The petrology and chronology of NWA 8009 impact melt breccia: Implication for early thermal and impact histories of Vesta

doi:10.1016/j.gca.2017.01.037

	The petrology and chronology of NWA 8009 impact melt breccia: Implication for early thermal and impact histories of Vesta
	Liao, Shiyong 1; Hsu, Weibiao 2
刊名	GEOCHIMICA ET COSMOCHIMICA ACTA
	2017-05-01
卷号	204 页码:159-178
关键词	Vesta HEDs Eucrite Impact Thermal Metamorphism
ISSN号	0016-7037
DOI	10.1016/j.gca.2017.01.037
通讯作者	Hsu, Weibiao(wbxu@pmo.ac.cn)
英文摘要	Studies of petrology, mineralogy and geochronology of eucrites are keys to reconstruct the thermal and impact history of 4 Vesta, the proposed parent body for HED meteorites. Here we report the petrography, mineralogy and geochemistry of NWA 8009, a newly found eucritic impact-melt breccia, and present SIMS U-Pb ages of zircon and phosphates. NWA 8009 consists of coarse-and fine-grained lithic and mineral clasts set in fine-grained recrystallized matrix. It was derived from a protolith of monomict non-cumulate eucrite. Evidence for intense shock metamorphism observed in NWA 8009 includes mosaicism, deformed exsolution lamellae and partial melting of pyroxene, melting and incipient flow of plagioclase, planar fractures and granular textures of zircon. These shock effects indicate NWA 8009 was subjected to an impact metamorphism with peak pressure of similar to 50-60 GPa and post-shock temperature of similar to 1160-1200 degrees C. NWA 8009 is among the most intensely shocked HEDs reported yet. After the impact, the sample was buried near the surface in target rocks and experienced rapid cooling (similar to 23 degrees C/h) and annealing, resulting in recrystallization of the matrix and devitrification of plagioclase and silica glasses. U-Pb isotopic system of apatite within plagioclase groundmass of lithic clasts is completely reset and constrains the timing of impact at 4143 +/- 61 Ma, providing a new robust impact age on Vesta. Combined with the presence of synchronous impact resetting events, especially those recorded by Lu-Hf, Sm-Nd, and Pb-Pb isotopic systems, we identified a period of high impacts flux at ca. 4.1-4.2 Ga on Vesta. This impact flux occurred coincident with the uptick at ca. 4.1-4.2 Ga in impact age spectra of the moon, probably reflects widespread intense bombardment throughout the inner solar system at ca. 4.1-4.2 Ga. Based on evidence from zircon chemical zoning, petrographic occurrences, as well as the distinctive Zr/Hf ratios, we suggested that zircons in NWA 8009 have had a metamorphic, instead of magmatic origin. They mainly crystallized from melts produced by partial melting of mesostasis area due to reheating event during early global thermal metamorphism, rather than by Zr release from Zr-rich minerals. The U-Pb isotopic system in zircons was not disturbed by subsequent impacts, the weighted-mean Pb-207/Pb-206 age of 4560 +/- 8 Ma represents the timing of zircon growth during thermal metamorphism. Zircons from NWA 8009 and other eucrites may share a common origin during metamorphic growth events, and constraining the global thermal metamorphism on Vesta at ca. 4.55 Ga. The main heat sources responsible for global metamorphism in basaltic crust of Vesta might be heating from the hot interior, especially heat flow related to magmatism, rather than impact. (C) 2017 Elsevier Ltd. All rights reserved.
WOS关键词	EUCRITE PARENT BODY ; GLOBAL CRUSTAL METAMORPHISM ; U-PB ; BASALTIC EUCRITES ; SHOCK METAMORPHISM ; SOLAR-SYSTEM ; MN-53-CR-53 SYSTEMATICS ; MINERAL CHEMISTRY ; PIPLIA KALAN ; ZIRCON
WOS研究方向	Geochemistry & Geophysics
语种	英语
出版者	PERGAMON-ELSEVIER SCIENCE LTD
WOS记录号	WOS:000402487100010
内容类型	期刊论文
源URL	[http://libir.pmo.ac.cn/handle/332002/23003]
专题	中国科学院紫金山天文台
通讯作者	Hsu, Weibiao
作者单位	1.Purple Mt Observ, Key Lab Planetary Sci, Nanjing, Jiangsu, Peoples R China 2.Macau Univ Sci & Technol, Space Sci Inst, Macau, Peoples R China
推荐引用方式 GB/T 7714	Liao, Shiyong,Hsu, Weibiao. The petrology and chronology of NWA 8009 impact melt breccia: Implication for early thermal and impact histories of Vesta[J]. GEOCHIMICA ET COSMOCHIMICA ACTA,2017,204:159-178.
APA	Liao, Shiyong,&Hsu, Weibiao.(2017).The petrology and chronology of NWA 8009 impact melt breccia: Implication for early thermal and impact histories of Vesta.GEOCHIMICA ET COSMOCHIMICA ACTA,204,159-178.
MLA	Liao, Shiyong,et al."The petrology and chronology of NWA 8009 impact melt breccia: Implication for early thermal and impact histories of Vesta".GEOCHIMICA ET COSMOCHIMICA ACTA 204(2017):159-178.