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您的检索式：作者名="D.Graham Pearson"
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| 1 | Deserpentinization and high-pressure(eclogite-facies) metamorphic features in the Eoarchean ultramafic body from Isua,Greenland显示文摘Discontinuous chains of ultramafic rock bodies form part of the 3800–3700 Ma Isua Supracrustal Belt(ISB),hosted in the Itsaq Gneiss Complex of southwestern Greenland.These bodies are among the world’s oldest outcrops of ultramafic rocks and hence an invaluable geologic record.Ultramafic rocks from Lens B in the northwestern limb of ISB show characteristics of several stages of serpentinization and deserpentinization forming prograde and retrograde mineral assemblages.Ti-rich humite-group minerals such as titanian chondrodite(Ti-Chn)and titanian clinohumite(Ti-Chu)often occur as accessory phases in the metamorphosed ultramafic rocks.The Ti-rich humite minerals are associated with metamorphic olivine.The host olivine is highly forsteritic(Fo96-98)with variable Mn O and Ni O contents.The concentrations of the rare-earth elements(REE)and high-field strength elements(HFSE)of the metamorphic olivine are higher than typical mantle olivine.The textural and chemical characteristics of the olivine indicate metamorphic origin as a result of deserpentinization of a serpentinized ultramafic protolith rather than primary assemblage reflecting mantle residues from high-degrees of partial melting.The close association of olivine,antigorite and intergrown Ti-Chn and Ti-Chu suggests pressure condition between$1.3–2.6 GPa within the antigorite stability field(<700°C).The overall petrological and geochemical features of Lens B ultramafic body within the Eoarchean ISB indicate that these are allochthonous ultramafic rocks that recorded serpentine dehydration at relatively lower temperature and reached eclogite facies condition during their complex metamorphic history similar to exhumed UHP ultramafic rocks in modern subduction zone channels. | Juan Miguel Guotana Tomoaki Morishita Ikuya Nishio Akihiro Tamura Tomoyuki Mizukami Kenichiro Tani Yumiko Harigane Kristoffer Szilas D.Graham Pearson | 2022 | Geoscience Frontiers2022，13，1： | 0 |
| 2 | Zircon within chromitite requires revision of the tectonic history of the Eoarchean Itsaq Gneiss complex,Greenland显示文摘The Ujaragssuit Nunat layered(UNL)unit in the Itsaq Gneiss Complex,west Greenland,has been consid-ered to contain one of the oldest chromitites on Earth based on~ca.4.1 Ga Hadean whole rock Pt-Os model ages and ca.3.81 Ga zircon U-Pb age of the surrounding orthogneiss.This study obtained zircon from the chromitite within this unit as well as granitoid sheets that intruded into the UNL unit.In-situ U-Pb-Hf-O isotope measurements were made on the zircons.Zircons from both the chromitite and the intrusive granitoids show concordant U-Pb ages of ca.2.97-2.95 Ga.In contrast,Hf and 0 isotopic anal-yses indicate that zircons in the chromitites have a different origin from those in the intrusive granitoids.Zircons from granitoids yielded Th/U ratios higher than 0.2,initial Hf isotope ratios of 0.2805-0.2807(i.e,initial:Hf value of-11 to-5),andδ^(18)O values of mostly 6.0‰-7.0‰,which are typical for felsic igneous rocks in Archean continental crust.The least altered zircons from a chromitite exhibited initial Hf isotope ratios of 0.28078-0.28084(i.e.,initial:Hf value of-1.1 to-0.4),close the chondritic value at ca.3.0 Ga and the depleted mantle at ca.3.2 Ga.These zircons also haveδ^(18)O values of 4.2‰6.1‰which correspond to typical mantle values.The other chromitite zircons yielded Th/U ratios lower than 0.1,and Hf and 0 isotopic compositions ranging between the least altered zircons and the intrusive granitoid zir-cons.These results indicate that the zircons in the chromitites crystallized before or during the 2.97-2.95 Ga granitoid intrusion and most of the zircons were altered by subsequent metasomatism.Furthermore,the present results suggest that zircons in the chromitites originally had depleted Hf iso-topic compositions at ca.3.2-3.0 Ga.This can be explained by two different models of the evolution of the UNL unit.One is that if the UNL unit was formed at>3.81 Ga as previously thought,with the zircons in the chromitites subsequently being precipitated by ca.3.2-2.95 Ga during metamorphism or metaso-matism.The other model is that the UNL unit itself was actually formed at ca.3.2-3.0 Ga,with zircon in the chromitite representing the crystallisation age of the unit,which was then tectonically incorporated into the ca.3.81 Ga orthogneiss prior to the 2.97-2.95 Ga granitoid intrusion event.In either case,our zircon analyses reveal significant evolutionary history prior to depleted mantle Hf model ages of 3.2-2.95 Ga.Revision of the geotectonic evolution of the UNL unit and the Itsaq Gneiss Complex is therefore required. | Hikaru Sawada Tomoaki Morishita Adrien Vezinet Richard Stern Kenichiro Tani Ikuya Nishio Kanta Takahashi D.Graham Pearson Kristoffer Szilas | 2023 | Geoscience Frontiers2023，14，6： | 0 |
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