[meteorite-list] Study Questions Dates for Cataclysms on Early Moon, Earth

[Ron Baalke]

http://news.wisc.edu/24103

Study questions dates for cataclysms on early moon, Earth
by David Tenenbaum
University of Wisconsin
October 16, 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.

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."