Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Study questions dates for cataclysms on early moon, Earth

19.10.2015

Phenomenally durable crystals called zircons are used to date some of the earliest and most dramatic cataclysms of the solar system. One is the super-duty collision that ejected material from Earth to form the moon roughly 50 million years after Earth formed. Another is the late heavy bombardment, a wave of impacts that may have created hellish surface conditions on the young Earth, about 4 billion years ago.

Both events are widely accepted but unproven, so geoscientists are eager for more details and better dates. Many of those dates come from zircons retrieved from the moon during NASA's Apollo voyages in the 1970s.


The deformed lunar zircon at center was brought from the moon by Apollo astronauts. The fractures characteristic of meteorite impact are not seen in most lunar zircons, so the ages they record probably reflect heating by molten rock, not impact.

Photo: Apollo 17/Nicholas E. Timms

A study of zircons from a gigantic meteorite impact in South Africa, now online in the journal Geology, casts doubt on the methods used to date lunar impacts. The critical problem, says lead author Aaron Cavosie, a visiting professor of geoscience and member of the NASA Astrobiology Institute at the University of Wisconsin-Madison, is the fact that lunar zircons are "ex situ," meaning removed from the rock in which they formed, which deprives geoscientists of corroborating evidence of impact.

"While zircon is one of the best isotopic clocks for dating many geological processes," Cavosie says, "our results show that it is very challenging to use ex situ zircon to date a large impact of known age."

Although many of their zircons show evidence of shock, "once separated from host rocks, ex situ shocked zircons lose critical contextual information," Cavosie says.

The "clock" in a zircon occurs as lead isotopes accumulate during radioactive decay of uranium. With precise measurements of isotopes scientists can calculate, based on the half life of uranium, how long lead has been accumulating.

If all lead was driven off during asteroid impact, the clock was reset, and the amount of accumulated lead should record exactly how long ago the impact occurred.

Studies of lunar zircons have followed this procedure to produce dates from 4.3 billion to 3.9 billion years ago for the late heavy bombardment.

To evaluate the assumption of clock-resetting by impact, Cavosie and colleagues gathered zircons near Earth's largest impact, located in South Africa and known to have occurred 2 billion years ago. The Vredefort impact structure is deeply eroded, and approximately 90 kilometers across, says Cavosie, who is also in the Department of Applied Geology at Curtin University in Perth, Australia. "The original size, estimated at 300 kilometers diameter, is modeled to result from an impactor 14 kilometers in diameter," he says.

The researchers searched for features within the zircons that are considered evidence of impact, and concluded that most of the ages reflect when the zircons formed in magma. The zircons from South Africa are "out of place grains that contain definitive evidence of shock deformation from the Vredefort impact," Cavosie says. "However, most of the shocked grains do not record the age of the impact but rather the age of the rocks they formed in, which are about 1 billion years older."

The story is different on Earth, says zircon expert John Valley, a professor of geoscience at UW-Madison. "Most zircons on Earth are found in granite, and they formed in the same process that formed the granite. This has led people to assume that all the zircons were reset by impact, so the ages they get from the Moon are impact ages. Aaron is saying to know that, you have to apply strict criteria, and that's not what people have been doing."

The accuracy of zircon dating affects our view of Earth's early history. The poorly understood late heavy bombardment, for example, likely influenced when life arose, so dating the bombardment topped a priority list of the National Academy of Sciences for lunar studies. Did the giant craters on the moon form during a brief wave or a steady rain of impacts? "It would be nice to know which," Valley says.

"The question of what resets the zircon clock has always been very complicated. For a long time people have been saying if zircon is really involved in a major impact shock, its age will be reset, so you can date the impact. Aaron has been saying, 'Yes, sometimes, but often what people see as a reset age may not really be reset.' Zircons are the gift that keep on giving, and this will not change that, but we need to be a lot more careful in analyzing what that gift is telling us."

###

CONTACT: Aaron Cavosie, aaron.cavosie@curtin.edu.au, +0061-4-5114-9039

David Tenenbaum, 608-265-8549, djtenenb@wisc.edu

DOWNLOAD PHOTOS: https://uwmadison.box.com/cavosie

Media Contact

Aaron Cavosie
aaron.cavosie@curtin.edu.au
61-451-149-039

 @UWMadScience

http://www.wisc.edu 

Aaron Cavosie | EurekAlert!

More articles from Physics and Astronomy:

nachricht Enhancing the quantum sensing capabilities of diamond
23.11.2017 | The Hebrew University of Jerusalem

nachricht Quantum optics allows us to abandon expensive lasers in spectroscopy
22.11.2017 | Lomonosov Moscow State University

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Frictional Heat Powers Hydrothermal Activity on Enceladus

Computer simulation shows how the icy moon heats water in a porous rock core

Heat from the friction of rocks caused by tidal forces could be the “engine” for the hydrothermal activity on Saturn's moon Enceladus. This presupposes that...

Im Focus: Nanoparticles help with malaria diagnosis – new rapid test in development

The WHO reports an estimated 429,000 malaria deaths each year. The disease mostly affects tropical and subtropical regions and in particular the African continent. The Fraunhofer Institute for Silicate Research ISC teamed up with the Fraunhofer Institute for Molecular Biology and Applied Ecology IME and the Institute of Tropical Medicine at the University of Tübingen for a new test method to detect malaria parasites in blood. The idea of the research project “NanoFRET” is to develop a highly sensitive and reliable rapid diagnostic test so that patient treatment can begin as early as possible.

Malaria is caused by parasites transmitted by mosquito bite. The most dangerous form of malaria is malaria tropica. Left untreated, it is fatal in most cases....

Im Focus: A “cosmic snake” reveals the structure of remote galaxies

The formation of stars in distant galaxies is still largely unexplored. For the first time, astron-omers at the University of Geneva have now been able to closely observe a star system six billion light-years away. In doing so, they are confirming earlier simulations made by the University of Zurich. One special effect is made possible by the multiple reflections of images that run through the cosmos like a snake.

Today, astronomers have a pretty accurate idea of how stars were formed in the recent cosmic past. But do these laws also apply to older galaxies? For around a...

Im Focus: Visual intelligence is not the same as IQ

Just because someone is smart and well-motivated doesn't mean he or she can learn the visual skills needed to excel at tasks like matching fingerprints, interpreting medical X-rays, keeping track of aircraft on radar displays or forensic face matching.

That is the implication of a new study which shows for the first time that there is a broad range of differences in people's visual ability and that these...

Im Focus: Novel Nano-CT device creates high-resolution 3D-X-rays of tiny velvet worm legs

Computer Tomography (CT) is a standard procedure in hospitals, but so far, the technology has not been suitable for imaging extremely small objects. In PNAS, a team from the Technical University of Munich (TUM) describes a Nano-CT device that creates three-dimensional x-ray images at resolutions up to 100 nanometers. The first test application: Together with colleagues from the University of Kassel and Helmholtz-Zentrum Geesthacht the researchers analyzed the locomotory system of a velvet worm.

During a CT analysis, the object under investigation is x-rayed and a detector measures the respective amount of radiation absorbed from various angles....

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Ecology Across Borders: International conference brings together 1,500 ecologists

15.11.2017 | Event News

Road into laboratory: Users discuss biaxial fatigue-testing for car and truck wheel

15.11.2017 | Event News

#Berlin5GWeek: The right network for Industry 4.0

30.10.2017 | Event News

 
Latest News

Underwater acoustic localization of marine mammals and vehicles

23.11.2017 | Information Technology

Enhancing the quantum sensing capabilities of diamond

23.11.2017 | Physics and Astronomy

Meadows beat out shrubs when it comes to storing carbon

23.11.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>