Olivine Grains Research Articles

Origins of olivine in Earth’s youngest kimberlite: Igwisi Hills volcanoes, Tanzania craton

Monomineralic millimeter-sized olivine nodules are common in kimberlites worldwide. It is generally thought that such ‘dunitic nodules’ originate from the base of the cratonic lithosphere and that their formation marks the onset of deep-rooted kimberlite magmatic plumbing systems. However, thermobarometric constraints to support such a model have been lacking thus far. This study focuses on the petrography and textures, as well as on pressure–temperature estimations, of well-preserved dunitic nodules from the Quaternary Igwisi Hills kimberlite lavas on the Tanzania craton, with the ultimate goal to constrain their origins. We utilize EBSD-determined textural information in combination with olivine geochemistry data determined by EPMA and LA-ICP-MS methods. We find that host olivine grains in these nodules are compositionally similar to olivine in garnet-facies cratonic mantle peridotites, and such an association is supported by garnet inclusions within olivine. Projection of Al-in-olivine temperatures onto a regional geotherm suggests that the host olivine grains equilibrated at ~ 100–145 km depth, which points to origins from mid-lithospheric levels down to the lower cratonic mantle if a depth of 160–180 km is considered for the lithosphere–asthenosphere transition beneath the Tanzania craton. These first pressure–temperature estimates for dunitic nodules in kimberlites suggest that their formation also occurs at much shallower depths than previously assumed. Recrystallized olivine grains (i.e., neoblasts) show random crystallographic orientations and are enriched in minor and trace elements (e.g., Ca, Al, Zn, Sc, V) compared to the host olivine grains. These features link neoblast formation to melt-assisted recrystallization of cratonic mantle peridotite, a process that persisted during kimberlite magma ascent through the lower half of thick continental lithosphere. Partial recrystallization of olivine-rich mantle xenoliths makes these materials texturally weaker and subsequent liberation of mineral grains promotes the assimilation of compositionally ‘unstable’ orthopyroxene in rising carbonate-rich melts, which is considered to be an important process in the evolution of kimberlite magmas. Dunitic nodules in kimberlites and related rocks may form as melt–rock equilibration zones along magmatic conduits within the lower half of the cratonic mantle column all the way up to mid-lithospheric depth. Such an origin potentially links dunitic nodules to olivine megacrysts, which are equally considered as melt/fluid-assisted recrystallization products of peridotitic mantle lithosphere along the ascent pathways of deep-sourced CO2–H2O-rich ultramafic melts.

Evidence from phosphorus X‐ray mapping for a multistep process in the formation of olivine phenocrysts in FeO‐rich porphyritic chondrules

AbstractPhosphorus X‐ray maps of olivine phenocrysts in many type II (FeO‐rich) porphyritic chondrules in LL3.00 Semarkona and CO3.05 Y 81020 reveal multiple sets of thin dark/bright (P‐poor/P‐rich) layers that resemble oscillatory zoning. Such discrete layers are generally not evident in BSE images or in Fe, Cr, Ca, Al, Mg, or Mn X‐ray maps because rapid diffusion of these cations in olivine at high temperatures smoothed out their initial distributions, thereby mimicking normal igneous zoning. In contrast, the relatively slow diffusion of P in olivine preserves original dendritic or hopper morphologies of olivine crystals; these skeletal structures formed during quenching after initial chondrule melting. The skeletal olivine crystals were filled in with low‐P olivine during cooling after one or more subsequent heating events, mainly involving the melting of mesostasis. Crystallization of mafic silicates depleted the mesostasis in FeO and MgO and enriched it in silico‐feldspathic components. Sectioning of the olivine grains at particular orientations can produce apparent oscillatory zoning in P. Strong evidence of a secondary melting event is evident in Semarkona chondrule H5k. Phenocryst H5k‐2 in this chondrule has a relict core (with rhythmic P zoning layers) that was fractured and severed; it is overlain by a set of differently oriented subparallel P‐poor olivine layers. Chondrule C6f from Y 81020 contains a large multi‐lobed olivine phenocryst that still preserves hopper cavities, partially outlined by P‐poor/P‐rich olivine layers. The thin P‐rich rims surrounding many olivine phenocrysts could reflect a short period of rapid grain growth after a late‐stage chondrule reheating event.

Petrology and mineralogy of an igneous clast in the Northwest Africa 1685 (LL4) chondrite: Comparison with alkali‐rich igneous clasts in LL‐chondritic breccias

AbstractWe have conducted petrological and mineralogical studies on an igneous clast in the Northwest Africa (NWA) 1685 (LL4) chondrite. This meteorite was described in the Meteoritical Bulletin as containing clasts similar to alkali‐rich clasts in LL chondritic breccias Yamato (Y)‐74442 (LL4), Bhola (LL3‐6), and Krähenberg (LL5). We carefully compared the textures, as well as mineral and matrix compositions, of the NWA 1685 clast with those of the previously described alkali‐rich clasts in the LL chondritic breccias. Olivine grains are embedded in glassy matrix and have no chemical zoning. Shock melt veins and fractures were observed only in olivine grains and did not continue into matrix. Potassium abundance of matrix glasses of the NWA 1685 clast is lower than those of alkali‐rich igneous clasts in Y‐74442, Bhola, and Krähenberg, indicating that the igneous clasts in NWA 1685 are different from the alkali‐rich clasts previously reported in the LL chondritic breccias. The NWA 1685 clast might have formed during an impact melting and quenching event on the LL‐chondrite parent body, and then been incorporated into a breccia.

Deformation of upper mantle rocks with contrasting initial fabrics in axial extension

To explore the role of the olivine grain size and crystal preferred orientation (CPO), on the evolution of the microstructure and the mechanical behavior of upper mantle rocks up to large strains, we performed axial extension experiments at 1200 °C, 300 MPa confining pressure, and constant displacement rate leading to strain rates ~10−5 s−1 on three natural peridotites: a fine-grained mylonitic harzburgite with a weak olivine CPO and two coarse-grained well-equilibrated dunites with olivine CPO of variable intensity. Despite the contrasting initial microstructures, initial flow stresses show a limited range of variation (115–165 ± 5 MPa), with the fine-grained harzburgite displaying the highest initial strength. However, the evolution of both mechanical behavior and microstructure differs markedly between fine and coarse-grained peridotites. In the fine-grained harzburgite, necking is associated with decrease in the apparent differential stress. Focusing of strain and stress resulted in increase of the olivine CPO intensity and recrystallized fraction and decrease of the recrystallized grain size in the neck. Analysis of the final stress and strain in the neck indicates softening in response to the evolution of the microstructure and CPO. In contrast, necking of the coarse-grained dunite samples is associated with either a weaker or no decrease in the apparent differential stress. This implies hardening, consistently with (1) the increase in bulk intragranular misorientation with increasing strain observed in these samples and (2) final stresses in the neck similar or higher than the initial ones. All coarse-grained dunites displayed a highly heterogeneous deformation. Crystals well oriented to deform by dislocation glide became elongated and developed marked undulose extinction, whereas crystals in hard orientations remained almost undeformed. In the neck, stress and strain concentration (local stresses and strains attained up to 365 ± 15 MPa and 240%, respectively) resulted in formation of kinks in “hard” crystals, dynamic recrystallization in “soft” crystals and, where the axial stress overcame the confining pressure, development of extensional fractures. We interpret the more effective strain-induced softening of the fine-grained peridotite as due to easier dynamic recrystallization, probably due to the higher proportion of grain boundaries acting as nucleation sites.

Textural evolution of metallic phases in a convecting magma ocean: A 3D microtomography study

The textures of solid and molten metal in the presence of varying fractions of silicate melt at high temperature have been investigated to shed light on differentiation processes occurring in magma oceans formed on rocky bodies of the early solar system. Analogue experiments have been performed in a three-phase system (composed of coexisting metal, forsterite and silicate melt) in both static (1 GPa, 1723 K) and dynamic (i.e. agitated, at 1 bar, 1713 K and 1743 K) conditions. Micro-textures were analyzed with SEM and EBSD techniques, while meso-textures of the metallic phase were analyzed using ex-situ 3D microtomography.Although all samples exhibit the same micro-scale organization consistent with the minimization of local interfacial energies, their meso-scale textures differ significantly. Static conditions produce metal grains that have shapes close to spherical, corresponding to the state predicted by the grain-scale minimization of interfacial energies. In contrast, under dynamic conditions and in the presence of high silicate melt fractions (≥50 vol%), molten metal coalesces to form pools with sizes that are several orders of magnitude larger than those predicted by grain growth mechanisms. Furthermore, in agreement with expectations based upon an interfacial energy budget, images show that nickel grains, whether solid or molten, do not occur surrounded entirely by silicate melt, but rather in contact with both forsterite crystals and silicate melt, leading to the formation of composite aggregates.Assuming that a magma ocean has less than 50 vol% of crystals (the upper limit that permits convective motion), thermodynamic calculations indicate that at the necessary temperatures, the metallic subsystem (Fe-Ni-S) of the planetesimal is entirely molten and the silicate residue is only composed of olivine. Convective motions in such a body will drive agitation, promoting the formation of composite aggregates of olivine and molten iron-sulfide, their initial coalescence and subsequent fragmentation. In detail, these composite aggregates have a reduced density contrast with the surrounding silicate melt that reduces their settling velocities compared to pure metal. They also entrain olivine during the downward migration of iron-sulfide pools. Olivine grains concentrate at the surface of the metallic pools, hindering coalescence between pools or with a pre-existing core. An alternative differentiation scenario for core formation is explored in which the simple compaction of partially molten mixtures in the basal non-convecting layer of the magma ocean expels the interstitial silicate melt upward, such that the local fraction of iron-sulfide increases by mass-balance, reaching its percolation threshold and allowing core formation. This process is not only limited to early accreted planetesimals but may also occur in terrestrial bodies.

Quantitative shock measurement of olivine in ureilite meteorites

AbstractUreilites are the second most abundant achondrite meteorite group, yet they are from an unknown source. They are ultramafic in composition and have undergone thermal metamorphism as well as shock deformation since their formation. In this work, olivine grains from six monomict ureilites, Northwest Africa 7059, Elephant Moraine 96042, Shişr 007 (all ˜shock level S3), Northwest Africa 2221 (S3‐S4), Larkman Nunatak 04315 (S5), and Allan Hills A81101 (˜S6) and one polymict ureilite, Elephant Moraine 87720 (S5), were examined by in situ micro‐X‐ray diffraction (XRD). It is observed in this study that with increasing shock, the samples develop more complicated 2‐D XRD patterns from diffracted spots, to streaks, to asterism, and “spotty” Debye rings, and these patterns correspond to shock metamorphic effects observed by optical microscopy from undulatory extinction to mosaicism and annealing or recrystallization textures. These 2‐D XRD patterns, representing olivine strain‐related mosaicity (SRM), as documented by given lattice planes, were each measured as the sum of the full‐width‐half‐maximum along the Debye ring or χ dimension (∑[FWHMχ]) to quantify the shock‐related deformation recorded in the olivine grains. In each meteorite, all olivine lattice planes show similar measured ∑(FWHMχ) values for the diffraction streaks. This work demonstrates an increasing trend of average ∑(FWHMχ) in degrees of arc, 4.0 ± 1.7°, 4.1 ± 1.4°, 3.7 ± 1.8° (all S3), 4.9 ± 2.1° (S3–S4), 7.0 ± 3.5° (S5), 13.4 ± 5.7° (S6), and 7.5 ± 3.3° (polymict, S5), which agrees well with petrographic observations. This quantitative method contributes to a more comprehensive shock classification system specifically for ureilites, complementing recently published shock classification systems for olivine‐bearing rocks.

Decorated dislocations in naturally deformed olivine with C-type fabric: A case study in the Lüliangshan garnet peridotite from the North Qaidam ultrahigh-pressure belt, NW China

This paper presents a detailed case study of decorated dislocations of olivine with C-type fabric in the Lüliangshan garnet peridotite from the North Qaidam ultrahigh-pressure (UHP) metamorphic belt in NW China. Electron backscatter diffraction (EBSD) measurements reveal a weak C-type fabric in olivine, i.e., the crystallographic [100] axes form a weak point maximum subnormal to the foliation while the [001] axes form a girdle in the foliation plane with a weak point maximum subparallel to the lineation. Oriented hydrous mineral of sodic amphibole and pyroxene were found in porphyroclastic garnet indicating considerable water content in the parental majoritic garnet at high pressure conditions. However, the water content of olivine and garnet is below the detection limit (less than a few ppm wt. H2O) of the instrument (a Nicolet 6700 FTIR with Continuum FTIR Microscope). Two distinctive deformation features of decorated olivine grains were recognized: (1) linear morphologies of many dislocations with long straight segments parallel to the [001] orientation, which are likely screw dislocations with b = [001]; (2) olivine grains with planar dislocation, subgrain boundaries or undulose extinction are scarce in thin sections under optical microscopy. The dislocation structures and densities in olivine are heterogeneous at multi-scales. The mean 3D dislocation density and the estimated differential stress based on the dislocation density piezometer are 0.4–0.8 × 10−12 m−2 and 65–108 MPa, respectively. Decorated dislocations and EBSD analyses suggest the most likely dominant activation slip systems with b = [001]. Considering that most C-type fabric of olivine are reported from UHP garnet peridotites, we emphasize the important effects of high pressure and relatively low temperature on the formation of the C-type fabric in olivine.

Shock‐Induced Incongruent Melting of Olivine in Kamargaon L6 Chondrite

AbstractIn this study, we report for the first‐time shock‐induced incongruent melting of olivine in an ordinary chondrite. Several olivine grains (Fo74), entrained in the shock‐melt vein of the Kamargaon L6 chondrite, were dissociated into magnesiowüstite (XFe = 0.71) and orthoenstatite (XFe = 0.22). We propose that the breakdown of olivine took place as a result of incongruent melting to produce magnesiowüstite and Mg‐rich liquid. We suggest that bridgmanite may have crystallized as the second phase from the olivine melt which back‐transformed to a low‐pressure phase of orthoenstatite from subsequent high‐temperature and low‐pressure events. In this case, olivine grains may have experienced pressure and temperature of ∼25 GPa and ∼2500°C, respectively. Our results suggest that the incongruent melting of olivine may possibly operate as one of the alternative mechanisms of dissociation reaction driving the phase transformation of olivine in the natural systems.

Chromitites from the Vavdos ophiolite (Chalkidiki, Greece): Petrogenesis and geotectonic settings; constrains from spinel, olivine composition, PGE mineralogy and geochemistry

Podiform chromitites of the Vavdos ophiolite are associated with dunite bodies hosted within residual mantle harzburgite. The mantle source is highly depleted and apparently developed at fore-arc settings as inferred from the presence of boninitic melts. The ascendance of these melts through dunite pathways and their subsequent interaction with the depleted harzburgite instigated chromitite mineralization within dunite. Spinel occurs in a) massive accumulations (Mg# = 0.56–0.68 and Cr# = 0.69–0.77), b) in non-massive disseminated textures (Mg# = 0.52–0.61 and Cr# = 0.80–0.81) and c) in schlieren chromitite deposits within dunite bodies (Mg# = 0.43–0.48 and Cr# = 0.79–0.82). Olivine is the main silicate inclusion in chromitites, whereas the absence of hydrous mineral phases is linked with crystallization at relatively deeper parts of the mantle.Olivine inclusions in spinel are enriched in Fo, Ni and Ca compared to the interstitial olivine grains. No compositional gaps were noted between the mineral analyses of chromitites and dunites; the calculated parental melts are also homogeneous, reinforcing the view for production of melt batches by similar processes. Platinum-group elements (PGE) concentrations and platinum-group minerals (PGM) demonstrate high (Os + Ir + Ru)/ (Rh + Pt + Pd) ratios, typical of many Tethyan ophiolitic chromitites. The main PGM phase is laurite that forms an extended solid solution series with substitution of Ru for Os + Ir, linked to the cooling of the system. Massive chromitites contain Ru-rich laurite and Os-Ir alloys whereas non-massive ores contain Ru-poor laurite. The PGM were included in fresh magnesiochromite favoring their magmatic origin and initial entrapment at ~ 1200 °C and low sulfur fugacity.

Oxygen isotope heterogeneity of olivine crystals in orogenic peridotites from Songshugou, North Qinling Orogen: Petrogenesis and geodynamic implications

AbstractOlivine grains from Songshugou mylonitized peridotite massif record δ18O both lower and higher than in pristine mantle samples in North Qinling Orogen, Central China. Olivines from dunites exhibit large variations in δ18O (4.03–7.07‰), and some porphyroclasts display negative correlations between δ18O and forsterite content {Fo; [100×Mg/(Mg+Fe2+)]}. The porphyroclast cores have low-δ18O values, indicating that they formed in the oceanic lithospheric mantle prior to subduction. We attribute low-δ18O values to seawater-peridotite interaction under high-temperature conditions. The porphyroclast rims and small olivines exhibit high-δ18O values. These features suggest that high-δ18O olivines formed during mylonitization in the exhumation process. Olivines reacted with 18O-rich melt/fluids released from subducted altered oceanic basalts and continental sediments at low temperature (<610–680 °C). The 18O-rich melt/fluids selectively affected porphyroclast rims and small olivine grains. Unlike the olivines in the dunites, the olivines and orthopyroxenes in the harzburgites show limited variations in δ18O (4.21–5.45‰ and 5.5–5.8‰, respectively), due to orthopyroxene exchange with melt/fluid at a slower rate than the coexisting olivine. The preservation of the low-δ18O signature in olivines indicates a short residence time (<20 Ma) for subducting peridotites to mantle depths.

Experimental measurements of anisotropic viscosity in naturally sourced dunite with a preexisting CPO

Naturally deformed dunite cores with a preexisting crystallographic preferred orientation (CPO), collected from a shear zone in the Josephine peridotite (southwest Oregon, USA), were deformed experimentally in triaxial compression. The compression axis was varied relative to the CPO geometry to measure the anisotropy in viscosity of these rocks. Deformation experiments were performed in a gas-medium apparatus at three constant displacement-rate steps, at a temperature of 1250 °C and a confining pressure of 300 MPa. Cores were dehydrated at 1200 °C in a controlled CO/CO2 atmosphere prior to deformation to achieve nominally dry conditions. Data from individual experiments were fit by a power-law, yielding a stress exponent of n ≈ 3.6, indicative of deformation by dislocation creep. The maximum difference in viscosity was a factor of 2.6 at constant stress. The CPO of the olivine grains was measured after deformation by electron backscatter diffraction (EBSD). The flow stress correlated with the mean Schmid factor (or resolved shear stress) on the easiest dislocation slip systems, calculated from grain orientations determined from EBSD analyses. Analysis of our data with a simplified effective-medium model, which incorporated constraints from published single-crystal flow laws, demonstrated the link between CPO and anisotropy in viscosity. Predictions from viscoplastic self-consistent (VPSC) models support our simplified analysis of the measured anisotropy.

Isua (Greenland) ~3700 Ma meta-serpentinite olivine Mg# and δ18O signatures show connection between the early mantle and hydrosphere: Geodynamic implications

The ~ 3700 Ma Inner Arc Group of the Isua supracrustal belt (Greenland) contains a 10 km long strip of ultramafic schists with two ≤ 1 km long meta-dunite lenses, preserving relict olivine + antigorite + titano-clinohumite and titano-chondrodite ~ 2.6 GPa ultra-high-pressure (UHP) assemblages. There are two distinct relict meta-peridotite variants: The southern lens ‘A’ variant is dominated by an aggregate of Fo91-92 olivine with δ18OVSMOW of + 5.4 ± 0.1‰ (95% confidence) in which are rare small clinopyroxene inclusions, plus rare interstitial accessory chrome spinel partly altered to magnetite. The olivine grains are bounded by serpentine domains, which are intergrown with the olivine margins with a marginally more iron-rich composition of Fo90.5. The northern lens ‘B’ contains coarser-grained Fo96-98 olivine which encloses magnetite as clusters and trails, some arranged with 60°-120° conjugate intersections. The Fo96-98 olivine has δ18OVSMOW of + 3.2 ± 0.2‰ and is in equilibrium with high-Al antigorite and the accessory UHP Ti-rich minerals.We interpret that the Fo90-92 versus the Fo96-98 olivine assemblage variants are caused by a varying degree of fluid influx during ca. 3700 Ma serpentinisation, prior to peak UHP metamorphism. The Fo90-92 variant was a rock-dominated system with a postulated early serpentine + ferroan brucite alteration assemblage, with abundant relict mantle-like δ18OVSMOW + 5.4‰ olivine. The Fo96-98 variant was a fluid-dominated system, where obliteration of the protolith olivine formed serpentine + magnetite + brucite ± magnesite. During prograde UHP metamorphism consumption of brucite by reaction with serpentine gave rise to a lower δ18OVSMOW Fo96-98 olivine + serpentine assemblage. Both varieties have similar bulk chemistry, interpreted as depleted mantle that was enriched in LILE and LREE by a melt or fluid, prior to varying degrees of serpentinisation and superimposed UHP metamorphism.In Phanerozoic abyssal peridotites similar textural, chemical and isotopic variations are the result of rock versus fluid-dominated serpentinisation and variable alteration temperatures. Analogously, the Fo96-98 Isua peridotites are interpreted as mantle peridotite entirely serpentinised by high temperature seawater in a fluid-dominated environment and then metamorphosed under UHP conditions at a convergent plate boundary. This indicates lateral lithosphere motions facilitating hydrosphere - mantle communication early in Earth’s history.

Presence of detrital olivine and serpentine minerals in the Dihing unit of upper Assam: Implication towards the source

A significant occurrence of olivine and serpentine (antigorite, lizardite) are reported for the first time in the Dihing sedimentary unit of upper Assam. Petrography of the Dihing Formation documents olivine, serpentine (antigorite, lizardite), pyroxene and major framework grains (quartz, feldspar and variety of lithic fragments). Based on modal composition, the Dihing unit samples are classified as lithic arenite. Heavy mineral analysis in addition to the above, provides evidence of other minerals such as zircon, tourmaline, rutile, garnet, epidote, amphibole, sphene, aluminosilicates, apatite, spinel, staurolite, chloritoid, chlorite and mica. The heavy mineral assemblage is dominated by olivine (36.9%) and pyroxene (16.5%). The low (10.7%) zircon–tourmaline–rutile (ZTR) index infers mineralogically immature nature of sediments. Raman spectroscopy confirms the presence of antigorite and electron microprobe analysis corroborates the existence of olivine, pyroxene, serpentine and chrome-spinel in the analyzed samples. The imbricated clasts and cross-bedding sedimentary structures indicate SSW to SW paleo-current direction. The peridotite and serpentinite rocks of the Tidding suture zone from the Lohit–Dibang valleys are suggested as the likely sediment provenance for olivine and serpentine minerals. We advocate that the Lohit and Dibang rivers earlier flowed close to the Indo-Burman Ranges (IBR) depositing the Plio-Pleistocene Dihing sediments before merging with the Brahmaputra river.

Strain Localization in the Root of Detachment Faults at a Melt‐Starved Mid‐Ocean Ridge: A Microstructural Study of Abyssal Peridotites From the Southwest Indian Ridge

AbstractDetachment faults that exhume mantle peridotites to the seafloor play a major role in the accommodation of plate divergence at slow‐spreading ridges. Using 99 samples of partially serpentinized peridotites dredged from a nearly amagmatic segment of the eastern part of the Southwest Indian Ridge, we characterize the deformation processes active in the root zone of detachment fault systems. The deformation is heterogeneous even at the sample scale and combines both brittle and crystal‐plastic mechanisms. Strain localization is initially controlled by strength contrasts at the grain scale between olivine and orthopyroxene and between variably oriented olivine crystals. Orthopyroxene deformation is primarily brittle (microfractures), but kink bands and dynamic recrystallization are locally observed. In contrast, olivine deforms primarily by dislocation creep with dynamic recrystallization under high deviatoric stresses (80–270 MPa). Olivine grains poorly oriented to deform by dislocation glide display kink bands and localized microfractures. Dynamic recrystallization controlled by strain and stress concentrations produce anastomosing zones of grain size reduction (GSR). GSR zones contain limited late to post‐kinematic amphibole, suggesting the presence of small volumes of hydrous fluids. Plagioclase, when present, is post‐kinematic. This heterogeneous high‐stress deformation is observed, with variable intensity, in every sample investigated, suggesting that it was pervasively distributed in the root region of axial detachments. Abyssal peridotite samples from more magmatically robust slow mid‐ocean ridges do not show this pervasive high stress deformation microstructure, implying magma, when present, tends to localize most of the strain at the root of axial detachment systems.

Characterizing the spectral, microstructural, and chemical effects of solar wind irradiation on the Murchison carbonaceous chondrite through coordinated analyses

We performed H+ and He+ irradiation experiments on slabs of the Murchison CM2 meteorite to simulate solar wind irradiation of carbonaceous asteroids. Two separate 6 mm × 6 mm regions were irradiated with 1 keV H+ and 4 keV He+, respectively, to fluences of 8.1 × 1017 ions/cm2 for H+ and 1 × 1018 ions/cm2 for He+. Unirradiated and irradiated surfaces were analyzed using X-ray photoelectron spectroscopy (XPS), visible to near-infrared spectroscopy (VNIR; 0.35–2.5 μm), and microprobe two-step laser-desorption mass spectrometry (μL2MS). We also performed analytical field-emission scanning transmission electron microscopy (FE-STEM) on focused ion beam (FIB) cross-sections extracted from olivine grains and matrix material within the H+- and He+-irradiated regions. In situ XPS analyses suggest that ion irradiation results in the removal of most surface carbon and the partial reduction of surface iron to lower oxidation states. In response to He+-irradiation, we observe brightening (longward of ~0.75 μm) and reddening of reflectance spectra, which is a departure from typical lunar-style space weathering. Additionally, H+- and He+-irradiation have opposing effects on organic carbon content: H+-irradiation increases the abundance of some low-molecular-weight free organic species by breaking down macromolecular material while He+-irradiation causes a decrease in overall organic content by cleaving bonds and sputtering constituent atoms. This suggests that solar wind H+-irradiation and solar wind He+-irradiation change the organic functional group chemistry of asteroidal regolith in different ways. In contrast to some previous experimental space weathering studies, we observe an increase in H2O and OH− abundances in our sample in response to both types of ion irradiation. FE-STEM and energy dispersive X-ray spectroscopy (EDX) analyses show complete amorphization of matrix phyllosilicates in ion-affected rims, partial amorphization of olivine, and changes in Si and Mg concentrations at and/or near the surface. We discuss the implications of these results for understanding the complex nature of space weathering on primitive, carbon-rich asteroids and for analyzing future returned samples from carbonaceous asteroids Bennu and Ryugu.

Short lifespans of serpentinization in the rocky core of Enceladus: Implications for hydrogen production

The discovery of a liquid ocean on Saturn's small moon Enceladus and evidence of modern hydrothermal activity provide an unexpected new environment in which to expand the search for life. However, as with the age of the moons themselves, the age of the liquid ocean and any hydrothermal activity therein remains an area of debate. Based on physical and chemical observations from the Cassini mission we can apply known mineral dissolution rates, estimated water-rock ratios from Enceladus' density, and variable water flow rates within the rocky core to constrain durations of active serpentinization. On this basis we developed a 1-D reactive transport model to compare the effect of initial olivine percentage, grain size, temperature, and flow rate on timespans of primary olivine alteration in a rocky core the size and density of Enceladus'. In most cases, olivine alteration and precipitation of hydrous secondary minerals results in a water-limited alteration regime. An alteration front that propagates in the direction of water flow then controls the overall rate of olivine alteration. Of the parameters explored, high initial olivine percentages and slow fluid flow rates were the strongest predictors of long serpentinization times, while temperature and grain size had a smaller effect. The annual global H2 production rate in all model cases (> 1 × 1012 mol yr−1) is several orders of magnitude greater than the minimum H2 release rate calculated from the observed H2 in Enceladus' plume (1 × 109 mol yr−1), suggesting that any ongoing active serpentinization processes in the core are likely nearing completion. The longest timescales indicate the potential for olivine alteration and H2 production for up to ~75 Myr, consistent with weathering rates of terrestrial peridotite massifs. If the H2 produced from Enceladus is sourced from primary mineral alteration, these results suggest that hydrothermal activity in the core of Enceladus may have developed only very recently – even as recent as within the past 100 Myr.

Microtextures in the Chelyabinsk impact breccia reveal the history of Phosphorus‐Olivine‐Assemblages in chondrites

AbstractThe geochemistry and textures of phosphate minerals can provide insights into the geological histories of parental asteroids, but the processes governing their formation and deformation remain poorly constrained. We assessed phosphorus‐bearing minerals in the three lithologies (light, dark, and melt) of the Chelyabinsk (LL5) ordinary chondrite using scanning electron microscope, electron microprobe, cathodoluminescence, and electron backscatter diffraction techniques. The majority of studied phosphate grains appear intergrown with olivine. However, microtextures of phosphates (apatite [Ca5(PO4)3(OH,Cl,F)] and merrillite [Ca9NaMg(PO4)7]) are extremely variable within and between the differently shocked lithologies investigated. We observe continuously strained as well as recrystallized strain‐free merrillite populations. Grains with strain‐free subdomains are present only in the more intensely shocked dark lithology, indicating that phosphate growth predates the development of primary shock‐metamorphic features. Complete melting of portions of the meteorite is recorded by the shock‐melt lithology, which contains a population of phosphorus‐rich olivine grains. The response of phosphorus‐bearing minerals to shock is therefore hugely variable throughout this monomict impact breccia. We propose a paragenetic history for P‐bearing phases in Chelyabinsk involving initial phosphate growth via P‐rich olivine replacement, followed by phosphate deformation during an early impact event. This event was also responsible for the local development of shock melt that lacks phosphate grains and instead contains P‐enriched olivine. We generalize our findings to propose a new classification scheme for Phosphorus‐Olivine‐Assemblages (Type I–III POAs). We highlight how POAs can be used to trace radiogenic metamorphism and shock metamorphic events that together span the entire geological history of chondritic asteroids.

The Critical Role of Fluid‐Mediated Diffusion in Anomalous Fe‐Mg‐O Isotope Fractionations in Ultramafic Rocks of Ophiolites

AbstractA number of bulk‐rock and mineral Fe and Mg isotope data have been increasingly reported from ophiolitic rocks, but their anomalous fractionations are still not well addressed. This study presents new Fe, Mg, and O isotopes and published Li isotope data of olivine and/or chromite in chromitite, dunite, and harzburgite from the Pozantı‐Karsantı ophiolite (southern Turkey) to reveal the mechanisms of inter‐ and intra‐mineral isotope fractionation during chromitite formation. In the studied samples, both olivine and chromite have highly variable δ56Fe ranging from −0.125‰ to 0.101‰ and −0.261‰ to 0.191‰, respectively, but with heavier olivine than coexisting chromite in most samples. In contrast, olivine records homogeneous Mg isotopic compositions with δ26Mg values of −0.36‰ to −0.26‰, whereas chromite grains have variable but relatively high δ26Mg values ranging from −0.02‰ to 0.45‰. The δ18O values of olivine range from 4.65‰ to 5.86‰ and co‐vary with δ7Li (−5.43‰ to 16.65‰), δ56Fe, and δ26Mg, particularly at lithological scale. These isotopic features cannot be explained solely by subsolidus diffusion between olivine and chromite, or by partial melting and fractional crystallization. Instead, they require fluids as a diffusive medium for high efficient element exchange between chromite and olivine. The fluids were likely the hydrous fluids that were collected on chromite grain surfaces during crystallization and released later from chromite piles during precipitation. The fluid‐olivine reaction preferentially transferred light Fe from olivine grains resulting in elevated δ56Fe, but caused small O and Mg isotopic variations in the olivine. The Li diffusion depends on Li concentration gradient between olivine and fluid and olivine/fluid ratio, which can account for large δ7Li variations in olivine between different rock types. As these reactions proceeded, they induced elemental and isotopic variations in the fluids. These fluids could continuously penetrate the harzburgites through the dunite envelopes around the chromitites, producing some of the observed anomalous geochemical features of the harzburgites. Thus, we conclude that the hydrous fluids acted as critical media for ion exchange between chromite and olivine and for significant modification of the host harzburgites.

Trace elements in olivine: Proxies for petrogenesis, mineralization and discrimination of mafic-ultramafic rocks

Olivine is a ubiquitous mineral in mafic-ultramafic rocks and has been widely used as a mineral marker in various geological processes. However, its development of trace elements is limited. Here we present newly-obtained trace element data 7Li, 27Al, 29Si, 31P, 43Ca, 45Sc, 49Ti, 51V, 53Cr, 55Mn, 59Co, 60Ni, and 66Zn of olivine in typical mantle xenoliths, mantle peridotites in ophiolites, and plutonic rocks from layered and Alaskan-type intrusions to develop trace element proxies for the petrogenesis, mineralization and discrimination of various mafic-ultramafic rocks. Residual olivine grains in mantle xenoliths and ophiolitic peridotites, which represent residues of mantle melting, have higher Ni/Co (>20) and Ni/Mn (>2) ratios than magmatic olivine (Ni/Co < 20, Ni/Mn < 2), which are consistent with the compatibilities of these elements during partial melting and magma differentiation. Lower Ni content, and lower Ni/Co and Ni/Mn ratios at a given Fo content can distinguish olivine in Alaskan-type intrusions from layered intrusions, reflecting the nature of their mantle sources. The V and Sc contents and V/Sc ratios in olivine can distinguish mantle xenoliths (V > 2 ppm, V/Sc > 0.5) from ophiolitic peridotites (V < 2 ppm, V/Sc < 0.5), indicating a more reduced state of continental lithospheric mantle compared to the oceanic lithospheric mantle. As a consequence, the four occurrences of mafic-ultramafic rocks can be distinguished by olivine with (Sc × 10)-(Ti × 2)-Zn and V/Sc-(Co/Ni × 2)-(Zn/Mn × 5) ternary diagrams. In addition, Li, Ti and P contents in olivine are good tracers of melt/fluid metasomatism, whereas Ni/Co, Ni/Mn and Mn/Zn ratios are indicators of chromite mineralization. Therefore, trace elements in olivine can be used as chemical proxies to distinguish the origin of various mafic-ultramafic rocks, as well as the processes by which they evolved.

Ultramagnesian Olivine in the Monchepluton (Fo96) and Pados-Tundra (Fo93) Layered Intrusions (Kola Peninsula)

Abstract —The paper focuses on compositional variations of olivine and chromian spinel in the Monchepluton and Pados-Tundra layered intrusions, which host significant chromitite mineralization. Ore-bearing dunite (with up to 25–30 vol.% Mcr) in the Sopcheozerskoe chromite deposit from the Monchepluton complex, Kola Peninsula, Russia, bears an assemblage of phases with exceptionally high magnesium contents: Fo96 + augite (Mg# = 94) + magnesiochromite, Mcr (Mg# ≈ 65); Mg# = 100·Mg/(Mg + Fe2+ + Mn). However, olivine in the host dunite has normal maximum values of Mg# comparable to those in cumulus olivine from layered intrusions worldwide (Fo≤91–92). The Fo96 phase in the Sopcheozerskoe deposit shows the most primitive composition ever reported from any layered intrusion. Magnesiochromite occurs as unzoned homogeneous euhedral crystals unaffected by subsolidus exchange or metasomatic effects. Olivine in ore-bearing dunite (20–25 vol.% magnesian chromite) from the Pados-Tundra complex attains Fo93, with the Mg# value notably higher than the range (Fo85.5–90.6) in olivine from orthopyroxenite, harzburgite, and dunite within the intrusion. Olivine and chromian spinel in the two complexes behave coherently, with covarying patterns of Mg# and Ni contents in olivine at R = 0.75 (n = 160) and positive correlation between Mg# in coexisting chromian spinel and olivine grains at R = 0.8 (n = 150). This behavior indicates that the two phases attained equilibrium during crystallization. It appears unlikely that the extremely high Mg enrichment in olivine (Fo96), as well as in all associated phases of the Monchepluton complex, would result from a subsolidus reaction between olivine and chromian spinel or low-temperature alteration of olivine. We suggest a more realistic explanation that the olivine (+ high-Mg augite)–chromian spinel assemblage crystallized from komatiitic magma under the conditions of progressively increasing oxygen fugacity (fO2). The high Mg# in the Mcr-chromite-enriched system, above the maximum values common in cumulus olivine from layered intrusions (up to Fo96 against Fo≤91–92), may be caused by shortage of ferrous iron.