Trapped nitrogen in individual chondrules: Nature of chondrule

precursors and clues to formation mechanisms

J. P. Das1,2 and S. V. S. Murty1

Received 3 July 2008; revised 25 September 2008; accepted 11 November 2008; published 30 January 2009.

[1] Nitrogen amount and isotopic composition have been measured in individualchondrules of six ordinary chondrites (OC) and two each of carbonaceous (CC) andenstatite chondrites (EC). The [N (ppm), d15Nt (%) {corrected for cosmogeniccontribution}] 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, d15N spans a wide range,in contrast to the narrow range (�25 to +14%) for bulk. For CC chondrules, N contentsare lower, while d15N is different and heavier than bulk. In contrast, EC chondrulesand bulk have comparable N and d15N values. Neither N nor d15Nt of OC and CCchondrules show any size (expected for nebular exchange) or chemical/mineralogicaldependence, ruling out the dominance of secondary effects of nebular or parent bodyorigin. The range of d15Nt among OC and CC chondrules represents d15Nt heterogeneityof chondrule precursors and their survival after a high-temperature chondrule-formingevent. On the basis of d15N of the chondrules, we propose the presence of at least threeN 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 OCand CC, which represents the different environment for their formation, compared to thebulk OC and CC. However, for EC chondrules, precursors as well as the formationenvironment are the same as those of enstatite chondrites.

Citation: Das, J. P., and S. V. S. Murty (2009), Trapped nitrogen in individual chondrules: Nature of chondrule precursors and clues

to formation mechanisms, J. Geophys. Res., 114, E01008, doi:10.1029/2008JE003232.

1. Introduction

[2] Chondrules are one of the earliest formed silicatespheres of about �1 mm average size and occur at anabundance of up to 80% by volume in chondrites [e.g.,Brearley and Jones, 1998; Rubin, 2000; Hewins et al., 2005;Lauretta et al., 2006]. Such high abundances of chondrulesimply that their formation was an important and commonprocess during early stages of the solar system [Hewins,1997]. Petrological features suggest that chondrules areformed by flash heating and rapid cooling of the preexistingsilicate precursors [e.g.,Rubin, 2000]. However, the nature ofchondrule precursors and environment(s) during chondruleformation for different types of chondrites remain to be fullyunderstood. Proposed mechanisms suggest either nebularenvironment or planetary surface as possible locations ofchondrule formation [Ciesla, 2005]. X-wind model [Shu etal., 2001] and nebular shock wave model [Connolly andLove, 1998] are the most favored nebular models forchondrule formation, whereas many different types ofmagmatic processes like volcanism and collisional events

between planetesimals (with solid, partially molten or fullymolten interiors) have also been proposed for chondruleformation [Sanders and Taylor, 2005; Ciesla, 2005].[3] In case of nebular origin of chondrules, interaction

between nebular gases and chondrules is expected duringchondrule formation. Oxygen isotopic composition of chon-drules has provided vital clues regarding nebular interactionwith chondrules. In a three-isotopic diagram (d18O versusd17O), bulk O isotopic compositions of most chondrules inordinary, enstatite, and carbonaceous chondrites plot above,along, and below the terrestrial fractionation line (with slope0.5 and result of mass-dependent fractionation betweenoxygen isotopes), respectively. This can be explained bythe exchange between two components of oxygen withdifferent abundances of 16O [Clayton et al., 1991; Clayton,1993; Krot et al., 2006; Chaussidon et al., 2008]. Studies ofType I chondrules suggest the important role of gas-meltinteraction during their formation and reveal the presence ofdifferent unequilibrated components [Libourel et al., 2006;Libourel and Krot, 2007; Chaussidon et al., 2008]. Undernebular conditions, nitrogen is most stable in the molecularform [Lewis and Prinn, 1980]. This inert form of molecularnitrogen can be trapped in chondrule precursors along withnoble gases, and as would be expected, usually shows acorrelation with trapped 36Ar. In addition, nitrogen may alsobe chemically bound, mostly as organic matter in chondrule

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1Planetary and Geosciences Division, Physical Research Laboratory,Ahmedabad, India.

2Now at Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan.

Copyright 2009 by the American Geophysical Union.0148-0227/09/2008JE003232$09.00

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