1,721,132 research outputs found
ANTARCTICA: A TREASURE–TROVE FOR PLANETARY SCIENCES. AUSTRALASIAN MICROTEKTITES FROM EAST ANTARCTICA
Full text linkThe planetary science community has benefitted greatly from research in Antarctica. The discovery of large accumulations of meteorites in blue ice fields on the polar plateau since 1969 and of cosmic dust in Antarctic ice, snow and supraglacial moraines as well as in loose sediment traps in the Transantarctic Mountains since the late 1980s has
had a tremendous impact on the planetary science community. Over the last 50 years tens of thousands of meteorite specimens and cosmic dust particles have been recovered by Japanese, US, European, Chinese and Korean polar programs. This enormous research effort has provided the international planetary science community with the opportunity to study an extraordinary number of samples from a large variety of planetary bodies, greatly advancing our knowledge of the origin and evolution of the solar system
Frontier Mountain meteorite specimens of the Acapulco-Lodranite clan: petrography, pairing and parent-rock lithology of an unusual intrusive rock
No full textIn this paper we reconstruct the heterogeneous lithology of an unusual intrusive rock from the acapulcoite-lodranite (AL) parent asteroid on the basis of the petrographic analysis of 5 small (<8.3 g) meteorite specimens from the Frontier Mountain ice field (Antarctica). Although these individual specimens may not be representative of the parent-rock lithology due to their relatively large grain size, by putting together evidence from various thin sections and literature data we conclude that Frontier Mountain (FRO) 90011, FRO 93001, FRO 99030, and FRO 03001 are paired fragments of a medium- to coarse-grained igneous rock which intrudes a lodranite and entrains xenoliths. The igneous matrix is composed of enstatite (Fs13.3±0.4 Wo3.1±0.2), Cr-rich augite (FS6.1±0.7 Wo42.3±0.9), and oligoclase (Ab80.5±3.3 Or3.2±0.6). The lodranitic xenoliths show a fine-grained (average grain size 488 ± 201 μm) granoblastic texture and consist of olivine Fa9.5±0.4 and Fe,Ni metal and minor amounts of enstatite Fs12.7±0.4 Wo1.8±0.1, troilite, chromite, schreibersite, and Ca-phosphates. Crystals of the igneous matrix and lodranitic xenoliths are devoid of shock features down to the scanning electron microscope scale. From a petrogenetic point of view, the lack of shock evidence in the lodranitic xenoliths of all the studied samples favors the magmatic rather than the impact melting origin of this rock. FRO 95029 is an acapulcoite and represents a separate fall from the AL parent asteroid, i.e., it is not a different clast entrained by the FRO 90011, FRO 93001, FRO 99030, and FRO 03001 melt, as in genomict breccias common in the meteoritic record. The specimen-to-meteorite ratio for the AL meteorites so far found at Frontier Mountain is thus 2.5
Discovery of shock-twinned zircon at Kamil Crater, Egypt
No full textWith an age of less than ~5000 yr and diameter of 45 m, Kamil crater in Egypt [1] is one of the youngest and smallest terrestrial impact craters. Abundant shock evidence has been reported from Kamil, including impact melt, coesite, stishovite, and PDF and PF in quartz, giving rise to estimates of shock pressures from 20-60 GPa [2,3]. Here we studied zircon in sample L23, a cm-sized clast comprised of shocked and partially melted sandstone ejecta, with lechatelierite, stishovite, coesite, and quartz with PDF and PF [2,3]. Four of twenty zircon grains analyzed by electron backscatter diffraction contain {112} deformation twin lamellae. Lamella lengths range from 1-2 μm, and thus are shorter than those reported from other impact structures, such as Vredefort [4], Sudbury [5], Rock Elm [6], and Santa Fe [7], where individual lamellae are 10s of μm in length. Empirical studies indicate formation of {112} twins at ca. 20 GPa [8], a finding supported by their co-existence (in some rocks) with high-pressure phases [6,9; this study]. The only available laboratory constraint is a diamond anvil cell (DAC) experiment that found {112} twins in zircon powder quenched at 20 GPa [10]. While the DAC experiment did not constrain the pressure at which twins formed, the authors cited plastic deformation of the twins as evidence that that may have formed <11 GPa. Given the porous nature of Kamil target rocks, magnification of a 5-10 GPa shock wave can readily produce >20 GPa conditions locally [11], along with high-temperature. The presence of coesite, stishovite, lechatelierite, and shocked quartz with PDF in sample L23 are consistent with empirically-derived pressure estimates for {112} twin formation in shocked zircon at Kamil crater. Kamil represents the smallest impact structure where shock-twinned zircon has been reported; given the apparent efficiency of {112} twin formation (20% of grains in sample L23), shock-twinned zircon is here shown to provide a robust record of diagnostic shock deformation at even the smallest known impact craters
The new CISUP facilities for Earth Science at the University of Pisa
Full text linkThe new CISUP facilities for Earth Science at the University of Pisa
Mugnaioli E.*1-2, Folco L.1-2, Masotta M.1-2, Biagioni C.1-2, Paoli G.1 & Capaccioli S.1-3
1 Centro per l'integrazione della strumentazione scientifica, Università di Pisa. 2 Dipartimento di Scienze della Terra, Università di Pisa. 3 Dipartimento di Fisica, Università di Pisa.
[email protected]
Innovative research in Earth sciences, particularly in the fields of mineralogy and petrography, requires the combination of more analytical techniques capable of high-accuracy and/or high spatial resolution. Still, it is rare that a single institution has all the necessary equipment and expertise for a thorough characterization of geological samples. This inevitably hinders the possibility to achieve cutting-edge results in a relatively short time, limiting the national/international attractivity of the facilities and the competitiveness with foreigner research groups.
In 2018, the University of Pisa established the Center for Instrument Sharing (CISUP), an interdepartmental body devoted to the creation of a network of existing facilities and to the pondered acquisition of new large analytical instrumentation (https://cisup.unipi.it/). Through CISUP, the Earth science researchers of the University of Pisa can presently benefit of:
a high-resolution field emission-scanning electron microscope (FE-SEM) operating under high, low and extended low vacuum modes with automated software for large area mapping;
a 193 nm ArF excimer laser ablation system coupled to an inductively coupled plasma-mass spectrometer (LA-ICP-MS);
a high-resolution field emission gun transmission electron microscope (HR-FEG-TEM) equipped with a large-area SDD EDS detector and state-of-the-art electron diffraction systems;
a single-crystal X-ray diffractometer (SC-XRD), with double source (Mo and Cu Kα radiation) and equipped with a detector having the largest active area so far available for lab-instruments.
Moreover, a focused-ion beam (FIB) SEM-FEG for high resolution imaging, EDS large area mapping system, 3D tomography, TEM and APT sample preparation and will be also installed in the next few months.
We believe that the recently installed and to-be-installed CISUP instrumentation will compose an analytical facility of national and international interest, able to valorize the consolidated tradition of the University of Pisa in the view of modern scientific challenges. This new facility will also favor the establishment of national and international scientific collaborations, possibly supporting the whole compartment of the Italian and European Earth sciences
Parentage Identification of Differentiated Achondritic Meteorites by Hand-held Energy Dispersive X-Ray Fluorescence Spectrometry
No full textWe evaluate the potential of a hand-held energy dispersive XRF spectrometer for the preliminary classification of non-chondritic differentiated meteorites. The studied achondrites include nine lunar meteorites, seventeen Martian meteorites, five angrites and eighteen meteorites from asteroid 4 Vesta. Analytical precision and accuracy was tested on thirty-nine terrestrial igneous rock slabs with a wide range of composition. Replicate analyses, performed on the studied meteorites, show that Fe/Mn values together with Si and Ca/K ratio can be used in the discrimination of different achondrite groups. Fusion crust's Fe/Mn values of meteorites from Vesta and Mars are indistinguishable from those of the interior implying that even measurements on the fusion-crusted external surface could be sufficient to pigeonhole non-chondritic meteorites. Hand-held energy dispersive XRF spectrometer is a non-destructive but very effective technique for preliminary classification of achondrites in the field and in laboratory and for the identification of mislabelled meteorites in museum collections
Direct quartz-coesite transformation in shocked porous sandstone from Kamil Crater (Egypt)
Full text linkCoesite, a high-pressure silica polymorph (pressure 3-10 GPa, temperature <3000 K), is a diagnostic feature of shock metamorphism associated with impact cratering on quartz-bearing target rocks. It is preserved as a metastable phase in sedimentary target rocks that experienced peak pressures in excess of similar to 10 GPa, where it typically occurs as intergranular polycrystalline aggregates of microcrystals embedded in silica glass known as "symplectic regions." The presence of coesite in the symplectic regions of rocks experiencing shock conditions beyond the limits of the coesite stability field is a controversial issue. Through a combined scanning and transmission electron microscopy and Raman spectroscopy study of shocked quartzarenites from the 45-m-diameter Kamil Crater (southwest Egypt), we show that coesite in symplectic regions forms through direct subsolidus transformation from quartz, in contrast with the prevailing hypothesis for crystalline targets. The quartz-to-coesite transformation takes place during localized shock-wave reverberation at the beginning of the pore collapse process. Complete pore collapse generates the high temperature regimes responsible for the subsequent production of the embedding silica melts, in part at the expense of the previously formed coesite. This work documents the role of pore collapse in producing localized pressure-temperature-time gradients in shocked porous targets, as predicted by numerical models in the literature
Formation of Impact coesite
Full text linkIntroduction: Coesite is one of the most common and reliable indicator of shock metamorphism associated with impact cratering in quartz-bearing target rocks. For this reason, coesite is the subject of numerous studies aiming to better understand how silica polymorphs react under sudden and extreme P-T increases. In impact rocks, coesite is preserved as a metastable phase in crystalline rocks that experienced peak shock pressures above ~30-40 GPa and in porous sedimentary rocks shocked at pressures as low as ~10 GPa. Furthermore, impact-coesite generally forms aggregates of microcrystalline grains scattered within silica amorphous material known as “symplectic regiorns”, and shows a characteristic polysynthetic twinning on (100) with the composition plane (010). There is a general consensus that the characteristic twinned impact-coesite is the result of crystallisation from a dense amorphous phase, either silica shock melt or highly densified diaplectic silica glass, during shock unloading, when the pressure release path passes through the coesite stability field. In contrast to these models, the coesite transformation mechanism through a direct solid-state transition from quartz was suggested the first time by [5] studying shocked Coconino sandstones from the Barringer crater (Arizona, USA). Our recent FESEM-TEM study, performed on impact ejecta from Kamil crater in Egypt and the Australasian tektite strewn field, confirms that the impact coesite forms through direct quartz-coesite transformation.
Samples and methods: Samples are from two different impact sites including a shocked porous sandstone from Kamil crater (Egypt) and two coesite-bearing quartz ejecta from the Australasian tektite/ microtektite strewn field (ODP site 1144A and Sonne Core SO95-17957-2). All samples were investigated using field emission gun – scanning electron microscopy (FEG-SEM), and then 5 electron-transparent lamellae from the Kamil crater shocked sandstone and 3 lamellae from the Australasian coesite-bearing ejecta were exctracted using focused-ion beam (FIB). These lamellae were investigated by transmission electron microscopy (TEM) and 3D electron diffraction (3D ED) for nano-petrographic and crystallographic analyses. Remarkably, both samples experienced relatively fast cooling, which preserved shock metamorphic mineralogies and textures only slightly altered by post-impact melting, (i.e., virtually unaffected by post-shock annealing and/or hydrothermal overprint).
Results and discussion: In Kamil ejecta we found rounded single-crystal coesite domains of 200 nm or less. These domains are surrounded by shocked quartz, without any amorphous phase in-between. We also observed larger coesite domains. The larger the domains, the more they appear fragmented and progressively dispersed and resorbed in amorphous silica. Our observations suggest that coesite seeds nucleate and grow inside quartz during pressure uploading, probably favored by shock-wave reverberation. Later, when pressure is released and temperature is still high, coesite domains fragment and melt giving rise to the “symplectic regions” observed – for instance - at Barringer, Yilan, Lonar, Cheasapeak Bay. In impact ejecta from the Australasian tektite strewn field, we again observe coesite crystals embedded in shocked quartz. Coesite crystals have well-developed euhedral habits, which grow at the expense of neighboring quartz and appear to postdate PDFs and other planar microstructures. In both ejecta, 3D electron diffraction reveals coesite grains displaying a recurrent crystallographic relation with quartz, with (010) coesite plane parallel to {10-11} or {-1011} of quartz. Such evidence suggests a topotactic relation between shocked quartz and impact coesite. The direct quartz to cosite subsolidus reaction is facilitated by the presence of pre-existing and shock-induced discontinuities in the target. Shock wave reverberations can provide pressure and time conditions for coesite nucleation and growth. Because discontinuities occur in both porous and non-porous rocks and the coesite formation mechanism appears similar for small and large impacts, we infer that the proposed subsolidus transformation model is valid for all types of quartz-bearing target rocks
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