GMPV4.2

EDI
Understanding planetary crusts and mantles: Recent advances in planetary sciences

Understanding planetary formation and evolution is a key endeavor in planetary science. Through the study of meteorites and returned samples from a range of solar system bodies, some of the key principles in how planets are built and are modified have been formulated. As analytical methods have continued to improve, so has our ability to gain new insights into this important extra-terrestrial sample set. This session encourages submissions on how recent developments in the field of geochemistry and petrology have advanced our understanding of planet-wide geological processes in the solar system. Specifically, we encourage submissions from a broad range of topics such as planetary accretion and differentiation, evolution of crust-mantle systems, and the role of impacts in shaping planetary evolution.

Public information:
For regular updates on running of the conference and potential alterations to the schedule please follow EGU @EuroGeosciences on Twitter.

This session is organised by the UK Planetary Forum (UKPF); for more information please follow the link. https://www.ukpf.org.uk/
Convener: John Pernet-Fisher | Co-conveners: Thomas BarrettECSECS, Enrica BonatoECSECS, Tara HaydenECSECS, Mark Nottingham
vPICO presentations
| Fri, 30 Apr, 11:00–11:45 (CEST)
Public information:
For regular updates on running of the conference and potential alterations to the schedule please follow EGU @EuroGeosciences on Twitter.

This session is organised by the UK Planetary Forum (UKPF); for more information please follow the link. https://www.ukpf.org.uk/

vPICO presentations: Fri, 30 Apr

Chairpersons: Thomas Barrett, Enrica Bonato, Tara Hayden
11:00–11:05
Lunar Petrology and Geochemistry
11:05–11:10
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EGU21-10624
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ECS
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solicited
Marissa Lo, Giuseppe La Spina, Katherine Joy, Margherita Polacci, and Mike Burton

Quantifying the volatile content of the lunar interior is valuable for understanding the formation, thermal evolution, and magmatic evolution of the Earth and Moon. Petrological modelling and geochemical measurements have been used to study the volatile composition of the lunar interior. Improvements to analytical instruments have facilitated more precise measurements of the volatile content of lunar samples and meteorites, however, several problems remain with these measurements, hence, the volatile content of lunar magmas has yet to be constrained with certainty. We propose a volcanological approach for inferring the volatile contents of different lunar magmas.

            A terrestrial magma ascent model has been modified for lunar applications. Numerous parameters were adjusted for lunar conditions, including: magma major element composition, from low-Ti (green and yellow glasses) to high-Ti (orange, red, and black glasses); H2O content; CO content; gravity; and pressure. The model calculated values for gas exsolution, viscosity, mass flow rate, and several other ascent processes, from a depth of 10 km to the surface. Using these results, we will assess the effect of varying magmatic volatile content on lunar magma ascent processes. We will also compare and contrast our results with existing models for lunar magma ascent, as well as models for magma ascent on other planetary bodies. Future work will involve modelling eruptions, using results from the magma ascent model, and verifying the results of the models using images and digital elevation models of the lunar surface.

How to cite: Lo, M., La Spina, G., Joy, K., Polacci, M., and Burton, M.: Modelling the ascent of picritic lunar magmas, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10624, https://doi.org/10.5194/egusphere-egu21-10624, 2021.

11:10–11:12
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EGU21-1123
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ECS
Tim Bögels and Razvan Caracas

The Earth-Moon system and its formation is a topic of great scientific interest, and great debate over the past decades. The giant impact hypothesis is the currently accepted model to explain the formation of our moon. Accordingly, a mars-sized impactor collides with the proto-earth. This giant impact vaporized a significant portion of the impactor and the proto-earth, creating a large accretionary disk from which the moon subsequently formed. Currently, there is a large effort to build reliable thermodynamic descriptors for the building materials of the two bodies involved in the impact. Understanding the behavior of major rock-forming minerals under these extreme conditions is vital for increasing the accuracy of these models.

Magnesium oxide, MgO, is one of the fundamental building blocks for rocky planets. It is an archetype material of ionic solids and a well-known refractory material. Because of its relevance it has been studied extensively; experimental and theoretical results have been produced up to pressures of 800 GPa and temperatures reaching 20000 K. These pressure and temperature regions are of great interest for the planetary sciences, studying planetary interiors. The transformation of the face-centered B1 phase to the body-centered B2 phase and the associated melting curve have been modelled numerous times. In contrast, we know very little of the liquid behaviour of MgO under pressure, let alone at the low pressures found in accretionary disks.

Here we investigate the low-density high-temperature regime characteristic of after-shock isentropic release. We explore the subcritical and the supercritical regimes of MgO using ab initio molecular dynamics. We determine the position of the critical point and examine the structural and transport properties in the sub- and supercritical regimes. We find an elevated critical temperature in comparison with previously studied magnesium-silicates, in agreement to the refractory nature of MgO. Furthermore, we provide insight into the speciation of liquid MgO and the liquid-gas separation. We see a shift in Mg-O speciation towards lower degrees of coordination as the temperature is increased from 4000K to 10000K. This shift in speciation is less pronounced at higher densities. The majority of the chemical species forming the incipient gas phase consist of isolated Mg and O ions and some MgO and O2.

This research was supported by the European Research Council under EU Horizon 2020 research and innovation program (grant agreement 681818 – IMPACT to RC). We acknowledge access to supercomputing facilities via eDARI stl2816, PRACE RA4947, and Uninet2 NN9697K grants.

How to cite: Bögels, T. and Caracas, R.: The behaviour of MgO in a giant impact setting, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1123, https://doi.org/10.5194/egusphere-egu21-1123, 2021.

11:12–11:14
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EGU21-1145
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ECS
Zhi Li, Razvan Caracas, and François Soubiran

The prevailing theory of the origin of the Moon is the giant impact hypothesis, in which a Mars-sized impactor collides with the proto-Earth in the late stage of accretion, and the Moon is subsequently formed from the proto-lunar disk made of the ejected materials. As the laboratory-scale experiments are not able to simulate planetary-scale impacts, our understanding of the giant impact mostly comes from hydrodynamic simulations. However, the results of these simulations heavily depend upon the available equation of state to describe the thermodynamic response of the constituent materials of the proto-Earth and impactor to shock waves.

 

Iron as a building block material of the terrestrial planets naturally received significant attention. But the major effort has been put to determine its phase diagram up to the Earth’s core conditions (126-360 GPa and 3000-7000 K) and beyond. The studies of iron at low densities are still scarce and the position of the critical point (CP) is uncertain. As the liquid-vapor dome ends at CP, the position of the latter determines the time evolution of the proto-lunar disk during its condensation.

 

In order to assess whether the core of the planets undergoes significant vaporization during a giant impact, we employ ab initio molecular-dynamics simulations to explore iron over a wide density region encompassing the critical point (CP) and the Hugoniot lines of the shocked iron cores. We determine the critical point of iron in the temperature range of 9000-9350 K, and the density range of 1.85-2.40 g/cm3, corresponding to a pressure range of 4-7 kbars [1]. This implies that the iron core of the proto-Earth may become supercritical after giant impacts. We show that the iron core of Theia partially vaporized during the Giant Impact. Part of this vapour may have remained in the disk, to eventually participate in the Moon’s small core. Similarly, during the late veneer stage a large fraction of the planetesimals have their cores undergoing partial vaporization. This would help to mix the highly siderophile elements into magma ponds or oceans.

 

References:

[1] Z. Li, R. Caracas, F. Soubiran, Partial core vaporization during Giant Impacts inferred from the entropy and the critical point of iron, Earth Planet. Sci. Letters, 2020, https://doi.org/10.1016/j.epsl.2020.116463

 

How to cite: Li, Z., Caracas, R., and Soubiran, F.: Partial core vaporization during giant impacts inferred from the entropy and the critical point of iron, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1145, https://doi.org/10.5194/egusphere-egu21-1145, 2021.

11:14–11:16
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EGU21-12352
John Pernet-Fisher, Margaret Hartley, and Kathrine Joy

Metamorphic rocks on the Moon are an important yet under-studied suite of lunar lithologies that have been identified in the Apollo and lunar meteorite collections [1]. These rocks, with granoblastic textures, are generally referred to as granulites; however, unlike their terrestrial counterparts, they are considered to represent the products of only high-temperature (> 1000 oC) thermal metamorphism that completely re-crystallised their protolith(s). Lunar granulites are commonly sub-divided into two main compositional groups related to their protolith lithologies. The Fe-granulites, found at most Apollo landing sites, are generally accepted to derive from metamorphosed plagioclase-rich igneous cumulates, termed the ferroan anorthosite (FAN) suite. The FAN suite are important lithologies as they represent products of the primary lunar crust. The Mg-granulites are found mostly at the Apollo 16 landing site and within lunar meteorite samples; the protolith(s) of this latter group is not well understood [2].  Early studies have linked the protolith to secondary magmatic intrusions into the primary anorthositic crust (termed the Mg-suite); however, recent studies have tentatively connected the protolith to a Mg-rich variation of the primary crustal plagioclase cumulates (termed the MAN suite). The occurrence of MANs is controversial, it is unclear how the MAN suite fits into canonical lunar crustal formation models [3]. To investigate the protoliths of the granulite suites, we report in situ trace- and minor-element abundances for olivine and pyroxene grains within Fe- and Mg-granulites, determined by LA-ICP-MS and EPMA respectively. Trace-element data presented here indicate that the Mg-granulites are compositionally similar to the MAN suite. Furthermore, by comparing plagioclase trace-element data with peak metamorphic temperatures (calculated using two-pyroxene thermometers [4]), we find no relationship between metamorphic temperature and diagnostic trace-element signatures suggesting that both granulite suites experienced similar thermal metamorphic conditions. Additionally, we estimate the duration of metamorphic heating using experimentally derived diffusion rates of minor elements in minerals,  (such as Ca in olivine [5]). Both the calculated cooling rates and peak metamorphic temperatures can set constraints on the metamorphic heat source responsible for thermally annealing the Fe- and Mg-granulites. Specifically, we are able to assess whether the granulites formed as a result of shallow (<1 km) burial of the protolith by impact melt sheets or hot, impact-generated fall-back breccias [6]; or deep (> 1km) contact metamorphism of the protolith due to the emplacement of magma chambers or upwelling plutons within the lunar crust [7].

 

[1] Lindstrom & Lindstrom, 1986, JGR, 91(B4), 263-276 [2] Treiman et al. 2010. MaPS, 45, 163-180. [3] Gross et al. 2014, EPSL, 388, 318-328. [4] Brey & Köhler, 1990, J Pet, 31, 1353-1378. [5] Dohmen et al, 2007, PCM, 34, 389-407. [6] Cushing et al. 1999, MaPS, 34, 185-195. [7] Hudgins et al. 2011, Am Min, 96, 1673-1685.

How to cite: Pernet-Fisher, J., Hartley, M., and Joy, K.: Trace and minor element variations in lunar granulites: Insights into lunar metamorphic conditions., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12352, https://doi.org/10.5194/egusphere-egu21-12352, 2021.

11:16–11:18
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EGU21-12247
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ECS
Mark C. Nottingham, Natalie M. Curran, John Pernet-Fisher, Ray Burgess, and Katherine H. Joy

The Apollo 16 landing site is dominated by regolith breccias; consolidated regolith palaeo-soils [5,7,8]. Each regolith soil (and, by extension, each regolith breccia) is composed of fragments of rock sourced from different impacts and lithological units [e.g. 2,3]. Because of this, these samples probe the impact history of the lunar surface across a wide range of time. McKay et al. (1986) reported the trapped argon isotope ratios (i.e., 40Ar/36ArTr) values of regolith breccias and used these values to semi-quantitatively model breccia formation ages [see also 4]. Two groups of regolith breccias were identified at the Apollo 16 landing site: (i) the ‘ancient’ group, lithified immature regolith (i.e., <30 Is/FeO), and (ii) a ‘younger’ group that generally have higher levels of maturity. Joy et al. (2011) used the 40Ar/36ArTr ratios to model that: (i) the ancient samples closed from soils to breccias between ~3.8 and 3.4 Ga, consistent with regolith developed and consolidated after the Imbrium basin-forming event, and during a time of declining basin-forming impacts, and (ii) that the young breccias were assembled in the Eratosthenian period between ~2.5 and 1.7 Ga, providing insight into post-basin bombardment impact processes.

A third set of regolith breccias identified by Jerde et al. (1987, 1990), (the soil-like breccias), have no reported noble gas or exposure age information. Joy et al. (2011) inferred that these samples were likely consolidated into breccias in the last 2 Ga (based on their Is/FeO maturity being similar to the Apollo 16 soils). They, therefore, may extend the current archive of impact and regolith processes into the Eratosthenian and Copernican periods.

Whole-rock samples were laser step heated and the extracted gases were measured using a Thermo Scientific Helix-MC noble gas magnetic sector mass spectrometer. Preliminary analysis of our data shows these breccias are dominated by a solar wind composition component, with minor spallation and radiogenic contributions. The concentrations of evolved gases suggest the samples are more similar in terms of noble gas budget to the present day Apollo 16 soil samples (based on analysis using data collated by Curran et al. 2020), than the ancient gas-poor Apollo 16 regolith breccias (McKay et al. 1986). Thus, these noble gas data are consistent with the petrological characterisation and Is/FeO classification [5,6] of these breccias being comparable to present day Apollo 16 soil samples. Solar wind composition gas concentrations comparable to present day soil samples suggest these new breccias represent consolidated regolith of comparable maturity, perhaps suggesting these soil-like breccias were formed around the same time period as the ‘younger’ group.

References: [1] Curran, N.M., et al., 2020, PSS, 182, 104823. [2] Donohue, P.H., et al., 2013, 44th LPSC, A#2897.; [3] Fagan, A.L., et al., 2013, GCA, 106, 429-445.; [4] Fagan, A.L., et al., 2014, Earth Moon Planets, 112, 59–71.; [5] Jerde, E.A., et al., 1987, J. Geophys. Res., 92(B4), E526– E536.; [6] Jerde, E.A., et al., 1990, EPSL, 98(1), 90-108.; [7] Joy, K.H., et al., 2011, GCA, 75(22), 7208-7225.; [8] McKay, D.S., et al., (1986), J. Geophys. Res., 91(B4), 277– 303.

How to cite: Nottingham, M. C., Curran, N. M., Pernet-Fisher, J., Burgess, R., and Joy, K. H.: Origins and Implications of the Apollo 16 Breccia Noble Gas Suite, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12247, https://doi.org/10.5194/egusphere-egu21-12247, 2021.

Petrology and Geochemistry of Solar System Materials
11:18–11:20
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EGU21-10792
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ECS
Leanne Staddon, James Darling, Winfried Schwarz, Natasha Stephen, Sheila Schuindt, Joseph Dunlop, and Kim Tait

Baddeleyite (monoclinic; m-ZrO2) is an important U-Pb chronometer within mafic lithologies from many planetary bodies. Recent in-situ U-Pb dating of micro-baddeleyite within shergottites has been key in confirming recent magmatic activity on Mars. However, despite a high U-Pb closure temperature (≥900 °C) and the retention of robust U-Pb isotope systematics to ~57 GPa within experimental studies, up to 80% Pb loss within baddeleyite has been reported from the highly-shocked shergottite Northwest Africa (NWA) 5298. Significantly, U-Pb isotopic disturbance has been shown to be strongly linked with baddeleyite internal microstructure, generated by partial to complete reversion from meta-stable, high P-T zirconia polymorphs during shock metamorphism. NWA 5298 has experienced elevated shock metamorphism, and particularly post-shock temperatures, in comparison to many other shergottites; in the absence of microstructural analyses, the magnitude of baddeleyite U-Pb isotopic disturbance within more moderately shocked shergottites remains unknown.

To address this, we combine electron backscatter diffraction (EBSD) microstructural analysis and in-situ U-Pb chronology of baddeleyite within three enriched shergottites: NWA 7257, NWA 8679 and Zagami. Studied samples have undergone shock conditions typical of shergottites, with complete transformation of plagioclase to maskelynite and pervasive fracturing of pyroxene, phosphates and oxides. Small veinlets of shock melt cross-cut NWA 8679 and Zagami, and shock melt pockets are present in all samples. Baddeleyite is abundant and ubiquitously associated with late-stage igneous assemblages, rather than shock melt.

We document a wide range of baddeleyite microstructures. These include crystal-plastically deformed magmatic twins, domains with a marked decrease in crystallinity, and complex, nanostructured domains with orthogonal orientation relationships that are interpreted to have resulted from complete reversion from high P-T polymorphs. Magmatic twins are only locally preserved due to shock heterogeneity. Despite this, and in contrast to NWA 5298, we find no link between baddeleyite microstructure and U-Pb isotope systematics. Analyses fall along well-defined discordia within Tera-Wasserburg plots for each sample, with the U-Pb isotopic composition of analyses controlled by overlap with surrounding phases and fractures rather than baddeleyite microstructure. We therefore determine two new, microstructurally constrained ages from discordia lower intercepts: 195 ± 15 Ma (95% confidence; MSWD 5.6) for NWA 7257 and 220 ± 23 Ma (95% confidence; MSWD 2.2) for NWA 8679. For Zagami, our findings support the previously reported magmatic crystallisation age of ~180 Ma. These results provide further confirmation that high post-shock temperatures are required to induce resolvable U-Pb isotopic disturbance baddeleyite, even within highly shocked samples, and that reversion from high P-T zirconia polymorphs alone does not necessitate U-Pb isotopic disturbance. While we caution the continued requirement for detailed microstructural analyses of baddeleyite prior to isotopic analyses, this study underlines the utility of baddeleyite chronology within martian meteorites and other shocked planetary materials.

How to cite: Staddon, L., Darling, J., Schwarz, W., Stephen, N., Schuindt, S., Dunlop, J., and Tait, K.: Combined microstructural analysis and in-situ U-Pb chronology of baddeleyite within shergottites Northwest Africa (NWA) 7257, NWA 8679 and Zagami , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10792, https://doi.org/10.5194/egusphere-egu21-10792, 2021.

11:20–11:22
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EGU21-12820
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ECS
Jasmeet K. Dhaliwal, James M.D. Day, John B. Creech, and Frederic Moynier

The moderately volatile elements, Cu and Zn, are not strongly affected by magmatic differentiation [1, 2] and are important tracers of volatile depletion in planetary bodies, particularly low-mass, airless bodies [3]. New isotopic ratio and abundance measurements for both Cu and Zn are presented for eucrites to more fully understand volatile depletion processes that affected the parent-body of the howardite-eucrite-diogenite (HED) meteorites, the asteroid 4-Vesta. Zinc isotope ratios are reported for twenty-eight eucrite samples, which along with prior data [4] yield a range of δ66Zn from -1.8 to +6.3 ‰, excluding one outlier, PCA 82502 (δ66Zn = -7.8 ‰) and a Zn concentration range from 0.3 to 3.8 p.p.m. Heavy Zn isotopic ratios (positive δ66Zn compositions) in eucrites form a negative trend with Zn concentration, reflecting volatile depletion processes on Vesta that are similar to the Moon [5, 6]. Within the combined sample set, eleven eucrites have light Zn isotopic compositions from δ66Zn of -0.02 to -7.8 ‰, with the majority having more negative compositions than likely chondritic precursors (maximum δ66Zn of ~ -0.2 ‰ [7]). These samples are interpreted to reflect condensates formed subsequent to surface volatilization and outgassing, such as during impact bombardment. Measurements of Cu compositions are also reported for nineteen of the samples, yielding a range of δ65Cu from -1.6 to +0.9 ‰, and range of Cu concentrations from 0.2 to 2.8 p.p.m., with the exception of Stannern (Cu > 10 ppm). As with Zn, negative Cu isotopic ratios that are lighter than chondritic compositions (δ65Cu ~ -0.5 ‰ [8]) are attributed to recondensation that occurred following impact-induced vaporization (cf. [9]). Within the wide ranges of Zn and Cu isotopic compositions measured in eucrites, most samples cluster within ~ 0 ‰ < δ66Zn < +3 ‰ and ~ 0.2 ‰ < δ65Cu < +0.9 ‰. This range is interpreted to reflect volatile depletion processes similar to those that affected the Moon (BSM: δ66Zn +1.4 ± 0.5‰ [5, 6, 10, 11] and δ65Cu = +0.92 ± 0.16‰ [9-11]). The greater heterogeneity in eucrite Zn and Cu isotopic compositions compared to lunar samples can be attributed to the smaller size of the HED parent asteroid, which may have experienced more limited homogenization of these signatures following volatile depletion and for eucrites which have experienced complex impact addition and metamorphic processes.  

[1] Chen et al. (2013) EPSL, 369, 34-42. [2] Savage et al. (2015) Geochemical Perspective Letters, 1, 53-64. [3] Day and Moynier (2014) Philisophical Transactions of the Royal Society A, 372, p.20130259. [4] Paniello et al. (2012) GCA, 86, 76-87. [5] Paniello et al. (2012) Nature, 490, 376-379. [6] Kato et al. 2015 Nature Communications, 6, 1-4. [7] Luck et al. (2005) GCA 69, 5351-5363. [8] Luck et al. (2003) GCA, 67¸143-151. [9] Day et al. (2019) GCA, 266, 131-143. [10] Moynier et al. (2006) GCA, 70, 6103-6117. [11] Herzog et al. (2009) GCA, 73, 5884-5904.

How to cite: Dhaliwal, J. K., Day, J. M. D., Creech, J. B., and Moynier, F.: Volatile depletion and evolution of Vesta from coupled Cu-Zn isotope systematics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12820, https://doi.org/10.5194/egusphere-egu21-12820, 2021.

11:22–11:24
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EGU21-1230
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ECS
Marion Auxerre, François Faure, and Delphine Lequin

Chondrules - major constituent of chondrites (primitive meteorites) - belong to the first object formed in the solar system. They are millimetre-sized igneous objects resulting from partial to complete fusion and are divided into main families: non-porphyritic and porphyritic (Gooding and Keil, 1981); the latter one is more abundant in chondrites. This study aims to reproduce thermal histories of macro-porphyritic olivine chondrules (PO) and to better constrain (thermal, temporal) the conditions reigning in the early solar system.

In general, PO chondrules are composed of numerous euhedral crystals of olivine and/or pyroxene suggesting an initially melting below their liquidus temperatures. By contrast, in our study, the macro-porphyritic olivine chondrule displays only one large euhedral olivine. The low number of olivine crystals indicates that chondrule suffered an initial step of superheating, limiting nucleation process (Lofgren, 1988; Hewins et al., 1988). Moreover, embayments observed in euhedral olivine show that olivine crystal began to growth rapidly and then the growth-rate decreased during the cooling. Therefore, our petrographic investigation proposes a first high temperature stage (ΔTliq = +10 °C) followed by a slow cooling.

To test this thermal history, experiments are performed to determine degree of superheating and cooling rate effect (i) on nucleation rate and (ii) on morphology of olivines formed during cooling. Preliminary results seem to confirm that macro-porphyritic olivine chondrules result from the slow cooling of a superheated initial chondritic liquid (Varela et al., 2006). Then these results allow to precise the beginning of the igneous processes (minimum thermal temperature and cooling rate) and to discuss the complete thermal evolution of the chondrule, by considering all other reaction textures observed in this chondrule: peritectic and oxidation reactions, quench texture and aqueous alteration.

 

References:

Gooding et al., (1981) Meteoritics, 16, No. I; Hewins et al., (1988) Meteoritics, 25, 309-318; Lofgren, (1988) Geochim. & Cosmochim. Ada, 50, 1715-1726; Varela et al., (2006) Icarus, 178 (2), 553–569.

How to cite: Auxerre, M., Faure, F., and Lequin, D.: Superheating and cooling rates effects on olivine growth in chondritic liquid: experimental and petrographic approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1230, https://doi.org/10.5194/egusphere-egu21-1230, 2021.

11:24–11:26
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EGU21-1751
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ECS
Dafilgo Fernandes

Extraterrestrial dust that reaches the Earth’s surface has shown to represent the diverse types of samples from different precursors, namely, asteroid complexes and cometary bodies from the solar system. A substantial amount of this dust that strikes the upper atmosphere is believed to have been lost due to frictional heating with air molecules. Cosmic spherules that are melted particles are some of the widely recognized micrometeorites that survived this catastrophic entry process; however, their primordial characteristics are altered from their precursors making it difficult to identify the precursors. An individual peculiar spherule MS-I35-P204 recovered from the Antarctica blue ice has been identified. The spherule has been segregated using magnetic separation method, mounted in epoxy, and examined using SEM, subsequently analysed under electron microprobe. It is surrounded by a thin magnetite rim, and also holds a single kamacite bead that protrudes out at its top. The interior mineralogy mostly constitutes of a bulk pyroxene normative glass (MnO>2wt%) with several vesicles. The rare mineral phase is a skeletal aggregate of free silica, bearing Fe nuggets embedded in a glass. An isolated narrow lath of forsterite appears to be chondritic and is observed as relict grain that is associated with an anomalous low Ca pyroxene (MnO ~1.3 wt%, FeO~13 wt%). Earlier, free silica has been reported in some chondritic meteorites particularly the Enstatite and Ordinary group, and also in some carbonaceous chondrites such as CM, CR, CH, and K. It profoundly forms a pod that encloses the ferromagnesian silicate in silica-bearing chondrules. The unusual mineral assemblage seen in this spherule thereby appears to constrain probably the unique type of its contributor which need to be studied.

How to cite: Fernandes, D.: A rare free silica-bearing micrometeorite, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1751, https://doi.org/10.5194/egusphere-egu21-1751, 2021.

11:26–11:45