WESTERN AUSTRALIAN DIVISION
West Australian Geologist (WAG)
Bi-monthly newsletter of the Western Australian Division of the Geological Society of Australia Inc.
Number 513: August ̶ September 2015 (2.8 Mb PDF file)
Time: 5.30 pm for 6.00 pm formal start (bar open upstairs before talk)
Venue: Irish Club of WA,
NEXT MONTHLY MEETING
Wednesday 5th August, 2015
Talk title: A Hadean zircon perspective on the early impact bombardment
Speaker: Dr Aaron J. Cavosie, Senior Research Fellow, Curtin University
Abstract: One enduring early Earth enigma is the absence of identified evidence in the terrestrial record for meteorite impact events during the Hadean, particularly during the so-called Late Heavy Bombardment from 4.0 to 3.8 Ga. Given the low likelihood of discovering an intact Hadean impact crater, the sedimentary record of shocked minerals created by the erosion of such craters may offer the best opportunity to discover direct evidence of the early impact history. Zircon is an excellent recorder of shock deformation and is a ubiquitous mineral in siliciclastic sediments of any age. The oldest known detrital zircons are found in the Jack Hills of Western Australia. To date >250,000 detrital zircons from the Jack Hills have been analyzed for U-Pb age, including the recent discovery of the oldest piece of Earth, a 4.374±0.006 Gyr zircon, whose age was confirmed by atom probe tomography. Of the 1000s of zircons older than ~3.8 Ga that have been identified, none have been reported as being shocked. In this talk I explore various explanations for the absence of shocked Hadean zircons in the context of the modern record.
Pervasively shocked detrital zircons have been shown to survive in recent siliciclastic sediments eroded and transported as far as >2000 km from their source craters, as demonstrated at the Vredefort (South Africa), Sudbury (Canada), and Santa Fe (USA) impact structures. They have also been found in lithified Paleozoic sediments, demonstrating their preservation over ‘Deep Time’. Thus, attributing the absence of shocked Hadean detrital zircons to their destruction during erosion, transport, and lithification is not consistent with the presence of shocked zircons in modern and ancient sedimentary rocks. Attributing the absence of Hadean detrital shocked zircons to wholesale destruction of early terrestrial crust by tectonics, burial, or erosion does not explain how unshocked Hadean detrital zircons with ages up to 4.4 Ga managed to survive. Attributing the absence of Hadean detrital shocked zircons to a paucity of zircon-saturated rocks on the early Earth is also not consistent with the preservation of unshocked Hadean zircons. Modern mafic oceanic crust contains abundant zircon; a large impact into a modern ocean basin would produce abundant detrital shocked zircons if the cratered ocean floor is later subjected to subaerial weathering. Unshocked Hadean detrital zircons do not appear to be related to impact processes, as their geochemistry is distinct from unshocked zircons in impact-generated melts. Simultaneous destruction of the early terrestrial impact record due to impact-related melting/assimilation does not explain the preservation of a global impact record on the Moon, including some shocked zircons. If our understanding of the early impact bombardment is correct, shocked Hadean detrital zircons should be preserved.
About the speaker: Aaron Cavosie is a petrologist and geochemist with broad interests in planetary science and the early Earth. Cavosie earned his BS and MS degrees in geology from the University of New Mexico. His PhD, from the University of Wisconsin-Madison, focused on the origin of detrital Hadean zircons from the Jack Hills of Western Australia. Before coming to Curtin University as a Senior Research Fellow in February 2015, Cavosie was a Professor of Geology at the University of Puerto Rico for 10 years. During this time he initiated the discipline of detrital shocked mineral studies, a new cross-disciplinary approach to identify evidence of ancient impact events, which he is continuing to advance at Curtin. Cavosie is a Science Editor at the Geological Society of America Bulletin and an Associate Editor at Geosphere. He currently has active federal research grants from the U.S. National Science Foundation, NASA Astrobiology, and the US National Oceanic and Atmospheric Administration (NOAA).
GSA-WA Student Bursary
The GSA-WA offers bursaries to assist students in their studies and research.
Up to $1000 can be granted for any meritorious project, for 1 or several students, such as field trips, for laboratory costs, for travel costs or conferences.
Applications close 31 March and 30 September each year.
View the flyer for further information and complete the application form to submit a nomination.
Bursary Flyer download: PDF doc
Bursary Application Conditions download: MS Word doc
Bursary Application Form download: MS Word doc
2015 Gibb Maitland Medal
The Gibb Maitland Medal is awarded by the Western Australia Division of the Geological Society of Australia in order to recognize individuals who have made substantial contributions to geoscience in Western Australia. It is named for Andrew Gibb Maitland, Government Geologist from 1896 to 1926, who established the Geological Survey of Western Australia. The Medal is usually awarded each year and nominations are now sought for the 2015 Gibb Maitland medallist.
2015 Gibb Maitland guidelines & nomination form download: MS Word doc
Nominations closed Monday 1 December 2014 and the winner was Dr Graham C Begg.
2014 GSA-WA Division AGM Minutes
The GSA-WA Division 2014 Annual General Meeting was held at the Irish Club, Subiaco on Wednesday 2 April 2014.
modified: 30 July 2015