FINE-GRAINED AGGREGATES IN EL3 MACALPINE HILLS 88136 … · amples of such aggregates in EL3...
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Introduction:. Certain EH3 chondrites have been described as primitive breccias because they contain fragments of chondrules, shock-melt veins of Fe metal and troilite with eutectoid intergrowths, and fine- grained aggregates that are interpretated as having formed through impact processes [1]. Impact-formed fine-grained aggregates within breccias are consistent with multiple episodes of parent-body disaggregation and reassembly [2] or perhaps regolith gardening not involving total disaggregation [3]. We report new ex- amples of such aggregates in EL3 MacAlpine Hills 88136 (MAC 88136) and EH3 Larkman Nunatuk 06252 (LAR 06252). The aggregates are similar in size to chondrules and metal-sulfide nodules, and consist of metal, sulfide, and silicate fragments (Fig. 1). Our goal is to describe these rounded aggregates and to interpret the implications of their occurrence and mineralogy for processes that may have occurred on the EH and EL parent bodies. Methods: Element maps and high-resolution im- ages of the aggregates were made with two field-emis- sion-source scanning electron microscopes (SEMs) equipped with energy-dispersive spectrometers. Quan- titative analysis was performed with a JEOL JXA-8600 electron microprobe using wavelength-dispersive spec- trometry. Results: The aggregates consist of 5 to 40 µm opaque and non-opaque fragments, which together form a fine-grained matrix (Fig. 1). The fragments in LAR 06252 are dominated by Fe-Ni metal, troilite, nin- ingerite, pyroxene, SiO 2 , and albite. Both high- and low-FeO pyroxene occur adjacent to one another. In MAC 88136, the fragments are pyroxene, troilite, and Fe-Ni metal. Pyroxene within the MAC 88136 aggre- gate contain ~4 wt. % Fe, whereas Fe is below detec- tion in pyroxene outside the aggregate. Most silicate fragments in the MAC 88136 aggre- gate are rounded or granular (Fig. 1A, B). In contrast, the silicate fragment shapes range from spheroidal to elongate and lath-like in LAR 06252 (Fig. 1C, D). The LAR 06252 aggregate consists of two distinct regions separated by a third region with higher concentration of kamacite and troilite. To the left of this region the opaque minerals are ~50% niningerite and troilite sur- rounding silicates with angular morphologies. To the right of this region the opaques are dominated by kama- cite, with minor troilite, and the silicates are more gran- ular (Fig. 1D). Discussion: Although these aggregates may result from multiple episodes of asteroid disaggregation and reassembly [2], smaller impacts could create them through the process of regolith gardening. Such primi- tive breccia has been proposed to represent immature regolith [3]. The mineralogical differences between the EH3 ag- gregates (present work and [1]) and the EL3 MAC 88136 aggregate shows that they formed in their re- spective EH and EL environments (i.e., they have the same redox state as the parent bodies and thus are not exotic objects). The EH3 meteorite as a whole and its aggregates are more reduced than the EL3 meteorite. Not only is the Si content of the metal higher in the EH3 aggregates than the EL3 aggregate, but the EL3 aggre- gate lacks Mg and Mn sulfides. Thermodynamic model- ling suggests such sulfides can form from silicates be- tween 1400 and 1600 K, with the fS 2 buffered by kamacite and troilite, but only in H-depleted environ- ments [4]. One such environment may be the EH rego- lith. The niningerite and silica replacing the more angu- lar and lath-like pyroxene in the left region of the EH3 aggregate suggest that the left side experienced higher fS 2 than the right side. The partial sulfidation of this ag- gregate indicates the process occurred only locally, per- haps as pockets of S-rich gas are generated by impact heating within regolith. Even smaller areas of sulfida- tion were observed in the fine-grained EH3 matrix [5]. Conclusions: The impact-formed aggregates con- sist of fragments reflecting the mineralogy of the much larger EH and EL chondrules and metal-sulfide nodules, respectively. The occurrence of these aggregates is in- terpreted as evidence for regolith gardening and possi- ble dissagregation that occurred prior to the final accre- tion and lithification of the meteorites. Such primitive regolith is one possible environment that could satisfy the conditions required for silicate sulfidation. References: [1] Lehner, S. W., and P. R. Buseck, (2009). LPSC 40 # 2154 [2] Stoffler, D., et al., (1988) in Meteorites and the Early Solar System, p. 165-202. [3] Scott, E.R.D., and G.J. Taylor, (1982) in Workshop on Lunar Breccias and Soils and Their Meteoritic Analogs, LPI p. 130-134. [4] Lehner, S. W., et al. (2013). GCA, 101: 34-56. [5] Lehner, S. W., Petaev, M. I., Buseck, P. R., (2012) LPSC 43, #2309. . FINE-GRAINED AGGREGATES IN EL3 MACALPINE HILLS 88136 AND EH3 LARKMAN NUNATUK 06252. S. W. Lehner 1 and P. R. Buseck 1 , 1 School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, ([email protected], [email protected]). 2463.pdf 47th Lunar and Planetary Science Conference (2016)
Transcript of FINE-GRAINED AGGREGATES IN EL3 MACALPINE HILLS 88136 … · amples of such aggregates in EL3...
Introduction:. Certain EH3 chondrites have been described as primitive breccias because they contain fragments of chondrules, shock-melt veins of Fe metal and troilite with eutectoid intergrowths, and fine-grained aggregates that are interpretated as having formed through impact processes [1]. Impact-formed fine-grained aggregates within breccias are consistent with multiple episodes of parent-body disaggregation and reassembly [2] or perhaps regolith gardening not involving total disaggregation [3]. We report new ex-amples of such aggregates in EL3 MacAlpine Hills 88136 (MAC 88136) and EH3 Larkman Nunatuk 06252 (LAR 06252). The aggregates are similar in size to chondrules and metal-sulfide nodules, and consist of metal, sulfide, and silicate fragments (Fig. 1). Our goal is to describe these rounded aggregates and to interpret the implications of their occurrence and mineralogy for processes that may have occurred on the EH and EL parent bodies.
Methods: Element maps and high-resolution im-ages of the aggregates were made with two field-emis-sion-source scanning electron microscopes (SEMs) equipped with energy-dispersive spectrometers. Quan-titative analysis was performed with a JEOL JXA-8600 electron microprobe using wavelength-dispersive spec-trometry.
Results: The aggregates consist of 5 to 40 µm opaque and non-opaque fragments, which together form a fine-grained matrix (Fig. 1). The fragments in LAR 06252 are dominated by Fe-Ni metal, troilite, nin-ingerite, pyroxene, SiO2, and albite. Both high- and low-FeO pyroxene occur adjacent to one another. In MAC 88136, the fragments are pyroxene, troilite, and Fe-Ni metal. Pyroxene within the MAC 88136 aggre-gate contain ~4 wt. % Fe, whereas Fe is below detec-tion in pyroxene outside the aggregate.
Most silicate fragments in the MAC 88136 aggre-gate are rounded or granular (Fig. 1A, B). In contrast, the silicate fragment shapes range from spheroidal to elongate and lath-like in LAR 06252 (Fig. 1C, D). The LAR 06252 aggregate consists of two distinct regions separated by a third region with higher concentration of kamacite and troilite. To the left of this region the opaque minerals are ~50% niningerite and troilite sur-rounding silicates with angular morphologies. To the right of this region the opaques are dominated by kama-cite, with minor troilite, and the silicates are more gran-ular (Fig. 1D).
Discussion: Although these aggregates may result
from multiple episodes of asteroid disaggregation and reassembly [2], smaller impacts could create them through the process of regolith gardening. Such primi-tive breccia has been proposed to represent immature regolith [3].
The mineralogical differences between the EH3 ag-gregates (present work and [1]) and the EL3 MAC 88136 aggregate shows that they formed in their re-spective EH and EL environments (i.e., they have the same redox state as the parent bodies and thus are not exotic objects). The EH3 meteorite as a whole and its aggregates are more reduced than the EL3 meteorite. Not only is the Si content of the metal higher in the EH3 aggregates than the EL3 aggregate, but the EL3 aggre-gate lacks Mg and Mn sulfides. Thermodynamic model-ling suggests such sulfides can form from silicates be-tween 1400 and 1600 K, with the fS2 buffered by kamacite and troilite, but only in H-depleted environ-ments [4]. One such environment may be the EH rego-lith.
The niningerite and silica replacing the more angu-lar and lath-like pyroxene in the left region of the EH3 aggregate suggest that the left side experienced higher fS2 than the right side. The partial sulfidation of this ag-gregate indicates the process occurred only locally, per-haps as pockets of S-rich gas are generated by impact heating within regolith. Even smaller areas of sulfida-tion were observed in the fine-grained EH3 matrix [5].
Conclusions: The impact-formed aggregates con-sist of fragments reflecting the mineralogy of the much larger EH and EL chondrules and metal-sulfide nodules, respectively. The occurrence of these aggregates is in-terpreted as evidence for regolith gardening and possi-ble dissagregation that occurred prior to the final accre-tion and lithification of the meteorites. Such primitive regolith is one possible environment that could satisfy the conditions required for silicate sulfidation.
References: [1] Lehner, S. W., and P. R. Buseck, (2009). LPSC 40 # 2154 [2] Stoffler, D., et al., (1988) in Meteorites and the Early Solar System, p. 165-202. [3] Scott, E.R.D., and G.J. Taylor, (1982) in Workshop on Lunar Breccias and Soils and Their Meteoritic Analogs, LPI p. 130-134. [4] Lehner, S. W., et al. (2013). GCA, 101: 34-56. [5] Lehner, S. W., Petaev, M. I., Buseck, P. R., (2012) LPSC 43, #2309.
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FINE-GRAINED AGGREGATES IN EL3 MACALPINE HILLS 88136 AND EH3 LARKMAN NUNATUK 06252. S. W. Lehner1 and P. R. Buseck1, 1School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, ([email protected], [email protected]).
2463.pdf47th Lunar and Planetary Science Conference (2016)
Kamacite TroiliteNiningerite
Fig.1. SEM of fine-grained aggregates. A: BSE image of spheroidal aggregate from EL3 MAC 88136 consisting of rounded silicates with interspersed Fe,Ni metal, troilite. En = enstatite, Tro = troilite, Kam = kamacite. B: Enlarge-ment of area inside the white rectangle in A showing the texture of interlocking silicate grains with kamacite and troilite. C: BSE image of aggregate from EH3 LAR 06252 showing variations in mineralogy from left to right sides. D: BSE image of same aggregate with Fe and S element maps superimposed showing higher sulfide concentration on the left side.
En
TroKamA
DC
B
Kam
Tro
En
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2463.pdf47th Lunar and Planetary Science Conference (2016)
