Mg In Olivine Research Articles

Titanium diffusion in olivine

Diffusion of Ti has been characterized in natural olivine and synthetic forsterite. Experiments on the natural olivines were run under buffered conditions (IW and NNO), and those on synthetic forsterite were run in air. Titanium diffusion appears relatively insensitive to crystallographic orientation and oxygen fugacity under the range of investigated conditions, and diffusivities are similar for Fe-bearing olivine and forsterite. For Ti diffusion in synthetic forsterite, we obtain the following Arrhenius relation for diffusion over the temperature range 900–1400°C: DForst=5.97×10-14exp(-203±19kJ mol-1/RT)m2s-1. For Ti diffusion in San Carlos olivine, we obtain the following “global” fit to all diffusion data (incorporating diffusion parallel to b and c-axes, and experiments run under IW and NNO buffers), over the temperature range 1050–1254°C: Dol-SC=2.11×10-14exp(-195±32kJ mol-1/RT)m2s-1. Titanium diffusivities in olivine are similar to those of the trivalent REEs, but considerably slower than those of Cr, Ca, and Fe–Mg in olivine. Titanium diffusivities in olivine, diopside and orthopyroxene are comparable over investigated temperature ranges, differing by about half a log unit at 900°C, but have increasing variance at lower temperatures given the higher activation energies (by 40–70kJmol−1) for Ti diffusion in pyroxene compared with olivine. These large differences in cation mobility among Ti, Cr, Ca, Fe–Mg and the REE in pyroxene and olivine may allow us to distinguish dominant processes that give rise to the chemical disequilibria in olivine and pyroxene. With decreasing temperature, Ti is preferentially partitioned from olivine to pyroxene in ultramafic rocks, giving rise to characteristic reversed zoning in olivine and orthopyroxene and normal zoning in clinopyroxene. Diffusive exchange models with temperature-dependent diffusion and partition coefficients have been developed for the olivine–pyroxene bi-mineralic and tri-mineralic systems, allowing us to assess cooling rates and closure temperatures of the ultramafic and mafic rocks.

Diffusion of helium in olivine at 1 atm and 2.7 GPa

Diffusion of helium has been characterized in natural Fe-bearing olivine (Fo90) and pure synthetic Mg2SiO4. Polished slabs of olivine, oriented normal to a, b or c-axes, were implanted with 3He, either at 100keV at a dose of 5×1015/cm2 or at 3.0MeV at a dose of 1×1016/cm2. A set of diffusion experiments on 3He implanted olivine were run in Pt capsules in 1-atm furnaces. In addition to the 1-atm experiments, experiments on implanted samples were also run at higher pressures (2.6–2.7GPa) to assess the potential effects of pressure on He diffusion and the applicability of the diffusion data in describing He transport in the mantle. The high-pressure experiments were conducted in a piston-cylinder apparatus using a special “soft” pressure cell, with the implanted forsterite samples directly surrounded by AgCl. 3He distributions following all experiments were measured with nuclear reaction analysis using the reaction 3He(d,p)4He. For diffusion in forsterite we obtain the following Arrhenius relations: D‖a=3.73×10-8exp(-135±5kJmol-1/RT)m2s-1,D‖b=1.33×10-8exp(-137±13kJmol-1/RT)m2s-1,D‖c=3.61×10-6exp(-159±4kJmol-1/RT)m2s-1.The data for diffusion parallel to the c-axis define a single Arrhenius line spanning more than 7 orders of magnitude in values of diffusion coefficients, and a 700°C range in temperature (250–950°C). Diffusion parallel to the a-axis appears slightly slower, and diffusion parallel to the b-axis is slower than diffusion parallel to the a-axis by about two-thirds of a log unit. This anisotropy is broadly consistent with observations for diffusion of Ni and Fe–Mg in olivine. Diffusion in Fe-bearing natural olivine (transport parallel to b) agrees within uncertainty with findings for He diffusion in the pure Mg end-member olivine. The higher-pressure experiments yield diffusivities in agreement with those from the 1-atm experiments, indicating that the results reported here can be reasonably applied to modeling He transport in the upper mantle. The insensitivity of He diffusion to pressure over the investigated range of conditions suggests that compression of the mineral lattice is not sufficient to significantly influence migration of the relatively small helium atoms, which likely diffuse via crystal interstices.The He diffusivities measured in this work are generally consistent with results from the study of Futagami et al. (1993), who measured He diffusion in natural olivine by outgassing 4He implanted samples, and with the diffusivities measured by bulk-release of He by Shuster et al. (2004), but are about 2 orders of magnitude slower than the recent findings of Tolstikhin et al. (2010) and Blard et al. (2008). An up-temperature extrapolation of our data also show reasonable agreement with the higher-temperature measurements of Hart (1984); the coincident diffusivities from these studies under the high-temperature conditions used by Hart et al. (2008) in their He diffusion/production modeling supports the applicability of their modeling results.

Implications of ferrous and ferric iron in antigorite

Microprobe analyses of antigorite show that (Al+Cr) and inferred Fe 3+ correlate inversely with Si apfu in a Tschermaks substitution. This observation suggests that the uptake of Fe 3+ is not simply related to f O 2 . For Si = 1.95 apfu estimated Fe 3+ = 0.032 apfu (or 0.95 wt% Fe 2 O 3 ). Such estimates of Fe 3+ require high analytical accuracy and precision, and assume a fixed polysomatic formula (e.g., m = 17) and freedom from interlayer sheet-silicate impurities. In many cases the estimates appear to be high. An alternative measure of Fe 3+ is provided by the partitioning of total Fe and Mg between antigorite and olivine in well-equilibrated natural antigorite-olivine-magnetite parageneses. Extrapolation of Nernst and Roozeboom partition plots to Fe-free olivine permits an estimate of the Fe 3+ content of the average antigorite in this paragenesis, namely 0.42 or 0.64 wt% Fe 2 O 3 . The partition estimates are in good agreement with the results of Mossbauer spectroscopy performed here on 14 antigorites from metaperidotites, together with four from the literature. These spectra reveal a range of 0.16 to 1.94 in wt% Fe 2 O 3 in metaperidotite antigorite, with an average of 0.83. In two olivine-bearing rocks, antigorite has Fe 3+ /∑Fe ratios of 0.13 and 0.15, which corresponds to wt% Fe 2 O 3 = 0.47 and 0.54, respectively. Larger amounts of Fe 2 O 3 occur in some, but not all, vein antigorites. The prograde formation of antigorite in serpentinite from lizardite is accompanied by loss of some cronstedtite component and the precipitation of additional magnetite. The Roozeboom Mg/Fe partition plot is concave down rather than up; in other words the partition coefficient K D is a function of the X Mg of olivine. This behavior has been found in other olivine-mineral pairs. It can be interpreted to reflect strongly non-ideal solution behavior of MgFe-olivine at low temperatures, viz. W G ≈ 8.5 kJ assuming a symmetrical solution. MgFe-brucite appears to be similarly non-ideal.

Variations of Li and Mg isotope ratios in bulk chondrites and mantle xenoliths

We present whole rock Li and Mg isotope analyses of 33 ultramafic xenoliths from the terrestrial mantle, which we compare with analyses of 30 (mostly chondritic) meteorites. The accuracy of our new Mg isotope ratio measurement protocol is substantiated by a combination of standard addition experiments, the absence of mass independent effects in terrestrial samples and our obtaining identical values for rock standards using two different separation chemistries and three different mass-spectrometric introduction systems. Carbonaceous, ordinary and enstatite chondrites have irresolvable mean stable Mg isotopic compositions (δ 25Mg = −0.14 ± 0.06; δ 26Mg = −0.27 ± 0.12‰, 2SD), but our enstatite chondrite samples have lighter δ 7Li (by up to ∼3‰) than our mean carbonaceous and ordinary chondrites (3.0 ± 1.5‰, 2SD), possibly as a result of spallation in the early solar system. Measurements of equilibrated, fertile peridotites give mean values of δ 7Li = 3.5 ± 0.5‰, δ 25Mg = −0.10 ± 0.03‰ and δ 26Mg = −0.21 ± 0.07‰. We believe these values provide a useful estimate of the primitive mantle and they are within error of our average of bulk carbonaceous and ordinary chondrites. A fuller range of fresh, terrestrial, ultramafic samples, covering a variety of geological histories, show a broad positive correlation between bulk δ 7Li and δ 26Mg, which vary from −3.7‰ to +14.5‰, and −0.36‰ to + 0.06‰, respectively. Values of δ 7Li and δ 26Mg lower than our estimate of primitive mantle are strongly linked to kinetic isotope fractionation, occurring during transport of the mantle xenoliths. We suggest Mg and Li diffusion into the xenoliths is coupled to H loss from nominally anhydrous minerals following degassing. Diffusion models suggest that the co-variation of Mg and Li isotopes requires comparable diffusivities of Li and Mg in olivine. The isotopically lightest samples require ∼5–10 years of diffusive ingress, which we interpret as a time since volatile loss in the host magma. Xenoliths erupted in pyroclastic flows appear to have retained their mantle isotope ratios, likely as a result of little prior degassing in these explosive events. High δ 7Li, coupled with high [Li], in rapidly cooled arc peridotites may indicate that these samples represent fragments of mantle wedge that has been metasomatised by heavy, slab-derived fluids. If such material is typically stirred back into the convecting mantle, it may account for the heavy δ 7Li seen in some oceanic basalts.

Infrared laboratory absorbance spectra of olivine: using classical dispersion analysis to extract peak parameters

Laboratory measurements quantifying the effect of Fe substituting for Mg in olivine are needed to distinguish compositional from temperature, grain size and grain shape effects in observational data. To address this need, we study room temperature absorption spectra of a large suite of olivines evenly spaced across Mg and Fe compositions. We apply the principle that classical dispersion theory may be used to determine peak positions as well as peak widths, strengths and possibly optical function (n(λ) and k(λ)) estimates from absorption spectra of thin film samples of these olivines and two additional isotropic and anisotropic minerals with varying hardness and numbers of spectral bands. For olivine, we find that this method provides good estimates of peak position and that accounting for asymmetric peak shapes in this way increases the error on full width at half-maximum and oscillator strengths. Values from classical dispersion fits better match published n and k derived from reflectivity of single crystals when the dust proxy is soft and the thickness of the sample is independently constrained. Electronic data and peak parameter trends for the laboratory olivine absorption spectra and the viability of the extracted n and k are discussed with regard to astronomy.

Subduction of mantle wedge peridotites: Evidence from the Higashi‐akaishi ultramafic body in the Sanbagawa metamorphic belt

AbstractThe Higashi‐akaishi garnet‐bearing ultramafic body in the Sanbagawa metamorphic belt, Southwest Japan, represents a rare example of oceanic‐type ultrahigh‐pressure metamorphism. The body of 2 km × 5 km is composed mostly of anhydrous dunite with volumetrically minor lenses of clinopyroxene‐rich rocks. Dunite samples contain high Ir‐type platinum group elements (PGE) and Cr in bulk rocks, high Mg and Ni in olivine, and high Cr in spinel. On the other hand, clinopyroxene‐rich rocks contain low concentrations of Ir‐type PGE and Cr, high concentrations of fluid‐mobile elements in bulk rocks, and low Ni and Mg in olivine. Clinopyroxene is diopsidic with low Al2O3. The compositions of bulk rocks and mineral chemistry of spinel, olivine, and clinopyroxene suggest that the olivine‐dominated rocks are residual mantle peridotites after high degrees of influx partial melting, and that the clinopyroxene‐rich rocks are cumulates of subduction‐related melts. Thus, the Higashi‐akaishi ultramafic body originated from the interior of the mantle wedge, most likely the forearc upper mantle. It was then incorporated into the Sanbagawa subduction channel by a mantle flow, and underwent high pressure metamorphism to a depth greater than 100 km. Such a strong active flow in the mantle wedge is likely facilitated by the lack of serpentinites along the interface between the slab and the overlying mantle, as it was too hot for serpentine. These unusually hot conditions and strong active mantle flow may reflect conditions in the earliest stage of development of subduction, and may have been maintained by massive upwelling and subsequent eastward flow of asthenospheric mantle in the northeastern Asian continent in Cretaceous time when the Sanbagawa belt began to form.

A translithospheric suture in the vanished 1-Ga lithospheric root of South India: Evidence from contrasting lithosphere sections in the Dharwar Craton

Garnet xenocrysts and eclogite xenoliths from 15 kimberlites (1.0–1.1 Ga in age) have been used to map the composition and structure of the lithospheric mantle along an 80-km traverse across the eastern margin of the Closepet Granite in Andhra Pradesh. The SCLM at the SW end of the traverse is more depleted (abundant harzburgites, mean X Mg of olivine ≈ 93.5, mean whole-rock Al 2O 3 ≈ 1.5%) than that at the NE end (fewer harzburgites, X Mg ≈ 92, Al 2O 3 > 2%). The depleted layer is ca 195 km thick in the SW, and ca 170 km in the NE, though geotherms are similarly low (ca 35–37 mW/m 2). The middle of the traverse is underlain by a strongly refertilised SCLM with a higher geotherm (ca 40 mW/m 2) and extensive evidence of metasomatism. At the SW end of the traverse, abundant eclogites are tightly concentrated in a layer from 175 to 190 km depth, coinciding with a zone of melt-related metasomatism. In the central part, eclogites are distributed through the highly metasomatised section from 90 to 160 km depth. These data suggest that the kimberlites at either end of the traverse sampled two distinct lithospheric blocks, perhaps coinciding with the Eastern and Western Dharwar Cratons. The zone of refertilised SCLM between them is interpreted as the cratonic suture, metasomatised by mafic melts (now eclogites). If this suture dips 70–80° to the east, its surface outcrop lies within the Closepet Granite. The “tilt” of the proposed cratonic suture may reflect overthrusting of the Eastern Dharwar Craton crust up to 100 km to the west. Recent geophysical data (seismic, MT) suggest that the depleted lithospheric root beneath the Dharwar Craton that was sampled by the 1.1 Ga kimberlites is no longer present. The removal or major modification of this root could have occurred during the breakup of Gondwanaland, and may help to explain India's rapid northward drift. India thus joins the North China Craton as an example of the destruction of an Archean continental keel.

Significance of the concentration gradients associated with dunite bodies in the Josephine and Trinity ophiolites

Detailed transects were sampled across dunite bodies in the Josephine and Trinity ophiolites. The major peridotite lithologies sampled in the Josephine transect are a sequence of dunite and harzburgite and in the Trinity transect a sequence of dunite, harzburgite, lherzolite, and plagioclase lherzolite (DHL‐PL). Major, minor, and selected trace element abundances in olivine, orthopyroxene, clinopyroxene, and spinel were measured. The composition profile from the Josephine transect has revealed a concentration gradient near the dunite‐harzburgite contact. The composition profile from the Trinity transect has revealed several concentration gradients: two within the dunite, one in the harzburgite, and at least two in the plagioclase lherzolite. The composition profiles record complex histories of melt transport, melt‐rock reaction, and subsequent subsolidus reequilibration. Analyses of closure distance suggest that compositional variation trends for a majority of major and minor elements in olivine, clinopyroxene, orthopyroxene, and spinel reported in this study were magmatic in origin. Subsolidus reequilibration may reduce the range or magnitude of variations for the 2+ cations such as Fe and Mg in olivine and spinel and significantly redistribute Ca and Li in coexisting minerals. Numerical simulations exploring the coupling of diffusion and advection in a porous matrix were used to explain compositional variations across the peridotite sequences. Melt flow from the host harzburgite into the dunite produces composition gradients near the dunite‐harzburgite contacts similar to those from the Josephine transect. In contrast, melt flow from the dunite into the surrounding peridotite lithologies can produce concentration gradients similar to those observed in the Trinity transect. At least two chemically distinct episodes of melt flow within the same dunite channel system are proposed. Results from this study show that concentration gradients developed around the dunite‐harzburgite and DHL‐PL sequences can be used to infer part of the melt flow history of the dunite channel systems in the mantle. Results from this study and those from other ophiolite studies also demonstrate that concentration profiles in dunite and the surrounding peridotite lithologies are highly variable even among differing peridotite sequences within the same ophiolite, suggesting that the composition of instantaneous melt flowing through individual dunite channels is quite variable and the mantle source regions are heterogeneous.

Composition and thermal evolution of the lithospheric mantle beneath the Harrat Uwayrid, eastern flank of the Red Sea rift (Saudi Arabia)

Harrat Uwayrid is one of several volcanic fields that developed along the western margin of the Arabian plate during opening of the Red Sea rift. Volcanic activity started in the late Miocene and Pliocene with the eruption of alkaline flood basalts. After a period of extensional faulting, numerous cinder cones formed in the central part of the volcanic field during the Quaternary. Xenoliths of both mantle and crustal origin are abundant. The mantle-derived suite is dominated by spinel lherzolite with X Mg of olivine and X Cr of spinel ranging from 0.868 to 0.911 and from 0.074 to 0.636, respectively. In addition to lherzolite, various types of pyroxenites and wehrlites occur. Most xenoliths contain primary and/or secondary amphibole (pargasite/kaersutite), indicating at least two enrichment events, both related to the formation of the Red Sea. Chemical zonation profiles of mineral grains constrain the thermal evolution of the xenoliths and their source regions in the shallow upper mantle. Ca contents in olivine are constant in the inner parts of the grains, but increase in the outermost zones (< 35 μm). X Mg values of spinel grains adjacent to olivine often show a rimward increase in their outer zones (< 150 μm). Both types of zonation patterns are interpreted to result from heating by the host magma during transport. Zonation patterns of Al and Ca/(1 − Na) in pyroxene grains record different thermal evolutions, such as (i) simple cooling, whereby a ‘near-equilibrium’ state was rarely achieved, (ii) cooling followed by heating, and (iii) simple heating. Diffusion modelling on measured zoning profiles suggests that cooling lasted for no more than ∼ 10 Myr. Temperature estimates for pyroxene core and rim compositions, calculated by using various pyroxene geothermometers range from ∼ 800 to 1140 °C (at an assumed pressure of 1.5 GPa). In conjunction with upper pressure limits derived from the composition of spinel, these values imply relatively high temperatures for the shallow upper mantle. This is corroborated by P– T estimates for spinel lherzolites obtained by combination of the two-pyroxene thermometer based on the solvus between enstatite and diopside and the Ca-in-olivine geothermobarometer. While the high- T state that preceded cooling was probably a regional phenomenon, renewed heating was of local nature, most probably related to advective heat transport by ascending and stagnating magmas.

Metasomatism of sub-arc mantle peridotites below southernmost South America: reduction of fO2 by slab-melt

Quaternary basalts in the Cerro del Fraile area contain two types of mantle xenoliths; coarse-grained (2–5 mm) C-type spinel harzburgites and lherzolites, and fine-grained (0.5–2 mm) intensely metasomatized F-type spinel lherzolites. C-type xenoliths have high Mg in olivine (Fo = 90–91) and a range in Cr# [Cr/ (Cr + Al) = 0.17–0.34] in spinel. Two C-type samples contain websterite veinlets and solidified patches of melt that is now composed of minute quenched grains of plagioclase + Cr-spinel + clinopyroxene + olivine. These patches of quenched melts are formed by decompression melting of pargasitic amphibole. High Ti contents and common occurrence of relic Cr-spinel in the quenched melts indicate that the amphibole is formed from spinel by interaction with the Ti-rich parental magma of the websterite veinlets. The fO2 values of these two C-type xenoliths range from ΔFMQ −0.2 to −0.4, which is consistent with their metasomatism by an asthenospheric mantle-derived melt. The rest of the C-type samples are free of “melt,” but show cryptic metasomatism by slab-derived aqueous fluids, which produced high concentrations of fluid-mobile elements in clinopyroxenes, and higher fO2 ranging from ΔFMQ +0.1 to +0.3. F-type lherzolites are intensely metasomatized to form spinel with low Cr# (∼0.13) and silicate minerals with low MgO, olivine (Fo = ∼84), orthpyroxene [Mg# = Mg/(Mg + ΣFe) = ∼0.86] and clinopyroxene (Mg# = ∼0.88). Patches of “melt” are common in all F-type samples and their compositions are similar to pargasitic amphibole with low TiO2 (<0.56 wt%), Cr2O3 (<0.55 wt%) and MgO (<16.3 wt%). Low Mg# values of silicate minerals, including the amphibole, suggest that the metasomatic agent is most likely a slab melt. This is supported by high ratios of Sr/Y and light rare earth elements (REE)/heavy REE in clinopyroxenes. F-type xenoliths show relatively low fO2 (ΔFMQ −0.9 to −1.1) compared to C-type xenoliths and this is explained by the fusion of organic-rich sediments overlying the slab during the slab melt. Trench-fill sediments in the area are high in organic matter. The fusion of such wet sediments likely produced CH4-rich fluids and reduced melts that mixed with the slab melt. High U and Th in bulk rocks and clinopyroxene in F-type xenoliths support the proposed interpretation.

The Quetico Intrusions of Western Superior Province: Neo-Archean examples of Alaskan/Ural-type mafic–ultramafic intrusions

The Quetico Intrusions are a series of ultramafic–mafic igneous rocks that occur along the northern Quetico subprovince boundary. These intrusions show lithological zoning from cores of olivine-bearing ultramafic rocks to margins of hornblendite and gabbroic rocks, and they are commonly accompanied by Cu–Ni–platinum group element (PGE) sulphide mineralization. We identified three additional Quetico Intrusions in the centre and southern part of the Quetico subprovince, bringing the entire Quetico Intrusion array to be over 125 km in length across the subprovince. The Samuels Lake intrusion in the centre of the subprovince has an U–Pb age of 2688 + 6/−5 Ma, which is similar to the age of ∼2690 Ma for the Quetico Intrusions along the northern subprovince boundary, suggesting that all the Quetico Intrusions are likely to be contemporaneous. The parental magmas of the intrusions were Mg-rich based on the high Mg of olivine and clinopyroxene, and they were hydrous as evidenced by abundant hornblende, early crystallization of clinopyroxene, and the common occurrence of mafic pegmatite. The bulk rocks show arc geochemical signatures, including a high concentration of large ion-lithophile elements combined with a low concentration of high field-strength elements. The mineral compositions of pyroxene and hornblende also plot in the field of arc igneous rocks. The data suggest the derivation of the parental magmas from a mantle wedge or a refractory mantle that was previously metasomatized in a subduction setting. The Quetico Intrusions display many similarities with Alaskan/Ural-type zoned mafic–ultramafic intrusions along major sutures of Phanerozoic orogenic belts. Common features include their geodynamic settings, internal lithological zoning, mineralogy and mineral chemistry, and common association with PGE mineralization. The striking similarities between the two indicate that the late Archean Quetico Intrusions likely formed in a tectonic setting similar to that of the Alaskan/Ural-type intrusions. We propose that the parental magmas for the Quetico Intrusions formed during the very early stage of accretion of the Wawa arc to the Wabigoon subprovince. Oblique accretion of the Wawa arc likely steepened the subduction angle, which allowed upwelling of hot asthenospheric mantle. This resulted in high degrees of partial melting in the mantle wedge to produce the Quetico Intrusions.

Oxygen isotopic evolution of amoeboid olivine aggregates in the reduced CV3 chondrites Efremovka, Vigarano, and Leoville1

Amoeboid olivine aggregates (AOAs) from the reduced CV chondrites Efremovka, Vigarano, and Leoville consist of forsteritic olivine, FeNi-metal and a refractory component composed of spinel, Al-diopside, ±anorthite. Secondary ferrous olivine and alkali-rich minerals (nepheline and sodalite), commonly observed in the oxidized CVs, are rare. Mineralogy and chemical compositions of AOAs are similar to those predicted by equilibrium thermodynamic condensation models, suggesting that AOAs formed primarily by gas-solid condensation over a narrow temperature range, slightly below the temperatures over which most Ca-Al-rich inclusions (CAIs) formed. AOAs in the reduced CVs preserve a 1 st -generation 16 O-rich signal (δ 17,18 O ∼ −40‰) similar to that observed in many CAIs, suggesting that these refractory objects originated from a common source in the solar nebula. In fact AOAs and many fine-grained CAIs may have formed by the same processes, but at slightly different temperatures, and can be considered a single class of refractory objects. Alteration of the AOAs is manifested by differing extents of 16 O-depletion in original AOA minerals, FeO-enrichment in olivine, and formation of interstitial very fine grained Na-bearing phases. From the six AOAs and one fine-grained, melilite-pyroxene-rich CAI examined in this study, five distinct patterns of alteration were identified. (1) One unaltered AOA from Vigarano is characterized by 16 O-rich forsterite without FeO-rich rims and interstitial Na-bearing phases. (2) Weak alteration in the melilite-pyroxene-rich CAI is characterized by incomplete 16 O-depletion in some melilite and precipitation of Na-bearing phases near the CAI rim. (3) Oxygen isotopic composition and mineralogy are correlated in two AOAs from Leoville with 16 O-rich olivine, 16 O-poor anorthite and a range of intermediate compositions in Al-diopside. This pattern is consistent with model diffusion between original grains and a 16 O-poor reservoir during a relatively short-term (<60 yr), high-temperature (900–1100°C) event. (4) Original forsterite has been enriched in FeO, but remained 16 O-rich in one AOA from Vigarano. This result is consistent with the slower rate of diffusion of O than Fe and Mg in olivine. At least some interstitial phases are 16 O-rich, and Na-bearing phases are abundant in this AOA. (5) In contrast, oxygen isotopic composition and Fo-content are correlated in two AOAs from Efremovka. The olivine in these AOAs tends to have forsteritic 16 O-rich cores and FeO-rich 16 O-depleted rims. The general correlation between oxygen isotopic composition and Fo-content is difficult to model by diffusion, and may have formed instead by aqueous dissolution and precipitation along the margins of preexisting olivine grains. Independent evidence for aqueous alteration of the Efremovka AOAs is provided by OH-rich signals detected during ion beam sputtering of some of the 16 O-poor olivine. Elevated 16 OH-count rates and order of magnitude increases in 16 OH detected during single analyses reflect trapping of an aqueous phase in 16 O-depleted olivine. An elevated 16 OH signal was also detected in one analysis of relatively 16 O-poor melilite in the melilite-pyroxene CAI from Vigarano, suggesting that this object also was altered by aqueous fluid.

Oxygen isotopic evolution of amoeboid olivine aggregates in the reduced CV3 chondrites Efremovka, Vigarano, and Leoville

Amoeboid olivine aggregates (AOAs) from the reduced CV chondrites Efremovka, Vigarano, and Leoville consist of forsteritic olivine, FeNi-metal and a refractory component composed of spinel, Al-diopside, ±anorthite. Secondary ferrous olivine and alkali-rich minerals (nepheline and sodalite), commonly observed in the oxidized CVs, are rare. Mineralogy and chemical compositions of AOAs are similar to those predicted by equilibrium thermodynamic condensation models, suggesting that AOAs formed primarily by gas-solid condensation over a narrow temperature range, slightly below the temperatures over which most Ca-Al-rich inclusions (CAIs) formed. AOAs in the reduced CVs preserve a 1 st-generation 16O-rich signal (δ 17,18O ∼ −40‰) similar to that observed in many CAIs, suggesting that these refractory objects originated from a common source in the solar nebula. In fact AOAs and many fine-grained CAIs may have formed by the same processes, but at slightly different temperatures, and can be considered a single class of refractory objects. Alteration of the AOAs is manifested by differing extents of 16O-depletion in original AOA minerals, FeO-enrichment in olivine, and formation of interstitial very fine grained Na-bearing phases. From the six AOAs and one fine-grained, melilite-pyroxene-rich CAI examined in this study, five distinct patterns of alteration were identified. (1) One unaltered AOA from Vigarano is characterized by 16O-rich forsterite without FeO-rich rims and interstitial Na-bearing phases. (2) Weak alteration in the melilite-pyroxene-rich CAI is characterized by incomplete 16O-depletion in some melilite and precipitation of Na-bearing phases near the CAI rim. (3) Oxygen isotopic composition and mineralogy are correlated in two AOAs from Leoville with 16O-rich olivine, 16O-poor anorthite and a range of intermediate compositions in Al-diopside. This pattern is consistent with model diffusion between original grains and a 16O-poor reservoir during a relatively short-term (<60 yr), high-temperature (900–1100°C) event. (4) Original forsterite has been enriched in FeO, but remained 16O-rich in one AOA from Vigarano. This result is consistent with the slower rate of diffusion of O than Fe and Mg in olivine. At least some interstitial phases are 16O-rich, and Na-bearing phases are abundant in this AOA. (5) In contrast, oxygen isotopic composition and Fo-content are correlated in two AOAs from Efremovka. The olivine in these AOAs tends to have forsteritic 16O-rich cores and FeO-rich 16O-depleted rims. The general correlation between oxygen isotopic composition and Fo-content is difficult to model by diffusion, and may have formed instead by aqueous dissolution and precipitation along the margins of preexisting olivine grains. Independent evidence for aqueous alteration of the Efremovka AOAs is provided by OH-rich signals detected during ion beam sputtering of some of the 16O-poor olivine. Elevated 16OH-count rates and order of magnitude increases in 16OH detected during single analyses reflect trapping of an aqueous phase in 16O-depleted olivine. An elevated 16OH signal was also detected in one analysis of relatively 16O-poor melilite in the melilite-pyroxene CAI from Vigarano, suggesting that this object also was altered by aqueous fluid.

Chemistry of glass inclusions in olivines of the CR chondrites Renazzo, Acfer 182, and El Djouf 001

Glass inclusions in olivines of the Renazzo, El Djouf 001, and Acfer 182 CR-type chondrites are chemically divers and can be classified into Al-rich, Al-poor, and Na-rich types. The chemical properties of the glasses are independent of the occurrence of the olivine (isolated or part of an aggregate or chondrule) and its composition. The glasses are silica-saturated (Al-rich) or oversaturated (Al-poor, 24% normative quartz). All glasses have chondritic CaO/Al 2O 3 ratios, unfractionated CI-normalized abundances of refractory trace elements and are depleted in moderately volatile and volatile elements. Thus the glasses are likely to be of a primitive condensate origin whose chemical composition has been established before chondrule formation and accretion, rather then the product of either crystal fractionation from chondrule melts or part melting of chondrules. Rare Na-rich glasses give evidence for elemental exchange between the glass and a vapor phase. Because they have Al 2O 3 contents and trace element abundances very similar to those of the Al-rich glasses, they likely were derived from the latter by Ca exchange (for Na) with the nebula. Elemental exchange reactions also have affected practically all olivines (e.g., exchange of Mg of olivine for Fe 2+, Mn 2+, and Cr 3+). Glasses formed contemporaneously with the host olivine. As the most likely process for growing nonskeletal olivines from a vapor we consider the VLS (vapor-liquid-solid) growth process, or liquid-phase epitaxy. Glasses are the possible remnants of the liquid interface between growing crystal and the vapor. Such liquids can form stably or metastably in regions with enhanced oxygen fugacity as compared to that of a nebula of solar composition.

Modeling the major-element evolution of olivine-hosted melt inclusions

This paper presents a detailed description of an approach for modeling the major element evolution of olivine-hosted melt inclusions. Numerical simulations constrained by experimental data on olivine/melt equilibrium and interdiffusion rates of Fe and Mg in olivine quantify the post-entrapment processes (crystallization, dissolution, Fe–Mg exchange with the host) that influence the major element compositions of included melts. Equilibrium at the olivine/melt interface is described by expressions for FeO and MgO partitioning calibrated using olivine–liquid pairs from the literature spanning temperatures of 1064 to 1950 °C and pressures of 1 bar to 70 kbar. The liquidus temperature and equilibrium olivine composition are calculated simultaneously for melt of a given composition using the partitioning expressions and olivine stoichiometry. Diffusion in the host olivine is modeled by the explicit finite-difference method. The main inputs for a simulation are the initial composition and size of the melt inclusion, the size of the host olivine, and the desired cooling path expressed as a constant cooling rate. The rate of olivine crystallization (d F/d T) within an inclusion is dynamically adjusted with each time step so that the observed Fe–Mg exchange coefficient ( K D Fe–Mg) at the inclusion/host interface matches the equilibrium value predicted by the partitioning equations. Application of the approach to model the major element evolution of an inclusion formed within the melting regime beneath an oceanic spreading center indicates that the composition of the included melt is significantly modified during transport to the surface along the mantle geotherm. Further, the compositional path followed by the included melt is dependent upon the capacity of the host olivine to maintain the inclusion at the pressure of entrapment.

Open system behavior of olivine-hosted melt inclusions

It is commonly assumed that an olivine-hosted melt inclusion is chemically isolated from the melt surrounding its host crystal. This assumption is probably valid for slow-diffusing, incompatible trace elements, but for readily exchanged major elements such as Fe and Mg, it is much more tenuous. We used numerical simulations constrained by experimental data on olivine–liquid equilibrium and interdiffusion rates of Fe and Mg in olivine to assess the extent to which post-entrapment processes alter the major element compositions of olivine-hosted melt inclusions. The results indicate that extensive diffusive communication between a melt inclusion and the melt surrounding its host olivine occurs at cooling rates as rapid as 1–2°C/yr. An included melt undergoes significant and irreversible compositional changes in order to maintain Fe–Mg exchange equilibrium with the fractionating external melt. When a magnesian olivine containing an exotic liquid, such as an ultra-depleted melt, becomes entrained in a basalt and equilibrates with the new external melt by diffusive exchange, the ensuing compositional changes to the included melt cannot be removed simply by adding olivine back into the final (measured) melt composition. Observable disequilibrium between an inclusion and its host olivine is likely to have been produced during cooling of the erupted lava. Simulations, supplemented by experiments carried out on inclusion-bearing olivines, demonstrate that laboratory heating can reverse the effects of syn-eruptive crystallization on melt inclusion composition, and the presence or absence of compositional zoning in the immediately adjacent olivine can provide an indication of how closely the experiment reproduced the pre-eruptive temperature.

Evaluation of diffusion coefficients of Fe-Mg and Ca in olivine.

We evaluated the diffusion coefficients of Fe–Mg and Ca in olivine reported in several literatures by analyzing the diffusion profiles experimentally produced. Both the Fe and Ca contents of olivine in run products drastically increase toward the rim within a few tens of micrometers probably due to atomic diffusion from the surrounding Fe- and Ca-rich melt. The chemical zoning profiles of olivine measured with an electron microprobe were compared with those calculated by numerically solving the diffusion equation by using different diffusion coefficients. The diffusion coefficient of Fe–Mg in olivine reported by Misener (1974) gives a better fit than that by Buening and Buseck (1973), that is, the latter is too large. The diffusion coefficient of CaO in olivine reported by Jurewicz and Watson (1988b) gives a better fit than that by Morioka (1981), that is, the latter is too small. Our calculations for analyzing chemical zoning on the basis of atomic diffusion successfully reproduce the curvature of a profile experimentally made by diffusional modification of initial uniform profile of olivine.

The Archean komatiite-related Inyala Chromitite, southern Zimbabwe

The chromitites at the Inyala mine, Zimbabwe, form part of a layered intrusion enclosed within the Archean granulite facies gneisses of the north marginal zone (NMZ) of the Limpopo belt. Chromitites occur within an olivine-orthopyroxene-chromite cumulate sequence which was derived from a komatiitic magma.An electron microprobe study and a comparison with the mineralogy of unaltered komatiites from the Belingwe greenstone belt, Zimbabwe, strongly suggest that some of the original olivine, chromite, and orthopyroxene mineral compositions are preserved. Olivine-chromite mineral pairs continued to equilibrate down-temperature until about 680 degrees C. Calculations based on the diffusion of Fe and Mg in olivine suggest that the cooling was rapid (several 1,000 degrees C/Ma), hence it is reasonable to expect igneous mineral compositions to be preserved. Calculations based on mineral-melt, Fe-Mg partitioning for olivine and orthopyroxene indicate that the parent komatiitie liquids varied in composition from 26 to 16 wt percent MgO. Thus some of the chromites crystallized from melts which are among the most magnesian komatiites known and which would have had an eruption temperature of ca. 1,520 degrees C.Orthopyroxenite cumulates, which are unusual in komatiitic sequences probably formed in response to assimilation of adjacent supracrustal rocks, possibly banded iron-formation. Oxygen activity calculations suggest that assimilation may have led to an increase in the oxygen activity in the melt which may be responsible for the crystallization of both chromite and orthopyroxene.

A thermal-tectonic model for high-pressure rocks in the Basal Gneiss Complex of western Norway

The Basal Gneiss Complex (BGC) of western Norway is a segment of continental crust that was subjected to eclogite facies metamorphism during the Caledonian Orogeny and then was overprinted by amphibolite facies conditions. Numerical methods have been used to construct a model for thermal evolution of the BGC. The calculated temperature-depth-time ( Tzt) paths for the BGC are in good agreement with the sequence of mineral assemblages that occur in metamorphosed mafic and ultramafic rocks. However, the thermal model indicates that retrograde zoning of minerals in the garnet-bearing assemblages of eclogite and ultramafite may have developed metastably at pressures below 10 kbar. Numerical modeling of iron diffusion in garnet grains adjacent to olivine inclusions was used to calculate zoning profiles based upon the thermal model. The calculated zoning profiles have shorter decay distances than the observed zoning profiles, which may be due to uncertainties in the diffusion coefficients. Also, discrepancies occur between the Tzt paths and specific PT values calculated for eclogite and garnet ultramafite due either to the preservation of pre-Caledonian mineral assemblages or to utilization of geobarometers and thermometers which are based on different elements in different phases (such as A1 in orthopyroxene and FeMg in olivine and garnet).

Thermoanalytical study on the chlorination of magnesium-containing ores: An application of a simple gas-flow differential thermal analysis technique

A simple gas-flow DTA apparatus has been designed to investigate the chlorination reaction of Mg-containing complex ores (olivine, protoenstatite and talc) with Cl 2-gas in a temperature range of 25–1000°C. The apparatus consists of two quartz tubes of i.d. = 10 mm containing 1 g of reference material (α-A1 20 3, —150 mesh) and 1 g of sample powders (—150 mesh) through which C1 2-gas can flow to allow the reaction. p]In the preliminary chlorination experiments with a constant Cl 2 flow-rate of 30–35 ml/min, the constituents of ores, MgO, FeO and Fe 2O 3, successfully showed the exothermic peak at 450, 150 and 450°C, respectively, and SiO 2 being inert against Cl 2 up to 950 and 750°C. Only MgO required carbon additive for the chlorination. p]Mg in the ores is chlorinated only when they are mixed with carbon. Olivine (Mg 2SiO 4) shows two exothermic peaks at 880 and 950°C, which indicate that Mg in olivine has different reactivities. Protoenstatite (MgSiO 3) and talc (Mg 3(Si 2O 5) 2(OH) 2) give peaks at 950 and 750°C, respectively. The results of X-ray diffraction and chemical analysis revealed that the chlorination exothermic peaks of olivine and protoenstatite corresponded to the following reactions, ▪ ▪ and that the chlorination of talc proceeded together with the decomposition reaction, ▪.