Scientists from Britain and America took the new technology-X radio originated from advanced photon source to research on the evolution of a 3 billion years old rock, and the relative paper was published in Nature.
The researchers took the advanced photon source to analyze the magmatic rock, and the photon source is a kind of large particle cyclotron several kilometers long located in Argonne National library of America, which was used for proving up the material structure.
The researchers took X-ray originated from the particle cyclotron to research the unusual magmatic rock--- komatiite, and the rock has been preserved for several billion years in crystaline, and formed by unusually high degrees of melting of mantle peridotite. Because of the previous poor technology, it is hard to know how to form komatiite. Now, with advanced detection instruments, the researchers found komatiite had been formed in the about 1700 0C mantle in 2.7 billion years ago, and the finding refute the view that the komatiite was formed in the cold environment. The researchers also found that the mantle has been cooling down nearly 300 0C in 2.7 billion years.
Andrew Berry, the writer of the paper, he said “nearly everyone in geology is seeking a chemical analysis of very small rock fragments for a long time, because this technology can provide important geological evidence and explain the early Earth's internal state. Our finding explains how komatiite to be formed, on the basis of it, we will have more and more findings about the history of the earth.” He also believed that the new technology can be used to explain the earth's internal activities; it also can build a new model to explain the details of planetary evolution; however, he added that he needed to do more experiments in order to fully understand the significance of this finding.
The abstract of the paper as follows:
Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle
Andrew J. Berry
Abstract: Komatiites are volcanic rocks mainly of Archaean age that formed by unusually high degrees of melting of mantle peridotite. Their origin is controversial and has been attributed to either anhydrous melting of anomalously hot mantle or hydrous melting at temperatures only modestly greater than those found today. Here we determine the original Fe3+/∑Fe ratio of 2.7-Gyr-old komatiitic magma from Belingwe, Zimbabwe, preserved as melt inclusions in olivine, to be 0.10 ± 0.02, using iron K-edge X-ray absorption near-edge structure spectroscopy. This value is consistent with near-anhydrous melting of a source with a similar oxidation state to the source of present-day mid-ocean-ridge basalt. Furthermore, this low Fe3+/Fe value, together with a water content of only 0.2–0.3 wt%, excludes the possibility that the trapped melt contained significantly more water that was subsequently lost from the inclusions by reduction to H2 and diffusion. Loss of only 1.5 wt% water by this mechanism would have resulted in complete oxidation of iron (that is, the Fe3+/∑Fe ratio would be ∼1). There is also no petrographic evidence for the loss of molecular water. Our results support the identification of the Belingwe komatiite as a product of high mantle temperatures (∼1,700 °C), rather than melting under hydrous conditions (3–5-wt% water), confirming the existence of anomalously hot mantle in the Archaean era.
If you want to know the more detail, please refer to <Nature>，455, 960-963，Andrew J. Berry，Stephen R. Sutton
The researchers took the advanced photon source to analyze the magmatic rock, and the photon source is a kind of large particle cyclotron several kilometers long located in Argonne National library of America, which was used for proving up the material structure.
The researchers took X-ray originated from the particle cyclotron to research the unusual magmatic rock--- komatiite, and the rock has been preserved for several billion years in crystaline, and formed by unusually high degrees of melting of mantle peridotite. Because of the previous poor technology, it is hard to know how to form komatiite. Now, with advanced detection instruments, the researchers found komatiite had been formed in the about 1700 0C mantle in 2.7 billion years ago, and the finding refute the view that the komatiite was formed in the cold environment. The researchers also found that the mantle has been cooling down nearly 300 0C in 2.7 billion years.
Andrew Berry, the writer of the paper, he said “nearly everyone in geology is seeking a chemical analysis of very small rock fragments for a long time, because this technology can provide important geological evidence and explain the early Earth's internal state. Our finding explains how komatiite to be formed, on the basis of it, we will have more and more findings about the history of the earth.” He also believed that the new technology can be used to explain the earth's internal activities; it also can build a new model to explain the details of planetary evolution; however, he added that he needed to do more experiments in order to fully understand the significance of this finding.
The abstract of the paper as follows:
Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle
Andrew J. Berry
Abstract: Komatiites are volcanic rocks mainly of Archaean age that formed by unusually high degrees of melting of mantle peridotite. Their origin is controversial and has been attributed to either anhydrous melting of anomalously hot mantle or hydrous melting at temperatures only modestly greater than those found today. Here we determine the original Fe3+/∑Fe ratio of 2.7-Gyr-old komatiitic magma from Belingwe, Zimbabwe, preserved as melt inclusions in olivine, to be 0.10 ± 0.02, using iron K-edge X-ray absorption near-edge structure spectroscopy. This value is consistent with near-anhydrous melting of a source with a similar oxidation state to the source of present-day mid-ocean-ridge basalt. Furthermore, this low Fe3+/Fe value, together with a water content of only 0.2–0.3 wt%, excludes the possibility that the trapped melt contained significantly more water that was subsequently lost from the inclusions by reduction to H2 and diffusion. Loss of only 1.5 wt% water by this mechanism would have resulted in complete oxidation of iron (that is, the Fe3+/∑Fe ratio would be ∼1). There is also no petrographic evidence for the loss of molecular water. Our results support the identification of the Belingwe komatiite as a product of high mantle temperatures (∼1,700 °C), rather than melting under hydrous conditions (3–5-wt% water), confirming the existence of anomalously hot mantle in the Archaean era.
If you want to know the more detail, please refer to <Nature>，455, 960-963，Andrew J. Berry，Stephen R. Sutton
Editor:sxya
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