Metasomatism in the early solar system: The record from chondritic meteorites

Abstract

Mineralogic, petrologic, and isotopic studies of chondritic meteorites have revealed a significant body of evidence of metasomatic processes during the earliest stages of Solar System evolution. The exact nature of these processes, as well as the conditions and environments where metasomatism occurred, are still the subject of vigorous debate. The interaction of aqueous fluids with early Solar System solids affected different chondrite groups to different degrees: even within a single chondrite group the effects of metasomatism can be highly variable. Among the carbonaceous chondrite groups, the CV (Vigarano-type) and CO (Ornans-type) chondrites show the best documented evidence of metasomatic effects. In the oxidized subgroup of the CV chondrites, Ca-Al-rich Inclusions (CAIs), Amoeboid Olivine Aggregates (AOAs), chondrules, and matrix have all been extensively affected by Fe-alkali-halogen metasomatism, that has resulted in the formation of a wide range of secondary, dominantly anhydrous minerals, including grossular, andradite, wollastonite, monticellite, anorthite, forsterite, ferroan olivine, corundum, Na-melilite, nepheline, sodalite, wadalite, Al-diopside, kushiroite, ferroan diopside − hedenbergite pyroxenes, ilmenite, phosphates, magnetite, awaruite, tetrataenite, and Fe,Ni sulfides. Hydrous phases are much rarer, but include, margarite, vesuvianite, and kaolinite. The mineral assemblages that form are highly dependent on the primary mineralogy of the host object: distinct mineral assemblages are produced by alteration of CAIs, chondrules, and matrix, for example. Nebular and asteroidal scenarios for these metasomatic effects have been extensively discussed in the literature for the metasomatism observed in the CV chondrites. Oxygen isotopic studies of the secondary minerals in CV chondrites, such as fayalite, magnetite and Ca, Fe-rich silicates, indicate formation at relatively low temperatures (<550 K) from aqueous solutions, consistent with an asteroidal environment. On the other hand, primary minerals in CAIs show oxygen isotopic heterogeneity, with melilite and anorthite exhibiting heavy isotope enrichments compared with spinel, hibonite, Al,Ti-diopside, and forsterite. The origin of this selective isotopic exchange is still the subject of debate; it may have occurred by gas–solid or gas–melt exchange in the solar nebula or by isotopic exchange with a 16O-depleted fluid in an asteroidal environment. The CO chondrites show significant evidence of metasomatic events, but the degree and extent of metasomatism is much less than that for the CV chondrites. Calcium-rich phases, such as melilite, plagioclase, and glassy mesostasis in CAIs, AOAs and chondrules have been affected the most and have been replaced by fine-grained alteration products. Although the secondary minerals in CO chondrites have not been characterized in as much detail as the CV chondrites, nepheline, sodalite, ilmenite, ferroan olivine, and ferroan diopside − hedenbergite pyroxenes have all been positively identified. Collectively the data indicate that Fe-alkali metasomatism has also affected the CO chondrites, but the involvement of halogens is much less extensive. Although the alteration of CAIs, AOAs and chondrules is extremely heterogeneous in CO chondrites, there is a general correlation between the degree of metasomatism and metamorphism, indicating that the metasomatism occurred dominantly within an asteroidal environment. However, some rare CAIs in type 3.0 CO chondrites and chondrules in higher petrologic CO3 subtypes contain metasomatic effects that may be best explained by alteration prior to asteroidal accretion. In comparison, the ordinary chondrites (H, L, and LL) show minimal evidence of metasomatic effects. Only a few unequilibrated ordinary chondrites show evidence of highly localized and minimal development of nepheline, sodalite, and scapolite that occurs only with chondrules that contain Al-rich phases such as plagioclase. The most extensively metasomatized ordinary chondrite is Tieschitz (H3.6), which contains a highly unusual component of matrix consisting of veins of nepheline and albite interstitial to chondrules. Chondrule glass in this meteorite has been leached extensively of alkalis and Al, indicating extensive interaction with an aqueous fluid. Finally, new evidence is coming to light which suggests that some more highly metamorphosed ordinary chondrites may have undergone metasomatism, indicated by partial albitization of plagioclase, formation of ferroan olivine replacing low-Ca pyroxene and remobilization of phosphate minerals. These limited data suggest that mineral-fluid interactions in the ordinary chondrites occurred late in the metamorphic history of these meteorites. Metasomatic effects may be much more extensively developed in the ordinary chondrites, but are cryptic in nature and have yet to be recognized. Research on the role of fluids in the geologic evolution of the ordinary chondrite parent bodies is therefore still in its infancy.