Trapped nitrogen in individual chondrules: nature of chondrule precursors and clues to formation mechanisms

Abstract

Nitrogen amount and isotopic composition have been measured in individual chondrules of six ordinary chondrites (OC) and two each of carbonaceous (CC) and enstatite chondrites (EC). The [N (ppm), δ ¹⁵N_t (%) {corrected for cosmogenic contribution}] for the chondrules have ranges of [0.8 to 61, -97 to +164], [19 to 140, -21 to -31], and [2 to 77, -27 to +116] among OC, EC, and CC, respectively. For OC, while N contents of chondrules fall in the range of bulk OC, δ¹⁵N spans a wide range, in contrast to the narrow range (-25 to +14%) for bulk. For CC chondrules, N contents are lower, while δ¹⁵N is different and heavier than bulk. In contrast, EC chondrules and bulk have comparable N and δ¹⁵N values. Neither N nor δ¹⁵N_t of OC and CC chondrules show any size (expected for nebular exchange) or chemical/mineralogical dependence, ruling out the dominance of secondary effects of nebular or parent body origin. The range of δ¹⁵N_t among OC and CC chondrules represents δ¹⁵N_t heterogeneity of chondrule precursors and their survival after a high-temperature chondrule-forming event. On the basis of δ¹⁵N of the chondrules, we propose the presence of at least three N components carried by phase Q, presolar diamonds (HL), and insoluble organic matter (IOM). The precursors of chondrules from OC and CC are different than the bulk OC and CC, which represents the different environment for their formation, compared to the bulk OC and CC. However, for EC chondrules, precursors as well as the formation environment are the same as those of enstatite chondrites.