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 From rocks in Colorado, evidence of a 'chaotic solar system'
23.02.2017 | University of Wisconsin-Madison

nachricht Prediction: More gas-giants will be found orbiting Sun-like stars
22.02.2017 | Carnegie Institution for Science

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: Breakthrough with a chain of gold atoms

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

Im Focus: DNA repair: a new letter in the cell alphabet

Results reveal how discoveries may be hidden in scientific “blind spots”

Cells need to repair damaged DNA in our genes to prevent the development of cancer and other diseases. Our cells therefore activate and send “repair-proteins”...

Im Focus: Dresdner scientists print tomorrow’s world

The Fraunhofer IWS Dresden and Technische Universität Dresden inaugurated their jointly operated Center for Additive Manufacturing Dresden (AMCD) with a festive ceremony on February 7, 2017. Scientists from various disciplines perform research on materials, additive manufacturing processes and innovative technologies, which build up components in a layer by layer process. This technology opens up new horizons for component design and combinations of functions. For example during fabrication, electrical conductors and sensors are already able to be additively manufactured into components. They provide information about stress conditions of a product during operation.

The 3D-printing technology, or additive manufacturing as it is often called, has long made the step out of scientific research laboratories into industrial...

Im Focus: Mimicking nature's cellular architectures via 3-D printing

Research offers new level of control over the structure of 3-D printed materials

Nature does amazing things with limited design materials. Grass, for example, can support its own weight, resist strong wind loads, and recover after being...

Im Focus: Three Magnetic States for Each Hole

Nanometer-scale magnetic perforated grids could create new possibilities for computing. Together with international colleagues, scientists from the Helmholtz Zentrum Dresden-Rossendorf (HZDR) have shown how a cobalt grid can be reliably programmed at room temperature. In addition they discovered that for every hole ("antidot") three magnetic states can be configured. The results have been published in the journal "Scientific Reports".

Physicist Dr. Rantej Bali from the HZDR, together with scientists from Singapore and Australia, designed a special grid structure in a thin layer of cobalt in...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Booth and panel discussion – The Lindau Nobel Laureate Meetings at the AAAS 2017 Annual Meeting

13.02.2017 | Event News

Complex Loading versus Hidden Reserves

10.02.2017 | Event News

International Conference on Crystal Growth in Freiburg

09.02.2017 | Event News

 
Latest News

Stingless bees have their nests protected by soldiers

24.02.2017 | Life Sciences

New risk factors for anxiety disorders

24.02.2017 | Life Sciences

MWC 2017: 5G Capital Berlin

24.02.2017 | Trade Fair News

VideoLinks
B2B-VideoLinks
More VideoLinks >>>