Olivine In Rocks Research Articles

Composition and Mineralogical Features of Ultramafic Rocks of Eastern Part of Southern Kraka Massif (South Urals)

The paper presents new data on the structure and composition of ultramafic rocks in the eastern part of the South Kraka massif. It is shown that the studied area is composed predominantly of spinel peridotites, among which the main role belongs to lherzolites with a low content of clinopyroxene (2–5%) and harzburgites, among which lens-shaped and bandshaped inclusions of monomineral olivine rocks – dunites – are quite often observed. The leading role in the structure of the rocks belongs to high-Mg olivine (Fo87-94), a secondary role (2016). Nature of the lithospheric mantle beneath the Arabian Shield and genesis of Al-spinel micropods: Evidence from the mantle xenoliths of Harrat is played by high-Mg orthopyroxene (enstatite) and Ca-Mg clinopyroxene diopside. Rare minerals of the mantle stage are amphibole and plagioclase. A typical accessory mineral of ultramafic rocks is spinel, the composition of which varies from high-Al in lherzolites (Cr# = 0.16–0.3), to moderate-Al in harzburgites (Cr# = 0.3–0.55) and to high-Cr in dunites (Cr# = 0.6–0.85). Of the rare accessory minerals in the rocks, the following were identified: native copper, pentlandite, awaruite and PGM (laurite, irarsite, Os-Ir-Ru-alloys). The section of mantle ultramafic rocks of the Sargan Range completed its high-temperature evolution to the levels of the plagioclase peridotite facies, partly in the transition zone from spinel to plagioclase facies. An assessment of oxygen fugacity allows us to speak about more reducing conditions for the formation of rocks of the studied area compared to other similar formations of upper mantle origin and allows us to classify the studied ultramafic rocks as moderately depleted restites of the upper mantle under the riftogenic structure of the Paleo-Ural basin. The geochemical specialization of PGM also indicates a restite origin of ultramafic rocks. At the stage of cooling and tectonic transformations in the crust, ultramafic rocks underwent serpentinization.

Green synthesis of nano-silica from olivine rock and its impact on the mechanical performance of geopolymer concrete composites

Green synthesis of nano-silica from olivine rock and its impact on the mechanical performance of geopolymer concrete composites

A nonlinear creep model of rocks based on memory-dependent derivative

The creep behavior of soil and rock exhibits significant time-dependence and non-linearity, which are difficult to capture using traditional composite element models. In the framework of memory-dependent derivative theory, a memory-dependent visco-pot element with non-linear creep behavior, no instantaneous deformation, elastic after-effect, and stress relaxation is proposed. By connecting a memory-dependent visco-pot element with strain triggering to the Nishihara model, a memory-dependent derivative creep model is developed and its constitutive equation is derived. The creep test data pertaining to coal, salt, sandstone, and olivine rock are used to validate the model, which can describe the initial creep, steady-state creep, and non-linear accelerated creep deformation characteristics of various soil and rock materials. Comparison with fractional-order models, improved Nishihara models, and Hehai models shows that our model has better fitting results and is more advantageous in characterizing the non-linear features of soil and rock during the accelerated creep stage.

The Monticellite-Bearing Rocks of the Krestovskaya Intrusion: the Genesis According to Melt Inclusion Study

The investigation of monticellitolites and olivine-monticellite rocks from the Krestovskaya Intrusion shows that the principal minerals (olivine and monticellite) contain higher amount of MgO than same minerals in olivinites and kugdites of the Intrusion. In the studied rocks olivine contains 90–93 mol. % Fo and monticellite has 41.6–42.3 mol. % Fo whereas in olivinites and kugdites olivine and monticellite contains 86–87 mol. % Fo, and 37.2–41.2 mol. % Fo, respectively. Melt inclusion study in minerals of monticellite rocks evidenced that the monticellite rocks of the Krestovskaya Intrusion were formed due to mixing melts different in composition and volatiles: K-rich high-iron low-alumina kamafugitic melt and Na-rich high-magnesia high-alumina picritic one. Minerals crystallized at high temperatures in the following sequence: perovskite I (1250–1230°C) → perovskite II (≥1200°C) ↔ olivine (1200°C) → monticellite (1150°C). Perovskite I in monticellite rocks, as well as olivine in olivinites, crystallized from K-rich high-iron (Mg # = MgO/(MgO + FeO) – 0.37), low-alumina kamafugitic melt. During crystallization of late perovskite II in monticellite rocks, the melt became more magnesian (Mg# = 0.41) and richer in Na2O and Al2O3, which is intermediate in composition between kamafugite and alkali picritoid. Olivine in monticellite rocks crystallized from melts similar in composition to melilitite, having a K-rich composition with Mg # = 0.39, whereas monticellite formed from a heterogeneous high-magnesian Si-undersaturated melt, which is highly enriched with volatile components (including H2O) and salts. The crystallization of minerals was accompanied by accumulation of volatile components in the mixing melts and as a result by the further processes of silicate-carbonate liquid immiscibility under 1250–1190°C and by multiphase carbonate-salt immiscibility under below 1190°C. In the latter event, the separated carbonate melt began to decompose into simpler immiscible fractions: alkali-sulfate-carbonate, alkali-phosphate-carbonate and calcio-carbonate.

Geochronology, geochemistry, Sr–Nd–Hf isotope composition of the late Permian adakite in West Ujimqin, Inner Mongolia: petrogenesis and tectonic implications

We report herein on new zircon U–Pb ages, the major and trace elements of whole-rock, and the Sr–Nd–Hf isotope composition for adakitic intrusives collected from the West Ujimqin district in the Southeast region of the Central Asian Orogenic Belt (CAOB). These data provide important constraints on the petrogenetic evolution and geodynamic setting of late Permian magmatism in the Southeast CAOB. The U–Pb dating of zircon shows that the ages of Seerbeng pluton and Nuhetingshala pluton in West Ujimqin are 255.3 ± 0.71 and 254.4 ± 1.2 Ma, respectively, which signifies that these are products of magmatic activity in the late Permian. The adakitic intrusives are characterized by high levels of Sr (Sr ≥ 741 ppm), low Y, low Yb, high Sr:Y ratios, and strongly fractionated rare earth elements (10.3 < LaN/YbN < 22.5), which is similar to the features of the adakite. The magmatic zircons exhibit positive Hf values (+8.1 to +13.3), and young two-stage model ages vary from 430 to 760 Ma. The high εNd(t) and low (87Sr:86Sr)i indicate that the adakitic granite derived from the partial melting of subducted oceanic slab. The high level of Mg# [100 × Mg/(Mg + Fe) in atomic number] and abundant Cr–Ni indicate that magmatic melts interacted with olivine rocks in the mantle. Considering these results and the regional rock assemblies, we conclude that the Paleo-Asian Ocean had not yet completely closed in the late Permian, and northward subduction continued, with the subducted slab possibly breaking off.

ARPA–E explores paths to emissions-free metal making

The agency is seeking advice from industry and academia on how to make mining, smelting, and remediation less polluting and more energy efficient.

Oxygen Isotope Fractionation between Phenocrysts and Melt: Equilibrium Estimation for the Alkaline Lavas of Changbaishan Volcano (Northeast China)

The mechanisms of the occurrence of nonequilibrium oxygen isotope fractionation between olivine and plagioclase phenocrysts and the groundmass of rocks (from alkaline basalts to alkaline rhyolites) of Changbaishan volcano are considered. The basaltic trachyandesite–trachyte–comendite–pantellerite rock series shows a systematic increase of the δ18O values of feldspar (from 5.4 to 7.8‰) and the rock groundmass (from 5.6 to 7.1‰). In contrast, the δ18О values of olivine decrease from 5.35‰ typical of mantle peridotites to significantly lower values (3.88–4.14‰). This is accompanied by changes in olivine composition from the Mg-rich (Fo = 74–79) to almost pure fayalite (Fo = 1), thus indicating a positive correlation between the Mg–number and δ18О of olivine. It is shown that feldspar and the rock groundmass are close to the oxygen isotopic equilibrium, whereas olivine–rock groundmass and olivine–feldspar pairs did not approach the oxygen isotopic equilibrium. The main mechanisms leading to the oxygen isotopic disequilibrium in the mineral–melt system are considered. An approach to the estimation of oxygen isotope equilibrium between the phenocrysts and melts is proposed. It takes into account the degree of melt polymerization (NBO/T ratio). This approach makes it possible to explain the positive correlation between δ18О values and olivine Mg-numbers in the Changbaishan lavas, as well as the occurrence of low δ18O values (down to +3‰) in ferrous olivine from alkaline–salic rocks. The δ18О values observed in the olivine and the rock groundmass reflect changes in the degree of melt polymerization during crystal fractionation. The proposed model does not require an additional geological process, in particular, the contribution of a low–δ18О material in the formation of the Changbaishan differentiated rock series.

Igneous Rocks, Fluidolites, and Rodingites of Paleocene Explosive Structures of the Taukha Terrane (Sikhote-Alin)

The Taukha terrane of the Sikhote-Alin hosts outcrops of rocks with unusual structural and textural features, the compositions of which vary from ultrabasic to intermediate and from subalkaline to alkaline. These are hornblendites, mica–feldspar, amphibole–feldspar, and carbonatite-like rocks of vent facies, a dike of phlogopite–olivine rocks, and a small intrusion of melanocratic amphibole gabbro. The magmatic rocks, together with fluidolites and rodingites, compose explosive structures of Paleocene age. Rodingites were formed via the development of explosive structures resulting from metasomatic replacement of basic vent-facies rocks and small intrusive bodies in sedimentary rocks. The Paleocene explosive structures of the Mokrushinskaya and Shirokopadninskaya areas of the Olginsky ore district and breaking of Triassic carbonaceous silicites are genetically related to Au–Pd–Pt mineralization, the specific features of which are “cupriferous gold” (including the Zn- and Ni-bearing varieties of cuproauride and auricupride), native elements, and intermetallic compounds.

Production and Application of Olivine Nano-Silica in Concrete

The aim of this research was to produce nano silica by synthesis of nano silica through extraction and dissolution of ground olivine rock, and applied the nano silica in the design concrete mix. The producing process of amorphous silica used sulfuric acid as the dissolution reagent. The separation of ground olivine rock occurred when the rock was heated in a batch reactor containing sulfuric acid. The results showed that the optimum mole ratio of olivine- acid was 1: 8 wherein the weight ratio of the highest nano silica generated. The heating temperature and acid concentration influenced the mass of silica produced, that was at temperature of 90 °C and 3 M acid giving the highest yield of 44.90%. Characterization using Fourier Transform Infrared (FTIR ) concluded that amorphous silica at a wavenumber of 1089 cm-1 indicated the presence of siloxane, Si-O-Si, stretching bond. Characterization using Scanning Electron Microscope – Energy Dispersive Spectroscopy (SEM-EDS) showed the surface and the size of the silica particles. The average size of silica particles was between 1-10 μm due to the rapid aggregation of the growing particles of nano silica into microparticles, caused of the pH control was not fully achieved.

Localized slip controlled by dehydration embrittlement of partly serpentinized dunites, Leka Ophiolite Complex, Norway

Dehydration of partly or completely serpentinized ultramafic rocks can increase the pore fluid pressure and induce brittle failure, a process referred to as dehydration embrittlement. However the extents of strain localization and unstable frictional sliding during deserpentinization are still under debate. In the layered ultramafic sections of the Leka Ophiolite Complex in the Central Norwegian Caledonides, prograde metamorphism of serpentinite veins led to local fluid production and to the growth of Mg-rich and coarse-grained olivine with abundant magnetite inclusions and δ18O values 1.0–1.5‰ below the host rock. Embrittlement associated with the dehydration caused faulting along highly localized (<10 μm-wide) slip planes near the centers of the original serpentinite veins and pulverization of wall rock olivine. These features along with an earthquake-like size distribution of fault offsets suggest unstable frictional sliding rather than slower creep. Structural heterogeneities in the form of serpentinite veins clearly have first-order controls on strain localization and frictional sliding during dehydration. As most of the oceanic lithosphere is incompletely serpentinized, heterogeneities represented by a non-uniform distribution of serpentinite are common and may increase the likelihood that dehydration embrittlement triggers earthquakes.

Heterogeneous mineral assemblages in martian meteorite Tissint as a result of a recent small impact event on Mars

The microtexture and mineralogy of shock melts in the Tissint martian meteorite were investigated using scanning electron microscopy, Raman spectroscopy, transmission electron microscopy and synchrotron micro X-ray diffraction to understand shock conditions and duration. Distinct mineral assemblages occur within and adjacent to the shock melts as a function of the thickness and hence cooling history. The matrix of thin veins and pockets of shock melt consists of clinopyroxene+ringwoodite±stishovite embedded in glass with minor Fe-sulfide. The margins of host rock olivine in contact with the melt, as well as entrained olivine fragments, are now amorphosed silicate perovskite+magnesiowüstite or clinopyroxene+magnesiowüstite. The pressure stabilities of these mineral assemblages are ∼15GPa and >19GPa, respectively. The ∼200-μm-wide margin of a thicker, mm-size (up to 1.4mm) shock melt vein contains clinopyroxene+olivine, with central regions comprising glass+vesicles+Fe-sulfide spheres. Fragments of host rock within the melt are polycrystalline olivine (after olivine) and tissintite+glass (after plagioclase). From these mineral assemblages the crystallization pressure at the vein edge was as high as 14GPa. The interior crystallized at ambient pressure. The shock melts in Tissint quench-crystallized during and after release from the peak shock pressure; crystallization pressures and those determined from olivine dissociation therefore represent the minimum shock loading. Shock deformation in host rock minerals and complete transformation of plagioclase to maskelynite suggest the peak shock pressure experienced by Tissint⩾29–30GPa. These pressure estimates support our assessment that the peak shock pressure in Tissint was significantly higher than the minimum 19GPa required to transform olivine to silicate perovskite plus magnesiowüstite.Small volumes of shock melt (<100μm) quench rapidly (0.01s), whereas thermal equilibration will occur within 1.2s in larger volumes of melt (1mm2). The apparent variation in shock pressure recorded by variable mineral assemblages within and around shock melts in Tissint is consistent with a shock pulse on the order of 10–20ms combined with a longer duration of post-shock cooling and complex thermal history. This implies that the impact on Mars that shocked and ejected Tissint at ∼1Ma was not exceptionally large.

The variation in composition of ultramafic rocks and the effect on their suitability for carbon dioxide sequestration by mineralization following acid leaching

AbstractCarbon dioxide capture and storage by mineralization has been proposed as a possible technology to contribute to the reduction of global CO2 levels. A main candidate as a feed material, to supply Mg cations for combination with CO2 to form carbonate, is the family of ultramafic rocks, Mg‐rich silicate rocks with a range of naturally occurring mineralogical compositions. A classification scheme is described and a diagram is proposed to display the full range of both fresh and altered ultramafic rock compositions. This is particularly for the benefit of technologists to raise the awareness of the variation in possible feedstock materials. A systematic set of acid leaching experiments, in the presence of recyclable ammonium bisulphate, has been carried out covering the range of ultramafic rock compositions. The results show that lizardite serpentinite releases the most Mg with 78% removed after 1 h, while an olivine rock (dunite) gave 55% and serpentinized peridotites intermediate values. Antigorite serpentinite only released 40% and pyroxene‐ and amphibole‐rich rocks only 25%, showing they are unsuitable for the acid leaching method used. This wide variation in rock compositions highlights the necessity for accurate mineralogical characterization of potential resources and for technologists to be aware of the impact of feed material variations on process efficiency and development.

Application of the Bammann inelasticity internal state variable constitutive model to geological materials

SUMMARY We describe how the Bammann internal state variable (ISV) constitutive approach, which has proven highly successful in modelling deformation processes in metals, can be applied with great benefit to silicate rocks and other geological materials in modelling their deformation dynamics. In its essence, ISV theory provides a constitutive framework to account for changing history states that arise from inelastic dissipative microstructural evolution of a polycrystalline solid. In this paper, we restrict our attention to a Bammann ISV elastic-viscoplastic model with temperature and strain rate dependence and use isotropic hardening and anisotropic hardening as our two ISVs. We show the Bammann model captures the inelastic behaviour of olivine aggregates (with and without water), lherzolite (with and without water), Carrara marble and rock salt using some experimental data found in the literature. These examples illustrate that when more experimental stress–strain data are gathered on other rock materials, much more realistic numerical simulation of rock behaviour becomes feasible. Though not available in the literature, we outline a set of experiments to obtain unique Bammann ISV model constants.

Combined U–Th/He and 40Ar/ 39Ar geochronology of post-shield lavas from the Mauna Kea and Kohala volcanoes, Hawaii

Late Quaternary, post-shield lavas from the Mauna Kea and Kohala volcanoes on the Big Island of Hawaii have been dated using the 40Ar/ 39Ar and U–Th/He methods. The objective of the study is to compare the recently demonstrated U–Th/He age method, which uses basaltic olivine phenocrysts, with 40Ar/ 39Ar ages measured on groundmass from the same samples. As a corollary, the age data also increase the precision of the chronology of volcanism on the Big Island. For the U–Th/He ages, U, Th and He concentrations and isotopes were measured to account for U-series disequilibrium and initial He. Single analyses U–Th/He ages for Hamakua lavas from Mauna Kea are 87 ± 40 to 119 ± 23 ka (2 σ uncertainties), which are in general equal to or younger than 40Ar/ 39Ar ages. Basalt from the Polulu sequence on Kohala gives a U–Th/He age of 354 ± 54 ka and a 40Ar/ 39Ar age of 450 ± 40 ka. All of the U–Th/He ages, and all but one spurious 40Ar/ 39Ar ages conform to the previously proposed stratigraphy and published 14C and K–Ar ages. The ages also compare favorably to U–Th whole rock–olivine ages calculated from 238U– 230Th disequilibria. The U–Th/He and 40Ar/ 39Ar results agree best where there is a relatively large amount of radiogenic 40Ar (>10%), and where the 40Ar/ 36Ar intercept calculated from the Ar isochron diagram is close to the atmospheric value. In two cases, it is not clear why U–Th/He and 40Ar/ 39Ar ages do not agree within uncertainty. U–Th/He and 40Ar/ 39Ar results diverge the most on a low-K transitional tholeiitic basalt with abundant olivine. For the most alkalic basalts with negligible olivine phenocrysts, U–Th/He ages were unattainable while 40Ar/ 39Ar results provide good precision even on ages as low as 19 ± 4 ka. Hence, the strengths and weaknesses of the U–Th/He and 40Ar/ 39Ar methods are complimentary for basalts with ages of order 100–500 ka.

Coastal spreading of olivine to control atmospheric CO 2 concentrations: A critical analysis of viability

Qualitative proposals to control atmospheric CO 2 concentrations by spreading crushed olivine rock along the Earth's coastlines, thereby accelerating weathering reactions, are presently attracting considerable attention. This paper provides a critical evaluation of the concept, demonstrating quantitatively whether or not it can contribute significantly to CO 2 sequestration. The feasibility of the concept depends on the rate of olivine dissolution, the sequestration capacity of the dominant reaction, and its CO 2 footprint. Kinetics calculations show that offsetting 30% of worldwide 1990 CO 2 emissions by beach weathering means distributing of 5.0 Gt of olivine per year. For mean seawater temperatures of 15–25 °C, olivine sand (300 μm grain size) takes 700–2100 years to reach the necessary steady state sequestration rate and is therefore of little practical value. To obtain useful, steady state CO 2 uptake rates within 15–20 years requires grain sizes <10 μm. However, the preparation and movement of the required material poses major economic, infrastructural and public health questions. We conclude that coastal spreading of olivine is not a viable method of CO 2 sequestration on the scale needed. The method certainly cannot replace CCS technologies as a means of controlling atmospheric CO 2 concentrations.

Subvolcanic bodies of the Bodrak Complex in the Southwestern Crimea: Structure, composition, and formation conditions

The structure and composition of three subvolcanic bodies of the Bodrak Complex are considered in detail. The data on their petrography and results of microprobe and chemical analyses are presented. We arrived at a conclusion about the comagmatic relations of two bodies and established different depths of their formation. The chemical composition of olivine in rocks of the Bodrak Complex was determined for the first time with a microprobe. By their chemical composition, the rocks are related to the island-arc tholeiitic series.

New data on the gold-antigorite association of the Urals

Many placers of the Urals spatially associated withmassifs of serpentinized ultramafic rocks contain gold–magnetite associations [7]. The bedrock analogs of thisassociation are rare, small, and interpreted as deriva-tives of granitoid magmatism. Gold–magnetite occur-rences in the Kagan alpinotype ultramafic massif in theSouth Urals, for example, served as a basis for distin-guishing the Au-productive serpentinite (antigorite)metasomatic association [9]. In addition to the spatialassociation of gold mineralization with antigorite ser-pentinites, the existence of this association is substanti-ated by the fact of significant redistribution and mobili-zation of Au during the development of antigorite,which allowed us to suggest that gold mineralization inthe antigorite serpentinites was formed during crustalregional or local metamorphism of the ultramafic mas-sifs under crustal conditions [6]. The present study ofgold–magnetite ores and altered rocks of the Kaganmassif revealed mineralogical and isotopic–geochemi-cal evidence for the metamorphic origin of mineraliza-tion. In this model, fluid and ore components (Fe, Cu,and Au) were mobilized from ultrabasic rocks.The Kagan Massif is located in the Vishnevogorsk–Il’menogorsk metamorphic complex in the SouthUrals. Lenslike ultrabasic bodies of the complex arecontrolled by deep-seated faults and traditionallyascribed to the Riphean riftogenic ophiolites, whichunderwent Late Cambrian regional zoned metamor-phism and Early Paleozoic siliceous metasomatism [2].According to these concepts, the ultramafic rockstogether with host volcanosedimentary rocks weretransformed with decreasing temperature into olivine–enstatite, talc–olivine, olivine–antigorite, and antigoriteserpentinites during high-grade metamorphism, whilelater metasomatism was responsible for the formationof enstatite, anthophyllite, and talc–carbonate rocks.According to Varlakov, rocks of the Kagan Massifunderwent zoned metamorphism. Its southern part iscomposed of talc–olivine and olivine–antigorite rocks,while the northern part consists of antigorite serpen-tinites with relicts of olivine–antigorite rocks (Fig. 1).The silicic metasomatism is weak and manifested in thedevelopment of anthophyllite in talc–olivine rocks andthe development of talc in olivine–antigorite rocks andantigorite serpentinites. The occurrences of massiveand vein–schlieren magnetite ores containing up to 2–3% sulfides are confined to the tectonic zone extendingover 2 km along the eastern contact of the northern partof the massif. Magnetite lenses up to 5–6 m long and0.2 m thick form chains rapidly pinching out along thetectonic zone. The ores are contained in reticulate ser-pentinite with abundant fine magnetite, which empha-sizes the affiliation of serpentine to chrysotile. In theimmediate contact with ore, the reticulate serpentinitecontains spots and veinlets of antigorite with large mag-netite, talc, chlorite, and amphibole.The gold–magnetite ores were developed by twosmall mines in the mid 20th century. Gold assayaccounts for 0.2–1.2 g/t, sharply increasing in the areaswith visible gold particles. The chemical–spectral anal-ysis of individual samples showed that the magnetiteores contain Au (up to 160 mg/t), Pd (up to 770 mg/t),and Pt (up to 20 mg/t) [5]. The contents of Rh, Ir, Os,and Ru are less than 10–20 mg/t.Magnetite of massive and disseminated ores is char-acterized by a ubiquitous admixture of Mg (1.0–2.4 wt % MgO). Magnetite from the wall rocks, inaddition, contains 0.4–2.0 wt % Cr

Compositional diversity among primitive lavas of Mauritius, Indian Ocean: Implications for mantle sources

Three distinct phases of basaltic volcanism (Older Series, 7.8–5.4 Ma; Intermediate Series, 3.5–1.9 Ma; Younger Series, 0.7–0.03 Ma), related to the Deccan–Réunion mantle–plume, occurred over a time span of nearly 8 million years on the island of Mauritius, Indian Ocean. Bulk rock compositions of 52 analyzed samples (including samples from 32 recent borehole sections), as well as the compositions of olivine and spinel phenocrysts, reveal significant temporal variations in the composition of the Mauritius magmas. Two distinct compositional groups are identified based on their major and trace element compositions. The Group 1 basalts comprise the Younger Series, Intermediate Series and the earlier products of the Older Series rocks, and have higher MgO (7.6–13.6 wt.%) and lower K 2O (0.2–0.5 wt.%) and La/Sm (2.1–3.5) compared to Group 2 basalts (MgO 6.2–9.2 wt.%; K 2O 1.0–1.5 wt.%; La/Sm 3.6–4.1), which exclusively encompass the later differentiation products erupted during the end of the Older Series volcanism, denoted as Older Differentiated (OD) Series. Petrographic and mineralogic studies further confirm the presence of two distinct groups. Relatively primitive olivine compositions (Fo: 87–80) in Group 1 rocks are characterized by increasing CaO (0.2 to 0.3 wt.%) with decreasing Fo content, whereas in Group 2 rocks olivine (Fo: 86–80) show decreasing CaO (0.4 to 0.3 wt.%) with decreasing Fo content. Olivine-hosted spinel inclusions also show strong compositional differences between groups 1 and 2. Unusually Al-rich spinels (compared to that usually found in plume derived magma) and the positive Sr anomalies in Group 1 rocks suggest assimilation of crustal gabbros by the plume magmas. We propose a laterally heterogeneous plume and the contributions from variable plume components to account for the compositional differences observed in the two Mauritian lava groups. The early-Shield building Older Series lavas (oldest Group1 lavas), characterized with a relatively less enriched trace element abundances, most likely derive from extensive melting of a depleted peridotite matrix in the core of the plume. Melting of this already depleted source produced the later Intermediate and Younger Series eruptions. Towards the late-shield stage, melts presumably derive from the enriched pyroxenite/eclogite components in the outer part of the plume.

Paleowattmeters: A scaling relation for dynamically recrystallized grain size

During dislocation creep, mineral grains often evolve to a stable size, dictated by the deformation conditions. We suggest that grain-size evolution during deformation is determined by the rate of mechanical work. Provided that other elements of microstructure have achieved steady state and that the dissipation rate is roughly constant, then changes in internal energy will be proportional to changes in grain-boundary area. If normal grain-growth and dynamic grain-size reduction occur simultaneously, then the steady-state grain size is determined by the balance of those rates. A scaling model using these assumptions and published grain-growth and mechanical relations matches stress–grain-size relations for quartz and olivine rocks with no fitting. For marbles, the model also explains scatter not rationalized by assuming that recrystallized grain size is a function of stress alone. When extrapolated to conditions typical for natural mylonites, the model is consistent with field constraints on stresses and strain rates.

Localized shock melting in lherzolitic shergottite Northwest Africa 1950: Comparison with Allan Hills 77005

Abstract— The lherzolitic Martian meteorite Northwest Africa (NWA) 1950 consists of two distinct zones: 1) low‐Ca pyroxene poikilically enclosing cumulate olivine (Fo70–75) and chromite, and 2) areas interstitial to the oikocrysts comprised of maskelynite, low‐ and high‐Ca pyroxene, cumulate olivine (Fo68–71) and chromite. Shock metamorphic effects, most likely associated with ejection from the Martian subsurface by large‐scale impact, include mechanical deformation of host rock olivine and pyroxene, transformation of plagioclase to maskelynite, and localized melting (pockets and veins). These shock effects indicate that NWA 1950 experienced an equilibration shock pressure of 35–45 GPa. Large (millimeter‐size) melt pockets have crystallized magnesian olivine (Fo78–87) and chromite, embedded in an Fe‐rich, Al‐poor basaltic to picro‐basaltic glass. Within the melt pockets strong thermal gradients (minimum 1 °C/μm) existed at the onset of crystallization, giving rise to a heterogeneous distribution of nucleation sites, resulting in gradational textures of olivine and chromite. Dendritic and skeletal olivine, crystallized in the melt pocket center, has a nucleation density (1.0 × 103 crystals/mm2) that is two orders of magnitude lower than olivine euhedra near the melt margin (1.6 × 105 crystals/mm2). Based on petrography and minor element abundances, melt pocket formation occurred by in situ melting of host rock constituents by shock, as opposed to melt injected into the lherzolitic target. Despite a common origin, NWA 1950 is shocked to a lesser extent compared to Allan Hills (ALH) 77005 (45–55 GPa). Assuming ejection in a single shock event by spallation, this places NWA 1950 near to ALH 77005, but at a shallower depth within the Martian subsurface. Extensive shock melt networks, the interconnectivity between melt pockets, and the ubiquitous presence of highly vesiculated plagioclase glass in ALH 77005 suggests that this meteorite may be transitional between discreet shock melting and bulk rock melting.