Olivine Textures Research Articles

Insights from olivine chemistry into crustal magmatic processes and the mantle source lithology of basalts from Hainan Island, China

Olivine in ocean island basalts can provide unique insights into lithological heterogeneity (i.e., peridotite versus pyroxenite) in the mantle. Here we present high-precision major and trace element data for olivine crystals in basalts from Hainan Island, China, and use these data to investigate their mantle source. Many of the olivines are in disequilibrium with their whole-rock compositions (i.e., Mg# values), and the basalts contain both normally and reversely zoned olivine crystals, which are indicative of magma mixing. The decoupled Fo–NiO profiles and concave-down trends defined by data in a Fo–NiO diagram suggest that diffusive re-equilibration controlled the olivine chemistry. The olivine compositions were significantly affected by crustal magmatic processes, such as fractional crystallization, magma mixing, and diffusive re-equilibration. However, the lack of clear correlations between Mn/Fe, Mn/Zn, Zn/Fe, and Ga/Sc ratios and olivine Fo contents suggests these trace element ratios were not significantly affected by crustal processes, and thus preserve information about the mantle source. The Mn/Zn, 100 × Mn/Fe and 10,000 × Zn/Fe ratios of the examined olivines indicated that the Hainan basalts had a mixed pyroxenite-peridotite mantle source. This is consistent with the whole-rock geochemical data and previously reported isotope geochemistry data for the Hainan Island basalts. However, crustal magmatic processes also affected the compositions of the primitive olivines that crystallized in the mantle-derived magma. Therefore, careful assessment of post-melting processes using olivine textures and minor element chemistry needs to be undertaken before discriminating between pyroxenitic and peridotitic source components based on olivine chemistry.

The effects of superheating and cooling rate on olivine growth in chondritic liquid

AbstractChondrules, the major constituent of chondrites, are millimeter‐sized igneous objects resulting from the crystallization of silicate liquids produced by the partial or complete melting of chondritic precursors, whose exact nature remains disputed. Various chondrule textures are observed as a function of the extent of the initial melting event. Here, we report dynamic crystallization experiments performed with a broad range of cooling rates (2–750 °C h−1) from superliquidus or subliquidus initial conditions to demonstrate the control of nucleation on the final chondrule texture. Classical crypto‐porphyritic, micro‐porphyritic, and porphyritic olivine textures were reproduced in subliquidus experiments in which heterogeneous nucleation dominates. In contrast, we were unable to reproduce barred olivine textures, regardless of the cooling rates investigated from superliquidus conditions; instead, macro‐porphyritic textures were systematically obtained at low cooling rates (<10 °C h−1). The small number and large size of crystals in the macro‐porphyritic texture are consistent with the initial step of superheating and the presence of long embayments that indicate an initial episode of rapid growth due to delayed nucleation. Crystals then acquired polyhedral shapes during a subsequent episode of slow growth. When the growth rate is too slow to decrease the degree of supersaturation in the liquid, a new episode of rapid growth produces a new generation of melt inclusions.

Intracrystalline melt migration in deformed olivine revealed by trace element compositions and polyphase solid inclusions

Abstract. Melt transport mechanisms have an important impact on the chemical composition of the percolated host rock and the migrating melts. Melt migration is usually assumed to occur at grain boundaries. However, microstructural studies revealed the occurrence of polyphase inclusions along dislocations, subgrain boundaries and microcracks in single mineral grains. The inclusions are interpreted as crystallized melt pockets suggesting that melts can migrate within deformed crystals. Intracrystalline melt migration and diffusive re-equilibration can lead to significant mineral trace element enrichments when associated with dissolution–precipitation reactions. In this contribution, we study a body of replacive troctolites associated with the Erro-Tobbio ophiolitic mantle peridotites (Ligurian Alps, Italy). The replacive formation of the olivine-rich troctolite involved extensive impregnation of a dunitic matrix, i.e. partial dissolution of olivine and concomitant crystallization of interstitial phases. The olivine matrix is characterized by two distinct olivine textures: (i) coarse deformed olivine, representing relicts of the pre-existing mantle dunite matrix (olivine1), and (ii) fine-grained undeformed olivine, a product of the melt–rock interaction process (olivine2). Previous studies documented a decoupling between olivine texture and trace element composition, namely enriched trace element compositions in olivine1 rather than in olivine2, as would be expected from the dissolution–precipitation process. Notably, the trace element enrichments in deformed olivines are correlated with the occurrence of elongated 10 µm size polyphase inclusions (clinopyroxene, Ti-pargasite, chromite) preferentially oriented along olivine crystallographic axes. These inclusions show irregular contacts and have no crystallographic preferred orientation with the host olivine, and the phases composing the inclusions show similar chemical compositions to the vermicular phases formed at the grain boundaries during late-stage reactive crystallization of the troctolite. This suggests that the investigated inclusions did not form as exsolutions of the host olivine but rather by input of metasomatic fluids percolating through the deformed olivine grains during closure of the magmatic system. We infer that strongly fractionated volatile-rich melts were incorporated in oriented microfractures within olivine1 and led to the crystallization of the polyphase inclusions. The presence of intracrystalline melt greatly enhanced diffusive re-equilibration between the evolved melt and the percolated olivine1, in turn acquiring the enriched character expected in neoformed olivine crystals. Intracrystalline melt percolation can have strong geochemical implications and can lead to efficient re-equilibration of percolated minerals and rocks.

Evidence for the presence of chondrule‐ and CAI‐derived material in an isotopically anomalous Antarctic micrometeorite

AbstractWe report the discovery of a unique, refractory phase‐bearing micrometeorite (WF1202A‐001) from the Sør Rondane Mountains, East Antarctica. A silicate‐rich cosmic spherule (~400 µm) displays a microporphyritic texture containing Ca‐Al‐rich inclusion (CAI)‐derived material (~5–10 area%), including high‐Mg forsterite (Fo98‐99) and enstatite (En98‐99, Wo0‐1). The micrometeorite also hosts a spherical inclusion (~209 µm), reminiscent of chondrules, displaying a barred olivine texture. Oxygen isotopic compositions of the micrometeorite groundmass (δ17O = –3.46‰, δ18O = 10.43‰, ∆17O = –1.96‰) are consistent with a carbonaceous chondrite precursor body. Yet, a relict forsterite grain is characterized by δ17O = –45.8‰, δ18O = –43.7‰, ∆17O = –23.1‰, compatible with CAIs. In contrast, a relict low‐Ca pyroxene grain (δ17O = –4.96‰, δ18O = –4.32‰, ∆17O = –2.71‰) presumably represents a first‐generation silicate grain that accreted 18O‐rich gas or dust in a transient melting scenario. The spherical inclusion displays anomalous oxygen isotope ratios (δ17O = –0.98‰, δ18O = –2.16‰, ∆17O = 0.15‰), comparable to anhydrous interplanetary dust particles (IDPs) and fragments from Comet 81P/Wild2. Based on its major element geochemistry, the chondrule size, and oxygen isotope systematics, micrometeorite WF1202A‐001 likely sampled a carbonaceous chondrite parent body similar to, but distinct from CM, CO, or CV chondrites. This observation may suggest that some carbonaceous chondrite bodies can be linked to comets. The reconstructed atmospheric entry parameters of micrometeorite WF1202A‐001 suggest that the precursor particle originated from a low‐inclination, low‐eccentricity source region, most likely either the main belt asteroids or Jupiter family comets (JFCs).

Evolving Viscous Anisotropy in the Upper Mantle and Its Geodynamic Implications

AbstractAsthenospheric shear causes some minerals, particularly olivine, to develop anisotropic textures that can be detected seismically. In laboratory experiments, these textures are also associated with anisotropic viscous behavior, which should be important for geodynamic processes. To examine the role of anisotropic viscosity for asthenospheric deformation, we developed a numerical model of coupled anisotropic texture development and anisotropic viscosity, both calibrated with laboratory measurements of olivine aggregates. This model characterizes the time‐dependent coupling between large‐scale formation of lattice‐preferred orientation (i.e., texture) and changes in asthenospheric viscosity for a series of simple deformation paths that represent upper mantle geodynamic processes. We find that texture development beneath a moving surface plate tends to align the a axes of olivine into the plate motion direction, which weakens the effective viscosity in this direction and increases plate velocity for a given driving force. Our models indicate that the effective viscosity increases for shear in the horizontal direction perpendicular to the a axes. This increase should slow plate motions and new texture development in this perpendicular direction and could impede changes to the plate motion direction for tens of millions of years. However, the same well‐developed asthenospheric texture may foster subduction initiation perpendicular to the plate motion and deformations related to transform faults, as shearing on vertical planes seems to be favored across a sublithospheric olivine texture. These end‐member cases examining shear deformation in the presence of a well‐formed asthenospheric texture illustrate the importance of the mean olivine orientation, and its associated viscous anisotropy, for a variety of geodynamic processes.

Oxygen isotope disequilibrium during serpentinite dehydration

AbstractTalc + olivine in metaperidotites result from the serpentinite breakdown due to increasing temperature in the Bergell contact aureole. Jack‐straw olivine textures are present in close proximity to the serpentine breakdown reaction. As the intrusion is approached, the number of olivine crystals increases while the size of the crystals decreases; this feature documents increased overstepping with increased heating rates. Talc veinlets are observed in the outer parts of the talc–olivine zone and are interpreted to be pathways of fluid produced during devolatilization of serpentine. None of the talc–olivine oxygen isotope pairs analysed are in isotopic equilibrium with respect to the peak contact temperature. This implies that escaping fluids cannot be in equilibrium with both phases. Hence, the fluid produced by serpentine reaction does not directly reflect the protolith composition, and attention must be given to the reaction mechanism before interpreting fluid isotope composition.

Axial‐type olivine crystallographic preferred orientations: The effect of strain geometry on mantle texture

AbstractThe effect of finite strain geometry on crystallographic preferred orientation (CPO) is poorly constrained in the upper mantle. Specifically, the relationship between shape preferred orientation (SPO) and CPO in mantle rocks remains unclear. We analyzed a suite of 40 spinel peridotite xenoliths from Marie Byrd Land, West Antarctica. X‐ray computed tomography allows for quantification of spinel SPO, which ranges from prolate to oblate shape. Electron backscatter diffraction analysis reveals a range of olivine CPO patterns, including A‐type, axial‐[010], axial‐[100], and B‐type patterns. Until now, these CPO types were associated with different deformation conditions, deformation mechanisms, or strain magnitudes. Microstructures and deformation mechanism maps suggest that deformation in all studied xenoliths is dominated by dislocation‐accommodated grain boundary sliding. For the range of temperatures (780–1200°C), extraction depths (39–72 km), differential stresses (2–60 MPa), and water content (up to 500 H/106Si) of the xenolith suite, variations in olivine CPO do not correlate with changes in deformation conditions. Here we establish for the first time in naturally deformed mantle rocks that finite strain geometry controls the development of axial‐type olivine CPOs; axial‐[010] and axial‐[100] CPOs form in relation to oblate and prolate fabric ellipsoids, respectively. Girdling of olivine crystal axes results from intracrystalline slip with activation of multiple slip systems and grain boundary sliding. Our results demonstrate that mantle deformation may deviate from simple shear. Olivine texture in field studies and seismic anisotropy in geophysical investigations can provide critical constraints for the 3‐D strain in the upper mantle.

Modeling olivine CPO evolution with complex deformation histories: Implications for the interpretation of seismic anisotropy in the mantle

AbstractRelating seismic anisotropy to mantle flow requires detailed understanding of the development and evolution of olivine crystallographic preferred orientation (CPO). Recent experimental and field studies have shown that olivine CPO evolution depends strongly on the integrated deformation history, which may lead to differences in how the corresponding seismic anisotropy should be interpreted. In this study, two widely used numerical models for CPO evolution—D‐Rex and VPSC—are evaluated to further examine the effect of deformation history on olivine texture and seismic anisotropy. Building on previous experimental work, models are initiated with several different CPOs to simulate unique deformation histories. Significantly, models initiated with a preexisting CPO evolve differently than the CPOs generated without preexisting texture. Moreover, the CPO in each model evolves differently as a function of strain. Numerical simulations are compared to laboratory experiments by Boneh and Skemer (2014). In general, the D‐Rex and VPSC models are able to reproduce the experimentally observed CPOs, although the models significantly over‐estimate the strength of the CPO and in some instances produce different CPO from what is observed experimentally. Based on comparison with experiments, recommended parameters for D‐Rex are: M* = 10, λ* = 5, and χ = 0.3, and for VPSC: α = 10–100. Numerical modeling confirms that CPO evolution in olivine is highly sensitive to the details of the initial CPO, even at strains greater than 2. These observations imply that there is a long transient interval of CPO realignment which must be considered carefully in the modeling or interpretation of seismic anisotropy in complex tectonic settings.

Characteristic Textures of Recrystallized, Peritectic, and Primary Magmatic Olivine in Experimental Samples and Natural Volcanic Rocks

Olivine textures are potentially important recorders of olivine origin and crystallization conditions. Primary magmatic and xenocrystic origins are commonly considered for olivine from ultramafic to intermediate magmas, whereas secondary olivine origins (i.e. crystals formed by recrystallization or peritectic reaction) are rarely considered in the interpretation of magmatic phenocrysts. The main aim of our study was to determine textures that are characteristic for secondary magmatic olivine and non-characteristic, or at least rare, for primary magmatic olivine. To characterize the textures of the different olivine types, we review previous experimental work and present new textural data for olivine from four melanorite melting and eight basalt crystallization experiments. We qualitatively characterize olivine textures using transmitted light and back-scattered electron microscopy, and semi-quantitatively characterize the 2D surface area of olivine branches and single crystals, their 2D and calculated 3D shapes, and 2D grain boundary segment lengths. Olivine recrystallization yields crystals with dendritic branches, whereas peritectic reaction produces olivine clusters with randomly oriented crystals. In agreement with previous studies, we find that olivine crystal and branch size, grain boundary segmentation, and inclusion relations cannot unequivocally distinguish between olivine of secondary and primary origins. However, recrystallized olivine typically has short prismatic branches, whereas primary magmatic olivine dendrites commonly have elongated branches. The peritectic crystals are closely comparable with glomerocrysts and single polyhedral primary magmatic crystals, but they commonly form small groups of touching crystals and clusters with crystal-poor cores. Spatial association and a comparison with xenocryst textures may further guide the interpretation of their origin, but detailed analyses of core compositions and zoning patterns appear necessary to firmly distinguish peritectic from primary magmatic crystals. Our comparison of experimental and natural samples suggests that dendritic olivine crystals and clusters of polyhedral olivine that commonly occur in mafic and ultramafic igneous rocks should be evaluated for possible primary as well as secondary magmatic origins.

Anisotropy of magnetic susceptibility in natural olivine single crystals

AbstractMantle flow dynamics can cause preferential alignment of olivine crystals that results in anisotropy of physical properties. To interpret anisotropy in mantle rocks, it is necessary to understand the anisotropy of olivine single crystals. We determined anisotropy of magnetic susceptibility (AMS) for natural olivine crystals. High‐field AMS allows for the isolation of the anisotropy due to olivine alone. The orientations of the principal susceptibility axes are related to the olivine's crystallographic structure as soon as it contains >3 wt % FeO. The maximum susceptibility is parallel to the c axis both at room temperature (RT) and at 77 K. The orientation of the minimum axis at RT depends on iron content; it is generally parallel to the a axis in crystals with 3–5 wt % FeO, and along b in samples with 6–10 wt % FeO. The AMS ellipsoid is prolate and the standard deviatoric susceptibility, k′, is on the order of 8*10−10 m3/kg for the samples with <1wt % FeO, and ranges from 3.1*10−9 m3/kg to 5.7*10−9 m3/kg for samples with 3–10 wt % FeO. At 77 K, the minimum susceptibility is along b, independent of iron content. The shape of the AMS ellipsoid is prolate for samples with <5 wt % FeO, but can be prolate or oblate for higher iron content. The degree of anisotropy increases at 77 K with p′77 = 7.1 ± 0.5. The results from this study will allow AMS fabrics to be used as a proxy for olivine texture in ultramafic rocks with high olivine content.

Khatyrka, a new CV3 find from the Koryak Mountains, Eastern Russia

AbstractA new meteorite find, named Khatyrka, was recovered from eastern Siberia as a result of a search for naturally occurring quasicrystals. The meteorite occurs as clastic grains within postglacial clay‐rich layers along the banks of a small stream in the Koryak Mountains, Chukotka Autonomous Okrug of far eastern Russia. Some of the grains are clearly chondritic and contain Type IA porphyritic olivine chondrules enclosed in matrices that have the characteristic platy olivine texture, matrix olivine composition, and mineralogy (olivine, pentlandite, nickel‐rich iron‐nickel metal, nepheline, and calcic pyroxene [diopside‐hedenbergite solid solution]) of oxidized‐subgroup CV3 chondrites. A few grains are fine‐grained spinel‐rich calcium‐aluminum‐rich inclusions with mineral oxygen isotopic compositions again typical of such objects in CV3 chondrites. The chondritic and CAI grains contain small fragments of metallic copper‐aluminum‐iron alloys that include the quasicrystalline phase icosahedrite. One grain is an achondritic intergrowth of Cu‐Al metal alloys and forsteritic olivine ± diopsidic pyroxene, both of which have meteoritic (CV3‐like) oxygen isotopic compositions. Finally, some grains consist almost entirely of metallic alloys of aluminum + copper ± iron. The Cu‐Al‐Fe metal alloys and the alloy‐bearing achondrite clast are interpreted to be an accretionary component of what otherwise is a fairly normal CV3 (oxidized) chondrite. This association of CV3 chondritic grains with metallic copper‐aluminum alloys makes Khatyrka a unique meteorite, perhaps best described as a complex CV3 (ox) breccia.

Deposition Mechanisms of Magmatic Sulphide Liquids: Evidence from High-Resolution X-Ray Computed Tomography and Trace Element Chemistry of Komatiite-hosted Disseminated Sulphides

Magmatic sulphides are a widespread component in mafic and ultramafic rocks and contain variable concentrations of nickel, copper and platinum-group elements. Previous literature has been concerned with the whole-rock geochemistry of magmatic sulphide ores and their host-rocks and relatively little attention has been paid to the physical nature of magmatic sulphide transport and accumulation. Our high-resolution X-ray computed tomography study quantifies for the first time the 2D and 3D size, shape and textural relationships, and distribution of disseminated magmatic sulphides and olivine in adcumulates from komatiites. These new data are combined with analysis of trace-element concentrations within sulphides to provide important information about the mechanisms of transport, deposition and post-accumulation migration of sulphide liquid in dynamic magmatic systems. Olivine shows evidence of textural maturation, with larger crystals growing at the expense of small ones to different degrees depending on the sulphide content of the rock. The olivine texture and the presence of poikilitic chromite provide evidence of in situ nucleation of olivine and chromite at the interface between a flowing magma and a basal pile of crystals. Disseminated to strongly interconnected base-metal sulphides are located at contacts between olivine crystals or in some cases can be entirely or partially enclosed within chromite. Based on their 3D morphologies, their size distribution and their Pd concentrations, the sulphides are divided into four main categories: finely disseminated sulphides; disseminated to slightly interconnected sulphides; disseminated to globular sulphides; disseminated to strongly interconnected sulphides. All samples contain a population of sub-spherical sulphide blebs ( 3 wt % and the sulphide bleb size population is multi-modal. The Pd content of the sulphide blebs is variable and positively correlated with the sulphide bleb size. The overall sulphide abundance, sulphide bleb size and Pd concentrations indicate that these sulphides have been transported in a flowing sulphur-saturated magma over some distance and accumulated at their present site by mechanical processes. Strongly interconnected network to matrix sulphides are observed in samples containing more than 5 wt % sulphide with small variability in Pd concentrations within and between blebs. These sulphides are interpreted to reflect the accumulation and coalescence (by film drainage) of small sulphide blebs. Overall our results show that komatiite-hosted disseminated sulphides form by a mechanical accumulation process that takes place against a background of steady-state in situ nucleation of small blebs along the olivine–sulphide liquid cotectic.

Petrological Insights into Shifts in Eruptive Styles at Volcán Llaima (Chile)

Tephra and lava pairs from two summit eruptions (AD 2008 and 1957) and a flank fissure eruption (� AD 1850) are compared in terms of textures, phenocryst contents, and mineral zoning patterns to shed light on processes responsible for the shifts in eruption style during typical eruptive episodes at Volcan Llaima (Andean SouthernVolcanic Zone, Chile).The mineralogy and whole-rock com- positions of tephra and lavas are similar within eruptive episodes, suggesting a common magma reservoir for Strombolian paroxysms and lava effusion. The zoning profiles and textures of plagioclase record successive and discrete intrusions of volatile-rich mafic magma accompanied by mixing of these recharge magmas with the resident basaltic-andesitic crystal mushes that are commonly present at shallow levels in the Llaima system. Each recharge event destabil- izes the plagioclase in equilibrium with the resident crystal mush melt and stabilizes relatively An-rich plagioclase, as is recorded by the numerous resorption zones. Lavas typically have � 15^20 vol. % more phenocrysts than the tephra. Differences in plagioclase and olivine textures and zoning, combined with different phenocryst contents, indicate that a greater volume fraction of recharge magma is present in the explosively erupted magma than in subsequent effu- sively erupted magma. We propose that Strombolian paroxysms at Volcan Llaima are triggered by interactions with large volume frac- tions of recharge magma, which decrease the bulk viscosity and in- crease the volatile contents of the erupted magmas, leading to the conditions required for the fragmentation of basaltic-andesite. Lava effusion ensues from reduced interactions with the recharge magma, after it has partially degassed and crystallized, thereby impeding rapid ascent. This process could be operating at other steady-state basaltic volcanoes, wherein shallow reservoirs are periodically re- filled by fresh, volatile-rich magmas.

Synthesis of a spinifex‐textured basalt as an analog to Gusev crater basalts, Mars

Abstract– Analyses by the Mars Exploration Rover (MER), Spirit, of Martian basalts from Gusev crater show that they are chemically very different from terrestrial basalts, being characterized in particular by high Mg‐ and Fe‐contents. To provide suitable analog basalts for the International Space Analogue Rockstore (ISAR), a collection of analog rocks and minerals for preparing in situ space missions, especially, the upcoming Mars mission MSL‐2011 and the future international Mars‐2018 mission, it is necessary to synthesize Martian basalts. The aim of this study was therefore to synthesize Martian basalt analogs to the Gusev crater basalts, based on the geochemical data from the MER rover Spirit. We present the results of two experiments, one producing a quench‐cooled basalt (<1 h) and one producing a more slowly cooled basalt (1 day). Pyroxene and olivine textures produced in the more slowly cooled basalt were surprisingly similar to spinifex textures in komatiites, a volcanic rock type very common on the early Earth. These kinds of ultramafic rocks and their associated alteration products may have important astrobiological implications when associated with aqueous environments. Such rocks could provide habitats for chemolithotrophic microorganisms, while the glass and phyllosilicate derivatives can fix organic compounds.

A petrological and chemical reexamination of Main Group pallasite formation

The large number of pallasite parent bodies suggests that their formation was a common event in asteroid evolution. New microprobe data has been generated for pallasite olivines and chromites that indicate subsolidus redox processes, while literature data has been collated and correlated to form new results and interpretations. The new data include information on metal compositions; the metal cooling rates; the formation of round and fragmental olivine; the close-packing of olivine; the crystallization of the phosphates, phosphoran olivine and orthopyroxene; and the bulk pallasite P and S contents. A model for Main Group pallasites has been developed using fractional melting of a chondritic precursor to make a multi-layered parent body. One portion of this contains a gradational metal–olivine (pallasite) texture. Concentric downward crystallization of the molten metal is indicated, based on cooling rates and metal compositions. Fractional crystallization of the residual silicate and metallic melts produces the minor phases on cooling. Comparisons to and a brief review of past models, particularly those utilizing mixing, have been made.

Investigations on a Large Collection of Cosmic Dust From the Central Indian Ocean

We collected 1,245 spherules from the Central Indian Ocean basin by Magnetic cosmic dust collection (MACDUC) experiment raking the deep sea floor. This collection ranks among the large deep sea collections of cosmic dust. For this study, 168 particles are analyzed with SEM-EDS to characterise their cosmic nature and identify the processes that their morphological features, textures and chemical compositions reveal. All the three basic types of cosmic spherules have been identified: I-type, S-type and the G-type. The silicate or the S-type spherules are dominant in this collection. In all, 115 spherules were sectioned, polished and analyzed for major elements. I-type spherules are mainly composed of Fe and Ni oxides, some have metallic cores where appreciable amounts of Co is observed in addition to glassy phases with lithophile elements are also observed in these spherules. These evidences are supportive of the view that the I-type spherules could be metal grains from carbonaceous/unequilibrated chondritic bodies. The S-type spherules show elemental composition of Mg, Al, Si, Ca, Fe, and Ni approximately similar to chondritic compositions. In addition, some other rare particles such as an S-type sphere which contains a large zoned relict chromite crystal, other spheres with a semi-porphyritic/barred olivine texture are also observed. While most the S-type spherules appear to have carbonaceous chondrites as their parent bodies, the relict grain bearing spherule shows distinctly an ordinary chondritic parent body.

Formation of barred olivine texture 4.6 billion years ago

Formation of barred olivine texture 4.6 billion years ago

Internal stratigraphic architecture of the komatiitic dunite-hosted MKD5 disseminated nickel sulfide deposit, Mount Keith Domain, Agnew-Wiluna Greenstone Belt, Western Australia

The MKD5 nickel deposit is hosted by the Mount Keith Ultramafic Complex (MKUC), a dunite body of komatiitic affinity located in the Agnew-Wiluna Greenstone Belt, Western Australia. The internal architecture of the MKUC comprises seven distinct internal units that range from extreme adcumulate dunite to relatively fractionated pyroxenitic and gabbroic horizons. The MKUC is divided into three packages of units. The main adcumulate domain (MAD), which is situated in the lower portion of the complex, contains the bulk of disseminated nickel sulfide and is dominated by coarse adcumulate olivine textures. Overlying the MAD is an upper fractionated zone, which is dominated by mesocumulate-to-orthocumulate peridotite with domains containing oikocrystic pyroxenite and gabbroic lenses. An aerially restricted unit comprising texturally and chemically distinct olivine cumulate rocks is known as the western mineralized zone (WMZ). The fractionation trend between the MAD and UPZ indicates a westerly facing for the MKUC, conformable with the bounding stratigraphy. In the MKUC, truncation of some of the uppermost internal stratigraphic units by the hangingwall contact indicates that the unit has undergone structural modification since its emplacement and suggests that a proportion of the upper section has been removed by faulting. Furthermore, vertical and lateral textural transitions within the internal stratigraphy suggest that the MKUC (excluding the WMZ) was emplaced from an essentially continuous magma flow, with the MAD representing the period of highest magma flux and the major constructional period of emplacement. Conversely, the WMZ is interpreted to represent a later pulse of ultramafic magma, emplaced stratigraphically above the main MKUC. Comparison of the MKUC with other komatiitic dunites from both within the Agnew-Wiluna Greenstone Belt and worldwide indicates that despite apparent geometrical differences between lens and sheet komatiitic dunites, a broad facies architecture can be defined. We suggest that the differences in geometry are related to differing degrees of flow localization within dunitic units and are a function of both differences in the thermal characteristics and the degree of litho-facies heterogeneity of the enclosing lithologies.

Isotopic compositions of oxygen, iron, chromium, and nickel in cosmic spherules: Toward a better comprehension of atmospheric entry heating effects

Large, correlated, mass-dependent enrichments in the heavier isotopes of O, Cr, Fe, and Ni are observed in type-I (metal/metal oxide) cosmic spherules collected from the deep sea. Limited intraparticle variability of oxygen isotope abundances, typically <5‰ in δ 18O, indicates good mixing of the melts and supports the application of the Rayleigh equation for the calculation of fractional evaporative losses during atmospheric entry. Fractional losses for oxygen evaporation from wüstite, assuming a starting isotopic composition equal to that of air (δ 18O = 23.5‰; δ 17O = 11.8‰), are in the range 55%–77%, and are systematically smaller than evaporative losses calculated for Fe (69%–85%), Cr (81%–95%), and especially Ni (45%–99%). However, as δ 18O values increase, fractional losses for oxygen approach those of Fe, Cr, and Ni indicating a shift in the evaporating species from metallic to oxidized forms as the spherules are progressively oxidized during entry heating. The observed unequal fractional losses of O and Fe can be reconciled by allowing for a kinetic isotope mass-dependent fractionation of atmospheric oxygen during the oxidation process and/or that some metallic Fe may have undergone Rayleigh evaporation before oxidation began. In situ measurements of oxygen isotopic abundances were also performed in 14 type-S (silicate) cosmic spherules, 13 from the Antarctic ice and one from the deep sea. Additional bulk Fe and Cr isotopic abundances were determined for two type-S deep-sea spherules. The isotopic fractionation of Cr isotopes suggest appreciable evaporative loss of Cr, perhaps as a sulfide. The oxygen isotopic compositions for the type-S spherules range from δ 18O = −2‰ to + 27‰. The intraspherule isotopic variations are typically small, ∼5% relative, except for the less-heated porphyritic spherules which have preserved large isotopic heterogeneities in at least one case. A plot of δ 17O vs. δ 18O values for these spherules defines a broad parallelogram bounded at higher values of δ 17O by the terrestrial fractionation line, and at lower values of δ 17O by a line parallel to it and anchored near the isotopic composition of δ 18O = −2.5‰ and δ 17O = −5‰. Lack of independent evidence for substantial evaporative losses suggests that much of this variation reflects the starting isotopic composition of the precursor materials, which likely resembled CO, CM, or CI chondrites. However, the enrichments in heavy isotopes indicate that some mixing with atmospheric oxygen was probably involved during atmospheric entry for some of the spherules. Isotopic fractionation due to evaporation of incoming grain is not required to explain most of the oxygen isotopic data for type-S spherules. However spherules with barred olivine textures that are thought to have experienced a more intense heating than the porphyritic ones might have undergone some distillation. Two cosmic spherules, one classified as a radial pyroxene type and the other showing a glassy texture, show unfractionated oxygen isotopic abundances. They are probably chondrule fragments that survived atmospheric entry unmelted. Possible reasons type-I spherules show larger degrees of isotopic fractionation than type-S spherules include: a) the short duration of the heating pulse associated with the high volatile content of the type-S spherule precursors compared to type-I spherules; b) higher evaporation temperatures for at least a refractory portion of the silicates compared to that of iron metal or oxide; c) lower duration of heating of type-S spherules compared to type-I spherules as a consequence of their lower densities.

Geochronology and Petrogenesis of the Cretaceous Antampombato–Ambatovy Complex and Associated Dyke Swarm, Madagascar

The Antampombato–Ambatovy complex is the largest intrusion in the central–eastern part of the Cretaceous flood basalt province of Madagascar, with an exposed surface area of about 80 km2. It has an 40Ar/39Ar incremental heating age of 89·9 ± 0·4 Ma and a U–Pb age of 90 ± 2 Ma. The outcropping plutonic rocks range from dunite and wehrlite, through clinopyroxenite and gabbro, to sodic syenite. A dyke swarm cross-cutting some of the above lithologies (and the nearby Precambrian basement rocks) is formed of picritic basalts, alkali to transitional basalts, benmoreites and rhyolites; some of the latter are peralkaline. A few basaltic dykes have cumulate olivine textures, with up to 26 wt % MgO and 1200 ppm Ni, whereas others have characteristics more akin to those of primitive liquids (9 wt % MgO; Mg-number 0·61; 500 ppm Cr; 200 ppm Ni). These basalts have relatively high TiO2 (2·2 wt %) and total iron (14 wt % as Fe2O3), and moderate contents of Nb (10–11 ppm) and Zr (c. 100 ppm). Initial (at 90 Ma) Sr- and Nd-isotope ratios of the clinopyroxenites and basalt dykes are 0·7030–0·7037 and 0·51290–0·51283, respectively. Syenites and peralkaline rhyolites have Sr- and Nd-isotope ratios of 0·7037–0·7039 and 0·51271–0·51274, respectively. The data suggest derivation of the parental magmas from a time-integrated depleted mantle source, combined with small amounts of crustal contamination in the petrogenesis of the more evolved magmas. The isotopic compositions of the mafic–ultramafic rocks are most similar to those of the mid-ocean ridge basalt (MORB)-like igneous rocks of eastern Madagascar, and suggest the existence of an isotopically ‘depleted’ component in the source of the entire Madagascar province, even though the Antampombato basalts are chemically unlike the lavas and dykes with the same depleted isotopic signature found in western Madagascar. If this depleted component is plume-related, this suggests that the plume has a broadly MORB-source mantle composition. The existence of isotopically more enriched magma types in the Madagascan province has several possible petrogenetic explanations, one of which could be the interaction of plume-related melts with the deep lithospheric mantle beneath the island.