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Late Archean impacts
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Asteroid Model Shows Early Life Suffered a Billion-Year Battering

Life was hard on early Earth, but geologists and planetary scientists at the Lunar and Planetary Science Conference reported evidence of even more biotic stress several billion years ago: a prolonged pummelling by huge asteroids that would have dwarfed the one that killed off the dinosaurs. The known impact beds suggest that there was a large impact every 40 million years on average in the Archean eon - the time from 3.8 billion years ago to 2.5 billion years ago - not every 500 million years, as has lately been the case.
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LATE ARCHEAN IMPACT
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Title: Geochemical evidence for an impact origin for a Late Archean spherule layer, Transvaal Supergroup, South Africa
Authors: Bruce M. Simonson, Christian Koeberl, Iain McDonald, Wolf Uwe Reimold

ABSTRACT
A Late Archean layer rich in sand-sized spherules of former silicate melt in the Monteville Formation (Transvaal Supergroup, South Africa) has Ir concentrations as high as 6.4 ppb and is clearly enriched in Ir relative to associated tuffs, carbonates, and shales. The Monteville spherule layer is also enriched in other siderophile elements, including the platinum group elements (PGEs). The PGEs in the spherule layer produce a flat (meteorite like) pattern when they are normalised to chondritic abundances. The abundances of Ir and other siderophile elements are similar to broadly contemporaneous spherule layers in the Hamersley basin of Western Australia. That the mineral compositions, textures, and sedimentary structures of the spherule layers in the Transvaal Supergroup and Hamersley basin are also very similar suggests that they were all formed by the same processes. We think that the best way to explain the high Ir concentrations and other characteristics of the Monteville spherule layer is that it represents distal impact ejecta. There are, however, significant differences between the Monteville spherule layer and Early Archean spherule layers in the Barberton greenstone belt, including much higher average and maximum concentrations of Ir in the latter. The data presented here clearly show that each Precambrian spherule layer is unique and needs to be characterised individually, as is true for the impact spherule layers of the Phanerozoic.

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Title: HOW MANY LATE ARCHEAN IMPACTS ARE RECORDED IN THE HAMERSLEY BASIN OF WESTERN AUSTRALIA?
Authors: B. M. Simonson, S. W. Hassler, J. Smit, and D. Sumner

Previous work: Various layers rich in sand-size spherules of former silicate melt and interpreted as impact ejecta have been identified in Precambrian successions. About half of the layers occur in the Hamersley basin of Western Australia, but there has been some uncertainty as to how many impacts they represent. One of the Hamersley spherule layers was described as early as 1966 [3], but they were not connected with impacts until much later. Shocked minerals have yet to be reported from any of the spherule layers, but an impact origin is indicated by numerous lines of geologic and geochemical evidence. Spherule layers were initially discovered in three units in the Hamersley basin: the Wittenoom Formation, the Carawine Dolomite, and the Dales Gorge Member of the Brockman Iron Formation. The Dales Gorge layer is the youngest and probably early Paleoproterozoic in age, whereas the Wittenoom and Carawine layers both formed in the late Achaean. The Wittenoom only occurs in the main part of the Hamersley basin whereas the Carawine is restricted to a separate area to the east known as the Oakover River area. The Wittenoom and Carawine are thought to be contemporaneous because they each represent the only thick succession of carbonates in their respective parts of the Hamersley basin. Given the rarity of impact ejecta layers and the petrographic similarity of the spherules, we interpreted the Wittenoom and Carawine spherule layers as products of a single impact which constituted a time plane across the Hamersley basin. This correlation was problematic because one layer is stratigraphically high in the Wittenoom whereas the other layer is stratigraphically low in the Carawine. Further searching yielded an additional spherule layer near the top of the stratigraphically lower Jeerinah Formation. The Jeerinah Formation extends throughout the entire Hamersley basin. We discovered the spherule layer in drill core FVG-1 in the eastern part of the main Hamersley basin and subsequently located it in a surface exposure at Hesta 60 km to the northwest. The layer is very different at these two sites, and in July 2001 we discovered another surface exposure of a spherule layer near the top of the Jeerinah which is again different from the other occurrences and the focus of this report.

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