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Project Progress

Understanding how quickly planetary surface environments evolve on newly accreted worlds is critical for predicting when habitable conditions are established (Cavosie, 2014). The meteorite impact history of the inner solar system strongly indicates that the Earth was subject to a global impact bombardment during the first few hundred million years after accretion. The scope, timing, and consequences of this profound event are hotly debated. In 2015 this project focused on the continued search for detrital shocked zircons in the Precambrian rock record; applications of detrital shocked zircons for dating the lunar impact history; a search for shocked zircons at a deformed impact crater; and concluded with a serendipitous discovery of a new locality for the rare high pressure ZrSiO₄ polymorph reidite.

Searching the Precambrian detrital zircon record- An example from the Superior Craton, Canada
In 2015, we investigated a suite of 1,000 detrital zircons from the Chelmsford Formation, a Precambrian turbidite deposited at ~1.75 Ga into the Sudbury impact basin in Ontario Canada. The rationale for investigating these zircons for the presence of shock microstructures is that the turbidite is a proximal deposit inside an impact basin, and thus has a high likelihood of containing shocked minerals from the local bedrock. Identification of detrital shocked zircon in this deposit would demonstrate the preservation of detrital shocked zircons in rocks that were deposited and lithified nearly 2 billion years ago. Instead, our survey of 1,000 grains by SEM did not yield any grains with shock microstructures. One interpretation is that the contribution from local rocks was limited to the Sudbury igneous complex, which does not contain shocked zircons. Another possibility is that the zircons are not locally derived.

A critical test for chronology of impacts on the Moon from detrital shocked zircons
One important and largely unconstrained aspect of the impact history of the Earth is the timing and flux of early impacts. As there are no intact Hadean-age impact craters on Earth, the impact history of the Moon is often used as a proxy for when Earth experienced large impacts. Many studies have interpreted U-Pb ages of lunar zircons as recording ages of impacts on the Moon. This assumes that U-Pb ages of lunar zircons were shock-reset and record the impact age. Our group has studied the U-Pb ages of shock-twinned detrital zircons eroded from the Vredefort Dome impact in South Africa (Fig. 1), and found that the vast majority of grains (>99%) do not record the known age of impact (2.02 Ga), and instead record bedrock crystallization age. Bedrock ages were found in 100% of detrital shocked zircons that contained various types of planar microstructures (Cavosie et al., 2015a). An impact age was only found in one grain, which preserved a microstructure of neoblastic zircon, formed as a consequence of recrystallization due to the high temperatures of impact. The results of this study demonstrate that ages of lunar zircons with planar microstructures are equivocal, and can not be assumed to record impact events in the absence of other microstructure evidence. These findings highlight the limitations of zircon for recreating the lunar impact history.

A search for shocked zircon at a deformed impact structure
One reality when searching the Precambrian or Archean record for evidence of ancient impact events is that little crust from this time remains intact. To date, there are no examples impact structure in rocks that have experienced tectonic deformation and/or metamorphism, as might be encountered in the Precambrian. To develop methods designed for this type of exploration, we conducted a search for shocked zircons at a tectonically deformed impact structure where they had not previously been reported, and chose the Santa Fe impact structure in New Mexico (USA). The Santa Fe structure is ideal, as it is a confirmed impact structure based on the presence of shocked quartz and shatter cones (Fackelman et al., 2008), but it is has been tectonically dismembered, and effectively only exists as one ~2 × 6 km fault block in near Santa Fe. There is no circular structure, no central uplift, no crater rim, no geophysical anomalies, or any other distinguishing feature beyond shocked quartz and shatter cones along a ~2 km roadcut in the area. Both the size (6-13 km) and age (1200-400 Ma) are poorly constrained.

In 2015, a graduate student in charge of this project, Pedro Montalvo, surveyed >6,000 detrital zircons from sediment samples collected from this mountainous area, and confirmed the presence of shocked zircons in this population. The implication of these results for studies of Precambrian rocks is that detrital shocked zircons can provide a record of impact craters that have not only eroded, but also ones that have been tectonically dismembered and may thus not be recognizable at the outcrop scale.

Discovery of the ultra-rare high pressure ZrSiO₄ polymorph reidite at Rock Elm.
The search for detrital shocked minerals in the Precambrian sedimentary record necessarily involves detailed investigation of siliciclastic rocks, generally sandstone, and the detrital zircons they contain. A field site in western Wisconsin, the Rock Elm impact crater, is a ~6 km diameter Ordovician age impact into the Cambrian Mt. Simon sandstone (French et al., 2004), and thus offers an opportunity to study shock metamorphism of sandstone at a deeply eroded impact structure, as might have once existed and perhaps been preserved in the Precambrian rock record.

During a field trip with NAI summer intern students at the Rock Elm impact structure in 2013, a sample of a complex polymictic impact breccia was collected to investigate if it contained shocked quartz. A petrographic study reporting shock-deformed quartz was presented at the LPSC in 2014 (Arana and Cavosie, 2014). Further investigation of the breccia sample revealed the presence of numerous, <50 µm-sized detrital zircons. A survey of ~100 zircons in the thin section revealed that one was intergrown with sub-um lamellae of a high atomic number phase (Fig. 2).

EBSD mapping of this phase confirmed that it is reidite, the rare high pressure polymorph that forms at pressures >30 GPa.