Strontium and neodynium isotope ratios in the Fraser River, British Columbia: a riverine transect across the Cordilleran orogen

Abstract

The Fraser is the largest undammed river to traverse the North American Cordillera. Its source is in Precambrian/Palaeozoic sedimentary rocks of the Rocky Mountains, at the margin of the North American Craton, from where it flows west across accreted ensimatic terranes, to the young magmatic arc of the Coast Range. 87 Sr 86 Sr and 143 Nd 144 Nd in suspended matter from the Fraser and its tributaries outline a mixing curve between two end members, (a) LILE-rich cratonic material with 87 Sr 86 Sr ∼ 0.77 and ϵ Nd (0) ∼ −23 and an Archaean crustal residence age and (b) material from mantle-derived igneous rocks with 87 Sr 86 Sr ∼ 0.704 and ϵ Nd (0) ∼ +4 and a crustal residence age of ∼ 0.6 Ga. In the eastern, ensialic region, dissolved Sr is less radiogenic than Sr in suspended matter, because of the contribution of dissolved Sr from Rb-poor carbonate rocks. As the Fraser flows west over younger terranes, this relationship inverts, with dissolved Sr more radiogenic. Inversion reflects a difference in the transport behaviour of dissolved and suspended material. Dissolved Sr from upstream, with a radiogenic signature, remains in solution, whereas the suspended load from upstream tends to fall out where the current decreases, to be replaced by local, less radiogenic material. For riverborne fluxes entering the ocean at active margins, compositions of conservative dissolved elements, such as Sr, represent both the interior craton and the outer orogen, while elements carried mainly in suspended form, such as Nd, are biased towards the outer orogen.