A primitive achondrite from early melting of the IIIAB iron parent body 

Rhiannon G. Mayne, Sarah L. Garcia, Timothy J. McCoy, Catherine M. Corrigan, Kathryn Gardner-Vandy, James M. D. Day and Richard C. Greenwood

Abstract
Enon is a primitive silicate bearing iron meteorite with a metal composition that matches the IIIAB group. Initially classified as a mesosiderite, Enon shows affinities to multiple meteorite groups, which has made deciphering its formational history complex. We present new analyses of major mineral chemistry, oxygen isotopes, and the thermal history of Enon, as well as the first modal mineralogy measurements, oxygen fugacity calculations and LA-ICP-MS analyses of the metal phases within the meteorite. Our conventional results match that of previous studies and confirm that Enon's silicate mineralogy is indicative of a chondritic precursor altered by reduction, while its metal phase is consistent with the IIIAB iron meteorite group. Evidence of silicate partial melting is seen throughout the sample, while the modal abundance remains mostly chondritic, suggesting that extensive melt migration did not occur. We hypothesize that the abundant metal and sulfide within Enon represent a bulk composition primitive Fe,Ni-FeS melt that never participated in IIIAB core formation. This model is tested by recalculating the measured composition of EnonŐs metal to include sulfur calculated from the modal abundance of troilite. This composition correlates with modeled values for the bulk composition of the IIIAB core. We propose that EnonŐs formation involved a primitive Fe,Ni-FeS melt from the IIIAB parent body becoming trapped within a cooler chondritic section, which then underwent reduction due to the abundance of S in the melt. Our results support the inefficient formation of protoplanetary cores in early planetesimals during early Solar System evolution.
Citation
Mayne, R.G., Garcia, S.L., McCoy, T.J., Corrigan, C.M., Gardner-Vandy, K., Day, J.M.D., Greenwood, R. Submitted. A primitive chondrite from early melting of the IIIAB iron parent body. Geochimica et Cosmochimica Acta.
 
Table of Contents
The document associated with this readme file (Mayneetal.docx) contains text-based versions of all tables in the manuscript:

Table 1: Modal Mineralogy of Enon
Table 2: Average composition measured for all silicate phases
Table 3: Average compositions measured for chromite, taenite, and kamacite
Table 4: LA-ICP-MS analyses of kamacite and taenite (in ?g/g)
Table 5: Modal abundance of silicate minerals in Enon and Puente del Zacate compared to mesosiderites
Table 6: The composition of kamacite and taenite in Enon, with bulk S (11.2 wt.%) calculated from the modal abundance of troilite in the sample, compared to the bulk composition for the IIIAB core (8, 9, and 10 wt.% S) modeled by Chabot and Zhang (2022)