Multistage Formation Research Articles

Age of tin ore from the Solnechnoe quartz-tourmaline-cassiterite deposit, the Khabarovsk krai, Russia from the results of Rb-Sr dating of quartz and adularia

The age of the main productive phase of ore formation at the large Solnechnoe tin deposit has been estimated for the first time based on the study of the Rb-Sr isotopic system of hydrothermal quartz and adularia from ore veins and metasomatic rocks. The Rb-Sr isochron age (84 ± 1 Ma) of mineralization coincides with the age of intrusive rocks pertaining to the third phase of the Silinka Complex, which control tin mineralization. The 87Sr/86Sr ratios of ore-forming solution and granitic rocks of the final intrusive phase are close to each other, indicating that the granitic melt was most likely one of the main sources of metals. The long and multistage formation history of the deposit could have been caused by complex geodynamic evolution of the Sikhote-Alin accretionary fold region in the Cretaceous.

Analysis of Micro-texture during Secondary Recrystallization in a Hi-B Electrical Steel

The understanding of Goss texture in Hi-B electrical steels possesses significant industrial and academic value, thus attracts worldwide attention. The prevailing models for sharp Goss texture formation during secondary recrystallization are CSL (coincident site lattice) boundary theory and HE (high energy) boundary theory. These theories stress the key factor of preferred growth and the difference between them only lies in the specific selection manner. This work examined the texture gradient in primarily recrystallized sheet and demonstrated its possible influence on the formation of secondary grains, and then determined the micro-texture during different stages of secondary recrystallization using EBSD (electron back-scattered diffraction) technique, finally analyzed a special type of secondary grains with near Brass orientation, which were detected in the later stage of secondary recrystallization, and discussed its origin and effect in terms of surface energy effect. The results indicate that texture gradient in primarily recrystallized sheet will lead to a multi-stage formation of Goss texture, namely, early stage of secondary grains with various orientations in subsurface region, intermediate stage of preferred growth of Goss grains into center layer and re-grow back to the surface and the final stage of Goss grain growth by swallowing slowly the island grains with the help of H 2 atmosphere.

Complex mineralization at large ore deposits in the Russian Far East

Genetic and mineralogical features of large deposits with complex Sn, W, and Mo mineralization in the Sikhote-Alin and Amur-Khingan metallogenic provinces are considered, as well as those of raremetal, rare earth, and uranium deposits in the Aldan-Stanovoi province. The spatiotemporal, geological, and mineralogical attributes of large deposits are set forth, and their geodynamic settings are determined. These attributes are exemplified in the large Tigriny Sn-W greisen-type deposit. The variation of regional tectonic settings and their spatial superposition are the main factor controlling formation of large deposits. Such a variation gives rise to multiple reactivation of the ore-magmatic system and long-term, multistage formation of deposits. Pulsatory mineralogical zoning with telescoped mineral assemblages related to different stages results in the formation of complex ores. The highest-grade zones of mass discharge of hydrothermal solutions are formed at the deposits. The promising greisen-type mineralization with complex Sn-W-Mo ore is suggested to be an additional source of tungsten and molybdenum. The Tigriny, Pravourminsky, and Arsen’evsky deposits, as well as deposits of the Komsomol’sk and Khingan-Olonoi ore districts are examples. Large and superlarge U, Ta, Nb, Be, and REE deposits are localized in the southeastern Aldan-Stanovoi Shield. The Ulkan and Arbarastakh ore districts attract special attention. The confirmed prospects of new large deposits with Sn, W, Mo, Ta, Nb, Be, REE, and U mineralization in the south of the Russian Far East assure expediency of further geological exploration in this territory.

Retrograde metamorphism of ultrahigh‐temperature granulites from the khondalite belt in Inner Mongolia, North China Craton: evidence from aluminous orthopyroxenes

AbstractOrthopyroxene‐sillimanite‐sapphirine‐spinel‐quartz granulites have been reported from the khondalite belt in Inner Mongolia, North China Craton (NCC), which record ultrahigh‐temperature (UHT) metamorphism. These rocks record metamorphic temperatures up to 1050°C and pressures up to 10 kbar during the peak metamorphism, as indicated by diagnostic UHT mineral assemblages such as equilibrium orthopyroxene + sillimanite + quartz, sapphirine + quartz and low‐Zn‐spinel + quartz. To well constrain the retrograde P–T conditions, we investigate orthopyroxenes in different textural settings in these UHT rocks and the results reveal marked variations in alumina contents, indicating a multistage formation. The core of coarse orthopyroxene porphyroblasts and some of the grains associated with sillimanite and quartz have the highest alumina content of up to 7.61–8.05 wt%. Rims of coarse orthopyroxene porphyroblasts and those occurring as coronas surrounding garnet grains have intermediate alumina value ranging from 6.21 to 7.61 wt%. Fine grained orthopyroxenes which form symplectite with cordierite have the lowest alumina content between 5.52 and 6.15 wt%. Based on the XAl–XMg isopleths, orthopyroxenes in different textural associations and textures indicate formation under varying temperature and pressure conditions, ranging from 950°C, 10.5 kbar to 840°C, 8.5 kbar. These results are in accordance with the P–T estimates from previous studies on these rocks using Ti‐in‐zircon geothermometry and pseudosection analysis. The reaction textures and orthopyroxene chemistry suggest an initial stage of isobaric cooling followed by isothermal decompression, and a final stage of cooling. Combined with the available age data which suggest the timing of the UHT event as 1.92 Ga, and the previous models on P–T evolution of the khondalites, our results suggest a complex exhumation of the UHT granulites within the Inner Mongolia Suture Zone associated with the final assembly of the NCC within the Columbia supercontinent during the Palaeoproterozoic. Copyright © 2010 John Wiley & Sons, Ltd.

Middle-proterozoic anorthosite–rapakivi granite complexes: An example of within-plate magmatism in abnormally thick crust: Evidence from the East European Craton

Large, bimodal, and multistage anorthosite–rapakivi granite complexes (ARGCs) of Middle-Proterozoic age (1.8–1.5 Ga) are distinct magmatic assemblages in the western part of the present-day European Craton. ARGCs formation commenced after stabilization of the Svecofennian orogen and relics of its abnormally thick crust occur in many places. In the eastern part of the Kola-Karelian domain and the Volga-Urals domain, where crustal thickness is normal ∼40 km, ARGCs are practically absent. Geological data evidence that melting processes occurred both in the crust and in the mantle during formation of the complexes. Geochemical peculiarities of ARGC rocks include enrichment in alkalis (mainly in K), Ti, Zn, Pb, Zr, Be, Sn, In, Y, Nb, Rb, F, Cu, W, Mo, sometimes Li and U. The negative ɛNd values and relatively high Th and Zn concentrations that are most frequently observed in the rocks imply that parental magmas were considerably contaminated by crustal components. According to the gravity and seismic profiles, ARGCs represent upper parts of large transcrustal magmatic systems, composed of alternating basic and silicic rocks layers. They are located over mantle uplift structures 10–20 km high, interpreted to represent former mantle plume heads. Origin of transcrustal systems was probably linked with penetrating sill-like bodies of Fe–Ti basalts, which after intruding into sialic crust, caused large-scale re-melting of the granitic material above them, forming two-layer magma chambers. Solidification of such chambers occurred in two stages: lower basic layers solidified first while heated from below, whereas the upper silicic layer crystallized later. Co-existing neighboring chambers led to gravitational instability, overturn, and mass redistribution in large volumes of crust. Furthermore, multistage formation of the ARGCs led to important petrological consequences: contamination of basic magmas by crustally derived SiO 2 and Al 2O 3, causing preferred precipitation of plagioclase and formation of plagioclase cumulates (anorthosites). Diffusion of Na into the basic melt and K into the silicic melt led to the appearance of potassic rapakivi granites. ARGCs on all Precambrian shields were developed in places with stabilized Paleoproterozoic orogens with atypical unusually thick crust compared with the Phanerozoic, and associated with large Mesoproterozoic belts of within-plate felsic volcanism. Under these conditions, the majority of mantle plume-derived mafic magmas were not able to reach the surface and were lost within the crust in the form of sill-like intrusions with zones of intense melting of a sialic roof. Evidently, ARGCs illustrate structure and processes that are not repeated in the Phanerozoic.

On concept of process signature in analysis of multistage surface formation

In this work, an attempt is made to develop a novel approach based on the concept of process signatures to investigate the correlation of the quality of manufactured surface and the process of its generation which is independent to manufacturing conditions. The process signature is the multiscale modifications of surface topography, which depicts the essential changes of the surface state produced on the original surface after abrasive machining. An advanced signal processing technique, the continuous wavelets transform is used to identify the multiscale transfer function of the process which represents the amount of surface modifications. The applicability of this methodology is demonstrated by several practical examples. It is shown that the signature is strongly influenced by abrasive dispersity and granulometry, process induced modes of deformation. The work demonstrates also that the process signatures respond in a predictable fashion to changes in process variables and combinations of stage.

Oxygen- and magnesium-isotope compositions of calcium–aluminum-rich inclusions from CR2 carbonaceous chondrites

We report both oxygen- and magnesium-isotope compositions measured in situ using a Cameca ims-1280 ion microprobe in 20 of 166 CAIs identified in 47 polished sections of 15 CR2 (Renazzo-type) carbonaceous chondrites. Two additional CAIs were measured for oxygen isotopes only. Most CR2 CAIs are mineralogically pristine; only few contain secondary phyllosilicates, sodalite, and carbonates – most likely products of aqueous alteration on the CR2 chondrite parent asteroid. Spinel, hibonite, grossite, anorthite, and melilite in 18 CAIs have 16O-rich (Δ 17O = −23.3 ± 1.9‰, 2 σ error) compositions and show no evidence for postcrystallization isotopic exchange commonly observed in CAIs from metamorphosed CV carbonaceous chondrites. The inferred initial 26Al/ 27Al ratios, ( 26Al/ 27Al) 0, in 15 of 16 16O-rich CAIs measured are consistent with the canonical value of (4.5–5) × 10 −5 and a short duration (<0.5 My) of CAI formation. These data do not support the “supra-canonical” values of ( 26Al/ 27Al) 0 [(5.85–7) × 10 −5] inferred from whole-rock and mineral isochrons of the CV CAIs. A hibonite–grossite-rich CAI El Djouf 001 MK #5 has uniformly 16O-rich (Δ 17O = −23.0 ± 1.7‰) composition, but shows a deficit of 26Mg and no evidence for 26Al. Because this inclusion is 16O-rich, like CAIs with the canonical ( 26Al/ 27Al) 0, we infer that it probably formed early, like typical CAIs, but from precursors with slightly nonsolar magnesium and lower-than-canonical 26Al abundance. Another 16O-enriched (Δ 17O = −20.3 ± 1.2‰) inclusion, a spinel–melilite CAI fragment Gao-Guenie (b) #3, has highly-fractionated oxygen- and magnesium-isotope compositions (∼11 and 23‰/amu, respectively), a deficit of 26Mg, and a relatively low ( 26Al/ 27Al) 0 = (2.0 ± 1.7) × 10 −5. This could be the first FUN ( Fractionation and Unidentified Nuclear effects) CAI found in CR2 chondrites. Because this inclusion is slightly 16O-depleted compared to most CR2 CAIs and has lower than the canonical ( 26Al/ 27Al) 0, it may have experienced multistage formation from precursors with nonsolar magnesium-isotope composition and recorded evolution of oxygen-isotope composition in the early solar nebula over 0.9 + 2.2 - 0.7 My. Eight of the 166 CR2 CAIs identified are associated with chondrule materials, indicating that they experienced late-stage, incomplete melting during chondrule formation. Three of these CAIs show large variations in oxygen-isotope compositions (Δ 17O ranges from −23.5‰ to −1.7‰), suggesting dilution by 16O-depleted chondrule material and possibly exchange with an 16O-poor (Δ 17O > −5‰) nebular gas. The low inferred ( 26Al/ 27Al) 0 ratios of these CAIs (<0.7 × 10 −5) indicate melting >2 My after crystallization of CAIs with the canonical ( 26Al/ 27Al) 0 and suggest evolution of the oxygen-isotope composition of the inner solar nebula on a similar or a shorter timescale. Because CAIs in CR2 and CV chondrites appear to have originated in a similarly 16O-rich reservoir and only a small number of CR2 and CV CAIs were affected by chondrule melting events in an 16O-poor gaseous reservoir, the commonly observed oxygen-isotope heterogeneity in CAIs from metamorphosed CV chondrites is most likely due to fluid–solid isotope exchange on the CV asteroidal body rather than gas–melt exchange. This conclusion does not preclude that some CV CAIs experienced oxygen-isotope exchange during remelting, instead it implies that such remelting is unlikely to be the dominant process responsible for oxygen-isotope heterogeneity in CV CAIs. The mineralogy, oxygen and magnesium-isotope compositions of CAIs in CR2 chondrites are different from those in the metal-rich, CH and CB carbonaceous chondrites, providing no justification for grouping CR, CH and CB chondrites into the CR clan.

Structure of melt flow channels in the mantle

Structural events during the formation of the mantle peridotite section in the Voikar-Syn’ya massif of the Polar Urals are considered. The structural units of the mantle section were formed during several deformation stages. Dunite bodies in restitic peridotites were formed in the course of deformation that completed the formation of large-scale folds of high-temperature plastic flow of mantle material. The final stage of deformation accompanied by migration of melt through harzburgite occurred in the shallow mantle in the setting of suprasubduction spreading related to the ascent of a mantle diapir. The rate of plastic deformation was relatively low. As a result, the intracrystal translation gliding of dislocations in olivine was the main mechanism in both harzburgite and dunite. The paths of focused melt flow are marked by dunite veins and associated pyroxenite and chromitite. It is suggested that stress concentration in fold hinges and their abrupt relaxation with formation of orthogonal network of weakened zones with high permeability was one of the possible mechanisms of the formation of melt conduits. The dispersed melt ascending from a great depth spontaneously migrated toward these zones. The distribution and structure of chromitite bodies reflect multistage formation of dunite, nonstationary dynamics of melt flow through restite, and abrupt variations of local stress fields in the areas adjacent to melt conduits.

Large‐scale spring deposits on Mars?

We present a large‐scale spring hypothesis for the formation of various enigmatic light‐toned deposits (LTDs) on Mars. Layered to massive LTDs occur extensively in Valles Marineris, chaotic terrains, and several large craters, in particular, those located in Arabia Terra. Most of these deposits are not easily explained with either a single process or multiple ones, either in combination or occurring sequentially. Spring deposits can have a very wide range of internal facies and exhibit complex architectural variations. We propose the concept of large‐scale spring deposits for explaining LTDs on Mars. Stable volcano‐tectonic settings, such as the ones typical on Mars, are compatible with a large‐scale, long‐term, multistage formation of spring deposits. The large‐scale spring deposit model can explain the formation of LTDs with a common process, although active in different times and locations, compatible with coeval local or regional processes and deposits, such as volcaniclastic ones. LTDs, if formed as spring deposits derived from subsurface fluids, could potentially offer favorable conditions both to life and to the fossilization of past life forms.

26Al– 26Mg and 207Pb– 206Pb systematics of Allende CAIs: Canonical solar initial 26Al/ 27Al ratio reinstated

The precise knowledge of the initial 26Al/ 27Al ratio [( 26Al/ 27Al) 0] is crucial if we are to use the very first solid objects formed in our Solar System, calcium–aluminum-rich inclusions (CAIs) as the “time zero” age-anchor and guide future work with other short-lived radio-chronometers in the early Solar System, as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation. New high-precision multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) measurements of 27Al/ 24Mg ratios and Mg-isotopic compositions of nine whole-rock CAIs (six mineralogically characterized fragments and three micro-drilled inclusions) from the CV carbonaceous chondrite, Allende yield a well-defined 26Al– 26Mg fossil isochron with an ( 26Al/ 27Al) 0 of (5.23 ± 0.13) × 10 − 5 . Internal mineral isochrons obtained for three of these CAIs ( A44A, AJEF, and A43) are consistent with the whole-rock CAI isochron. The mineral isochron of AJEF with ( 26Al/ 27Al) 0 = (4.96 ± 0.25) × 10 − 5 , anchored to our precisely determined absolute 207Pb– 206Pb age of 4567.60 ± 0.36 Ma for the same mineral separates, reinstate the “canonical” ( 26Al/ 27Al) 0 of 5 × 10 − 5 for the early Solar System. The uncertainty in ( 26Al/ 27Al) 0 corresponds to a maximum time span of ± 20 Ka (thousand years), suggesting that the Allende CAI formation events were culminated within this time span. Although all Allende CAIs studied experienced multistage formation history, including melting and evaporation in the solar nebula and post-crystallization alteration likely on the asteroidal parent body, the 26Al– 26Mg and U–Pb-isotopic systematics of the mineral separates and bulk CAIs behaved largely as closed-system since their formation. Our data do not support the “supra-canonical” 26Al/ 27Al ratio of individual minerals or their mixtures in CV CAIs, suggesting that the supra-canonical 26Al/ 27Al ratio in the CV CAIs may have resulted from post-crystallization inter-mineral redistribution of Mg isotopes within an individual inclusion. This redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk CAI and bulk mineral data obtained by MC-ICP-MS. The radiogenic 208Pb⁎/ 206Pb⁎ ratio obtained as a by-product from the Pb–Pb age dating is used to estimate time-integrated 232Th/ 238U ratio ( κ value) of CAIs. Limited κ variations among the minerals within a single CAI, contrasted by much larger variations among the bulk CAIs, suggest Th/U fractionation occurred prior to crystallization of igneous CAIs. If interpreted as primordial heterogeneity, the κ value can be used to calculate the mean age of the interstellar dust from which the CAIs condensed.

Chemical and boron isotopic variations of tourmaline in the Hnilec granite-related hydrothermal system, Slovakia: Constraints on magmatic and metamorphic fluid evolution

Multi-stage formation of tourmaline occurs in the Hnilec granite-related hydrothermal tin mineralisation system from the Western Carpathians, Slovakia. The tourmalines belong to the schorl–dravite series and have two major stages of formation: the majority crystallized during the first stage (defined as M-stage), forming zoned tourmaline crystals with the cores being generally more Fe, Al, and Mn rich than the rims. During the second stage (defined as L-stage), tourmaline formed as small veins or irregular patches along fractures and cracks in the M-stage tourmaline grains. In the contact metapelites near the granite body, the L-stage tourmalines are more Mg-rich and Fe, Al, Mn depleted than the M-stage tourmalines. In the granites, the L-stage tourmalines have generally similar compositions to those of the M-stage tourmaline rims. The boron isotopic compositions of the M-stage tourmalines vary from − 10.3‰ to − 15.4‰; with no clear variation between the cores and the rims, however, some of the tourmaline grains from the contact metapelites show a slightly higher δ 11B in the cores than in the rims. The L-stage tourmalines have lower δ 11B value of − 16.0‰ to − 17.1‰. We suggest that these trends reflect a changing fluid source from a dominant magmatic–hydrothermal fluid derived from the granites to a late-stage metamorphic fluid derived from the regional metamorphism (chlorite and biotite zone) in the metapelites. The significantly higher Fe 3+ in the L-stage than the M-stage tourmalines reflect changing redox conditions towards a more oxidising environment. This redox condition change may have important implications for the hydrothermal tin mineralisation in the area.

Fluid Inclusion Thermobarometry as a Tracer for Magmatic Processes

Research Article| January 01, 2008 Fluid Inclusion Thermobarometry as a Tracer for Magmatic Processes Thor H. Hansteen; Thor H. Hansteen IFM-GEOMAR, Leibniz-Institute for Marine Sciences, Dynamics of the Ocean Floor, D-24148 Kiel, Germany, thansteen@ifm-geomar.de Search for other works by this author on: GSW Google Scholar Andreas Klügel Andreas Klügel Fachbereich Geowissenschaften, Universität Bremen, D-28334 Bremen, Germany, akluegel@uni-bremen.de Search for other works by this author on: GSW Google Scholar Author and Article Information Thor H. Hansteen IFM-GEOMAR, Leibniz-Institute for Marine Sciences, Dynamics of the Ocean Floor, D-24148 Kiel, Germany, thansteen@ifm-geomar.de Andreas Klügel Fachbereich Geowissenschaften, Universität Bremen, D-28334 Bremen, Germany, akluegel@uni-bremen.de Publisher: Mineralogical Society of America First Online: 13 Jul 2017 © The Mineralogical Society Of America Reviews in Mineralogy and Geochemistry (2008) 69 (1): 143–177. https://doi.org/10.2138/rmg.2008.69.5 Article history First Online: 13 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Thor H. Hansteen, Andreas Klügel; Fluid Inclusion Thermobarometry as a Tracer for Magmatic Processes. Reviews in Mineralogy and Geochemistry 2008;; 69 (1): 143–177. doi: https://doi.org/10.2138/rmg.2008.69.5 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Fluid inclusions in minerals may form in any type of volcanic or plutonic rock ranging from mafic to silicic compositions. Because all igneous rocks reach fluid saturation at some stage during their evolution, fluids trapped as inclusions in magmatic minerals belong to a certain paragenesis or phase assemblage, which may include minerals, melts and one or more fluid phases. These fluid inclusions reflect one or more stages during rock evolution, and can be used to constrain multistage formation and evolution processes including ascent histories, magma chamber processes and crystallization behavior. Fluid inclusions can provide thermobarometric data on various timescales. During... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Sirente crater, Italy: Impact versus mud volcano origins

Abstract— The Sirente crater is a circular structure with a diameter of ˜80 m. The rim deposit is an inverse‐graded, matrix‐supported breccia. Sedimentological features of the rim deposit suggest that the crater is not related to an explosion or violent mechanical displacement. The structure and texture of the deposit exhibit a primary sedimentary character. The rim deposits do not contain artifacts and do not show evidence of reworking. A multistage formation is reconstructed for the rim growth and associated deposits. The geometry and sedimentology of the deposits indicate that they were produced by the extrusion and accumulation of mudflow deposits. The dominant ejection mechanism was low mud fountains and the transport medium was water. Petrographic and geochemical evidence does not indicate any physical or cryptic trace of an extraterrestrial body.The most realistic agent that explains the observed effects is a rapid local emission of mud and/or water. Geological processes capable of producing these features include piping sinkholes or, more probably, “caldera”‐type mud volcanoes, which may result from underground water‐table perturbation and/or decompression of deep CO2/hydrocarbon gas reservoirs due to tectonic deformation or faulting activity during a seismic event. In both cases, the name “crater” for this geological form may be maintained, but there is no compelling evidence for an impact origin.In this paper, the scientific literature on the Sirente crater is reconsidered in the light of new morphological, sedimentological, geochemical, and archaeological data. A new mechanism is proposed involving mud‐fountaining.

Trace and Rare Earth Elements in Weathering Crusts of the Kursk Group of the Kursk Magnetic Anomaly

The behavior of trace and rare earth elements (REE) in weathering crusts (WC) developed after metasedimentary rocks of the Kursk Group of the Kursk Magnetic Anomaly (KMA) is considered. Specific features of their distribution in the weathering profile are noted and intervals of their concentration are outlined. It is suggested that the database of these elements can be used as indicators of the multistage formation and respective age of the WC.

Redox evolution of the Earth caused by a multi-stage formation of its core

A model of multi-stage formation of the Earth's core is presented allowing consistent solution to the problem of deficiency of heat sources in the Earth's thermal balance and problem of redox evolution of the Earth's mantle. The additional heat yield and oxidative evolution of the mantle are coordinated with continued growth of the core. It is suggested that after early formation of the major part of the core (ca. 95%) – within the first 100 million years of Earth's life – the growth of the core slowly continued. Following the exhaustion of native metal and sulfide iron, the core build-up proceeded at the expense of the mantle FeO. As partial disproportionation of iron in Fe 2+O to Fe 0 and Fe 2 3+O 3 may occur in the deep mantle and at the core–mantle boundary, this brought about the gradual oxidization of the mantle. This stage lasted the next 150–300 million years. By the end of this period the mantle redox potential transformed so that the state of oxidization of the upper mantle approached the level of QFM buffer. At the same period the composition of the atmosphere must have been changed from a reduced (containing CH 4, CO, NH 3) to a neutral one (containing CO 2, N 2). The final and the longest stage of the core build-up did not involve significant changes in its redox potential and is still under way. The slow core growth is implemented due to the mantle resources in FeO. The process of dissolution of FeO, which is delivered to the core boundary by the mantle convective downflow, ensures generation of heat maintaining the super-adiabatic temperature gradient in the mantle and eliminating apparent deficiency of heat sources in the Earth's thermal balance. Some other geological consequences of the model are considered.

Fine‐grained, spinel‐rich inclusions from the reduced CV chondrites Efremovka and Leoville: I. Mineralogy, petrology, and bulk chemistry

Abstract— Fine‐grained, spinel‐rich inclusions in the reduced CV chondrites Efremovka and Leoville consist of spinel, melilite, anorthite, Al‐diopside, and minor hibonite and perovskite; forsterite is very rare. Several CAIs are surrounded by forsterite‐rich accretionary rims. In contrast to heavily altered fine‐grained CAIs in the oxidized CV chondrite Allende, those in the reduced CVs experienced very little alteration (secondary nepheline and sodalite are rare). The Efremovka and Leoville fine‐grained CAIs are 16O‐enriched and, like their Allende counterparts, generally have volatility fractionated group II rare earth element patterns. Three out of 13 fine‐grained CAIs we studied are structurally uniform and consist of small concentrically zoned nodules having spinel ± hibonite ± perovskite cores surrounded by layers of melilite and Al‐diopside. Other fine‐grained CAIs show an overall structural zonation defined by modal mineralogy differences between the inclusion cores and mantles. The cores are melilite‐free and consist of tiny spinel ± hibonite ± perovskite grains surrounded by layers of anorthite and Al‐diopside. The mantles are calcium‐enriched, magnesium‐depleted and coarsergrained relative to the cores; they generally contain abundant melilite but have less spinel and anorthite than the cores. The bulk compositions of fine‐grained CAIs generally show significant fractionation of Al from Ca and Ti, with Ca and Ti being depleted relative to Al; they are similar to those of coarsegrained, type C igneous CAIs, and thus are reasonable candidate precursors for the latter. The finegrained CAIs originally formed as aggregates of spinel‐perovskite‐melilite ± hibonite gas‐solid condensates from a reservoir that was 16O‐enriched but depleted in the most refractory REEs. These aggregates later experienced low‐temperature gas‐solid nebular reactions with gaseous SiO and Mg to form Al‐diopside and ±anorthite. The zoned structures of many of the fine‐grained inclusions may be the result of subsequent reheating that resulted in the evaporative loss of SiO and Mg and the formation of melilite. The inferred multi‐stage formation history of fine‐grained inclusions in Efremovka and Leoville is consistent with a complex formation history of coarse‐grained CAIs in CV chondrites.

Efremovka 101.1: a CAI with ultrarefractory REE patterns and enormous enrichments of Sc, Zr, and Y in Fassaite and Perovskite

Inclusion 101.1 from the CV3 carbonaceous chondrite Efremovka is a compact Type A Ca-Al-rich inclusion (CAI) highly enriched in ultrarefractory (UR) oxides. It is the first complete CAI with a UR rare earth element (REE) pattern found in a CV3 chondrite. The inclusion is petrographically complex and was formed in a multistage process. It consists of several lithologically unrelated units.The core contains abundant Y- and Zr-perovskite, Sc- and Zr-rich fassaite, and metallic FeNi enclosed in melilite. All mineral species (except spinel) in all lithological units exhibit the same basic UR REE pattern. Four different populations of perovskites are distinguished by different Y/Zr ratios. A few of the perovskites have Y/Zr ratios similar to those obtained from crystal/liquid fractionation experiments. Perovskites from the other three populations have either chondritic, lower than chondritic Y/Zr ratios or extremely low Zr contents. Ca isotopic ratios differ among three perovskites from different populations, demonstrating a variety of sources and formational processes. Most fassaites crystallized in situ through reaction between the CAI liquid and preexisting perovskites. This process induced redistribution of Zr, Y, Sc, and V between perovskite and fassaite, thus overprinting the original abundances in perovskite. Fassaite reaction rims around FeNi metals are also encountered. They are enriched in V, which was gained from the metal through oxidation of V in metal during fassaite crystallization. The relative abundances of Zr, Y, and Sc in perovskites are complementary to the abundances of these elements in Sc- and Zr-fassaite, indicating subsolidus partitioning of these elements between the two phases. Perovskites are enriched in Y and depleted in Sc and Zr in comparison to fassaites.The core contains two complete captured CAIs, several sinuous fragments, and fine-grained polygonal refractory fragments. An assemblage of andradite-wollastonite-hedenbergite and pure metallic iron is encountered as enclaves in the interior of some sinuous fragments. Metallic Fe and wollastonite formed by reduction of preexisting andradite and hedenbergite nebular alteration products upon inclusion in the highly reduced CAI melt. Numerous spinel clusters and framboids with varying V2O3 and Cr2O3 concentrations are enclosed in individual melilite crystals in the host CAI and captured CAIs. The rim sequence of the host consists of six layers (from the inside outward): (a) FeO-poor spinel, (b) Sc-bearing fassaite, (c) Al-diopside, (d) Al- and Ca-bearing olivine, (e) pure diopside, and (f) Ca-poor olivine. Like the constituents of the CAI core, all mineral layers of the rim sequence, except spinel, have the same UR REE pattern. However, the total REE abundances decrease systematically by 1 order of magnitude from layer 2 to layer 6. This feature strongly suggests formation of the rim sequence by successive condensation from a unique reservoir enriched in UR elements and excludes formation by flash heating. Petrography, mineral chemistry, REE, refractory lithophile element abundances, and Ca isotopic compositions demonstrate the complex multistage formation history of a CAI that on the surface looks like a regular Type A inclusion.

Plagioclase‐rich chondrules in the reduced CV chondrites: Evidence for complex formation history and genetic links between calcium‐aluminum‐rich inclusions and ferromagnesian chondrules

Abstract— Plagioclase‐rich chondrules (PRCs) in the reduced CV chondrites Efremovka, Leoville, Vigarano and Grosvenor Mountains (GRO) 94329 consist of magnesian low‐Ca pyroxene, Al‐Ti‐Cr‐rich pigeonite and augite, forsterite, anorthitic plagioclase, FeNi‐metal‐sulfide nodules, and crystalline mesostasis composed of silica, anorthitic plagioclase and Al‐Ti‐Cr‐rich augite. The silica grains in the mesostases of the CV PRCs are typically replaced by hedenbergitic pyroxenes, whereas anorthitic plagioclase is replaced by feldspathoids (nepheline and minor sodalite). Some of the PRCs contain regions that are texturally and mineralogically similar to type I chondrules and consist of forsterite, low‐Ca pyroxene and abundant FeNi‐metal nodules. Several PRCs are surrounded by igneous rims or form independent compound objects. Twelve PRCs contain relic calcium‐aluminum‐rich inclusions (CAIs) composed of anorthite, spinel, high‐Ca pyroxene, ± forsterite, and ± Al‐rich low‐Ca pyroxene. Anorthite of these CAIs is generally more heavily replaced by feldspathoids than anorthitic plagioclase of the host chondrules. This suggests that either the alteration predated formation of the PRCs or that anorthite of the relic CAIs was more susceptible to the alteration than anorthitic plagioclase of the host chondrules. These observations and the presence of igneous rims around PRCs and independent compound PRCs suggest that the CV PRCs may have had a complex, multistage formation history compared to a more simple formation history of the CR PRCs.Relatively high abundances of moderately‐volatile elements such as Cr, Mn and Si in the PRCs suggests that these chondrules could not have been produced by volatilization of ferromagnesian chondrule precursors or by melting of refractory materials only. We infer instead that PRCs in carbonaceous chondrites formed by melting of the reduced chondrule precursors (magnesian olivine and pyroxene, FeNi‐metal) mixed with refractory materials (relic CAIs) composed of anorthite, spinel, high‐Ca pyroxene, and forsterite. The mineralogical, chemical and textural similarities of the PRCs in several carbonaceous chondrite groups (CV, CO, CH, CR) and common presence of relic CAIs in these chondrules suggest that PRCs may have formed in the region(s) intermediate between the regions where CAIs and ferromagnesian chondrules originated.

High time resolution by use of the 26Al chronometer in the multistage formation of a CAI

The Allende calcium-rich inclusion (CAI) 5241 has been found to contain distinct initial 26Al/ 27Al in the three consecutive igneous zones that have been identified by extensive petrogenetic studies to have formed in three distinct crystallization events. The zones in order of sequence of formation from the petrologic observations are: (1) spinel-free islands (SFI) included in (2) a pyroxene–spinel-rich core (SRC) which in turn is included in (3) a melilite mantle (MM). The initial ( 26Al/ 27Al) values of these zones are respectively (4.6–5.0)×10 −5, 4.3×10 −5, and 3.3×10 −5. It is argued that these distinct ( 26Al/ 27Al) 0 values are not the result of metamorphism but reflect the relative times of formation by crystallization from melts. Relative to the canonical value of ( 26Al/ 27Al) 0=5×10 −5, we find the following chronology: t SFI≈0, t SRC≈10 5 yr and t MM≈4×10 5 yr. The three-layer CAI is inferred to have sampled a reservoir with an initial uniform 26Al/ 27Al ratio that decreased in value due to radioactive decay. From these observations we conclude that we have resolved time differences of a few hundred thousand years at the very early stages of formation of the solar system. The SFI, SRC, and MM zones reflect sequential addition of molten CAI material which crystallized rapidly without seriously metamorphosing the previously formed material. These additions took place over a time of about 400 000 years. We believe that these sequential events are not compatible with condensation in a hot region of the solar nebula. It is proposed that a scenario involving stages of protoplanetary accretion and of melt generation in protoplanetary sites heated by 26Al might provide a possible source for CAIs and chondrules.

The giant chromite deposits at Kempirsai, Urals: constraints from trace element (PGE, REE) and isotope data

The investigation of stable and radiogenic isotopes and of platinum-group (PGE) and rare earth elements (REE) in chromitites and associated ultramafic rocks of the Kempirsai Massif, southern Urals, gives strong evidence for a multistage formation of giant ophiolitic-podiform chromite deposits present in the southeastern part of the massif. The Kempirsai ophiolite massif is divided by a shear zone into two parts: in the northwestern area, small bodies of Al-rich chromite formed from basaltic melts between 420 to 400 Ma, according to Sm-Nd mineral isochrons of harzburgite, pyroxenite, websterite and gabbro. Harzburgites and pyroxenites in this area are enriched in light REE and have ɛNd(400) > +6 and ɛSr(400) ∼ +5. Chromitites have scattered PGE distributions (Pd/Ir, 0.4–7.0), being partly enriched in Pd and Pt. γOs(400) of one chromitite is −4.4. The southeastern part of the Kempirsai Massif, well-known for its world-class deposits of podiform low-Al magnesiochromite, is characterized by harzburgite and dunite enriched in light REE with very low ɛNd(400) (+4.3 to –17.1) and positive ɛSr(400) (>+10) values. Chromitites are strongly enriched in Ir, Os and Ru and depleted in Pd and Pt. γOs(400) of three chromitites is uniform and approaches C1 and DMM compositions. In veins and pods postdating crystallization of massive chromite, pargasitic amphibole formed in equilibrium with fluid-inclusion-bearing chromite at temperatures close to 1000 °C. These amphiboles give 40Ar/39Ar stepwise heating ages of 365 to 385 Ma and are characterized by low ɛNd(400) (+0.6 to −4.6) and general enrichment in REE. The cooling ages correspond to a 379.3 ± 1.6 Ma Rb-Sr mineral isochron produced from amphibole and phlogopite of a pyroxenite vein in the western part of the massif. From these data it is concluded that parts of the Kempirsai Massif have been pervasively metasomatized by large amounts of fluids and melts derived from a subducted slab composed of oceanic crust and sediments. Subduction occurred at least 15–35 Ma after a melting event that produced a typical ophiolitic sequence in the Paleozoic Sakmara Zone. We conclude that large chromite orebodies formed from second-stage high-Mg melts that interacted with depleted mantle and fluids on their way upward in a suprasubduction zone regime, and in a fore-arc position to the Magnitogorsk island arc.