Re-Assessment of Cascade Arc Mantle Heterogeneity and Slab Inputs using High-Precision Pb-Hf-Sr-Nd Isotopic Data

Abstract:

In the Cascade Arc of western North America, several primitive magma lineages are distinguished by major and trace elements: calc-alkaline basalt (CAB), high-alumina olivine tholeiite (HAOT) and relatively minor intraplate basalt (IPB). Previous studies have concluded that these basalt groups represent distinct mantle sources ¹. However, new high precision Sr-Nd-Hf-Pb isotope data for primitive magmas from 7 High Cascades volcanic centers show that CAB and HAOT are derived from the same isotopically depleted mantle, with the exception of Mt. Adams-Simcoe backarc basalts. In isotope space, High Cascades CAB and HAOT have similar compositional ranges, forming a single mixing array between two end members that coincide with Juan de Fuca (JdF) MORB and bulk average northern Cascadia sediment². The High Cascades array is consistent with a depleted sub-arc mantle similar to JdF MORB-source, modified by a homogenized subducted sediment component. The High Cascades array does not intersect Astoria Fan compositions, consistent with the young depositional age of this sediment³. Trace element data for CAB also indicate contributions from a third end member that is a match to fluid derived from subducting JdF MORB. Glacier Peak, the southernmost Garibaldi Belt center, also plots on the High Cascades array. More northerly Garibaldi Belt basalts have lower ²⁰⁸Pb*/²⁰⁶Pb* and εHf, reflecting influx of enriched mantle at the northern slab edge that generates IPB⁴. Mt. Adams-Simcoe HAOT and IPB tap a second enriched mantle component that is consistent with a slab tear in this backarc region. The most isotopically ‘enriched’ High Cascades CAB and HAOT overlap in isotope and trace element compositions with the Imnaha (C2) component of the Columbia River basalts⁵, indicating that this mantle is a widespread and long-lived feature in the Pacific Northwest.

¹Schmidt et al. 2008, EPSL 266, 166. ²Carpentier et al. 2014, Chem. Geol. 382, 67. ³Prytulak et al. 2006, Chem. Geol. 233, 276. ⁴Mullen & Weis 2015, EPSL 414, 100. ⁵Wolff et al. 2008, Nat. Geosci. 1, 177.