Time-scales for magmatic differentiation at the Snaefellsjökull central volcano, western Iceland: Constraints from U–Th–Pa–Ra disequilibria in post-glacial lavas

Abstract

We present major and trace element and Sr–Nd–Pb and U–Th–Pa–Ra isotope data for a small sample suite of primarily post-glacial, mildly alkalic volcanic rocks from the Snaefellsjökull central volcano situated off the main rift systems in western Iceland. The volcanic rocks are crystal-poor and range from olivine alkali basalt to trachyte and show tight correlations of major and trace elements that are explained by fractional crystallization involving removal of olivine, plagioclase, clinopyroxene, Fe–Ti oxide and apatite. Sr–Nd–Pb isotopes are practically invariant, consistent with derivation from the same source region. During fractionation from primitive basalt to evolved trachyte, ( 230Th/ 232Th), ( 230Th/ 238U) and ( 231Pa/ 235U) decrease progressively at broadly constant ( 238U/ 232Th). A continuous closed-system fractionation model that assumes constant initial ( 230Th/ 232Th) in the basaltic precursor melt indicates that hawaiite was derived from olivine basalt by ∼50% fractional crystallization within 12.1 - 5.2 + 5.1 kyr and trachyte by ∼80% fractionation within 53.9 - 6.7 + 7.0 kyr . An overrepresentation of evolved basalts and hawaiites with young inferred magma ages in the dataset is consistent with the parental precursor to these magmas intruded into the sub-volcanic magma plumbing system as a consequence of lithospheric rebound caused by deglaciation. Lavas affected by apatite removal have higher ( 231Pa/ 235U) than predicted for simple radioactive decay, suggesting apatite significantly fractionates U from Pa. The proposed fractionation model consistently explains our U-series data assuming D Pa apa ⩽ 0.5 and D U apa ∼ 3.2 and D Th apa ∼ 3.0 . If applicable, these D values would indicate that the effect of apatite fractionation must be adequately considered when assessing differentiation time scales using ( 231Pa/ 235U) disequilibria data.