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Geotimes
 Published by the American Geological Institute
Newsmagazine of the Earth Sciences

August 2000


News Notes
 Astrogeochemistry

Briny beginning

A salt-containing meteorite that fell in Morocco in 1998 may contain the oldest mineral ever discovered in our solar system, providing a glimpse into a short period of time during which water existed in the asteroid belt 4.571 billion years ago.

“Quite amazing,” is how James Whitby of the University of Manchester, describes the discovery of halite crystals with water-bearing fluid inclusions in the chondrite Zag (H3-6). He believes the salt formed by the evaporation of mineral-rich water within 2 million years of the age of the oldest known solar system minerals. Whitby and his colleagues published their work in the June 9 Science. “I think at the moment we’ve got the oldest mineral on Earth,” Whitby says, in light of recent work by Charles Hohenberg at the University of Washington in St. Louis. Magnetite from the Murchison meteorite held the honor of being the oldest mineral ever discovered until Hohenberg recalculated its age and found it to be younger than was previously thought.

Scientists believe that the Zag meteorite was part of one of the large asteroids that provided building material for planets. Whitby believes that the fragment under study broke off its parent body approximately 4 million years ago. If the halite did indeed precipitate from Zag’s short-lived hydrothermal system it would indicate that water existed at least once in the earliest years of the solar system. “Nobody has found water as such in these things before,” he says. The only prior evidence of water has been from aqueous alteration that can occur when extraterrestrial minerals are contaminated with water from Earth.

Nearly all of the xenon found in Zag’s halite crystals is the isotope xenon-129. This fact, paired with the absence of xenon-132, supports the argument that the xenon was not carried from some other source by contaminant water. If the mineralized water that precipitated the salt had remobilized xenon on Earth, it would most likely have contained more xenon-132.
 
Whitby recognized the halite as unusual right away by its discoloration from radiation that would not likely have occurred on Earth. To be sure that the halite was of extraterrestrial origin he turned to radioisotope dating.

Isotopic analysis of a small sample of the halite revealed an abundance of pure xenon-129, the radioactive decay product of iodine-129. Iodine-129 is known to have existed in the early solar system, but is no longer found on Earth. It has a half-life of 15.7 million years and can only be used to date very old minerals from the first 100 million years of the solar system’s existence, according to Whitby. Using relative age dating, he and his colleagues estimated that the crystals formed 4.57 billion years ago — the commonly accepted age of Earth is 4.56 billion years. Since these results were published in June, Whitby and his colleagues calculated the absolute age of the halite using data from Hohenberg’s research to give the chondrite an age of 4.571 billion years.


    Enlarged photograph of the Zag meteorite. The scratches are saw marks 
    from the preparation of the slice. The halite is purple due to radiation 
    damage during its long history. This slice is held by the Natural History 
    Museum, London. John Bridges, Natural History Museum, London.

To explain how water may have come to exist so early in the solar system’s history, Whitby offers a relatively simple explanation. Imagine a planetary body at the dawn of the solar system. It might have been as few as 10 kilometers in diameter, its interior heating up from the radioactive decay of minerals such as aluminum-26. The internal heat and pressure may have created the ideal conditions to form a transitory period in which liquid water existed for a geologic blink of an eye.

While Whitby and his team aren’t searching for extraterrestrial life, these findings provide a glimmer of hope for those who are. “Wherever you have liquid water, the common view is that life is soon to follow,” Whitby says. “It would have to be quick though, because water couldn’t have existed [on Zag] for very long.”
 

Laura Wright
 



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