NEW INSIGHTS ON CASCADE ARC MANTLE HETEROGENEITY FROM BASALT GEOCHEMISTRY

To what extent is the compositional diversity of primary arc magmas related to mantle heterogeneity? In the Cascade Arc of western North America, three main primitive magma types are distinguished by major and trace elements: calc-alkaline basalt (CAB), high-alumina olivine tholeiite (HAOT), and relatively minor intraplate basalt (IPB). Many studies have concluded that these basalt groups are associated with three distinct mantle sources [e.g., 1]. However, new high precision Sr-Nd-Hf-Pb isotope and trace element data for basalts from 7 High Cascade volcanoes and the Garibaldi Volcanic Belt (GVB) reveal that all High Cascade CAB and HAOT (except Mt. Adams HAOT) are derived from a single ambient mantle end member variably modified by slab input. High Cascade isotopic data define a linear array between high precision Juan de Fuca MORB data and a narrowly constrained sediment end member with the composition of bulk Northern Cascadia sediment [2], indicating efficient homogenization of the diverse subducting sediment package. The High Cascade array does not intersect the field defined by Astoria Fan sediment from the Columbia River [3], consistent with its young depositional age. High Cascade HAOT and CAB overlap isotopically, although about half the HAOT have slightly lower ²⁰⁷Pb/²⁰⁴Pb and higher ε_Hf than CAB. Glacier Peak, commonly classified as the southernmost GVB volcano, plots on the High Cascade array. The rest of the GVB extends towards a second mantle component with lower ²⁰⁸Pb*/²⁰⁶Pb* and ε_Hf that produces alkalic IPB above the northern GVB slab edge [4]. Finally, rear-arc Mt. Adams HAOT and neighboring Simcoe IPB define a third isotopic array that is parallel to MORB. Sediment input is greatly reduced in both GVB and Adams-Simcoe basalts.

Our new data imply that much of western North America is underlain by depleted mantle with common isotopic characteristics. Factors other than mantle heterogeneity are required to account for the major/trace element differences between Cascade HAOT and CAB. These processes may include recent episodes of melt extraction and/or closed-system refertilization of the mantle prior to basalt generation.

[1] Schmidt et al. (2008) EPSL 266, 166-181 [2] Carpentier et al. (2014) Chem Geol 382, 67-82 [3] Prytulak et al. (2006) Chem Geol 233, 276-292 [4] Mullen & Weis (2013) G3 14, 3126-3155