Ca-poor Pyroxene Research Articles

Ferro-chloro-winchite in Northwest Africa (NWA) 998 apatite-hosted melt inclusion: New insights into the nakhlite parent melt

Melt inclusions are of major significance because they can constrain the volatile abundances in magmas. Here, we report the discovery of the first melt inclusion in a martian apatite containing the first chloro-amphibole reported in Northwest Africa (NWA) 998, a sample that crystallized early from the nakhlite-source. The amphibole is also the first sodic-calcic amphibole in a nakhlite, identified as ferro-chloro-winchite (4.75 wt% Cl) by FIB-TEM. The melt inclusion is present in a euhedral, cumulus apatite grain (Cl/F = 2.11) and is surrounded by a shell of voids. Evidence indicates that the melt inclusion remained as a closed system although syn- and post-entrapment processes may have modified the chemical composition of the original trapped melt. The inclusion also contains Fe-rich, Ca-poor pyroxene and a residual silicate melt consisting of pyroxene and interstitial K-rich glass. Additionally, Cl-enriched apatite is present within the boundaries of the melt inclusion. This apatite could result from cracking of host-apatite during contraction of the melt inclusion glass or represent daughter apatite crystallizing on the walls of the inclusion. Given that Cl-enrichments are found in the host apatite outside the melt inclusion, it is inferred that a later, fluid alteration event locally modified the composition of the apatite in and/or around the melt inclusion. The calculated bulk composition of the melt inclusion is generally consistent with previous work in other nakhlites. Prior to this study, Cl-rich amphiboles have only been found within olivine- and pyroxene-hosted melt inclusions in the later-formed nakhlites. The present study thus demonstrates that (i) the nakhlites record magmatic mixing with a Cl-rich exogenous component that is absent within olivine-hosted melt inclusions in the chassignites, (ii) Cl-rich amphiboles were able to crystallize from the earliest nakhlite parent melt, and (iii) the presence of a Cl-rich fluid was not required to stabilize chloro-amphiboles. We also conclude that magmatic martian amphiboles likely stabilized at lower pressures (and temperatures) than terrestrial amphiboles due to their higher Cl contents.

Formation of Siberian cratonic mantle websterites from high-Mg magmas

Garnet–(olivine) websterite xenoliths from the lithospheric mantle of the central and northeastern parts of the Siberian Craton contain exsolution microstructures after Si- and Ti-rich precursor garnets. We petrographically, geochemically, and thermobarometrically investigated 13 such xenoliths from the Mir, Obnazhennaya, and Udachnaya kimberlite pipes. All samples contain garnet grains with needle- to lamellae-shaped precipitates (up to 3.0 vol%), including Ti-oxide and/or pyroxene. Orthopyroxene and clinopyroxene grains host oriented lamellae of complementary Ca-rich and Ca-poor pyroxene, respectively, in addition to lamellae of garnet and Ti- and/or Cr-oxides. The common exsolution lamellae assemblages in garnet and pyroxene imply that exsolution occurred during cooling from high-temperature precursors. Exsolution is unlikely to have resulted from variations in pressure, given experimental and thermodynamic constraints. Host mineral partitioning of transition metal and lanthanide elements with different diffusivities record temperatures that range between those of local geotherms and a dry pyroxenite solidus. Inferred magmatic minimum temperatures of 1500–1700 °C satisfy the physical conditions predicted from experimental studies of the solubility of excess Si and Ti in garnet. Granular inclusions of all major minerals within each other imply an overlapping crystallisation history. The reconstructed compositions of the websterite whole-rocks have high MgO contents (15.7–35.7 wt%). A plot of MgO/SiO2 versus SiO2 forms an array, apart from the compositions of natural websterites that formed by interaction of peridotite with basaltic or siliceous melts. The array overlaps the compositional range of komatiite flows from Commondale and Barberton, South Africa, including spinifex, massive, and cumulate subtypes of komatiites. Other major and minor element abundances and ratios of the Siberian websterite suite resemble those of South African Al-enriched komatiites and are distinct from melt–rock reaction websterites. Therefore, the mineral microstructures and geochemistry of the Siberian websterites are suggestive of the former presence of a thermal anomaly. We propose that mantle plume activity or a similar form of lower-mantle ascent played a major role in stabilising cratonic nuclei before amalgamation of the present-day Siberian Craton.

A magmatic origin for silica-rich glass inclusions hosted in porphyritic magnesian olivines in chondrules: An experimental study

Rare silica-rich glass inclusions (69<SiO2<82wt.%) are described within magnesian olivines of porphyritic Type IA chondrules. These glass inclusion compositions are clearly out of equilibrium with their host Mg-olivines and their presence within the olivines is generally attributed to an unclear secondary process such as a late interaction with nebular gases. We performed dynamic crystallisation experiments that demonstrate that these Si-rich glass inclusions are actually magmatic in origin and were trapped inside olivines that crystallized slowly from a magma with a CI, i.e. solar, composition. Their silica-rich compositions are the consequence of the small volumes of inclusions, which inhibit the nucleation of secondary crystalline phase (Ca-poor pyroxene) but allow olivine to continue to crystallize metastably on the walls of the inclusions. We suggest that Si-rich glass inclusions could be the only reliable relicts of what were the first magmas of the solar system, exhibiting a CI, i.e. non-fractionated, composition.

Plagioclase Fractionation in Troctolitic Magma

Troctolitic intrusive rocks poor in augite are common in certain Proterozoic anorthosite complexes and related rocks.The Lower Zone of the Kiglapait intrusion, Labrador, consists of 1570 km troctolite today and possibly 2900 km before erosion. In this augite-free Lower Zone, plagioclase fractionation is as low as 1 6%An km of cumulate thickness and averages 3%An km.When augite crystallizes after 84% of the magma has crystallized, the fractionation becomes as much as 17%An km of cumulate.Why such a difference? It is clear from first principles of phase equilibria that fractionation accelerates with saturation in augite, but not so clear that the difference should be so great.The answer is to be found in the silica-poor nature of troctolitic magma that is critically undersaturated in silica. This low-silica effect reduces the activity of the NaSi albite component relative to the CaAl anorthite component in the plagioclase, thereby favoring the An component of the liquid and crystals and weakening the fractionation process. As the normative augite component in the magma rises from the base of the Lower Zone to the base of the Upper Zone, the activity of silica also rises slightly and its consequent effect on plagioclase composition tends to diminish. Liquid fractionation paths derived from observed crystal paths, when plotted in the system Diopside^Anorthite^Albite, rise across the liquidus fractionation lines toward diopside and reach augite saturation near the 1atm cotectic. They produce plagioclase compositions 10 mol % higher in An than pure liquidus fractionation lines predict. The key criterion for the troctolitic fractionation of plagioclase composition is the absence of Ca-poor pyroxene from the rocks. Noritic magmas, by contrast, have higher activities of silica and more effective fractionation of plagioclase. A parallel fractionation of olivine is also retarded in the Lower Zone by the accumulation of ferric iron in the liquid until augite and titanomagnetite crystallize in the Upper Zone. KEY WORD: plagioclase fractionation; troctolite melts; silica activity; layered intrusions; Kiglapait intrusion

Shock veins in the Sahara 02500 ordinary chondrite

Shock veins in the Sahara 02500 ordinary chondriteA specimen of the Sahara 02500 ordinary chondrite contains shock-produced veins consisting of recrystallised fine-grained pyroxenes that include small droplets of Ni-rich metal. Non-melted olivines and pyroxenes show planar deformations filled by shock-melted and -polluted metal and troilite. Shock-melted feldspathic glass is present close to the shock veins. Geothermometric estimations indicate that the meteorite locally experienced moderate shock metamorphism with a minimum local peak temperature above 1400°C, resulting in partial melting of Ca-poor pyroxene and full melting of feldspars, metal and sulphides. The mineral assemblage in the shock veins suggests a pressure during melt recrystallisation below 10 GPa.

Fractional crystallization of olivine melt inclusion in shock-induced chondritic melt vein

The formation of ringwoodite, wadsleyite and majorite from their parental low-pressure polymorphs in melt veins in chondritic meteorites is usually interpreted to be the result of shock-induced solid-state phase transformation. Formation and survival of individual mineral melt enclaves in the chondritic high-pressure melt was not considered a viable possibility. We report evidence for melting of individual large olivine fragments entrained in melt veins, their survival as melt enclaves in the chondritic melts and their subsequent fractional crystallization at high-pressures and temperatures. The fractionally crystallized olivine melt enclaves appear to be ubiquitous in chondrites. In contrast, Ca-poor pyroxene fragments in the same veins and Ca-poor pyroxene in chondrules entrained do not show any sign of melting. Texture and compositions of olivine fragments are indicative of fractional crystallization from individual olivine melts alone. Fragments of original unzoned olivine (Fa 24–26) melted, and melts subsequently fractionally crystallized to Mg-rich wadsleyite (Fa 6–10) and Mg-poor ringwoodite (Fa 28–33) with a compositional gap of ≤26 mol% fayalite. In contrast, compositions of ringwoodite and wadsleyite that emerged from solid-sate phase transformations are identical to that of parental olivine thus erasing any source of enigma. The olivine monomineralic melts barely show any signs of mixing with the chondritic liquid prior to or during their individual fractional crystallization. Our findings demonstrate that the formation of high-pressure minerals during shock events in asteroids also results from melting and fractional crystallization from some individual mineral melts that barely mixed with the chondritic melt host, a mechanism previously not recognized or accepted.

Relict silicate inclusions in extraterrestrial chromite and their use in the classification of fossil chondritic material

Chromite is the only common meteoritic mineral surviving long-term exposure on Earth, however, the present study of relict chromite from numerous Ordovician (470 Ma) fossil meteorites and micrometeorites from Sweden, reveals that when encapsulated in chromite, other minerals can survive for hundreds of millions of years maintaining their primary composition. The most common minerals identified, in the form of small (<1–10 μm) anhedral inclusions, are olivine and pyroxene. In addition, sporadic merrillite and plagioclase were found. Analyses of recent meteorites, holding both inclusions in chromite and corresponding matrix minerals, show that for olivine and pyroxene inclusions, sub-solidus re-equilibration between inclusion and host chromite during entrapment has led to an increase in chromium in the former. In the case of olivine, the re-equilibration has also affected the fayalite (Fa) content, lowering it with an average of 14% in inclusions. For Ca-poor pyroxene the ferrosilite (Fs) content is more or less identical in inclusions and matrix. By these studies an analogue to the commonly applied classification system for ordinary chondritic matrix, based on Fa in olivine and Fs in Ca-poor pyroxene, can be established also for inclusions in chromite. All olivine and Ca-poor pyroxene inclusions (>1.5 μm) in chromite from the Ordovician fossil chondritic material plot within the L-chondrite field, which is in accordance with previous classifications. The concordance in classification together with the fact that inclusions are relatively common makes them an accurate and useful tool in the classification of extraterrestrial material that lacks matrix silicates, such as fossil meteorites and sediment-dispersed chromite grains originating primarily from decomposed micrometeorites but also from larger impacts.

Trace-element partitioning between garnet, clinopyroxene and Fe-rich picritic melts at 3 to 7 GPa

We have determined mineral-melt partition coefficients (D values) for 20 trace elements in garnet-pyroxenite run products, generated in 3 to 7 GPa, 1,425–1,750°C experiments on a high-Fe mantle melt (97SB68) from the Parana-Etendeka continental-flood-basalt (CFB) province. D values for both garnet (∼Py63Al25Gr12) and clinopyroxene (∼Ca0.2Mg0.6Fe0.2Si2O6) show a large variation with temperature but are less dependent on pressure. At 3 GPa, Dcpx/liq values for pyroxenes in garnet-pyroxenite run products are generally lower than those reported from Ca-rich pyroxenes generated in melting experiments on eclogites and basalts (∼Ca0.3–0.5Mg0.3–0.6Fe0.07–0.2Si2O6) but higher than those for Ca-poor pyroxenes from peridotites (∼Ca0.2Mg0.7Fe0.1Si2O6). Dgrt/liq values for light and heavy rare-earth elements are ≤0.07 and >0.8, respectively, and are similar to those for peridotitic garnets that have comparable grossular but higher pyrope contents (Py70–88All7–20Gr8–14). 97SB68 DLREEgrt/liq values are higher and DHREEgrt/liq values lower than those for eclogitic garnets which generally have higher grossular contents but lower pyrope contents (Py20–70Al10–50Gr10–55). D values agree with those predicted by lattice strain modelling and suggest that equilibrium was closely approached for all of our experimental runs. Correlations of D values with lattice-strain parameters and major-element contents suggest that the wollastonite component and pyrope:grossular ratio exert major controls on 97SB68 clinopyroxene and garnet partitioning, respectively. These are controlled by the prevailing pressure and temperature conditions for a given bulk-composition. The composition of co-existing melt was found to have a relatively minor effect on 97SB68 D values. The variations in D values displayed by different mantle lithologies are subtle and our study confirms previous investigations which have suggested that the modal proportions of garnet and clinopyroxene are by far the most influential factor in determining incompatible trace-element concentrations in mantle melts. The trace-element partition coefficients we have determined may be used to place high-pressure constraints on garnet-pyroxenite melting models.

Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway). Part I: Constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent

Whole-rock major element compositions are investigated in 99 cumulates from the Proterozoic Bjerkreim–Sokndal layered intrusion (Rogaland Anorthosite Province, SW Norway), which results from the crystallization of a jotunite (Fe–Ti–P-rich hypersthene monzodiorite) parental magma. The scattering of cumulate compositions covers three types of cumulates: (1) ilmenite–leuconorite with plagioclase, ilmenite and Ca-poor pyroxene as cumulus minerals, (2) magnetite–leuconorite with the same minerals plus magnetite, and (3) gabbronorite made up of plagioclase, Ca-poor and Ca-rich pyroxenes, ilmenite, Ti-magnetite and apatite. Each type of cumulate displays a linear trend in variation diagrams. One pole of the linear trends is represented by plagioclase, and the other by a mixture of the mafic minerals in constant proportion. The mafic minerals were not sorted during cumulate formation though they display large density differences. This suggests that crystal settling did not operate during cumulate formation, and that in situ crystallization with variable nucleation rate for plagioclase was the dominant formation mechanism. The trapped liquid fraction of the cumulate plays a negligible role for the cumulate major element composition. Each linear trend is a locus for the cotectic composition of the cumulates. This property permits reconstruction by graphical mass balance calculation of the first two stages of the liquid line of descent, starting from a primitive jotunite, the Tjörn parental magma. Another type of cumulate, called jotunite cumulate and defined by the mineral association from the Transition Zone of the intrusion, has to be subtracted to simulate the most evolved part of the liquid line of descent. The proposed model demonstrates that average cumulate compositions represent cotectic compositions when the number of samples is large (> 40). The model, however, does not account for the K 2O evolution, suggesting that the system was open to contamination by roof melts. The liquid line of descent corresponding to the Bjerkreim–Sokndal cumulates differs slightly from that obtained for jotunitic dykes in that the most Ti-, P- and Fe-rich melts (evolved jotunite) are lacking. The constant composition of the mafic poles during intervals where cryptic layering is conspicuous is explained by a compositional balance between the Fe–Ti oxide minerals, which decrease in Fe content in favour of Ti, and the pyroxenes which increase in Fe.

Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway): Part II. REE and the trapped liquid fraction

Rare earth elements in bulk cumulates and in separated minerals (plagioclase, apatite, Ca-poor and Ca-rich pyroxenes, ilmenite and magnetite) from the Bjerkreim–Sokndal layered intrusion (Rogaland Anorthosite Province, SW Norway) are investigated to better define the proportion of trapped liquid and its influence on bulk cumulate composition. In leuconoritic rocks (made up of plagioclase, Ca-poor pyroxene, ilmenite, ±magnetite, ±olivine), where apatite is an intercumulus phase, even a small fraction of trapped liquid significantly affects the REE pattern of the bulk cumulate, together with cumulus minerals proportion and composition. Contrastingly, in gabbronoritic cumulates characterized by the presence of cumulus Ca-rich pyroxene and apatite, cumulus apatite buffers the REE content. La/Sm and Eu/Eu* vs. P 2O 5 variations in leuconorites display mixing trends between a pure adcumulate and the composition of the trapped liquid, assumed to be similar to the parental magma. Assessment of the trapped liquid fraction in leuconorites ranges from 2 to 25% and is systematically higher in the north-eastern part of the intrusion. The likely reason for this wide range of TLF is different cooling rates in different parts of the intrusion depending on the distance to the gneissic margins. The REE patterns of liquids in equilibrium with primitive cumulates are calculated with mass balance equations. Major elements modelling ( Duchesne, J.C., Charlier, B., 2005. Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway): Part I. Constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent. Lithos. 83, 299–254) permits calculation of the REE content of melt in equilibrium with gabbronorites. Partition coefficients for REE between cumulus minerals and a jotunitic liquid are then calculated. Calculated liquids from the most primitive cumulates are similar to a primitive jotunite representing the parental magma of the intrusion, taking into account the trapped liquid fraction calculated from the P 2O 5 content. Consistent results demonstrate the reliability of liquid compositions calculated from bulk cumulates and confirm the hypothesis that the trapped liquid has crystallized as a closed-system without subsequent mobility of REE in a migrating interstitial liquid.

Relationship between the Layered Series and the overlying evolved rocks in the Bjerkreim-Sokndal Intrusion, southern Norway

The Bjerkreim-Sokndal layered intrusion (BKSK) consists of a > 7000-m-thick Layered Series comprising anorthosites, leuconorites, troctolites, norites, gabbronorites and jotunites (hypersthene monzodiorites), overlain by an unknown thickness of massive, evolved rocks: mangerites (hypersthene monzonites; MG), quartz mangerites (QMG) and charnockites (CH). The Layered Series is subdivided into six megacyclic units that represent the crystallisation products of successive major influxes of magma. We have studied a ca. 2000-m-thick section that straddles the sequence from the uppermost part of the Layered Series to the QMG in the northern part of the intrusion. Mineral compositions in 37 samples change continuously in the lower part of the sequence up to the middle of the MG-unit (plagioclase An37-18; olivine Fo40-7; Ca-poor pyroxene Mg#57-15; Ca-rich pyroxene Mg#65-21). Above this compositions are essentially constant in the upper part of the MG-unit and in the QMG (An21-13; Fo6-4; Mg#opx17-13; Mg#cpx25-20). The amount of interstitial quartz and the amount of normative orthoclase, however, both increase systematically upwards through the QMG-unit, implying that these rocks are cumulates. There is no evidence of a compositional break in the MG-QMG sequence that could reflect influx of relatively primitive magma. Two types of QMG/CH are known in the uppermost part of BKSK. Olivine-bearing types are comagmatic with the underlying Layered Series; the studied stratigraphic sequence belongs to this suite. Two-pyroxene QMG and amphibole CH define a separate compositional lineage related to jotunites. An intrusive unit of dominantly two-pyroxene QMG is discordant to the olivine-bearing jotunite-MG-QMG sequence near Rapstad, confirming the presence of two compositionally distinct suites of QMG and related lithologies in the upper part of BKSK. A xenolith-rich unit near the olivine-bearing MG-QMG boundary represents a major collapse of the roof of the magma chamber during the final stages of crystallisation.

Petrografia e química mineral dos diques máficos da região Crixás-Goiás, estado de Goiás

Paleoproterozoic mafic dikes intrude Archean rocks of the Goiás Block along two main trends, NE and NW. Based on petrographic features they are classified in three groups: diabase, metabasite and amphibolite. Chemical analyses display tholeiitic affinity and permit the great majority of rocks to be classified as basalt. Some samples, mainly diabase, are basaltic andesites. Diabase, metabasite, and at least part of the amphibolites seem to be cogenetic. Diabase and metabasite contain Carich (augite) and Capoor pyroxenes (enstatite and pigeonite). In both types the wollastonite content (Wo) decreases and the ferrosilite component (Fs) increases from core to rim of the pyroxenes. This compositional variation is less pronounced in the Capoor pyroxenes. Cores and rims of plagioclases of diabase and metabasite vary from labradorite to bytownite. In the amphibolites the plagioclase ranges from oligoclase to andesine. Several geothermometers indicate similar crystallization temperatures for diabase and metabasite. Data from pyroxene and plagioclase point out average values of 1100ºC - 1200ºC and 1100°C respectively. Igneous and secondary amphiboles (derived from pyroxenes) exhibit a large compositional range, mainly due to variable contents of FeO and MgO. Chemical analyses of opaque minerals identified magnetite and ilmenite.

FeO-rich silicates and Ca, Al-rich inclusions in Qingzhen and Yamato 691 (EH3) meteorites: Evidence for migration of mass in the solar nebula

The Qingzhen and Yamato 691 (EH3) enstatite chondrites, which formed under extremely reducing conditions, are studied using the scanning electron microscope and electron probe microanalyzer. Both meteorites contain FeO-rich silicates and minor Ca, Al-rich inclusions. Most FeO-rich silicates are Ca-poor pyroxenes and occur as fragments in matrix. A few grains of FeO-rich silicates were found in chondrules, and FeO-rich olivine is rare. In Qingzhen, FeO-rich silicates commonly contain abundant dust-like Ni-poor metals, which probably formed through reduction of FeO. In contrast, only a few fragments of FeO-rich silicates in Yamato 691 enclose dust-like metals. This difference is consistent with a more reducing condition of Qingzhen than Yamato 691. Ca, Al-rich inclusions have similar modal compositions and mineral chemistry as their counterparts in carbonaceous chondrites. We suggest that (1) the FeO-rich silicates probably formed in oxidized regions of the solar nebula, and then moved into the enstatite chondrite locations; and (2) Ca, Al-rich inclusions in both enstatite chondrites and carbonaceous chondrites may have similar origins and reservoirs.

TEXTURAL EVOLUTION OF THE PYROXENE-SPINEL SYMPLECTITE FROM THE HOROMAN PERIDOTITE COMPLEX, HOKKAIDO, JAPAN

The Horoman peridotite contains pyroxene-spinel symplectites, which are thought to be after garnet (Takahashi and Arai, 1989). The symplectite consists of clinopyroxene, orthopyroxene and spinel and typically occurs in spinel lherzolite lithologies. Examination of the bulk composition of the symplectite reveals that their composition is well expressed in terms of mixtures of pyropic garnet and forsterite components. Although the mineral and bulk chemical compositions of the symplectite are rather constant among studied samples, their texture is variable in terms of grain size, grain shape and spatial relationships among the constituent minerals. We tried to quantitatively characterize these textural features and variations by means of SEM observations combined with digital image analysis aided with a personal computer. In particular, spatial correlations among minerals were measured using the method of Morishita and Obata (1995). Based on the scale parameters defined from the spatial correlations, a hypothesis on the structural evolution of the symplectite is proposed as follows. The primary garnet was first transformed into a single phased, highly-aluminous pyroxene, which was thermodynamically metastable. Subsequently this aluminous pyroxene started to be decomposed into two-phase mixtures of Ca-rich and Ca-poor pyroxenes. The pyroxene domain structure continues to grow as time goes. At some time during the coarsening of the pyroxene composites, Al-Cr spinel started to nucleate preferentially on the grain boundaries of the two pyroxenes and rapidly grew along the grain boundaries. The excess Al and Cr that was desolved in the pyroxenes reacted with the forsterite component supplied by diffusion from the surroundings to form the Al-Cr spinel. Since the appearance of the spinel and the establishment of phase equilibrium in spinel lherzolite facies, the texture continued to evolve. The pyroxene domain structure continued to grow, while spinel grains became more spherical to minimize the surface energy without significantly changing their positions, resulting in a gradual loss of spatial correlations between the spinel and the two-pyroxene grain boundaries. The proposed evolutionary scheme well explains the textural diversity observed among the Horoman symplectites.

Minor and trace element partitioning between pyroxene and melt in rapidly cooled chondrules

Other| June 01, 1997 Minor and trace element partitioning between pyroxene and melt in rapidly cooled chondrules Rhian H. Jones; Rhian H. Jones University of New Mexico, Department of Earth and Planetary Sciences, Albuquerque, NM, United States Search for other works by this author on: GSW Google Scholar Graham D. Layne Graham D. Layne Woods Hole Oceanographic Institution, United States Search for other works by this author on: GSW Google Scholar Author and Article Information Rhian H. Jones University of New Mexico, Department of Earth and Planetary Sciences, Albuquerque, NM, United States Graham D. Layne Woods Hole Oceanographic Institution, United States Publisher: Mineralogical Society of America First Online: 02 Mar 2017 Online Issn: 1945-3027 Print Issn: 0003-004X Copyright © 1981 by the Mineralogical Society of America American Mineralogist (1997) 82 (5-6): 534–545. https://doi.org/10.2138/am-1997-5-613 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Rhian H. Jones, Graham D. Layne; Minor and trace element partitioning between pyroxene and melt in rapidly cooled chondrules. American Mineralogist 1997;; 82 (5-6): 534–545. doi: https://doi.org/10.2138/am-1997-5-613 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 SocietyAmerican Mineralogist Search Advanced Search Abstract We present minor and trace element (REE, Sr, Y, and Zr) data for pyroxenes and mesostases in four porphyritic chondrules from the Semarkona ordinary chondrite. Apparent partition coefficients for clinoenstatite, orthoenstatite, pigeonite, and augite are compared with experimental and petrologic data from the literature, and the effects on apparent partition coefficients of the rapid cooling rates at which chondrules crystallized are evaluated. For most elements, the effects of cooling at rates of hundreds of degrees per hour cannot be distinguished from variations in equilibrium data resulting from differences in temperature or composition. However, for LREE apparent partition coefficients are significantly higher than comparable equilibrium data, and the ratio of HREE/LREE partition coefficients is lower, particularly for Ca-poor pyroxene. We attribute this flattening of REE patterns to the effect of rapid cooling. Apparent partition coefficients of all REE and Y in augite are higher than equilibrium data, particularly in one chondrule with a high Al2O3 content. We suggest that this may be attributed to an increase in the uptake of trivalent trace element cations in the pyroxene crystal structure as a result of charge-balanced substitutions with Al3+ cations. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Petrology of the Treknattan intrusion in the Fongen-Hyllingen complex, Trondheim region, Norway: a late intrusion into an evolved layered complex

The basic-ultrabasic Treknattan intrusion is an important example of a late intrusion in a solidified, evolved, layered complex and sheds light on possible mechanisms by which such associations may develop. The Treknattan intrusion, emplaced into the basic Fongen-Hyllingen intrusion shortly after the latter had solidified, consists mainly of massive or weakly layered peridotite (olivine ± Cr-spinel cumulate) and troctolite (plagioclase + olivine ± Cr-spinel cumulate). The mineral compositional range partially overlaps the most primitive end of the much larger variation-interval in the Fongen-Hyllingen intrusion. The margin of the Treknattan intrusion is sometimes outlined by massive feldspathic websterite which appears to have formed by reaction between magma and melts of gabbroic country rock. The parental magma appears to have been a relatively water-rich picritic basalt with a possible genetic relationship to the magma parental to the enveloping Fongen-Hyllingen intrusion, both displaying tholeiitic relationship between olivine and Ca-poor pyroxene, and having crystallized from relatively water-rich magmas with an early crystallization order of olivine ± Cr-spinel-plagioclase-Ca-rich pyroxene. The recognition of the Treknattan intrusion as a separate body suggests that the bulk composition of the Fongen-Hyllingen intrusion is dioritic rather than gabbroic as previously thought.

Melting of a chromite-bearing harzburgite and generation of boninitic melts at low pressures under controlled oxygen fugacity

Compositions of melts and coexisting olivine, chromite and Ca-poor pyroxene have been determined in the temperature range 1242 °–1405 °C at 1 atm and at 10 −10–10 −5 atm oxygen fugacity (fO 2). Experiments were also conducted at 5 kbar to examine the effect of pressure on the composition of the melts. Starting materials are mixtures of olivine, enstatite, diopside and chromite separated from peridotites and a chromitite. The compositions of melts coexisting with olivine, Ca-poor pyroxene and chromite are relatively silica- and MgO-rich (SiO 2, 53–57; MgO, 11–20 wt.%) and are similar to those of some boninites. Melts formed at 5 kbar from the same starting material are also boninitic, although the silica contents are slightly lower than those formed at 1 atm. The effect of fO 2 on the composition of melts is small; the total iron content in the melts slightly increases with increasing fO 2. The Cr 2O 3 contents in the melts saturated with chromite range from 0.34–1.82 wt.%. The experimental results suggest that boni nitic magmas can be formed even under anhydrous conditions, if magmas are equilibrated with harzburgite at shallow levels. Some boninitic lavas reported in ophiolite complexes may have been formed under such conditions. The composition of chromite coexisting with olivine and Ca-poor pyroxene becomes more iron-rich with increasing fO 2. Magnetite component in chromite increases from 0.4–2.2 mole% at 10 −10 atm fO 2 to 8–10% at 10 −5 atm. fO 2 at temperatures between 1240 and 1350 °C. The Cr (Cr + Al) ratio of chromite varies from 0.52 at about 1250 °C to 0.79 at about 1400 °C. The partition coefficient for the Mg-Fe +2 distribution between coexisting chromite and olivine [( Fe 2+ Mg ) chr ( Fe 2+ Mg ) ol] has a range of 3.25–4.03 in the temperature range of the present experiments and varies as a function of temperature and Cr (Cr + Al) ratio of chromite. The isotherms for 1250 °, 1300 °, 1350 ° and 1400 °C are drawn from the present experimental results. These results combined with those of the previous experiments are applicable to the estimation of temperature and oxygen fugacity of the crystallization of chromite in mafic magmas.

Evolution of tholeiitic diabase sheet systems in the eastern United States: examples from the Culpeper Basin, Virginia-Maryland, and the Gettysburg Basin, Pennsylvania

High-TiO 2, quartz-normative (HTQ) tholeiite sheets of Early Jurassic age have intruded mainly Late Triassic sedimentary rocks in several early Mesozoic basins in the eastern United States. Field observations, petrographic study, geochemical analyses and stable isotope data from three HTQ sheet systems in the Culpeper basin of Virginia and Maryland and the Gettysburg basin of Pennsylvania were used to develop a general model of magmatic differentiation and magmatic-hydrothermal interaction for HTQ sheets. The three sheet systems have remarkably similar major-oxide and trace-element compositions. Cumulus and evolved diabase in comagmatic sheets separated by tens of kilometers are related by igneous differentiation. Differentiated diabase in all three sheets have petrographic and geochemical signatures and fluid inclusions indicating hydrothermal alteration beginning near magmatic temperatures and continuing to relatively low temperatures. Sulfur and oxygen isotope data are consistent with a magmatic origin for the hydrothermal fluid. The three sheet systems examined apparently all had a similar style of crystal-liquid fractionation that requires significant lateral migration of residual magmatic liquid. The proposed magmatic model for HTQ sheets suggests that bronzite-laden magma was intruded in an upper crustal magma chamber, with bronzite phenocrysts collecting in the lower part of the magma chamber near the feeder dike. Early crystallization of augite and Ca-poor pyroxene before significant plagioclase crystallization resulted in density-driven migration of lighter residual magmatic liquids along lateral and vertical pressure gradients towards the upper part of the sheet. The influence of water on the physical properties of the residual liquid, including density, viscosity and liquidus temperature, may have facilitated the lateral movement more than 15 km up dip in the sheets. Exsolution of a Cl- and S-rich metal-bearing aqueous fluid from residual magma resulted in concentration and redistribution of incompatible and aqueoussoluble elements in late-stage differentiated rocks. This proposed hydrothermal mechanism has important economic implications as it exerts a strong control on the final distribution of noble metals in these types of diabase sheets.

Shock-induced black veins and organic compounds in ordinary chondrites

Abstract A detailed study of Barbotan (H5) and Charsonville (H6), and a partial study of AA Pervomaisky (L6) indicates that shock-induced black veins differ from the host part of chondrites by the following peculiarities: 1. (1) smaller quantities of Na and quite possible Fe, Ni, Co, and in some cases Ir; 2. (2) the presence of a major portion of glass and melted structures of troilite-metal mixture. 3. (3) different composition of metal, troilite, Ca-poor pyroxene, feldspar, and some grains of olivine. 4. (4) the presence of organic compounds. Organic matter in H-chondrites is identical and corresponds to paraffins and in L-chondrite to aliphatic hydrocarbons and carbonyl and N-containing compounds. Their presence in black veins may be related to two possible sources: 1. (1) contamination or, 2. (2) cosmic origin. The character of distribution and disposition of organic compounds in the close pores of veins and their chemical interrelation with chemical group of chondrites allows speculation about a cosmic origin of organic matter related to shock metamorphism.

The transition from porous to channelized flow due to melt/rock reaction during melt migration

The effect of melt/rock reaction on the mechanisms and kinetics of melt migration is investigated through a series of melt migration experiments in which melt saturated only in olivine infiltrates synthetic peridotites containing different proportions of olivine and Ca‐poor pyroxene. The microstructures that develop are significantly different from those formed during melt migration in a chemically equilibrated system. A reaction zone develops in which all pyroxene is dissolved, the melt fraction is increased, and olivine grain size is increased. The reaction front is unstable so that finger‐like structures or channels with higher melt content develop. The morphology of the instability depends on the initial pyroxene content of the infiltrated rock. This reactive‐infiltration instability with the resulting formation of melt fingers could facilitate the transition from porous to channelized flow in the mantle.