Oxygen isotope evidence for large-scale hybridization of the lower crust during magmatic underplating

Abstract

Lower crustal granulite xenoliths from eight localities worldwide exhibit an extremely large range in O-isotope ratios ( δ 18O = +5.4 to + 13.5%.) with a concentration of δ 18O values around +7.0%.. Mafic granulites with Mg numbers greater than 0.7 (group Ml) exhibit the smallest range and lowest average δ 18O value ( +6.7 ± 0.6%.) of all granulite types. Mafic granulites with Mg numbers less than 0.7 (groups M2 and M3) and intermediate composition granulites have higher average δ 18O values that are statistically indistinguishable at +8.0 ± 1.1%. (M2), +7.7 ± 1.2%. (M3) and +8.1 ± 1.8%., respectively. Silicic metaigneous granulites ( + 10.0 ± 1.9%.) and metasediments ( + 10.2 ± 2.2%.) are significantly enriched in 18O relative to other granulite groups and exhibit the widest range of δ 18O values (6.9%.). The overall range in 18O/ 16O ratios of ca. 8%. emphasizes the O-isotope heterogeneity of the present-day deep continental crust and demonstrates that pervasive deep crustal fluid flow and isotopic homogenization is not a major process operating on a regional scale. Contamination of the order of 20–50% by an 18O-enriched component (such as the metasedimentary and/or silicic metaigneous granulites) with an average δ 18O value of ca. +10%. is required to account for the 18O/ 16O ratios of most mafic xenoliths. Although such an explanation may apply to some M2 and M3 granulites, it is not applicable to M1 and M2 group granulites with strongly mafic major element compositions. Instead, many of these samples may represent the culmination of a large-scale hybridization process involving differentiating mantle-derived mafic melts and mafic restite residual after metasediment anatexis and granite genesis. This hypothesis can explain the rarity of identified restites in lower crustal xenolith suites because such rocks have been modified and/or consumed by interaction with mafic magmas during and/or following episodes of large-scale intracrustal melting. Delamination of mafic lower crust following transient overthickening events at convergent plate margins may be an important means of introducing surficially derived oxygen into the region of the crust/mantle boundary.