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Pdf/14/1/286/4120162/286.Pdf 286 by Guest on 02 October 2021 Research Paper Research Paper GEOSPHERE Alteration, mass analysis, and magmatic compositions of the Sentinel Bluffs Member, Columbia River flood basalt province GEOSPHERE; v. 14, no. 1 Michael G. Sawlan U.S. Geological Survey, 345 Middlefield Road, MS 973, Menlo Park, California 94025, USA doi:10.1130/GES01188.1 13 figures; 4 supplemental files ABSTRACT The CRBG stratigraphy has been developed from numerous studies in which chemical composition, magnetic polarity, petrography, physical flow CORRESPONDENCE: msawlan@usgs .gov Accurate characterization of the magmatic compositions of flood basalt characteristics, and stratigraphic position have been used to distinguish indi- lavas is fundamental to interpretations of magma genesis, stratigraphy, and vidual flows or packages of compositionally similar flows (e.g., Wright et al., CITATION: Sawlan, M.G., 2018, Alteration, mass correlation across these extensive provinces. Analysis of the geochemistry of 1973; Swanson et al., 1979; Mangan et al., 1986; Reidel et al., 1989, 2013; Reidel analysis, and magmatic compositions of the Sentinel Bluffs Member, Columbia River flood basalt province: the Sentinel Bluffs Member of the Grande Ronde Basalt, Columbia River Basalt and Tolan, 2013). Although nonchemical characteristics have been useful in Geosphere, v. 14, no. 1, p. 286–303, doi: 10 .1130 Group (northwestern USA), demonstrates that a mass-based methodology, distinguishing some post–Grande Ronde Basalt (GRB) lavas, field identifica- /GES01188.1. similar to those routinely used in studies of weathering and soil formation, tion of GRB flows is problematic in that, with few exceptions, most are aphyric enables the identification of subtle and previously unrecognized low-tem- or rarely to sparsely porphyritic (see Reidel and Tolan, 2013, table 2 therein). Science Editor: Shanaka de Silva perature alteration, and the determination of primary magmatic geochemical Swanson et al. (1979) could reliably subdivide and map the GRB only on the Associate Editor: Julie Roberge characteristics in rocks modified by secondary processes. This methodology, basis of four magnetic polarity intervals. Mangan et al. (1986) and Reidel et al. here termed mass analysis, employs concentrations and ratios of immobile (1989) later subdivided GRB lavas within the four polarity intervals on the basis Received 20 March 2015 Revision received 25 July 2017 elements, which are not transported by low-temperature alteration processes, of chemical differences and stratigraphic position, and Reidel and Tolan (2013) Accepted 9 October 2017 to show that alteration has resulted in loss of rock mass due to mineral dis- recognized several additional members within the GRB. Increased resolution Published online 8 December 2017 solution in anoxic groundwater. Immobile element abundances corrected for of the GRB chemostratigraphy has corresponded to improvements in analyti- mass loss permit the identification and province-wide correlation of individual cal precision of several generations of analytical instruments over the past four flows and flow packages, even for rocks that have undergone nearly 50% mass decades. Chemical criteria have therefore emerged as the defining criteria for loss. The methodology developed with Sentinel Bluffs lavas is applicable to characterizing GRB stratigraphic units. other lavas of the Columbia River flood basalt province, and most likely to The use of chemical criteria to discriminate among stratigraphic units is other volcanic provinces in which lavas have undergone long-term interaction straightforward where the chemical differences between units exceed the vari- with groundwater. ations within members. This applies to many post-GRB lavas (Wanapum Ba- salt and Saddle Mountains Basalt). Chemical differences between GRB mem- INTRODUCTION bers, as currently defined, vary from distinct to ambiguous (see Reidel and Tolan, 2013, figure 7 therein). For example, within the GRB 2R polarity interval, There is perhaps no more important aspect to understanding flood basalt the Meyer Ridge Member is readily distinguished from other R2 flows by its volcanism, and to applying this knowledge to derivative geologic applications, markedly higher MgO and compatible trace elements (e.g., Cr). However, a OLD G than establishing the magmatic compositions of the lavas. The ability to cor- TiO2 threshold was adopted to distinguish the Grouse Creek and Wapshilla relate lavas, determine their chemostratigraphy, and understand their petro- Ridge members among a compositional continuum among low-Mg R2 flows genesis relies on the accurate characterization of their magmatic chemistry. (Reidel and Tolan, 2013). It is evident from the current chemostratigraphic cri- The Columbia River Basalt Group (CRBG; northwestern USA) is the young- teria that the chemical variations within GRB members, and even between OPEN ACCESS est and, due to its excellent exposure, preservation, and access, the most some members, are not yet understood, and that the distinguishing chemical thoroughly studied continental flood basalt province on Earth. Despite being identity of individual flows within GRB members, including the Sentinel Bluffs the smallest province in terms of erupted magma volume, the CRBG includes Member (SB), cannot be reliably determined. Accordingly, the correlation of some of the most far-traveled lavas known. Several dozen lava flows (flow GRB flows across the nearly 600 km extent of the GRB (Fig. 1) has mainly been fields) span ~600 km from eastern Washington and Oregon or western Idaho limited to GRB members comprising multiple flows. to the Pacific Ocean. The CRBG also includes some of the largest eruptions of The problem of distinguishing individual flows within a GRB member is This paper is published under the terms of the basaltic magma; some eruptions correspond to supereruptions (magnitude, illustrated in Figure 2, which shows the TiO2-MgO distribution of SB samples CC‑BY‑NC license. M >8) (Self, 2006; Bryan et al., 2010). from earlier data sets. These analyses are from samples collected during the © 2017 The Authors GEOSPHERE | Volume 14 | Number 1 Sawlan | Alteration, mass analysis, and Sentinel Bluffs geochemistry Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/1/286/4120162/286.pdf 286 by guest on 02 October 2021 Research Paper 125° 123° 121° 119° 117° 115° CN WA ID 48° Sentinel Blus 48° U.S. MT Member ID Seattle Spokane Wenatchee Figure 1. Map showing approximate ex- tents of the Grande Ronde Basalt and the Sentinel Bluffs Member (SB) (after Reidel SG and Tolan, 2013), and the locations where N Pasco Pullman SB lavas were sampled for this study. Basin Filled triangles—continuous stratigraphic RANGE Pasco sections in which two or more SB flows CEA PG were sampled. Red two-letter identifiers 46° O WA 46° St. Helens for the continuously sampled sections BG DH OR are explained in Supplemental File 3 (see Pendleton footnote 4). Open triangles—approximate Gorge The Dalles locations of one or more samples from C . R. Portland Col WC La Grande BL AC nearby locations (generally within 10 km) Fossil for which constraints on stratigraphic po- E CIFI sition relative to other analyzed SB lavas . are lacking; these locations mostly repre- PA sent 1–3 samples, but 12 samples from 8 locations are represented by the symbol Eugene Grande Ronde located west of Saint Helens, Oregon. 44° 44° CN—Canada; ID—Idaho; MT—Montana; RANGillamette Vly Basalt T W OR—Oregon; WA—Washington. COAS 0100 200 CASCADES km OR ID Alteration, mass analysis,and magmatic compositions of the Sentinel Bluffs Member, Columbia River flood basalt province 124° 122° 120° 118° 116° Michael G. Sawlan U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 MS-973 2.1 Basalt Waste Isolation Project (BWIP1) (e.g., Landon and Long, 1989), geo- SUPPLEMENTAL FILE 1 logic mapping studies (e.g., Hooper and Gillespie, 1996; Reidel, 1988), theses, Previously Published Analyses of Sentinel Bluffs Member Samples and previously unpublished analyses, and are mainly from the compilation 2.0 Figure 2. TiO2-MgO distribution of pre- The primary source of the earlier SB data is the compilation of Reidel and Valenta (2000), of Reidel and Valenta (2000). Because this compilation was titled prelimi- which includes 777unique analyses (not including replicate analyses). In addition, thirty viously published analyses for Senti- previously unreported analyses from Reidel (2005) and a total of seven analyses from Derkey et nel Bluffs Member lavas. Most analy- nary, these data were reviewed to ensure that only SB lavas would be con- al. (1999, 2004) were also considered.These analyses were performed at the GeoAnalytical Lab, ses are from the compilation of Reidel 2 WSU, prior to a change in instrumentation (and in some analytical procedures) in mid-2004. 1.9 sidered here. As described in Supplemental File 1 , this review showed that and Valenta (2000); a small number of Supplemental File 3describes differences in the accuracy and precision of these analyses n analyses are from Reidel (2005) and some analyses from the older data set are not of SB lavas and these samples compared to the more recent analyses.Among the analyses in Reidel and Valenta (2000), most TiO2 arefrom samples collected for the Basalt Waste Isolation Project (BWIP).Other data sources Derkey et al. (1999, 2004). Analyses were excluded from consideration. These earlier SB data define a continuum referenced in Reidel and Valenta (2000) (see their Appendix A) includegeologic mapping 1.8 exclude outliers
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