What Kind of Asteroids Impacted the Earth During the PaleoProterozoic?

Chromium isotope evidence in ejecta deposits for the nature of Paleoproterozoic impactors

Authors:

Mougel et al

Abstract:

Non-mass dependent chromium isotopic signatures have been successfully used to determine the presence and identification of extra-terrestrial materials in terrestrial impact rocks. Paleoproterozoic spherule layers from Greenland (Graenseso) and Russia (Zaonega), as well as some distal ejecta deposits (Lake Superior region) from the Sudbury impact (1,849 +/- 0.3 Ma) event, have been analyzed for their Cr isotope compositions. Our results suggest that 1) these distal ejecta deposits are all of impact origin, 2) the Graenseso and Zaonega spherule layers contain a distinct carbonaceous chondrite component, and are possibly related to the same impact event, which could be Vredefort (2,023 +/- 4 Ma) or another not yet identified large impact event from that of similar age, and 3) the Sudbury ejecta record a complex meteoritic signature, which is different from the Graenseso and Zaonega spherule layers, and could indicate the impact of a heterogeneous chondritic body.

Just how big WAS the PaleoArchean Vredefort Impact?

A massive asteroid almost as wide as Rhode Island and about three to five times larger than the rock thought to have wiped out the dinosaurs slams into Earth. The collision punches a crater into the planet's crust that's nearly 500 kilometers (about 300 miles) across: greater than the distance from Washington, D.C. to New York City, and up to two and a half times larger in diameter than the hole formed by the dinosaur-killing asteroid. Seismic waves bigger than any recorded earthquakes shake the planet for about half an hour at any one location – about six times longer than the huge earthquake that struck Japan three years ago. The impact also sets off tsunamis many times deeper than the one that followed the Japanese quake.

Although scientists had previously hypothesized enormous ancient impacts, much greater than the one that may have eliminated the dinosaurs 65 million years ago, now a new study reveals the power and scale of a cataclysmic event some 3.26 billion years ago which is thought to have created geological features found in a South African region known as the Barberton greenstone belt. The research has been accepted for publication in Geochemistry, Geophysics, Geosystems, a journal of the American Geophysical Union.

The huge impactor – between 37 and 58 kilometers (23 to 36 miles) wide – collided with the planet at 20 kilometers per second (12 miles per second). The jolt, bigger than a 10.8 magnitude earthquake, propelled seismic waves hundreds of kilometers through the Earth, breaking rocks and setting off other large earthquakes. Tsunamis thousands of meters deep – far bigger than recent tsunamis generated by earthquakes -- swept across the oceans that covered most of the Earth at that time.

"We knew it was big, but we didn't know how big," Donald Lowe, a geologist at Stanford University and a co-author of the study, said of the asteroid.

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New Type of Evidence for Ancient Impacts?

Discovery of mafic impact melt in the center of the Vredefort dome: Archetype for continental residua of early Earth cratering?

Authors:

Cupelli et al

Abstract:

Melting by impact heating is thought to have been a significant process in the modification of early planetary crusts; however, crustally derived melt bodies in ancient terrestrial crust are frequently presumed to be absent due to erosion. Here we demonstrate that in the central basement uplift of the 2.020 Ga Vredefort impact basin (South Africa), components of mafic impact melt have survived amid Archean gneiss as decimeter-scale dikes and lenses of variably foliated gabbronorite. Zircon microstructural, trace element, and isotopic analyses (U-Pb, Lu-Hf) of the gabbronorite reveal a dominant population of 2.02 Ga unshocked igneous zircon with apparent Ti-in-zircon temperatures of 800–900 °C, similar to those from the mafic sublayer of the Sudbury impact melt sheet. Highly negative subchondritic εHf values of –1.4 ± 1.1 to –7.9 ± 1.4 are consistent with a depleted mantle model age of ca. 3 Ga and gabbronorite derivation from the once superjacent Witwatersrand basin lithologies. The recrystallized igneous mineral textures and Archean felsic gneiss inclusions in the gabbronorite are attributable to the effects of emplacement and crater modification following ∼20 km elevation of the central uplift. Long mistaken as preimpact basement, the setting and characteristics of the Vredefort gabbronorite may provide new benchmarks in the search for remnants of large cratering events and melt residua on Earth's cratons.

Possible Spherule Beds From Orosiria PaleoProterozoic Vredefort Impact

Impact spherules from Karelia, Russia: Possible ejecta from the 2.02 Ga Vredefort impact event

Authors:

Huber et al

Abstract:

Spherule beds of possible impact origin have been discovered in two drill cores in the Paleoproterozoic Zaonega Formation, Karelia, northwest Russia. Spherules are found within the dolomite matrix of in-situ brecciated sedimentary dolostones. Spherules are millimeter size and generally round, although teardrop and dumbbell morphologies are present. Spherules contain up to 0.75 ppb Ir, with a Ru/Ir of 2, indicating a mixing of target rocks with a minor chondritic component. The age of the Zaonega Formation is constrained between limits of 1.975 ± 0.024 Ga (Sm-Nd) and 1.980 ± 0.057 Ga (Pb-Pb), and 2.050 Ga (Re-Os), which brackets the age of the 2020 Ma Vredefort impact structure in South Africa, and suggests that the spherule beds could represent ejecta from that event. If the link is confirmed, the size of the spherules and thickness of the beds suggest that the distance from the impact site was less than 2500 km, thereby constraining the paleogeographic distance between the Fennoscandian Shield and Kaapvaal craton during the late Paleoproterozoic.