Modification of a rare-earth element deposit by low -temperature partial melting during metamorphic overprinting: Norra K?rr alkaline complex, southern Sweden

被引:6
作者
Sjoqvist, Axel S. L. [1 ]
Zack, Thomas [1 ,2 ]
Honn, Denise K. [3 ,4 ]
Baxter, Ethan F. [3 ,5 ]
机构
[1] Univ Gothenburg, Dept Earth Sci, POB 460, SE-40530 Gothenburg, Sweden
[2] Univ Adelaide, Dept Earth Sci, Adelaide, SA, Australia
[3] Boston Univ, Dept Earth Sci, 685 Commonwealth Ave, Boston, MA 02215 USA
[4] McCrone Associates Inc, 850 Pasquinelli Dr, Westmont, IL USA
[5] Boston Coll, Dept Earth & Environm Sci, 140 Commonwealth Ave, Chestnut Hill, MA 02167 USA
关键词
ILIMAUSSAQ COMPLEX; U-PB; GREENLAND IMPLICATIONS; AGPAITIC ROCKS; ICP-MS; EUDIALYTE; HF; EVOLUTION; KARR; MINERALIZATION;
D O I
10.1016/j.chemgeo.2020.119640
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
Rare-earth elements play a crucial role in modern technologies and are necessary for a transition to a green economy. Potentially economic deposits of these elements are typically hosted in minerals such as monazite, bastnäsite, and eudialyte (a complex Na-Ca-Fe-Zr silicate mineral with Cl), making these prime targets for geological research. Globally, rare-earth mineral deposits commonly show evidence of polyphase development and mineralisation processes, which need to be better understood to improve exploration strategies. The Norra Kärr alkaline complex (Sweden) contains a globally significant deposit of rare-earth elements, hosted in the mineral eudialyte. In this study, we focussed on eudialyte crystals in undeformed, cross-cutting pegmatoid veins from Norra Kärr. In order to determine their age, we refined an established micromilling method to enable sampling of minerals rich in rare-earth elements for precise analysis of major and trace elements, Nd isotope ratios, and Sm-Nd geochronology down to a scale of <200 μm. Mineral samples were subjected to detailed textural and chemical characterisation by backscattered electron imaging and laser ablation inductively coupled plasma mass spectrometry, by which precise and accurate Sm/Nd ratios were determined to steer subsequent micromill sampling for small-aliquot Sm-Nd isotope analysis by isotope dilution thermal ionisation mass spectrometry. Given enough internal spread in Sm/Nd ratios, reliable Sm-Nd isochrons can be derived from discrete textural domains within a single crystal. This provided an age of 1.144 ± 0.053 Ga (95% confidence); approximately 350 million years younger than the magmatic intrusion of the alkaline complex (ca. 1.49 Ga). Primary compositional sector and oscillatory zoning in these eudialyte crystals shows core-to-rim enrichment in rare-earth elements and significant fractionation of K/Rb, Y/Ho, Zr/Hf, and Nb/Ta, which we attribute to crystallisation under influence of complexing ligands in a confined volume. We argue that these mineralised pegmatoid veins formed by low-temperature (<550 °C) partial melting of the agpaitic host rock during an early Sveconorwegian (Grenvillian) metamorphic overprinting event. Given the challenge of directly dating rare-earth ore minerals by conventional methods, modification of rare-earth mineral deposits may be more widespread than already assumed, which shows the importance of investigations that date the rare-earth minerals themselves. © 2020
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页数:13
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