ALBERT

Description: Petrographic and geochemical profiles collected through a ~250-m-thick Nipigon diabase sill near Black Sturgeon Lake, Ontario, constrain the complex emplacement and differentiation processes that controlled the magmatic evolution of these rocks: several nested reinjections of phenocryst-bearing magma, compactiondriven redistribution of mildly evolved interstitial liquids in the central portion of the sill, and segregation of highly-evolved interstitial liquids into micropegmatitic veins near the upper contact. These processes impact not only the silicate mineralogy of the rocks, but also distribution of sulfide phases. Both petrographic and geochemical data demonstrate that the sill was gradually inflated via several discrete injections of olivine-bearing magma. Plagioclase crystal size trends exhibit several reversals that are correlated with increases in olivine abundance and bulk-rock MgO content. These textural and compositional reversals are attributed to reinjections of olivine-bearing magma into partially solidified magma from earlier injections. Mixing between evolved resident magma and reinjections of less-evolved magma is commonly proposed as a mechanism for triggering sulfide mineralization. In this sill, which contains no significant mineralization, localized minor enrichments and depletions in copper content are associated with the margins of a well-constrained reinjection horizon. Chemical differentiation of the magma was controlled by three distinct processes: flow differentiation, crystal-mush compaction, and solidification front instability. Although the mineralogy of the sill is dominated by plagioclase and clinopyroxene, major-oxide compositional variations are largely controlled by the mechanical concentration of olivine via flow differentiation or phenocryst redistribution within the individual reinjection pulses. This redistribution of phenocryst phases is the primary factor controlling the major-element composition of the rocks in most of the sill. In the central portion of the sill, the abundances of incompatible trace elements are influenced by the compaction of partially crystalline mush and the expulsion of incompatible-rich interstitial liquid. This is recognized by a negative correlation between the alignment of plagioclase crystals and concentrations of incompatible trace elements such as Zr. Finally, there is a ~2.5-m-thick silicic segregation approximately 32 m below the upper contact, whose composition is consistent with residual liquid after ~65 to 85% fractional crystallization of magma with a composition equivalent to that of the marginal rocks. Similar veins and patches are common throughout the upper 50 m of the sill, indicating that this segregation process was common and widespread during the cooling and crystallization of the sill. The complexity of intrusion and differentiation in this sill, despite its generally unremarkable texture and composition, suggests that many diabase sills may have experienced a range of processes that have previously been recognized to impact the petrologic evolution of larger igneous systems, and that these processes can be identified in minor intrusions such as sills and dikes using simple and robust analytical techniques.

Description: The Beacon Sill, a member of the Jurassic Ferrar group of the Transantarctic Mountains, South Victoria Land, is a 150 m thick tholeiitic diabase intrusion. Uniform on a field and macroscopic scale, it displays chemical and textural variability indicative of a sustained and complex history of emplacement and differentiation. Emplacement of the sill consisted of at least two, and probably four, discrete massive injections of magma, each averaging ~35 m thick, over a time span of ~100 years. The final injection event is marked by a 30 m thick interval of significantly finer-grained rock at the sill center. These fine-grained textures can be successfully reproduced by combining the results of thermal and crystal growth models, but only if the sill was a multiple intrusion. With the exception of a minor reinjection event captured in the chilled margin of the sill, earlier reinjection events are not evident texturally, most probably because of textural overprinting during prolonged cooling after initial crystallization. The dominant process involved in the post-emplacement differentiation of the Beacon Sill was compaction-driven redistribution of interstitial liquid. Transfer of residual liquid from the compacting lower solidification front to the dilating upper solidification front resulted in characteristic chemical and mineralogical effects, such as the depletion of the lower half of the sill and the enrichment of the upper half of the sill in incompatible elements (e.g. TiO 2 , Zr) and modal granophyre. Based on thermodynamic models, most of the compaction occurred at a crystallinity of roughly 33%. Geochemical profiles are distinctly segmented, suggesting that the sill was repeatedly split and reinjected with fresh magma near the center after compaction had redistributed the interstitial liquid within the partially solidified magma. The recognition of evidence for reinjection and compaction in a macroscopically uniform, relatively quickly cooled sill suggests that these processes may be common in the construction of sills and other mesoscale igneous intrusions in general.

Description: Most chondrules preserve, in their texture, a record of their precursor material and the nature and intensity of the nebular events in which they were formed. We have used crystal size distributions (CSDs) together with crystallization experiments to explore the textures of natural chondrules and their relationships to natural formation conditions. Many careful experimental studies have demonstrated fundamental relationships between the texture of a chondrule and its precursor material, melting intensity, and cooling rate. However, until recently most studies have focused on distinctions between textural types (e.g., barred, radial, porphyritic). Previous work has shown that CSDs provide precise, reproducible characterizations of chondrule-scale textures, and can be used together with crystallization experiments to estimate chondrule formation conditions. Here, we expand this study to investigate the link between texture and formation conditions for a range of natural porphyritic olivine (PO) chondrules.