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Post Info TOPIC: Zircon Thermometer
Blobrana


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Zircon Thermometer
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Title: Post-Hadean transitions in Jack Hills zircon provenance: A signal of the Late Heavy Bombardment?
Authors: Elizabeth A. Bell, T. Mark Harrison

Hadean Jack Hills (Western Australia) detrital zircons represent the best documented terrestrial resource with which to observe the pre-4 Ga Earth. The >4 Ga component of this semi-continuous 4.38 to 3.0 Ga zircon record has been investigated in detail for age, 18O, Lu-Hf systematics, and Ti thermometry. The more abundant post-Hadean population is less well-characterised, but a previous study (Bell et al., 2011) suggests a more restricted range of 18O source materials together with a ca. 4.0-3.6 Ga discontinuity in Lu-Hf evolution. These differences could reflect a transformation in the character of the older zircon source region or their sourcing from different terranes entirely. The relative scarcity of 4.0-3.6 Ga zircons corresponds to a discontinuity in Lu-Hf evolution after which 176Hf/177Hf in zircon reverts to more radiogenic values relative to the >4 Ga population. We present new oxygen isotope, titanium, and trace element results for 4.0-3.6 Ga Jack Hills zircons in a search for apparent transitions in petrological conditions. Post-3.8 Ga zircons show a marked decrease in the occurrence of heavy oxygen (>6.5%), but remain close to the average of the Hadean distribution despite their restricted range. This may point to the decreased importance of sedimentary materials in post-3.8 Ga magmas. Ca. 3.9 Ga zircons fall into two categories: "Group I" displays temperatures and compositions similar to the Hadean zircons whereas "Group II" zircons have higher U and Hf, and lower (Th/U), Ce and P. Group II zircons also have anomalously low Ti, and are remarkably concordant in the U-Pb system. Group II's geochemical characteristics are consistent with formation by transgressive recrystallisation (Hoskin and Black, 2000), in which non-essential structural constituents are purged during high-grade thermal metamorphism. The restricted age range of Group II occurrence (3.91-3.84) and its coincidence with the postulated intense bolide flux in the inner solar system (i.e., Late Heavy Bombardment; 3.95.-3.85) may have causal significance.

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Blobrana


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Lunar zircon
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Geologists uncover how meteorites struck moon

A wealth of evidence, provided by lunar rock samples, shows how meteorites struck the moon.
The study, headed by microstructural geology experts Nick Timms and Steven Reddy, professor, Western Australian School of Mines (WASM), documents the discovery of impact-related shock features in lunar zircon
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Blobrana


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Zircon
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Zircon is a mineral that exists in nature from colourless to reddish-brown, yellow, green or blue. Zircon may also be transparent making its visual appearance similar to diamond. Zircon is a common accessory mineral in many rocks of the continental crust. Their chemical and physical durability allow this mineral to survive even under the pressure and temperature conditions of the Earth's upper mantle. These characteristics challenged a group of scientists from Tübingen (Germany), Los Angeles (USA), Perth (Australia) and Hefei (China) to take a closer look on zircons from basaltic fields in north-eastern Bavaria.
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Blobrana


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RE: Zircon Thermometer
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Zircon is a mineral that exists in nature from colourless to reddish-brown, yellow, green or blue. Zircon may also be transparent making its visual appearance similar to diamond. Zircon is a common accessory mineral in many rocks of the continental crust. Their chemical and physical durability allow this mineral to survive even under the pressure and temperature conditions of the Earth's upper mantle.
These characteristics challenged a group of scientists from Tübingen (Germany), Los Angeles (USA), Perth (Australia) and Hefei (China) to take a closer look on zircons from basaltic fields in north-eastern Bavaria. Their results show that these zircons were formed in the Earth mantle and were stored in this environment many million of years before they were delivered to the surface by basalt lava flows with which they are associated. Their study appears online in Nature Geoscience.
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Blobrana


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Lunar zircon
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Dating of the oldest-known piece of lunar zircon, brought back from our nearest neighbour in 1972, has pushed back the time when the moon's surface first formed.
The landmark find, published online in Nature Geoscience, has allowed the team of German, Australian and US scientists to give a "precise younger age limit" for the solidification of the moon's surface.
Lead author Associate Professor Alexander Nemchin, of Curtin University of Technology, says the moon is generally believed to have formed from the debris of a collision between the earth and a Mars-sized body more than 4.5 billion years ago.

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Blobrana


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RE: Zircon Thermometer
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A new analysis of ancient minerals called zircons suggests that a harsh climate may have scoured and possibly even destroyed the surface of the Earth's earliest continents.
Zircons, the oldest known materials on Earth, offer a window in time back as far as 4.4 billion years ago, when the planet was a mere 150 million years old. Because these crystals are exceptionally resistant to chemical changes, they have become the gold standard for determining the age of ancient rocks.

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Tiny diamonds found in Australia suggest the early Earth was not a hellish world for as long as previously supposed, the journal Nature reports.
The miniature gems, from Jack Hills in the west of the country, are encased in zircon crystals that have been dated up to 4.25 billion years ago.

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Earth is roughly 4.5 billion years old, but her early eons were tempestuous. Not even rock survives from the first 500 million years of her lifean eon known as the Hadeanbecause geologists speculate the planet's surface boiled and bubbled with molten lava under a steady bombardment of comets and meteorites.

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Diamonds are indeed forever, or at least nearly as old as the Earth, a new study shows.
Scientists have unearthed diamonds more than 4 billion years old and trapped inside crystals of zircon in the Jack Hills region in Western Australia. Nearly as old as Earth itself and considered the oldest terrestrial diamonds ever discovered, the gems could give insights into the early evolution of our planet's crust.

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"Jack Hills is the only place on Earth that can give us this kind of information about the formation of the Earth... We're dealing with the oldest material on the planet" - study team member Alexander Nemchin, a geochemist at Curtin University of Technology, Western Australia.


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Jack Hills
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