The Distribution of Troilite in Chondrules from Type-3 Ordinary Chondrites: Implications for the Duration of Chondrule Formation

An important constraint on the nature of the chondrule-formation mechanism is the amount of time this mechanism was active. If chondrule formation occurred only at high ambient temperatures in a canonical cooling solar nebula, chondrules should be free of low-temperature phases (e.g., troilite, magnetite) unless they were introduced by thermal or shock metamorphism or aqueous alteration on the parent asteroid. In contrast, if chondrule formation persisted until nebular temperatures were <650 K (the 50-percent condensation temperature of S; [1]), at least some chondrules should contain primary troilite. The occurrence of enveloping compound chondrules, chondrules with igneous rims, and chondrules containing relict grains indicates that many chondrules were melted multiple times [2]. Chondrules that formed at high temperatures (and thus had low FeO and no troilite) may have later remelted and acquired troilite and additional FeO from adhering fine-grained material that had equilibrated with nebular gas at lower temperatures. Previous workers found low troilite contents in low-FeO chondrules [3] and suggested that troilite was introduced during metamorphism [4]. In order to constrain the duration of the chondrule-formation mechanism and determine if any troilite in chondrules is primary, we made petrographic observations of 139 intact, spheroidal chondrules of all textural types from LL3.1 Bishunpur, L/LL3.4 Chainpur, LL3.1 Krymka, LL3.4 Piancaldoli, LL3.3 St. Mary's County and H/L3.6 Tieschitz. We noted the presence or absence of troilite, and, where present, the nature of associated phases and the location of the troilite grains within the chondrules. Chondrule olivine and low-Ca pyroxene compositions were determined by electron microprobe. A total of 123 chondrules (88%) contain troilite; these chondrules represent all textural types and contain variable amounts of FeO (i.e., Fa0.6-38). In 40% of these chondrules, one or more troilite grains occur near the center of the chondrule and are either completely surrounded by mesostasis or included within mafic silicate phenocrysts. Because these troilite grains are inaccessible to the matrix except through sluggish body diffusion and the chondrites themselves experienced little thermal or shock metamorphism, it is unlikely that these troilite grains were introduced into the chondrules from the matrix by parent-body processes. The chondrules probably formed after nebular temperatures cooled below ~650 K. Although some of the chondrules may have melted several times, the most recent melting episode occurred after troilite had condensed. Of the 16 chondrules (12%) that, in section, are free of troilite, 4 have extremely low FeO (Fa<0.4 or Fs<0.4), 8 have Fa≤0.8 or Fs≤0.8, and 13 have Fa<2.5 or Fs<2.5; 9 of these chondrules contain blebs of kamacite. Chondrule types include BO, C, GOP and type-I PO and POP varieties. We suggest that the most magnesian chondrules formed at high nebular temperatures before FeS or much FeO had condensed; the maximum ambient nebular temperature at equilibrium for a chondrule containing olivine with Fa0.25 (equivalent to the most reduced chondrule in our set) is ~1120 K (J.T. Wasson, pers. commun., 1997). The three remaining troilite-free chondrules are FeO-rich droplet (BO and C) chondrules that lack opaque phases; these chondrules were once completely molten and may have lost their opaques as a dense immiscible liquid. Chondrule formation thus appears to have persisted during the period in which ambient nebular gas cooled from ~1120 K to <650 K. During this period, many chondrules were heated multiple times [2]. Chondrule-formation mechanisms that satisfy these constraints include nebular lightning, solar flares and nebular shock waves.