Earth's 'Time Capsules' May Be Flawed
by Sid Perkins on 17 November 2011, 2:07 PM
Found in rocks throughout Earth's crust, zircons are some of the oldest bits of mineral on Earth. These tiny crystals are so durable—and some are so ancient, dating to just 150 million years or so after our world formed—that geologists have long viewed the tiny bits of minerals embedded within them as a kind of time capsule, offering a peek at conditions on the early Earth. But a new study suggests that these so-called inclusions are not as pristine as scientists thought, raising doubts about conclusions that researchers have drawn from them, from the rise of early oceans to the movements of the ancient continents.
Time capsule? Material in an inclusion (white mass at center, left image) is about 700 million years younger than the 3.4-billion-year-old zircon surrounding it, a sign that mineral-rich fluids somehow found a way to infiltrate this zircon in the Jack Hills of Western Australia (right).
Credit: Rasmussen et al., Geology; (scenery, inset) Bruce Watson
In the new study, researchers led by geologist Birger Rasmussen of Curtin University in Bentley, Australia, analyzed more than 7000 zircons from a portion of the Jack Hills of Western Australia, where rocks are between 2.65 billion and 3.05 billion years old. These zircons ended up in the Jack Hills rocks after eroding from rocks that were even more ancient, and the researchers painstakingly sorted the individual crystals from other bits of minerals. Many of the individual, centimeter-sized pebbles in the silicate-rich conglomerate have been heavily metamorphosed—stretched, flattened, and sometimes chemically altered when tectonic activity carried them deep within Earth, where pressures and temperatures are hellish. Zircons in the rocks were exposed to the same conditions. "These zircons have been absolutely hammered," Rasmussen says.
A total of 485 zircons held inclusions, and about a dozen or so of these contained radioactive trace elements that allowed the researchers to determine their ages. Those ages fell into two clumps—one of about 2.68 billion years and another of about 800 million years. "This was a big surprise to us," Rasmussen says, especially because the zircons themselves ranged in age from 3.34 billion and 4.24 billion years old.
Rather than matching the ages of the zircons, the researchers note, the ages of the inclusions matched the ages of the metamorphic minerals surrounding the zircons. Some of those inclusions lie along hairline fractures in the zircons, a route by which mineral-rich fluids could have infiltrated, Rasmussen says. But other inclusions appear to be entirely enclosed. In those cases, the fluids may have traveled along defects in the zircon's crystal structure caused by radioactive decay or along pathways that are either too small to see or oriented such that they're invisible.
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Read more here/Leia mais aqui: Science Now
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Metamorphic replacement of mineral inclusions in detrital zircon from Jack Hills, Australia: Implications for the Hadean Earth
Birger Rasmussen1, Ian R. Fletcher1, Janet R. Muhling2, Courtney J. Gregory1 and Simon A. Wilde1
Author Affiliations
1Department of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia
2Centre for Microscopy, Characterisation and Analysis, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
Abstract
The Hadean (before 4.0 Ga) crust has long been considered to comprise mainly primitive mafic and ultramafic rocks. However, mineral inclusions in detrital zircons as old as 4.4 Ga from Jack Hills, Australia, have been interpreted to be magmatic and to provide evidence for extensive granitic crust. In situ U-Pb dating of monazite and xenotime inclusions in 4.25–3.35 Ga detrital zircons from Jack Hills shows that these inclusions are not magmatic, but formed during metamorphism at either 2.68 Ga or 0.8 Ga. Monazite-xenotime thermometry of intergrowths in the inclusions and the quartz-muscovite rock matrix constrain temperatures to between 420–475 °C, corresponding with conditions during peak regional metamorphism. Petrography and U-Pb geochronology of zircon inclusions from other localities show that the replacement of primary inclusions may commence in the igneous host rock and continue through weathering, sedimentation, and diagenesis. With increasing metamorphic grade, the inclusion assemblage increasingly reflects the composition of the rock matrix. In Jack Hills, most of the inclusions have the same composition and abundances as the metamorphic matrix, consistent with their formation during metamorphism. The titanium content of quartz inclusions indicates formation temperatures of 350–490 °C, supporting a metamorphic origin. Several lines of evidence indicate that at least some of the muscovite inclusions are also secondary. The lack of apatite inclusions in zircons from Jack Hills, relative to zircon in common granitic rocks, suggests that secondary minerals may have replaced primary apatite. Thus, detrital zircon may not be impermeable to post-depositional fluids, raising doubts about the use of the mineral inclusions they contain to infer initial magma chemistry. These results call for a reassessment of the source melts of the Hadean zircons and the composition of the earliest crust.
Received 14 June 2011.
Revision received 5 July 2011.
Accepted 11 July 2011.
© 2011 Geological Society of America
Subscription or payment needed/Necessário assinatura ou pagamento: Geology
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Metamorphic replacement of mineral inclusions in detrital zircon from Jack Hills, Australia: Implications for the Hadean Earth
Birger Rasmussen1, Ian R. Fletcher1, Janet R. Muhling2, Courtney J. Gregory1 and Simon A. Wilde1
Author Affiliations
1Department of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia
2Centre for Microscopy, Characterisation and Analysis, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
Abstract
The Hadean (before 4.0 Ga) crust has long been considered to comprise mainly primitive mafic and ultramafic rocks. However, mineral inclusions in detrital zircons as old as 4.4 Ga from Jack Hills, Australia, have been interpreted to be magmatic and to provide evidence for extensive granitic crust. In situ U-Pb dating of monazite and xenotime inclusions in 4.25–3.35 Ga detrital zircons from Jack Hills shows that these inclusions are not magmatic, but formed during metamorphism at either 2.68 Ga or 0.8 Ga. Monazite-xenotime thermometry of intergrowths in the inclusions and the quartz-muscovite rock matrix constrain temperatures to between 420–475 °C, corresponding with conditions during peak regional metamorphism. Petrography and U-Pb geochronology of zircon inclusions from other localities show that the replacement of primary inclusions may commence in the igneous host rock and continue through weathering, sedimentation, and diagenesis. With increasing metamorphic grade, the inclusion assemblage increasingly reflects the composition of the rock matrix. In Jack Hills, most of the inclusions have the same composition and abundances as the metamorphic matrix, consistent with their formation during metamorphism. The titanium content of quartz inclusions indicates formation temperatures of 350–490 °C, supporting a metamorphic origin. Several lines of evidence indicate that at least some of the muscovite inclusions are also secondary. The lack of apatite inclusions in zircons from Jack Hills, relative to zircon in common granitic rocks, suggests that secondary minerals may have replaced primary apatite. Thus, detrital zircon may not be impermeable to post-depositional fluids, raising doubts about the use of the mineral inclusions they contain to infer initial magma chemistry. These results call for a reassessment of the source melts of the Hadean zircons and the composition of the earliest crust.
Received 14 June 2011.
Revision received 5 July 2011.
Accepted 11 July 2011.
© 2011 Geological Society of America
Subscription or payment needed/Necessário assinatura ou pagamento: Geology
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