TY - JOUR
T1 - Detrital K-feldspar as a novel archive of continental crustal evolution using coupled in situ Rb–Sr dating and Pb isotope analysis
AU - Bevan, Dan
AU - Stubbs, Daniel
AU - Coath, Christopher D.
AU - Lewis, Jamie
AU - Elliott, Tim
N1 - Publisher Copyright:
© 2024
PY - 2024/12/20
Y1 - 2024/12/20
N2 - Here we present in situ Rb–Sr and Pb isotopic analysis of K-feldspar from the Shap Granite, (Cumbria, UK) and detrital K-feldspar from the associated Shap Wells Conglomerate Formation, to investigate the potential of the detrital K-feldspar record as an archive of crustal composition. We use a unique, collision cell – multi collector inductively coupled plasma tandem mass spectrometer to make the in situ isotope measurements, using ‘chemical resolution’ of 87Rb+ and 204Hg+ isobaric interferences on 87Sr+ and 204Pb+, achieved with SF6 and NH3 reaction gases, respectively. Using this technique, we successfully demonstrate that the majority of detrital K-feldspar Rb–Sr ages, as well as initial Sr and Pb isotope ratios, from the Shap Wells Conglomerate are within uncertainty of those for parental Shap Granite K-feldspar (398 ± 1 Ma, n = 6). Some detrital K-feldspar grains record perturbed Rb–Sr ages, reflecting post depositional alteration events. The majority of samples with disturbed Rb–Sr ages carry distinct petrographic features evident in SEM imaging, prior to in situ dating, which indicates a means to help screen for reliable ages. We further illustrate the potential value of combined Rb–Sr and Pb isotopic information preserved in K-feldspar to estimate the timing and chemical composition of juvenile continental crust formed in the past. Where U–Pb has been fractionated during juvenile crust formation, the initial Pb isotope ratio provided by the K-feldspar yields model crustal U/Pb (diagnostic of tectonic setting) and extraction age. The contrast in the U/Pb composition of juvenile crust formed in intraplate or subduction settings allows the model U/Pb ratios of the detrital K-feldspars to be used to constrain the style of juvenile crust formation. Additionally, the initial 87Sr/86Sr isotope ratio measured in detrital K-feldspar can be combined with the calculated crustal extraction age and Rb–Sr K-feldspar age to model the Rb/Sr of the juvenile crustal protolith.
AB - Here we present in situ Rb–Sr and Pb isotopic analysis of K-feldspar from the Shap Granite, (Cumbria, UK) and detrital K-feldspar from the associated Shap Wells Conglomerate Formation, to investigate the potential of the detrital K-feldspar record as an archive of crustal composition. We use a unique, collision cell – multi collector inductively coupled plasma tandem mass spectrometer to make the in situ isotope measurements, using ‘chemical resolution’ of 87Rb+ and 204Hg+ isobaric interferences on 87Sr+ and 204Pb+, achieved with SF6 and NH3 reaction gases, respectively. Using this technique, we successfully demonstrate that the majority of detrital K-feldspar Rb–Sr ages, as well as initial Sr and Pb isotope ratios, from the Shap Wells Conglomerate are within uncertainty of those for parental Shap Granite K-feldspar (398 ± 1 Ma, n = 6). Some detrital K-feldspar grains record perturbed Rb–Sr ages, reflecting post depositional alteration events. The majority of samples with disturbed Rb–Sr ages carry distinct petrographic features evident in SEM imaging, prior to in situ dating, which indicates a means to help screen for reliable ages. We further illustrate the potential value of combined Rb–Sr and Pb isotopic information preserved in K-feldspar to estimate the timing and chemical composition of juvenile continental crust formed in the past. Where U–Pb has been fractionated during juvenile crust formation, the initial Pb isotope ratio provided by the K-feldspar yields model crustal U/Pb (diagnostic of tectonic setting) and extraction age. The contrast in the U/Pb composition of juvenile crust formed in intraplate or subduction settings allows the model U/Pb ratios of the detrital K-feldspars to be used to constrain the style of juvenile crust formation. Additionally, the initial 87Sr/86Sr isotope ratio measured in detrital K-feldspar can be combined with the calculated crustal extraction age and Rb–Sr K-feldspar age to model the Rb/Sr of the juvenile crustal protolith.
KW - Crustal evolution
KW - Geochronology
KW - In situ Rb–Sr dating
KW - Isotope geochemistry
KW - K-feldspar
UR - http://www.scopus.com/inward/record.url?scp=85203500111&partnerID=8YFLogxK
U2 - 10.1016/j.chemgeo.2024.122379
DO - 10.1016/j.chemgeo.2024.122379
M3 - Article
AN - SCOPUS:85203500111
SN - 0009-2541
VL - 670
JO - Chemical Geology
JF - Chemical Geology
M1 - 122379
ER -