Doreen E. Ames

Dating of a regional hydrothermal system induced by the 1850 Ma Sudbury impact event

New U-Pb geochronologic data from the Sudbury structure directly link extensive hydrothermal activity in crater-fill breccias with the 1850 Ma impact event, and constrain complex impact-induced processes to <4 m.y. Semiconformable alteration of breccias in the Onaping Formation includes silicification, albitization, chloritization, calcitization, and complex feldspathization, which directly underlies Zn-Cu-Pb ore deposits. Geochronologic data demonstrate a middle to lower crustal source for the crater-fill deposit that was subsequently affected by the hydrothermal event at 1848.4 +3.8/−1.8 Ma. These and other data confirm that the crater-fill breccia, hydrothermal system, Sudbury igneous complex, and its sublayer Ni-ore-bearing units were emplaced as a result of impact. The ages of events associated with the Sudbury structure are now more closely constrained than in any other meteorite impact structure.

Geochemical evidence from the Sudbury structure for crustal redistribution by large bolide impacts

Deformation and melting of the crust during the formation of large impact craters must have been important during the Earths early evolution, but such processes remain poorly understood. The 1.8-billion-year-old Sudbury structure in Ontario, Canada, is greater than 200 km in diameter and preserves a complete impact section, including shocked basement rocks, an impact melt sheet and fallback material. It has generally been thought that the most voluminous impact melts represent the average composition of the continental crust, but here we show that the melt sheet now preserved as the Sudbury Igneous Complex is derived predominantly from the lower crust. We therefore infer that the hypervelocity impact caused a partial inversion of the compositional layering of the continental crust. Using geochemical data, including platinum-group-element abundances, we also show that the matrix of the overlying clast-laden Onaping Formation represents a mixture of the original surficial sedimentary strata, shock-melted lower crust and the impactor itself.

Impact-Generated Hydrothermal System — Constraints from the Large Paleoproterozoic Sudbury Crater, Canada

The 1848 Ma impact-generated hydrothermal system in the ∼200-km-diameter Sudbury structure in Canada is sexceptionally well preserved and provides the opportunity to study potential fossil ecosystems associated with impact craters. The hydrothermal alteration fingerprint at the Sudbury impact site is preserved for ∼1 km below the melt sheet and ∼2 km above. The system was capable of producing sufficient heat and fluid flow to form sinter deposits on the crater-floor. Fluid-rock interaction and resultant alteration mineral products record the waxing and waning phases of a complex hydrothermal system within the impact crater with temperatures in the basin ranging from 250–300°C down to ambient. Below the melt sheet fluid-rock interaction took place at <420°C. The exceptional preservation of the Sudbury impact structure including fractured and shocked basement rocks, melt sheet, impact-related crater-fill breccias, chemical sediments on the crater-floor and post impact sedimentation, yields significant new insights into the physical, chemical and potentially the biological framework of impact-generated hydrothermal systems in large craters. Significant to the development of microbial niches is defining the lower temperature regimes (<120°C) of the habitable zone. In the Sudbury basin from base to top, lies a 1.4-km-thick sequence of suevite (Onaping Formation) that has undergone extensive water-rock interaction manifested as regionally extensive semiconformable alteration zones, a thin ∼ 14-m-thick exhalative-sedimentary sequence (Vermilion Formation) and in a metal-enriched hydrothermal plume extending another <1 km into the post-impact basin sediments (Onwatin Formation). The hydrothermal signature includes basin-wide semiconformable alteration zones defined by silicification, albitization, carbonatechlorite alteration in the Onaping Formation. Also present are discordant alteration zones with focussed fluid flow which produced local higher temperature perturbations imposed on the more extensive lower temperature (<250°C) alteration zones within the crater-fill sequence. The Vermilion Formation represents a subaqueous hydrothermal vent complex with a proximal hydrothermal Ca-Fe-Mg-Mn carbonate mound facies containing replacement type Zn-Pb-Cu-Fe mineralization, a distal finely laminated carbonate facies, or “carbonate-facies iron formation”, buried by distal turbidite sediments. Prolonged post-mineralization diffuse fluid flow and unfocussed low temperature emanation of hydrothermal plumes and the Fe-Mn-rich distal carbonates produce favourable habitats for thermophilic microorganisms.

Chicxulub impact ejecta deposits in southern Quintana Roo, México, and central Belize

Discoveries of Chicxulub impact ejecta of the Albion Formation in road cuts and quarries in southern Quintana Roo, Mexico and Belize, broaden our understanding of ejecta depositional processes in large impacts. There are numerous new exposures of ejecta near the Rio Hondo in Quintana Roo Mexico, located at distances of 330– 350 km from the center of the Chicxulub crater. A single ejecta exposure was discovered near Armenia in central Belize, 470 km from Chicxulub. The Albion Formation is composed of two lithostratigraphic units: the spheroid bed and diamictite bed, originally identifi ed at Albion Island, Belize. The new spheroid bed exposures range from

Protracted volcanism after large impacts: evidence from the Sudbury impact basin

Morphological studies of large impact structures on Mercury, Venus, Mars, and the Moon suggest that volcanism within impact craters may not be confined to the shock melting of target rocks. This possibility prompted reinvestigation of the 1.85Ga subaqueous Sudbury impact structure, specifically its 1.5km thick immediate basin fill (Onaping Formation). Historically, breccias of this formation were debated in the context of an endogenic versus an impact-fallback origin. New field, petrographic, and in situ geochemical data document an array of igneous features, including vitric shards, bombs, sheet-like intrusions, and peperites, preserved in exquisite textural detail. The geochemistry of vitric materials is affected by alteration, as expected for subaqueous magmatic products. Earlier studies proposed an overall andesitic chemistry for all magmatic products, sourced from the underlying impact melt sheet. The new data, however, suggest progressive involvement of an additional, more magnesian, and volatile-rich magma source with time. We propose a new working model in which only the lower part of the Onaping Formation was derived by explosive melt-fuel-coolant interaction when seawater flooded onto the impact melt sheet in the basin floor. By contrast, we suggest that the upper 1000m were deposited during protracted submarine volcanism and sedimentary reworking. Magma was initially sourced from the impact melt sheet and up stratigraphy, from reservoirs at greater depth. It follows that volcanic deposits in large impact basins may be related to magmatism caused by the impact but not directly associated with the impact-generated melt sheet.