TY - JOUR
T1 - Molybdenum and Boron Isotope Evidence for Fluid-fluxed Melting of Intraplate Upper Mantle Beneath the Eastern North China Craton
AU - Li, Hong-Yan
AU - Li, Jie
AU - Ryan, Jeffrey G.
AU - Li, Xiang
AU - Zhao, Rui-Peng
AU - Ma, Liang
AU - Xu, Yi-Gang
PY - 2019/8/15
Y1 - 2019/8/15
N2 - Fluid-fluxed melting of the mantle is the principal mechanism for the generation of arc magmas above subduction zones, but it has rarely been documented as important in intraplate settings. Here, we present new molybdenum (Mo) isotopic data from a suite of well-characterized Cenozoic basalts from the eastern North China Craton (NCC) to constrain the mechanisms of mantle melting in the region. These basalts represent mixtures of three components, namely, nephelinites and basanites that variably mixed with alkali basalts. The alkali basalts have relatively low δ 98/95 Mo ranging from −0.56‰ to −0.22‰ relative to the NIST3134 standard, suggesting the presence of dehydrated pelagic sediments in their mantle source rocks. Interestingly, the basanites range to higher δ 98/95 Mo (−0.04 ± 0.02‰) compared with the nephelinites (−0.31‰ to −0.22‰) with no associated changes in their radiogenic isotopes, and they show trends of decreasing Dy/Yb, Ce/Mo and δ 11 B with increasing Ba/Nb and B/Nb ratios. These correlations indicate that Mo behaves as a fluid-mobile trace element in the intraplate mantle beneath the NCC, and that the mantle sources of the nephelinites and basanites were infiltrated by this fluid component. Existing data show that this fluid component has lower H 2 O/Ce ratios (110–130) than that of mid-ocean ridge basalts (H 2 O/Ce ≈ 200). This result, together with the low-SiO 2 contents, relative depletion of Zr–Hf–Ti, and high δ 66 Zn characteristics of these strongly alkaline rocks, suggests derivation from a carbonated mantle source. Mo and B isotope systematics thus reveal carbonated fluid-fluxed mantle melting occurred beneath the eastern NCC. Increasing fluid inputs led to increases in the degree of melting of the mantle source that generated melts ranging in composition from nephelinite through basanite. The origin of such a fluid flux is enigmatic in an intraplate setting, but could be related to the decarbonization of subducted slabs stalled in the deep mantle.
AB - Fluid-fluxed melting of the mantle is the principal mechanism for the generation of arc magmas above subduction zones, but it has rarely been documented as important in intraplate settings. Here, we present new molybdenum (Mo) isotopic data from a suite of well-characterized Cenozoic basalts from the eastern North China Craton (NCC) to constrain the mechanisms of mantle melting in the region. These basalts represent mixtures of three components, namely, nephelinites and basanites that variably mixed with alkali basalts. The alkali basalts have relatively low δ 98/95 Mo ranging from −0.56‰ to −0.22‰ relative to the NIST3134 standard, suggesting the presence of dehydrated pelagic sediments in their mantle source rocks. Interestingly, the basanites range to higher δ 98/95 Mo (−0.04 ± 0.02‰) compared with the nephelinites (−0.31‰ to −0.22‰) with no associated changes in their radiogenic isotopes, and they show trends of decreasing Dy/Yb, Ce/Mo and δ 11 B with increasing Ba/Nb and B/Nb ratios. These correlations indicate that Mo behaves as a fluid-mobile trace element in the intraplate mantle beneath the NCC, and that the mantle sources of the nephelinites and basanites were infiltrated by this fluid component. Existing data show that this fluid component has lower H 2 O/Ce ratios (110–130) than that of mid-ocean ridge basalts (H 2 O/Ce ≈ 200). This result, together with the low-SiO 2 contents, relative depletion of Zr–Hf–Ti, and high δ 66 Zn characteristics of these strongly alkaline rocks, suggests derivation from a carbonated mantle source. Mo and B isotope systematics thus reveal carbonated fluid-fluxed mantle melting occurred beneath the eastern NCC. Increasing fluid inputs led to increases in the degree of melting of the mantle source that generated melts ranging in composition from nephelinite through basanite. The origin of such a fluid flux is enigmatic in an intraplate setting, but could be related to the decarbonization of subducted slabs stalled in the deep mantle.
KW - Mo–B isotopes
KW - intracontinental basalts
KW - North China Craton
KW - fluid-fluxed melting
KW - carbonated fluid
UR - https://digitalcommons.usf.edu/geo_facpub/1776
UR - https://doi.org/10.1016/j.epsl.2019.05.038
UR - https://digitalcommons.usf.edu/geo_facpub/2153
U2 - 10.1016/j.epsl.2019.05.038
DO - 10.1016/j.epsl.2019.05.038
M3 - Article
VL - 520
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
ER -