Invited research articlePrimitive arc magma diversity: New geochemical insights in the Cascade Arc
Graphical abstract
Introduction
Primary magmas in subduction settings encompass a wide range of compositional variability on local to global scales (e.g., Kelemen et al., 2014, Arculus, 1994). Deciphering the factors responsible for this diversity is difficult because these magmas are likely to contain contributions from a compositionally heterogeneous array of fluids and/or melts derived from several reservoirs in the subducting plate during prograde metamorphism. These reservoirs include altered oceanic crust, overlying sediment, and underlying hydrated mantle lithosphere (e.g., Schmidt and Poli, 2014). Pre-existing compositional variabilities within the mantle wedge are an additional potential source of diversity among primary arc magmas. Many studies concur that the sub-arc mantle primarily consists of depleted mantle that is similar in composition to the sources of mid-ocean ridge basalts (MORBs) (e.g., Green, 1976, Pearce and Peate, 1995, Perfit et al., 1980). However, enriched ocean island basalt (OIB) source mantle has been proposed to play an important role in controlling magma compositions in some arcs (Central America: Reagan and Gill, 1989; Mexico: Petrone et al., 2003; Sulu: Macpherson et al., 2010, Castillo et al., 2007; Izu-Bonin: Hochstaedter et al., 2000; Marianas: Woodhead et al., 2012; Aleutians: Morris and Hart, 1983; Fiji: Gill, 1984, Ellam and Hawkesworth, 1988).
The Cascade Arc of western North America offers a particularly interesting case in which to examine the factors contributing to arc magma compositional diversity because several genetically distinct primitive magma lineages have been identified on the basis of major and trace elements. The most widespread of these suites are referred to in this paper as high alumina olivine tholeiite (HAOT), calc-alkaline basalt (CAB), and intraplate-type basalt (IPB). Previous studies have concluded that these lineages reflect a sub-arc mantle that is composed of geochemically and mineralogically distinct domains of lithospheric and asthenospheric mantle, ranging in composition from MORB- to OIB-source-type, that have been modified by a spectrum of slab-derived components on multiple occasions (Righter, 2000, Hildreth, 2007, Bacon et al., 1997, Leeman et al., 1990, Leeman et al., 2005, Borg et al., 1997, Borg et al., 2002, Conrey et al., 1997, Grove et al., 2002, Schmidt et al., 2008, Green and Harry, 1999, Hughes, 1990, Baker et al., 1994). The Cascades is also considered an endmember ‘hot’ subduction zone, and the relatively high temperatures predicted for the subducting Juan de Fuca plate (Syracuse et al., 2010, Wada and Wang, 2009) imply nearly complete dehydration of the slab at sub-arc depths (e.g., van Keken et al., 2011, Hacker et al., 2003, Harry and Green, 1999). In turn, several workers have proposed that some Cascade Arc magmas may be little-influenced by modern slab-derived components (e.g., Leeman et al., 1990, Leeman et al., 2004, Leeman et al., 2005, Conrey et al., 1997, Borg et al., 1997, Borg et al., 2002). In this study, we use new high-precision Sr-Nd-Hf-Pb isotope and trace element data to test the hypothesis that the major endmember primary magma suites in the Cascade Arc represent isotopically distinct mantle sources on a regional scale. The high precision geochemical data that have recently become available for subducting sediment on the Juan de Fuca plate (Carpentier et al., 2013, Carpentier et al., 2014, Prytulak et al., 2006) and oceanic crust of the Explorer plate (Cousens et al., in review) further allow us to ascertain which slab-derived components have contributed to Cascade Arc magmas.
Radiogenic isotope ratios can be effective tracers of potential mantle heterogeneities and the fate of slab-derived materials, particularly when measured with modern analytical techniques that offer higher accuracy and precision. For example, multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) coupled with advances in sample preparation techniques have led to significant improvements over TIMS (thermal ionization mass spectrometry) in accuracy and precision, particularly in the case of lead isotopes, and to the routine analysis of Hf isotopic ratios (Albarède et al., 2004, Blichert-Toft et al., 1997, Weis et al., 2006). In the Garibaldi Volcanic Belt (GVB), the northern segment of the Cascade Arc (Fig. 1), high-precision Sr-Nd-Hf-Pb isotopic studies have demonstrated that IPBs and CABs tap two geochemically distinct mantle sources (Mullen and Weis, 2013, Mullen and Weis, 2015). In the main segment of the arc, the High Cascades (Fig. 1), numerous Sr-Nd-Pb radiogenic isotope studies have been conducted over the past 4 + decades. However, it has not been possible to conclusively resolve whether the Cascade primary magma groups originate from isotopically distinct mantle domains on a whole-arc scale because few of the data are associated with the requisite precision. Published high-precision Pb isotope data are limited to the Mt. Rainier area (Sisson et al., 2014), and Hf isotope data, which can be particularly useful in arc settings because Hf is mobilized to a lesser extent in slab-derived fluids than are Sr, Nd and Pb (Woodhead et al., 2012), are available only for Lassen Peak and Mt. Adams (Borg et al., 2002, Jicha et al., 2009a).
We have measured new high-precision Sr-Nd-Hf-Pb isotopic and trace element data for magmas from seven High Cascades volcanic fields. The samples analyzed from Lassen Peak, Medicine Lake, Crater Lake, Mt. Shasta, Mt. Adams, and Simcoe (Fig. 1) were identified by Bacon et al. (1997) as representative of the most primitive magmas in the High Cascades. We also analyzed a suite of the distinctive North Sister-type basaltic andesites from the Three Sisters region (Fig. 1) to examine the isotopic effects of crustal contamination because these magmas record pervasive assimilation of mafic deep crust (Schmidt and Grunder, 2011, Schmidt et al., 2013). We also present several new analyses of GVB magmas to augment the existing high-precision isotopic database (Mullen and Weis, 2013, Mullen and Weis, 2015, Mullen and McCallum, 2014) and to strengthen our comparison between the GVB and High Cascades segments. The new GVB dataset includes the first published geochemical data for the Silverthrone volcanic field, which was proposed to be the northernmost volcano in the Cascades by Green et al. (1988) but has been excluded from the arc by others (e.g., Hildreth, 2007).
Our new isotopic data differ significantly from previously published data on the same samples, particularly in Pb isotopes, which are associated with a ten-fold increase in analytical precision. Most importantly, the new data show that all CABs and HAOTs from the High Cascades are derived from a single isotopic mantle endmember that has been variably modified by subduction components. Factors other than mantle heterogeneity must be responsible for the differences between HAOTs and CABs. In contrast, IPBs sample isotopically distinct mantle components.
Section snippets
Cascadia subduction system: High Cascades and Garibaldi Volcanic Belt
The active Cascade Arc extends ~ 1300 km between southwestern British Columbia and northern California (Fig. 1). Magmatism initiated at ~ 4 Ma (du Bray and John, 2011) and is a consequence of northeasterly subduction of the Juan de Fuca plate system beneath the North American plate. Rates of convergence increase northward along the trench from 30 to 45 mm/year (Wilson, 2002). The southern portion of the Juan de Fuca plate, known as the Gorda sub-plate, has been internally deforming for the past ~ 3 Ma
Cascade Arc primitive magmas
Primary partial melts of the mantle, which have undergone no differentiation since separating from their sources, would ideally be used to investigate mantle sources but such melts are rare, especially in arc settings. The magmas most closely approaching primary compositions (i.e., most magnesian) must therefore be used instead, and we refer here to such magmas as “primitive” (e.g., Kelemen et al., 2014). Previous studies have established that the most primitive magmas in the Cascades have Mg#
Samples analyzed
In an effort to evaluate the numerous hypotheses proposed for the origin of Cascade primitive magma diversity, we analyzed 53 samples from 11 Quaternary volcanic centers for Sr-Nd-Hf-Pb isotope ratios and trace elements. Although the samples are assigned the name of the nearest major composite volcano, most of the magmas were erupted from peripheral vents as is typical for basalts in the Cascades (Hildreth, 2007). Thirty-six of the samples are re-analyzed splits of the same sample powders used
Analytical techniques
Sr-Nd-Hf-Pb isotopic and trace element analyses were conducted at the Pacific Centre for Isotope and Geochemical Research (PCIGR) at The University of British Columbia. All chemical separations and mass spectrometric analyses were carried out in Class 100 and 10,000 clean laboratories. Sample powders (~ 0.1 g) were digested in 15 mL screw-top Savillex beakers on a hotplate at 120–130 °C for 2–3 days in a 10:1 mixture of concentrated sub-boiled HF and HNO3, with periodic ultrasonication. Samples were
Trace elements
Trace element data are listed in Table 1, Table 2. The data are compared to previous analyses of the same samples by Bacon et al. (1997) in Fig. A1 and Table A4. Most REEs are within 10% of the previous values that were measured by isotope dilution (Fig. A1; Table A4). Pb concentrations are uniformly higher in the new dataset, although Bacon et al. previously analyzed Pb by isotope dilution. For elements previously measured by XRF, ICP-AES, or INAA, the new data are typically within ± 30%, with
New vs. previous isotope data
The significant difference between our new Pb isotope ratios and those previously published is attributable to the nearly 10 times better reproducibility of MC-ICP-MS analyses as compared to the previous TIMS measurements (~ 0.1% external 2σ reproducibility), as well as the employment of acid leaching in this study. In the case of Nd isotopes, although MC-ICP-MS and TIMS data can, in principle, be associated with similar external reproducibilities, our new MC-ICP-MS data are outside the 2σ
Conclusions
New high precision Pb-Sr-Nd-Hf isotope and trace element data for magmas from seven volcanic centers in the High Cascade segment of the Cascade Arc, coupled with new and published high precision isotopic data from the GVB segment, permit a re-evaluation of models proposed for the petrogenesis of compositionally diverse Cascade Arc primitive magmas from three endmember groups, CABs, HAOTs, and IPBs. The most widespread basalts in the High Cascades, CABs and HAOTs, are distinct in major and trace
Acknowledgements
We thank T. Horscroft for encouraging us to submit this invited paper and C. Chauvel for efficient editorial handling. The comments of G. Jacques and an anonymous reviewer improved the presentation and helped us to clarify our scientific arguments. C. Bacon, A. Grunder, G. Woodsworth, N. Vigouroux, and P. Adam are acknowledged for generously providing the samples from the High Cascades, North Sister, Silverthrone, Mt. Garibaldi, and the Bridge River Cones and Elaho Valley areas, respectively.
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