sheet-like magma body formed a cumulus pile and an overlying crystal-free melt layer. After the formation of the compound structure with partial freezing of the upper and lower marginal zones, the internal part of the intrusion slowly solidified. During this period the melt in the central part of the intrusive sheet underwent fractional crystallization. This is shown by patterns of vertical variation in the composition of the interstitial melt. The overall patterns are inverted (mirrorimage) S-shaped and S-shaped for incompatible and compatible elements, respectively. The incompatible element abundances show maxima near the bottom of the middle zone, which corresponds to the final solidification horizon. They show minima in the cumulate zone, which suggests discharge of an evolved melt from the crystal pile. Fractionation is inferred to have taken place via compositional convection without settling of crystals grown after the intrusion event or collapse of the upper boundary layer.The melt in the central melt layer descended into the permeable crystal pile counterbalancing discharge of buoyant evolved melt formed by crystallization of the pore melt in the crystal pile.This conclusion is based on the following observations. (1) There is no accumulation of crystals grown after the intrusion of the magma above the cumulate zone. (2) Leucocratic pipe-like structures, which represent relict pathways for the evolved melt, are developed in the cumulate zone.These structures increase in diameter, decrease in number density, and increase in abundance of incompatible elements with height. (3) Cumulus augite in the cumulate zone has Ca-rich and Al-poor dissolved rims. The extent of dissolution increases with stratigraphic height, suggesting downwelling of an H2O-rich melt from the overlying melt layer. (4) There is no evidence for extensive compaction accompanying pore melt crystallization in the main part of the cumulate zone. (5) The initial melt composition estimated from the groundmass of the chilled margin rocks and the average composition of the material interstitial to the primocrysts for the whole sill are statistically identical. A mass-balance model for compositional convection was constructed to quantify the melt transportation processes. Model parameters were optimized by fitting the observed vertical variation of the interstitial melt compositions. The optimized results suggest that the evolved melts discharged from the crystal pile and rose through the central melt layer as plumes with minor mingling with the surrounding melt. The initially formed compound structure is a highly preferential environment for compositional convection.

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.

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.

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.

Chemical assembly of Archean anorthosites from amphibolite- and granulite-facies terranes, SW Greenland

We report whole-rock, major- and trace-element compositions (obtained by XRF and INA methods) for the amphibolite-facies Buksefjorden and granulite-facies Nordland anorthosites, SW Greenland. In a previous petrologic study on the same sample suite, we documented differences in texture, mineralogy, and mineral compositions between these two anorthosite bodies. Chemical analyses confirm differences in composition between the two bodies, but these differences cannot be explained by variations in metamorphic conditions, and point towards differences in the nature of their protoliths. Analyzed Nordland samples are anorthosites and leucogabbros with 88–98% normative plagioclase, whereas those from Buksefjorden include anorthosite, leucogabbro, and gabbro with ~55–95% normative plagioclase. Two or more compositional groupings can be recognized at each site, which correspond to differences in color and mineralogy of the hand samples. Samples from Buksefjorden are mainly quartz-normative, whereas those from Nordland are olivine (± nepheline) normative. Other differences include higher Ni/Co ratio and REE contents in the granulite-facies anorthosites from Nordland. REE pattern shapes are similar, however, being moderately fractionated at ~0.5–10× chondrites with positive Eu-anomalies. Calculated equilibrium melt patterns are similar for both anorthosites, being relatively flat at ~50–150× chondrites, suggesting unfractionated (but evolved) parental magmas. Olivine must have been present in the protoliths of the Nordland rocks compared with Buksefjorden. Otherwise, the protoliths contained plagioclase with variable An-content (~An62–An92) and a mafic component with variable Fe/Mg (mg ~0.3–0.8). This mafic component was either hornblende or a combination of ortho- and clinopyroxene in fixed proportions, plus a small amount of magnetite. Mixing calculations demonstrate that some Buksefjorden anorthosites contain two varieties of plagioclase: a calcic type that may correspond to cumulus crystals, and a sodic-type that may correspond to a trapped-melt component. On plots of normative whole-rock An versus mg, compositions of the Buksefjorden and Nordland anorthosites form crude negative arrays that differ from the generally positive trends of mafic layered intrusions (Kiglapait, Skaergaard) and from the generally flats trends of plagioclase-rich cumulate rocks (St. Urbain and Stillwater anorthosites). This difference provides further evidence for the distinctive nature of Archean calcic-anorthosite complexes compared with other types of mafic intrusions. Moreover, this distribution of data points is consistent with the assembly of the protolith of the SW Greenland anorthosites mainly as mixtures of plagioclase and hornblende. Finally, the field for the Buksefjorden and Nordland anorthosites overlaps only slightly with that for the Fiskenaesset Complex, thus extending the known range of compositions for Archean anorthosites in West Greenland.

Prolonged reactions between harzburgite xenoliths and silica-undersaturated melt: implications for dissolution and Fe-Mg interdiffusion rates of orthopyroxene

Orthopyroxene and olivine exposed along the rim of a harzburgite xenolith from La Palma (Canary Islands) show polycrystalline selvages and diffusion zones that result from contact with mafic, alkaline, silica-undersaturated melts during at least 10–100 years before eruption. The zoned selvages consist of a fine-grained reaction rim towards the xenolith and a coarser grained, cumulate-like layer towards the melt contact. The diffusion zones are characterized by decreasing magnesium number from about 89–91 in the xenolith interior to 79–85 at the rims, and clearly result from Fe-Mg exchange with surrounding mafic melt. The width of the diffusion zones is 80–200 µm in orthopyroxene and 1,020–1,730 µm in olivine. Orthopyroxene also shows decreasing Al2O3 and Cr2O3 and increasing MnO and TiO2 towards the reaction rims. Textural relations and comparisons with dissolution experiments suggest that orthopyroxene dissolution by silica-undersaturated melt essentially ceased after days to weeks of melt contact, possibly because of decreasing temperature and formation of the reaction rims. The short dissolution phase was followed by prolonged growth of diffusion zones through cation exchange between xenolith minerals and melt across the reaction rims, and by the growth of cumulus crystals. The observations indicate that orthopyroxene xenocrysts and harzburgite xenoliths can survive in mafic, silica-undersaturated, subliquidus magmas at 1,050–1,200 °C and 200–800 MPa for tens of years. Modeling and comparison of the diffusion zones indicate that the average Fe-Mg interdiffusion coefficient D FeMg in orthopyroxene is 2 log units lower than that in olivine; at 1,130 °C and QFM-buffered oxygen fugacity, $$D_{FeMg}^{opx} = 3 \times 10^{ - 19} \,{\rm m}^2 \,{\rm s}^{{\rm - 1}} $$ . The new data overlap well with recently published data for D FeMg in diopside, and indicate that $$D_{FeMg\,}^{opx} $$ (as predicted by previous authors) may be extrapolated to higher temperatures and oxygen fugacities. It is suggested that $$ D_{FeMg\,}^{opx} $$ and D FeMg in Mn-poor ferromagnesian garnet are similar within 0.5 log units at temperatures between 1,050 and 1,200 °C.

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.

Petrology of the late-Carboniferous Punta Falcone gabbroic complex, nothern Sardinia, Italy

The Punta Falcone gabbroic complex represents an evolved high-alumina basalt which rose from the mantle through the lower crust, and subsequently intruded a granite magma in middle crustal levels, during the calc-alkaline magmatic activity which took place in the Sardinian and Corsican islands in the Carboniferous. The gabbroic complex has a stratified sub-vertical structure, and consists of three zones developing from the bottom to the top of the magma chamber. An interaction zone can be recognized along contacts with the surrounding granite stock, and it is characterized by finer-grained and more evolved rocks than the interior of the gabbroic complex. Processes occurring in its interior zone have been substantially different from those occurring in its marginal interaction zone. Petrographical and geochemical features indicate that the differentiation of the interior of the gabbroic complex can be accounted for by low pressure, closed-system in-situ crystallization. The different gabbroic units represent mixtures between cumulus phases and trapped liquid. Plagioclase + pyroxenes, and successively plagioclase + calcic amphibole + oxides nucleated and grew in-situ on the floor and walls of the chamber. Floating of plagioclase towards the top of the magma chamber resulted in the accumulation of the denser liquid at the bottom. Compaction phenomena and convective fractionation processes permitted the development of the pile of cumulus crystals with their trapped liquid, and the migration of part of this evolved liquid towards the top of the magma chamber. On the basis of major and trace element modelling a mathematical artifice has been developed to evaluate cumulus-intercumulus processes that occurred in the interior of the gabbroic complex. Accordingly, the formation of the different units can be modelled by mixtures between the parental magma and different percentages of minerals formed during the first stages of crystallization. Contemporaneously with the differentiation of the interior zone, the envelope of fine-grained rocks enclosing and grading into the coarser inner part of the gabbroic complex experienced both chemical and physical processes. Chemical processes resulted in the evolution of the marginal interaction zone by crystal fractionation plus contamination by the acid magma. Physical processes were closely related to the thermodynamic instability of this marginal zone, and consisted of mingling and back veining phenomena which developed interdigitations of granite veins along contacts. In addition, an increase of the melt fraction of the granite magma, superheated by the latent heat of crystallization of the mafic magma, caused the occurrence of tilting of the mafic magma chamber, and resulted in the development of the sub-vertical structure of the gabbroic complex.

Trace element evidence for plagioclase recycling in calc-alkaline magmas

Trace element zoning in plutonic plagioclase feldspars from the Val Fredda Complex, southern Adamello Massif, Italy has been measured using secondary-ion mass spectrometry (SIMS). The studied samples range from hornblende-gabbro through diorite to granitoid with an overall range in plagioclase composition from An 96 to An 15. Individual plagioclases display extensive zonation in anorthite through a considerable variety of zoning textures. There is equally extensive zoning in trace elements, notably Sr and Ba. Broadly similar trace element zoning patterns and concentrations are displayed by all plagioclases irrespective of host rock composition. Using a relationship between X An and the distribution coefficients D Sr and D Ba trace element concentrations in plagioclase are converted to those of the melt which precipitated each crystal zone. The calculated variations in Ba and Sr are greater than can be attributed to changes in pressure or temperature alone, indicating that compositional variation in the coexisting melt was the principal cause of zoning. The Ba Sr melt trend for each crystal describes a two-stage crystallisation history. The calcic cores of all crystals show melt trends of increasing Sr and Ba, which are attributed to mixing and fractionation in parental hydrous picrobasalt magmas. In contrast the melt trends recorded by the rims vary from sample to sample, but essentially show decreasing Sr and Ba consistent with fractionation of sodic plagioclase, alkali feldspar and/or biotite. The plagioclase cores are thought to have crystallised in a common magma chamber at 6–10 kbar pressure, while the rims crystallised at the emplacement level (1.5 kbar) from interstitial melt whose composition is determined by that of the host magma. The data suggest that early-formed calcic plagioclase was “recycled” by the magma during fractionation. Vigorous convection is invoked to both retain relatively dense calcic plagioclase within the evolving magma and to periodically re-entrain cumulus crystals into more evolved magmas. The occurrence of calcic plagioclase cores in felsic derivative magmas indicates that fractionation from picrobasalt to tonalite occurred on a relatively short time scale.

Rare earth elements in minerals of the ALHA77005 shergottite and implications for its parent magma and crystallization history

Rare earth element (REE) and selected trace element abundances were measured in individual grains of pyroxenes, whitlockite, maskelynite and olivine of the ALHA77005 shergottite. Minor (~ 1%) whitlockite, which displays a negative Eu anomaly indicative of co-crystallization with plagioclase, dominates the bulk-rock REE pattern. Detectable Ce anomalies in ~20% of the pyroxene grains analyzed appear to be the result of weathering in the Antarctic environment and indicate that pyroxene is particularly susceptible to element remobilization. Mineral compositions and textural observations suggest that ALHA77005 is a cumulate with ~50% cumulus material (olivine + chromite). Olivine re-equilibrated with intercumulus liquid on cooling. The intercumulus liquid then crystallized as a closed system to form successively poikilitic low-Ca and high-Ca pyroxene, followed by interstitial pyroxenes, plagioclase, whitlockite (and other minor phases), and reacted with cumulus crystals to form rims of different composition. This crystallization sequence provides a constraint on the shergottite source region composition. The ALHA77005 and Shergotty intercumulus liquids had similar REE compositions; however, as opposed to Shergotty, if ALHA77005 was formed during the last 0.72 Ga, its parent melt was not contaminated by LREE-enriched material.

Cumulate maturation and melt migration in a temperature gradient

A potential driving force for cumulate compaction and melt segregation in magmas arises from the change in crystal solubility with temperature. Spatial thermal variation in a cumulus crystal pile sets up a gradient in interstitial melt composition, which in turn provides a diffusion potential for silicate liquid species. Mass transport in response to this potential, referred to as thermal migration, drives adcumulus mineral growth in the cooler regions of a cumulus crystal pile and segregation of interstitial melt toward warmer regions. Heat loss through the cumulate mush zone to the country rock promotes expulsion of intercumulus liquid back toward the magma reservoir. Heat loss from the mush to cooler convective draughts within the chamber promotes trapping of intercumulus liquid. Liquid state Soret diffusion alters this mechanism only in minor chemical detail except when Soret fractionation produces changes in liquid chemistry that exceed those produced by crystal fractionation. This can cause solubility curves to be intersected at high temperature rather than low T, burying the intercumulus liquid in the boundary layer and preventing its escape back into the magma. Quartz saturation in silicic systems may be affected in this way. We demonstrate this process with laboratory experiments in the systems Mg2SiO4‐SiO2 and mid‐ocean ridge basalt. Cumulate compaction rates by thermal migration are limited by diffusion rate, but length scales for compaction increase linearly in time as long as the saturation conditions and temperature are unchanged. Adcumulus crystal growth remote from the accumulation front by thermal migration, although locally at equilibrium with the intercumulus melt, is not in chemical equilibrium with the main magma reservoir. The operation of thermal migration can reconcile two commonly observed, but seemingly contradictory, features of adcumulate rocks: unzoned cumulus crystals (reflecting isothermal crystallization at one place) within cryptically varying mesoscopic layers (reflecting spatial variations of temperature). The chemical details of this process are complicated by selective interdiffusion of components in the intercumulus liquid along compositional/thermal gradients. Compatible and incompatible elements show anomalous enrichments, thus complicating estimates of residual intercumulus liquid porosity and magma composition from cumulus rock chemistry. Thermal migration does not operate independently of magma body cooling or to the exclusion of the other processes of cumulate crystal compaction. Physically, the most promising opportunities for thermal migration to produce observable results occur within individual layers of large layered sequences. Large mafic intrusions cool slowly enough that mass diffusion in the intercumulus melt can observably rearrange constituents of individual layers of cumulus crystal pile in chemistry, modal mineralogy, and texture. Some of the expected consequences of the process are found in layered sequences of mafic intrusions.

Metasomatism of cumulus magnesian olivine by iron-rich postcumulus liquids in the upper Critical Zone of the Bushveld Complex

AbstractHybrid olivine grains from locally concordant contact zones between bodies of iron-rich ultramafic pegmatite (postcumulate) and layered harzburgite (cumulate) are distinguished from cumulus olivine grains by petrographic features and compositional differences. The hybrid olivines, which in comparison with the cumulus crystals are Fe-Mn-rich and Mg-Ni-poor, represent an arrested stage of replacement and exhibit unusually high NiO/MgO ratios. These features are explained by a disequilibrium process of cation-for-cation exchange between crystals and silicate liquid i.e. magmatic metasomatism. Plots of cations across a contact zone give straight line relationships, a function of the extent to which the metasomatizing liquid infiltrated the cumulate layer. A plot of NiO against MgO enables a distinction to be made between metasomatic olivine and olivine that has fractionally crystallized from a tholeiitic magma. These metasomatic olivines all formed by the replacement of pre-existing cumulus olivine and no chemical evidence has been found to support the formation of olivine in iron-rich ultramafic pegmatite bodies by metasomatism of other phases.

Loveringite from the koitelainen layered intrusion, Northern Finland

Conditions favouring the crystallization of loveringite, (Ca, REE) (Ti, Fe, Cr)21 038 in Koitelainen were sustained from the base of the lower zone to the upper part of the main zone. Loveringite is most common in rocks with much intercumulus quartz, potassium feldspar, and phlogopite, and is accompanied by accessories such as zircon, chlorapatite, baddeleyite and thorite. It is particularly common among pyroxene cumulates in the upper part of the lower zone. Here, and apparently also in plagioclaseorthopyroxene-clinopyroxene cumulates of the main zone, loveringite occurs as a cumulus phase. The presence of Cr in the intercumulus liquid is not essential to the stability of loveringite; instead, petrographic evidence and the presence of incompatible elements (Zr, Hf, REE, U, Th) contained in loveringite point to the crucial role of crustal contaminants. Loveringite crystallized in parts of the magma chamber heavily contaminated by acid crustal anatectic melt, and eventually settled from density flows loaded with cumulus crystals. Postcumulus reactions produced concentric ilmenitezircon mantles around loveringites and thus contributed to the compositional heterogeneity of the mineral. We suggest that much of the loveringite that once crystallized in layered intrusions may have been consumed through postcumulus reactions. Loveringite is easy to detect under the reflected light microscope and can be readily distinguished from rutile, chromite and ilmenite, but not from some other members of the crichtonite group. The composition of the Koitelainen loveringite is comparable to that of other occurrences. The spectral reflectance values are 2–3% lower as compared to ilmenite. The reflectance data indicate that the mineral is anisotropic. The VHN100 values, 792–933, are significantly higher than those published heretofore.[/p]

The effect of trapped liquid crystallization on cumulus mineral compositions in layered intrusions

A series of calculations have been carried out to evaluate the effect on cumulus mineral compositions of solidification of trapped intercumulus liquid in orthocumulates. The calculation assumes local equilibrium between phases, and that the system remains chemically closed during crystallization of the trapped liquid. The latter assumption is held to be valid on a scale of tens to hundreds of centimeters. It is not necessary to know the composition of the trapped liquid, as the calculation only requires an estimate of FeO content and trapping temperature. The change in composition of a mineral from that of the initially precipitated cumulus crystals to the final composition after complete solidification is termed the “trapped liquid shift”. Its magnitude depends on the modal proportions of cumulus phases and the initial porosity, and is only weakly dependent on initial phase compositions. Trapped liquid shifts are significant when compared with mineral composition changes occurring during fractional crystallization. Crystallization of 30% trapped liquid gives rise to shifts of up to 10 mol. percent in Mg number of olivine or pyroxene. The size of the shift becomes greater when the initial cumulus assemblage has a lower total FeO+MgO content, and vice versa. As a result of the relationship between trapped liquid shift and cumulus mode, mineral composition variations and trends may be generated in sequences of cumulates which originally had constant compositions of cumulus minerals. For example, in a cyclic unit grading from a pyroxenitic base to an anorthositic top, crystallization of a uniform proportion of trapped liquid will result in an apparent iron enrichment trend from bottom to top of the cycle, as has been observed in the Upper Critical Zone of the Bushveld Complex.