[meteorite-list] ALH84001 Not As Old And Still No Fossils

[JoshuaTreeMuseum]

I completely agree with Alan Treiman on this one.  I couldn't believe when 
McKay and team made the same unfounded claims 10+ years later with no new 
evidence to back them up.  I think their 15 minutes of fame is finally over.


http://www.wired.com/wiredscience/2010/04/allan-hills-meteorite-age/




The Allan Hills meteorite, named for the site where it was found in 
Antarctica, was once thought to contain fossil traces of life. That idea has 
been mostly dismissed, and now the rock also appears to be not quite as old 
as previously thought.

The oldest known Martian meteorite isn't so old after all. Though it's still 
the oldest chunk of Mars scientists have ever found, new research suggests 
the Allan Hills meteorite - officially known as ALH84001 - is about 400 
million years younger than previously estimated.

A new analysis published in the April 15 Science pegs the meteorite's age at 
a mere 4.091 billion years. Previously the meteorite was commonly accepted 
to have formed 4.51 billion years ago, when the planet's surface was still 
solidifying out of its primordial magma ocean. But the new age indicates the 
rock would have formed during a later, chaotic period when Mars was being 
pummeled by meteorites that fractured and shocked the planet's solid 
surface.

The Allan Hills meteorite has been a lightning rod for controversy since 
scientists announced in 1996 that it might hold fossils of Martian bacteria. 
The scientific community has since mostly abandoned that idea, as one by one 
every line of evidence for life has been given a non-biological explanation.


"People usually ask me about the life aspect, and I'm so sick to death of 
that," says Allan Treiman of the Lunar and Planetary Institute in Houston, 
who was not involved in the new work. Treiman and others now believe that 
what once looked like fossils is actually rock that was shaped by ordinary 
geological activity.

The previously accepted age of 4.51 billion years old was calculated in 1995 
by measuring radioactive isotopes of samarium and neodymium. Radioactive 
elements decay from a "parent" isotope (in this case, samarium) to a 
"daughter" isotope (neodymium) at a set rate. By comparing the amount of the 
parent element to the daughter element, scientists can infer how long a rock 
has been around.

"To understand how the Martian mantle has evolved, it's critical to get 
samples that are old, to see what the mantle sources were early in the 
planet's history," says Thomas Lapen of the University of Houston, a 
coauthor of the new study. "This is the only sample in that age range."

Lapen and his colleagues used radioactive isotope dating to calculate the 
age of the meteorite, using different elements than the 1995 analysis did. 
Lapen says that the elements used back then were mostly found in minerals 
called phosphates, which succumb relatively quickly to weathering and 
geological processes. Like hair dye or a fake ID, weathering could disguise 
the rock's age in some ways, but not so thoroughly that more reliable 
indicators are obscured.

"If it's subject to weathering, the phosphate would be the first to be 
disturbed," Lapen says. "Then ages dependent on the phosphates are altered."

Instead of elements found in phosphates, Lapen's group used lutetium and 
hafnium, elements that are mostly found in more change-resistant components 
of the rock. This method showed that the meteorite is just 4.091 billion 
years old.

Surprisingly, the researchers also found that several younger meteorites 
have essentially the same composition as the Allan Hills meteorite, meaning 
some of the same basic geologic processes have been at work on Mars for 
almost its entire history.

"That connection is perhaps the most amazing outcome of this research," 
Lapen says. "Mars is a very steady state planet. Igneous processes were 
happening the same way four billion years ago as they are happening right 
now."

The new age places the rock's birth date right at a period in the solar 
system's history when all of the inner planets were being bombarded with 
meteorites. That could clear up some confusion about the meteorite, Treiman 
says. Parts of the rock show signs of having been melted and reformed a 
second time since its birth, which would have been tough to explain if the 
rock were all original Martian crust.

"That had been a bit of a problem," Treiman says. "You'd have to do whatever 
mantle processing, whatever happened on the planet, before this rock came to 
be formed. There's not a lot of time for that."



Read More 
http://www.wired.com/wiredscience/2010/04/allan-hills-meteorite-age/#ixzz0lHih0pTn

Phil Whitmer