A petrographic survey of > 1600 chondrules in thin‐sections of 12 different mildly to highly unequilibrated H‐, L‐, and LL‐chondrites, as well as morphological and textural study of 141 whole chondrules separated from 11 of the same chondrites, was used to determine the relative abundances of definable chondrule primary textural types. Percentage abundances of various chondrule types are remarkably similar in all chondrites studied and are ∼ 47–52 porphyritic olivine‐pyroxene (POP), 15–27 porphyritic olivine (PO), 9–11 porphyritic pyroxene (PP), 3–4 barred olivine (BO), 7–9 radial pyroxene (RP), 2–5 granular olivine‐pyroxene (GOP), 3–5 cryptocrystalline (C), and ≤ 1 metallic (M). Neither chondrule size nor shape is strongly correlated with textural type. Compound and cratered chondrules, which are interpreted as products of collisions between plastic chondrules, comprise ∼ 2–28% of nonporphyritic (RP, GOP, C) but only ∼ 2–9% of porphyritic (POP, PO, PP, BO) chondrules, leading to a model‐dependent implication that nonporphyritic chondrules evolved at number densities (chondrules per unit volume of space) which were 102 to 104 times greater than those which prevailed during porphyritic chondrule formation (total range of ∼ 1 to ∼ 106 m−3). Distinctive “rims” of fine‐grained sulfides and/or silicates occur on both porphyritic and nonporphyritic types and appear to post‐date chondrule formation. Apparently, either the same process(es) contributed chondrules to all unequilibrated ordinary chondrites or, if genetically different, the various chondrule types were well mixed before incorporation into chondrites. Melting of pre‐existing materials is the mechanism favored for chondrule formation.
The Lafayette meteorite, a nakhlite of the SNC (Martian?) group, contains hydrous alteration materials as intergranular films and as veinlets and patches replacing olivine, pyroxenes, and high-Si glass. The alteration materials ("iddingsite") consist of ferroan smectite clays, magnetite (or maghemite), and femhydrite, as shown by SEM and TEM. Three varieties of veinlets are present and formed in the order: coarse phyllosilicate; fine-grained (phyllosilicate-oxide), and porous oxide. Veinlets of finegrained material cross-cut coarse phyllosilicate veinlets. The alteration materials are preterrestrial, as they are older than Lafayette's fusion crust, which is glassy and not affected by any alterations. Approaching the crust, the veinlets are progressively modified to the point of melting, and progressively depleted in adsorbed volatile constituents (S, C1, and P). Bulk compositions of the alteration veinlets (SEM and TEM EDX) are consistent with the observed mineralogy, and suggest: that the smectite contains significant adsorbed S and C1; that the femhydrite contains significant adsorbed S, but not C1; that rare grains of sulfate (a?) and chloride (Na or K?) are present; and that the compositions of the alteration materials are approximated by Lafayette's olivine + high-Si glass + water. We estimate that Lafayette's alteration materials formed at less than 100 "c. The oxidation potential of the water was near or slightly below that of the magnetite-hematite buffer. The presence of sulfate and chloride in discrete phases and as adsorbates on femhydrite and smectite suggests that the altering solutions were saline. However, relatively little mass was transferred into or out of Lafayette because the bulk composition of the alteration materials is nearly isochemical with a mixture of magmatic silicate phases and water. Chemical transport within Lafayette was also limited, as alteration materials preserve some chemical signature of their host minerals. Presence of alteration materials along only some grain boundaries and some cracks suggests that Lafayette was not soaked in fluid. These last two inferences suggest that the alteration of Lafayette took place during episodic infiltrations of small volumes of saline water.
Abstract— Interior samples of three different Nakhla specimens contain an iron‐rich silicate “rust” (which includes a tentatively identified smectite), Ca‐carbonate (probably calcite), Ca‐sulfate (possibly gypsum or bassanite), Mg‐sulfate (possibly epsomite or kieserite), and NaCl (halite); the total abundance of these phases is estimated as <0.01 weight percent of the bulk meteorite. Rust veins are truncated and decrepitated by fusion crust and are preserved as faulted segments in partially healed olivine crystals, indicating that the rust is pre‐terrestrial in origin. Because Ca‐carbonate and Ca‐sulfate are intergrown with the rust, they are also indicated to be of pre‐terrestrial origin. Similar textural evidence regarding origins of the NaCl and Mg‐sulfate is lacking. Impure and poorly crystallized sulfates and halides on the fusion crust of the meteorite suggest leaching of interior (pre‐terrestrial) salts from the interior after Nakhla arrived on Earth but coincidental addition of these same salts by terrestrial contamination cannot be excluded. At least the clay‐like silicate “rust,” Ca‐carbonate, and Ca‐sulfate were formed by precipitation from water‐based solutions on the Nakhla parent planet although temperature and pressure conditions of aqueous precipitation are unconstrained by currently available data. It is possible that aqueous alteration on the parent body was responsible for the previously observed disturbance of the Rb‐Sr geochronometer in Nakhla at or near 1.3 Ga.
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