Microstructures of Hibonite From an ALH A77307 (CO3.0) CAI: Evidence for Evaporative Loss of Calcium

Hibonite is a comparatively rare, primary phase found in some CAIs from different chondrite groups and is also common in Wark-Lovering rims [1]. Hibonite is predicted to be one of the earliest refractory phases to form by equilibrium condensation from a cooling gas of solar composition [2] and, therefore, can be a potential recorder of very early solar system processes. In this study, we describe the microstructures of hibonite from one CAI in ALH A77307 (CO3.0) using FIB/TEM techniques in order to reconstruct its formational history

New observations on high-pressure phases in a shock melt vein in the Villalbeto de la Pena meteorite: insights into the shock behavior of diopside

The petrology and mineralogy of shock melt veins in the L6 ordinary chondrite host of Villalbeto de la Peña, a highly shocked, L chondrite polymict breccia, have been investigated in detail using scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and electron probe microanalysis. Entrained olivine, enstatite, diopside, and plagioclase are transformed into ringwoodite, low-Ca majorite, high-Ca majorite, and an assemblage of jadeite-lingunite, respectively, in several shock melt veins and pockets. We have focused on the shock behavior of diopside in a particularly large shock melt vein (10 mm long and up to 4 mm wide) in order to provide additional insights into its high-pressure polymorphic phase transformation mechanisms. We report the first evidence of diopside undergoing shock-induced melting, and the occurrence of natural Ca-majorite formed by solid-state transformation from diopside. Magnesiowüstite has also been found as veins injected into diopside in the form of nanocrystalline grains that crystallized from a melt and also occurs interstitially between majorite-pyrope grains in the melt-vein matrix. In addition, we have observed compositional zoning in majorite-pyrope grains in the matrix of the shock-melt vein, which has not been described previously in any shocked meteorite. Collectively, all these different lines of evidence are suggestive of a major shock event with high cooling rates. The minimum peak shock conditions are difficult to constrain, because of the uncertainties in applying experimentally determined high-pressure phase equilibria to complex natural systems. However, our results suggest that conditions between 16 and 28 GPa and 2000–2200 °C were reached.Postprint (author's final draft

Magma Mixing at OlDoinyo Lengai: A Mineralogical and Trace Element Analysis of the 2007-8 Eruption.

Chapter I: The 2007-8 eruption at Lengai was highly explosive, reaching plinian proportions, and the anhydrous nature of the nephelinite magma at Lengai, does not explain this highly volatile behavior. The increase in volatiles in a low H2O nephelinite magma could occur from decompression melting of magma injection from a deeper source. Two distinct nephelinite compositions were identified in a mineralogical analyses of the ash erupta: a highly evolved nephelinite (OL2), with less than 3% glass from the magma chamber, as indicated by the highly peralkalinic feldspathoid: combeite (Na2Ca2Si3O9), commonly found in Lengai eruptive products (Dawson 1966, 1998), and a less evolved nephelinite magma, with up to 17% glass (ASHES) that did not contain combeite, with significantly higher Si, Al, Mg, and Mn content. Phase abundances, mineral formulas and endmember components are calculated for both assemblages. Phenocrysts encountered in both nephelinite assemblages are nepheline, augite (CPX), titanium andradite, wollastonite, apatite, and iron oxides. Magma mixing of the two nephelinites are evidenced by sudden changes in the melt chemistry in both ash sample sets. In the combeite-wollastonite-nephelinite (OL2), combeite microlites exhibit resorbtion rims indicative of mineral instability, and nepheline from this assemblage has a distinct chemical boundary withinrim, evidenced by Mg overgrowth. The wollastonite-nephelinite contains almost fully resorbed CPX, and resorbtion rims on Ti-andradite. Chemical changes resulting from a decrease of in Ca in the melt were detected in the rims of the wollastonite via electron microprobe WDS mapping. Two large CPX mineral grains with very differing composition and crystallization histories were found alongside each other in the wollastonite-nephelinite. Primary compositional differences between the two CPX grains are Ti and Mg content, the CPX mineral grain exhibiting disequilibrium features (ASH15-DISEQ) had higher total Mg (Mg content as high as 0.87 c.p.f.u., with an average of 0.72 c.p.f.u. as opposed to an average of 0.52 c.p.f.u.) and lower Ti (on average 0.00 c.p.f.u., as opposed to 0.02 c.p.f.u. in the second grain), than the zoned CPX (ASH15-EQUIL). The Ti-enriched CPX (ASH15-EQUIL) exhibits oscillatory compositional zoning, with few inclusions. The second (ASH15-DISEQ) is richer in Mg, and contains abundant inclusions, suggesting a high degree of disequilibrium. Both CPX and nepheline microlites and rims are enriched in Al, Mg and Mn, elements typically depleted in the highly peralkaline magma chamber. For both ash types the crystal size distribution is bimodal indicative of two stage cooling: an initial stage of slow cooling, with low nucleation and high growth rates producing large crystals (longest axes up to 1.5mm), followed by a stage of rapid cooling with high nucleation and low growth rates as the magma migrated to the surface. The large volume of visible interstitial glass vesicles in OL2 scoria is indicative of rapid degassing and subsequent crystallization in the magma chamber. Chapter II: Oldoinyo Lengai is the world’s only active natrocarbonatite-nephelinite mixed-magma system on earth. Recent volcanic activity and geochemical studies suggest there may be two nephelinite magmas mixing prior to the 2007-8 eruption. In this study, we present scanning electron microscope (SEM) analyses from 2006 natrocarbonatite deposits, electron microprobe (EMPA) melt analyses for the 2007-8 eruptive nephelinite deposits: combeite-wollastonite nephelinite (CWN) and wollastonite nephelinite (WN). We also present laser ablation inductively-coupled plasma mass spectrometry (LA-ICPMS) trace and rare earth element data (ppm) for a xenolith sample (consisting of CPX and apatite), melt phenocrysts (andradite, and CPX), and matrix (a non-vitrified, non-crystalline, ultrafine ash representative of the pre-eruptive melt composition). Rare earth and trace element data presented for: V, Cr, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Pb, Th, and U. In addition, mineral/matrix partition coefficients (Kds) are presented for andradite and CPX. From the total alkali vs. silicate (TAS) diagram and Harker’s diagrams two distinct melt compositions were identified. These two melt compositions are characterized by different REE and trace element abundance patterns for the melt and phenocrysts, both of which demonstrate differences of up to 3 orders of magnitude in concentration (ppm), especially in the LREE. Similarity in trace- and rare-earth-element-normalized abundance patterns for both matrix and andradite phenocryst analyses suggest they share a common source and may originate from the same parental magma. However the broad range in values suggests that the WN may be more recently evolved from the parental magma than the CWN, which demonstrates evidence of contact with natrocarbonatite in the form of resulting enrichments of HREE, Th and U. However, interaction with the natrocarbonatite was not indicated by the CPX patterns, which show significant differences in concentration (ppm; normalized to CI chondrite), in addition to a pronounced negative K anomaly and a positive Y anomaly displayed by some samples. Overlap in melt compositions is interpreted as the chemical signature of magma mixing, especially in combination with evidence of other disequilibrium features, as documented by Thomas et al (2018), such as CPX and garnet resorbtion, zoning, and the two differentmineral assemblages (CWN and WN). The data from this study support the presence of a deeper nephelinite source (WN) injecting Lengai’s primary nephelinite chamber (CWN) causing the 2007-8 eruption. A time series of seismic and eruptive events at Lengai supports the hypothesis that all explosive eruptions are triggered by injection of deeper magma (WN) which is preempted by a series of significant seismic events (ISC., 2001, Baer et al., 2008, and GVP., 2014), as supported in the most recent eruption by InSAR studies (Biggs et al 2009., 2013)

Mineralogy and Petrology of COMET WILD2 Nucleus Samples

The sample return capsule of the Stardust spacecraft will be recovered in northern Utah on January 15, 2006, and under nominal conditions it will be delivered to the new Stardust Curation Laboratory at the Johnson Space Center two days later. Within the first week we plan to begin the harvesting of aerogel cells, and the comet nucleus samples they contain for detailed analysis. By the time of the LPSC meeting we will have been analyzing selected removed grains for more than one month. This presentation will present the first results from the mineralogical and petrological analyses that will have been performed

Aluminum-26 chronology of dust coagulation and early solar system evolution

Dust condensation and coagulation in the early solar system are the first steps toward forming the terrestrial planets, but the time scales of these processes remain poorly constrained. Through isotopic analysis of small Ca-Al–rich inclusions (CAIs) (30 to 100 μm in size) found in one of the most pristine chondrites, Allan Hills A77307 (CO3.0), for the short-lived 26Al-26Mg [t1/2 = 0.72 million years (Ma)] system, we have identified two main populations of samples characterized by well-defined 26Al/27Al = 5.40 (±0.13) × 10−5 and 4.89 (±0.10) × 10−5. The result of the first population suggests a 50,000-year time scale between the condensation of micrometer-sized dust and formation of inclusions tens of micrometers in size.M.-C. Liu, J. Han, A. J. Brearley, and A. T. Hertwi

Mineralogy and petrology of comet 81P/wild 2 nucleus samples

The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk

Magnetite in ALH 84001: An origin by shock-induced thermal decomposition of iron carbonate

In martian orthopyroxenite ALH 84001, pockets of feldspathic glass frequently contain carbonate masses that have been disrupted and dispersed within feldspathic shock melt as a result of impact(s). Transmission electron microscope studies of carbonate fragments embedded within feldspathic glass show that the fragments contain myriad, nanometer-sized magnetite particles with cuboid, irregular, and teardrop morphologies, frequently associated with voids. The fragments of carbonate must have been incorporated into the melt at temperatures of ~900 degrees C, well above the upper thermal stability of siderite (FeCO3), which decomposes to produce magnetite and CO2 below ~450 degrees C. These observations suggest that most, if not all, of the fine-grained magnetite associated with Fe-bearing carbonate in ALH 84001 could have been formed as result of the thermal decomposition of the siderite (FeCO3) component of the carbonate and is not due to biological activity.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

An experimental and kinetic study of the breakdown of aluminous biotite at 800 °C : reaction microstructures and mineral chemistry

The disequilibrium breakdown of natural aluminous, iron-rich biotite has been investigated experimentally using cores of a well-characterized biotite schist as a starting material. Experiments have been carried out at 800 °C, 1 kbar for run durations between 2 days and 8 weeks. All the run products have been studied by light and transmission electron microscopy (TEM). The reaction is characterized by the progressive darkening and pseudomorphing of the biotite by an intergrowth of extremely fine-grained reaction products. TEM observations show that the reaction can be schematically described as : Al-biotite = Al-orthopyroxene + hercynitic spinel + magnetite + melt and occurs by two mechanisms which are controlled by the original microstructures of the biotite. Electron diffraction studies have shown that in the early stages of the reaction both the hercynitic spinel and the pyroxene have orientations which are strongly controlled by the host biotite, but these crystallographic relationships are lost as the melt fraction increases. Analysis of the coexisting phases by analytical electron microscopy suggests that the earliest spinels and pyroxenes to nucleate have metastable compositions which move towards the stable equilibrium compositions as a function of increasing run time.On a étudié expérimentalement la dégradation hors d'équilibre de biotite naturelle alumineuse et riche en fer en utilisant, comme matériel initial, des éprouvettes d'un schiste de biotite bien caractérisé. Les expériences ont été réalisées à 800 °C, 1 kbar, pendant 2 jours à 8 semaines. Tous les produits de la réaction ont été étudiés par microscopie photonique et microscopie électronique par transmission. La réaction est caractérisée par un noircissement progressif de la biotite, qui est pseudomorphosée par une intercroissance de produits de réaction à grain très fin. Les observations en MET montrent que la réaction peut schématiquement être décrite ainsi : Biotite-Al = orthopyroxène-Al + spinelle hercynitique + magnétite + produits fondus. Elle procède par deux mécanismes différents selon le site de la biotite où elle se déroule. Dans les premiers stades de la réaction, le spinelle hercynitique et le pyroxène ont des orientations qui sont fortement contrôlées par la biotite-hôte, mais ces relations cristallographiques disparaissent lorsque la proportion de produits fondus augmente. L'analyse des phases présentes par microscopie électronique analytique suggère que les premiers spinelles et pyroxènes qui nucléent ont des compositions métastables qui évoluent vers les compositions stables quand la durée de l'expérience augmenteBrearley Adrian J. An experimental and kinetic study of the breakdown of aluminous biotite at 800 °C : reaction microstructures and mineral chemistry. In: Bulletin de Minéralogie, volume 110, 5, 1987. Minéralogie et géochimie expérimentales [1st Experimental Mineralogy, Petrology and Geochemistry (EMPG) symposium

Episodic carbonate precipitation in the CM chondrite ALH 84049: An ion microprobe analysis of O and C isotopes

We have determined the O and C isotope compositions of dolomite grains and the C isotope compositions of calcite grains in the highly altered CM1 chondrite, ALH 84049, using Secondary Ion Mass Spectrometry (SIMS). Chemically-zoned dolomite constitutes 0.8 volume percent (vol%) of the sample and calcite 0.9 vol%. Thirteen separate dolomite grains have δ13C values that range from 37 to 60 (±2) ‰, δ^(18)O values from 25 to 32 (±3) ‰, and δ^(17)O values from 10 to 16 (±3) ‰ (VSMOW). Intragrain δ^(13)C values in dolomite vary up to 10‰. The δ^(13)C values of three calcite grains are distinct from those of dolomite and range from 10 to 13 (±2) ‰ (PDB). Calcite and dolomite appear to record different precipitation episodes. Carbon isotope values of both dolomite and calcite in this single sample encompass much of the reported range for CM chondrites; our results imply that bulk carbonate C and O isotope analyses may oversimplify the history of carbonate precipitation. Multiple generations of carbonates with variable isotope compositions exist in ALH 84049 and, perhaps, in many CM chondrites. This work shows that one should exercise caution when using a clumped isotope approach to determine the original temperature and the isotopic compositions of water for CM chondrite carbonates. Less altered CM meteorites with more-homogeneous C isotope compositions, however, may be suitable for bulk-carbonate analyses, but detailed carbonate petrologic and isotopic characterization of individual samples is advised