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Phosphate minerals in ordinary chondrites provide a record of fluids that were present during metamorphic heating of the chondrite parent asteroids. We have carried out a petrographic study of the phosphate minerals, merrillite and apatite, in metamorphosed H group ordinary chondrites of petrologic type 4-6, to understand development of phosphate minerals and associated fluid evolution during metamorphism. In unbrecciated chondrites, apatite is Cl rich and shows textural evolution from fine-grained apatite-merrillite assemblages in type 4 toward larger, uniform grains in type 6. The Cl/F ratio in apatite shows a similar degree of heterogeneity in all petrologic types, and no systematic change in compositions with metamorphic grade, which suggests that compositions in each meteorite are dictated by localized conditions, possibly because of a limited fluid/rock ratio. The development of phosphate minerals in H chondrites is similar to that of L and LL chondrites, despite the fact that feldspar equilibration resulting from albitization is complete in H4 chondrites but not in L4 or LL4 chondrites. This suggests that albitization took place during an earlier period of the metamorphic history than that recorded by preserved apatite compositions, and chemical equilibrium was not achieved throughout the H chondrite parent body or bodies during the late stages of metamorphism. A relict igneous clast in the H5 chondrite, Oro Grande has apatite rims on relict phenocrysts of (possibly) diopside that have equilibrated with the host chondrite. Apatite in the Zag H3-6 regolith breccia records a complex fluid history, which is likely related to the presence of halite in this meteorite. The porous dark H4 matrix of Zag, where halite is observed, has a high apatite/merrillite ratio, and apatite is extremely Cl rich. One light H6 clast contains similarly Cl-rich apatite. In a second light H6 clast, apatite compositions are very heterogeneous and more F-rich. Apatites in both H4 matrix and H6 clasts have very low H2O contents. Heterogeneous apatite compositions in Zag record multiple stages of regolith processing and shock at the surface of the H chondrite parent body, and apatite records either the passage of fluids of variable compositions resulting from different impact-related processes, or the passage of a single fluid whose composition evolved as it interacted with the chondrite regolith. Unraveling the history of apatite can potentially help to interpret the internal structure of chondrite parent bodies, with implications for physical and mechanical properties of chondritic asteroids. The behavior of halogens recorded by apatite is important for understanding the behavior of volatile elements in general: if impact-melt materials close to the surface of a chondritic asteroid are readily degassed, the volatile inventories of terrestrial planets could be considerably more depleted than the CI carbonaceous chondrite abundances that are commonly assumed.

Magnesium-rich forsterite grains (>Fo98) in FeO-poor chondrules show variations in cathodoluminescence (CL) intensity that are associated closely with variations in Al2O3and TiO2contents. CL thus can be used to map the distribution of Al and Ti in individual grains. We describe several categories of CL distribution, including grains that are homogeneous, grains that are zoned continuously, grains that show oscillatory zoning, and grains that show a heterogeneous distribution of CL intensity. Heterogeneous CL intensity is interpreted as the result of disequilibrium growth from the chondrule melt. CL surveys of individual chondrules help to identify forsterite grains that are potentially relics in their host chondrules, i.e., grains that did not crystallize in situ from the chondrule melt. Several of these grains show minor-element variations that differ from the majority of melt-grown grains in their host chondrules: this observation supports the inference that they are relics. Little evidence exists for growth of individual grains in multiple brief heating episodes. Identification of the abundance of relict grains is important for determining the thermal histories of chondrules as well as the nature of the precursor assemblages from which they formed.

We present minor and trace element (REE, Sr, Y, and Zr) data for pyroxenes and mesostases in four porphyritic chondrules from the Semarkona ordinary chondrite. Apparent partition coefficients for clinoenstatite, orthoenstatite, pigeonite, and augite are compared with experimental and petrologic data from the literature, and the effects on apparent partition coefficients of the rapid cooling rates at which chondrules crystallized are evaluated. For most elements, the effects of cooling at rates of hundreds of degrees per hour cannot be distinguished from variations in equilibrium data resulting from differences in temperature or composition. However, for LREE apparent partition coefficients are significantly higher than comparable equilibrium data, and the ratio of HREE/LREE partition coefficients is lower, particularly for Ca-poor pyroxene. We attribute this flattening of REE patterns to the effect of rapid cooling. Apparent partition coefficients of all REE and Y in augite are higher than equilibrium data, particularly in one chondrule with a high Al2O3content. We suggest that this may be attributed to an increase in the uptake of trivalent trace element cations in the pyroxene crystal structure as a result of charge-balanced substitutions with Al3+cations.