52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548)

1434.pdf

RELICT FORSTERITE IN UNEQUILIBRATED ENSTATITE CHONDRITES. L. C. Zenie1, N. V. Almeida1,A. J. King1, I. A. Franchi2, P. F. Schofield1 and S. S. Russell1, 1 Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, UK, 2 Planetary and Space Sciences, The Open University, Walton Hall,
Milton Keynes, MK7 6AA, UK.

Introduction: Enstatite chondrites (ECs) arethought to represent the material present in the accretion
regions of the terrestrial planets [1]. However, ECs contain unique mineral assemblages which indicate thatthey formed under much more reduced conditions thanthe Earth. Their major silicate phase is near-endmember
enstatite [2, 3] of which two types have been identified[4]: primary enstatite which crystallised directly fromchondrule melts and secondary enstatite which formedby reduction of ferroan olivine. Olivine in unequilibrated ECs (<8 vol.%) is almost pure forsterite (Fa<1)[5], and found almost exclusively inside chondrules.
Forsterite grains which experienced low degrees ofmelting and/or short heating durations above solidus
temperatures have been found to survive as relict grains[6]; relicts are grains that did not crystallise in situ with
their host chondrules. It is hypothesized that most forsterite grains within type I chondrules are relicts [7].
The detection and abundance of relict grains in porphyritic chondrules is important for determining the natureof their precursors and their mode of formation [8]; theypreserve the imprint of processes that occurred in thesolar nebula [9].
High resolution cathodoluminescence (CL) imaging
increases significantly the probability of finding relict
grains [10, 11]. Two types of low-FeO (<3 wt.% FeO)
silicates have been described in the literature [12] andcan be identified with CL imaging; pure, Fe-free,endmember minerals with below detectable (<0.04wt.%) levels of trace elements exhibit blue luminescence, whereas if trace minerals are present (>0.04wt.%) they may appear red [13].
It has long been assumed that ECs formed from material condensed at supersolar C/O ratios, however, recent trace element work suggests that ECs may have instead formed from precursors that formed in a more oxidising environment [14]. Later processing of these precursors in an unusually S (and other volatile)-rich andO-poor environment may then explain the highly reduced nature of ECs [14, 15]. Exposure to such environments could explain the reduction of olivine to Mg-richpyroxenes. ECs may, therefore, record an evolving nebular composition from oxidising to reducing conditions.
Following on from an earlier study [16], we have analysed several relict olivines in primitive unequilibrated
ECs to explore this hypothesis.
Method: A polished section of Kota Kota (EH3;BM.1905, 355; P22811) was studied. Element mappingby EDS (Zeiss EVO 15LS SEM) was used to locate olivine grains, followed by high-resolution CL imaging(512×512 pixels). Mosaics were collected with a Gatan
ChromaCL imaging system, at an accelerating voltageof 10kV, beam current of 3nA and a distance of ~1 mmfrom the CL detector.
Findings: Forsterite grains were identified in 24
chondrules in the Kota Kota sample (~25% of all chondrules in the sample). They occur either as anhedral tosubhedral grains in the centre of chondrules surroundedby enstatite or as mottled grains within larger enstatite
phenocrysts. Olivine abundance in the chondrules
ranged from ≤ 1% to > 30% with the largest grainmeasuring ~ 1.2 mm × 1.2 mm. Based on the size andmorphology of the forsterite grains, ten porphyritic olivine-pyroxene (POP) chondrules were prioritized for
CL imaging; two of which are shown in Figs. 1 and 2.
The analysed chondrules are type IB and, therefore,
it is likely that all forsterite grains located within them
are relicts. The chondrules were found to exhibit different CL properties that can be divided into six groups: (i)bright red CL throughout the grain, often with varying
intensity; (ii) bright red CL interiors rimmed by blue
CL; (iii) bright red CL containing areas of enstatite
showing blue CL; (iv) dark red CL around the edges of
the chondrules; (v) mixed CL appearing purple; and (vi)consistently blue CL throughout.
Red and blue CL observed in the chondrules indicate
the presence of forsterite and enstatite with varying concentrations of trace minerals. Many of the red luminescing forsterite grains exhibit intensity differences. Theselikely represent the heterogeneity and complex zoningof trace element distributions in individual grains [10],however, they may also be the result of shock. The enstatite showing dark red luminescence around the edgesof the chondrules likely represents a later generation ofchondrule growth (Figs. 1 and 2).
Implications: The existence of silicates exhibiting
different CL indicates these grains may have different
precursors, evolutionary paths or may have formed atdifferent times in the same evolving region of the nebula. It is possible that blue luminescing enstatite formedby reduction of olivine. This supports the hypothesis ofan evolving solar nebula and the suggestion that the precursors of EC chondrules may have formed in a moreoxidized environment.
EC chondrule precursors, therefore, may have been
more similar to other chondrite groups and terrestrial

52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548)

planets than previously thought. Indeed, the relict olivine grains of our study may give us insight into thecomposition and nature of the building blocks of Earth.
Our work will advance to analysis on the electron
microprobe to confirm the CL intensity variations are
due to complex zoning of trace elements in these grains
as opposed to being shock induced. This analysis should
aid in providing further understanding of the origin andhistory of relict grains.

101-115. [6] Nagahara, H. (1983) In: Kring EA (Ed.). Chondrules andtheir origins, Houston, Texas: LPI, 211-222. [7] Chaussidon, M., et al.
(2008) GCA 72, 1924-1938. [8] Tenner, T. J., et al. (2013) GCA 102,226-245. [9] Rambaldi, E. R. (1981) Nature 293, 558-561. [10] Steele,
I. M. (1986a) Am. Mineralogist, 71, 966-970. [11] Steele, I. M.
(1986b) GCA 50, 1379-1395. [12] Lusby, D., et al. (1987) Proc. Lunar
Planet. Sci. Conf. 17, J Geophys. Res. 92: E679-E695. [13] Leitch, C.
A. and Smith, J. V. (1982) GCA 46, 2083-2097. [14] Jacquet, E., etal. (2018) In: Russell, S. S., Connolly Jr, H. C. and Krot, A. N. (Eds)
Chondrules: Records of Protoplanetary Disk Processes. Cambridge,
UK: Cambridge University Press. [15] Weisberg, M. K. and Kimura,
M. (2012) Chemie der Erde, 72, 101-115. [16] Almeida, N. V., et al.
(2020) BPSC 2020 abstract.

Figure 1 – Composite element map (top) and CL image (bottom) of chondrule C1 (Olivine < 20%; Fa0.42).
References: [1] Bizzarro M., et al. (2017) In: Pessah M. and Gressel O. (Eds) Formation, Evolution, and Dynamics of Young Solar
Systems. Switzerland; Springer. [2] Keil, K. (1968) J. Geophys.
Res. 73, 6945-6976. [3] Sears, D. W. G., et al. (1982) GCA 46, 597608. [4] Ikeda, Y. (1989a) Proc. NIPR Symp. Antarct. Met. 2, 75-108.
[5] Weisberg, M. K. and Kimura, M. (2012) Chemie der Erde, 72,1434.pdf

Figure 2 – Composite element map (top) and CL image (bottom) of chondrule C2 (Olivine ~ 30%; Fa1.17).