CHONDRULES AND FINE-GRAINED RIMS IN AGUAS ZARCAS CARBONACEOUS CHONDRITE (CM2)

Dominant phases within chondritic meteorites include chondrules (sub-millimeter- to centimeter-sized silicate melt droplets), and calcium-aluminum-rich (CAls) inclusions. These phases are typically surrounded by a fine-grained silicic matrix. In addition to its presence as interstitial matrix, fine-grained material also occurs as rims of substantial thickness (up to 100 μm) around these major phases. Fine-grained rims (FGRs) are dust-sized material that surround chondrules and refractory inclusions and can usually be distinguished from the matrix in optical and scanning electron microscopy (SEM) images [1]. The origin of FGRs and the relationship between FGRs and their encompassing chondrules or associated matrix remains unclear, with two general formation scenarios: FGRs were formed within chondrite parent bodies; FGRs formed within the solar nebula. Studying these materials can thus help better characterize conditions in the early Solar System.

Here, we report on our study of multiple chondrules and FGRs within a sample of a CM2 chondrite, Aguas Zarcas (AZ), that was recovered on April 2019. Using optical microscopy, SEM and electron probe microanalysis techniques (EPMA), we observed two distinct areas within our sample – a chondrule rich area with larger FGR:chondrule surface area ratio and a chondrule poor area with smaller FGR:chondrule surface area ratio. Utilizing EPMA wavelength-dispersive (WDS) analyses, no evidence for different chemical compositions in chondrules between the two areas was detected. On the contrary, energy-dispersive (EDS) mapping revealed a discrepancy in certain element distributions among chondrules and their FGRs. Previous studies (e.g. [2][3]) indicate the existence of various reservoirs within the protoplanetary disk but the extent of diversity in chemical and physical parameters between these reservoirs remains unclear. Hanna and Ketcham [4] discuss the possibility that CM Murchison’s FGRs formed in separate areas with varying degrees of nebular turbulence, and mixed afterwards to accrete on the CM parent body. So far, our findings support the case for the formation of FGRs within the solar nebula in areas with varying dust content that are responsible for the different FGR growth rate.

References: [1] Lauretta et al. 2006, Met. and the Ear. Sol. Sys. II. [2] Clayton 2003, Oxygen isotopes in meteorites. [3] Jones 2012, Met. & Plan. Sci., v. 47. [4] Hanna & Ketcham 2018, Earth and Plan. Sci. Let., v. 481.