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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

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125° 123° 121° 119° 117° 115°

CN WA ID

48° Sentinel Blu s 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 that are most likely studies, theses,and previously unpublished data from several contributors. of compositions, which, at higher MgO values, form a point cloud; with de- from samples of other Grande Ronde Basalt members (Supplemental File 1; creasing MgO, two diffuse, inversely correlated arrays are apparent. A key 2Supplemental File 1. Describes previously published see footnote 2). Oxide abundances objective of this study is to assess whether this variation truly represents a analyses of Sentinel Bluffs Member (SB) samples, the 1.7 (in wt%) from analyses normalized criteria applied in validating samples as SB samples, to 100% volatile free, as indicated by 100 1 MI Al2O3-TiO2 baseline determinations for these The Basalt Waste Isolation Project (1976 to 1988) conducted studies to evaluate the suitability of superscript n. samples, and the basis to identify samples collected GRB lava as a deep subsurface repository for nuclear waste beneath the Hanford site located in 1.6 from boreholes. Please visit http://doi .org /10 .1130 4.04.4 4.85.2 5.6 the Pasco Basin, south-central Washington (Fig. 1). See Dahlem (1987) for a summary descrip- /GES01188.S1 or the full-text article on www.gsapubs n tion of these studies. The BWIP followed preliminary feasibility studies conducted from 1968 to .org to view Supplemental File 1. MgO 1972 (e.g., Atlantic Richfield Hanford Company, 1976).

continuum in SB magmatic compositions, or whether this variation results are common in surface exposures of lava in those areas not scoured by the from postemplacement modification of a limited number of chemically dis- Missoula floods. tinct magmatic compositions. This question has implications beyond under- CRBG lavas generally have been considered to be chemically homo standing the lava chemistry and chemostratigraphy of the SB. The stratig- geneous (e.g., Wright et al., 1973; Hooper, 1984, 1988, 2000; Reidel et al., 1989; raphy of the entire GRB is based largely on chemical differences between Tolan et al., 2009), although a few exceptions have been reported (Reidel and

lavas, mainly in TiO2 and MgO abundances, in concert with magnetic polarity Fecht, 1987; Reidel, 1998, 2005; Vye-Brown et al., 2013a; Reidel and Tolan, and stratigraphic position (see Reidel and Tolan, 2013, table 2 and figure 7 2013). Chemical variations within a flow or package of flows are also observed. therein). For example, among stratigraphic units of the Saddle Mountains Basalt, vari- Due to their great expanse and rapid emplacement, individual GRB lavas ations in Ti and P abundances are commonly 5%–10% (e.g., Hooper, 2000, fig- can serve as strain markers in documenting regional structure, identifying ure 3 therein), and such variations have gone unexplained. Intraflow chemical faults and their displacements, and defining the geometries of CRBG-hosted differences have been reported in several studies (Mangan et al., 1986; Reidel, aquifers. Regional aquifer systems hosted by GRB lavas (Kahle et al., 2011; 1998, 2005; Vye-Brown et al., 2013a; Reidel and Fecht, 1987). In accounting for Burns et al., 2011; Conlon et al., 2005) support agriculture province-wide, and such variations, several studies (Reidel and Fecht, 1987; Reidel, 1998, 2005) supply the greater Portland-Vancouver metropolitan area. Previously consid- have appealed to mixing, distal to vents, of lava flows that were simultane- ered as potential repositories for nuclear waste under the BWIP and predeces- ously erupted but independently sourced. Vye-Brown et al. (2013a) attributed sor studies conducted from 1968 to 1988 (Atlantic Richfield Hanford Company, intraflow variations to inflation of lava fed by chemically zoned magma. 1976; Myers and Price, 1979, 1981; Landon and Long, 1989), GRB lavas have Along with an updated assessment of SB geochemistry and chemostratig-

recently been evaluated as sites for CO2 sequestration (McGrail et al., 2011, raphy, the development of a mass-based methodology is presented to account 2014; Zakharova et al., 2012). The precision in the use of GRB lavas as strain for changes in SB compositions resulting from alteration. This methodology, markers is dependent on the level of specificity in the identification of chemo- referred to as mass analysis, derives from quantitative relationships among stratigraphic units. immobile elements. This report focuses on the immobile element variations, Although CRBG lavas are commonly regarded as well preserved, several using mass analysis, to define the magmatic chemical compositions and lines of evidence suggest that they have undergone water-rock interaction, chemostratigraphy of SB lavas, and how these enable province-wide correla- which potentially can modify their chemical composition. Secondary min- tions of individual flows. Mobile element variations are discussed here only erals in lavas from surface exposures and the subsurface have been amply to a limited extent where they bear on the conditions of alteration or on crite- documented (Ames, 1980; Benson and Teague, 1982; Hearn et al., 1985). R.E. ria used for chemostratigraphic distinctions. The variations in mobile element Evarts (2008, personal commun.) and Wells et al. (2009) recognized chemical abundances due to alteration are sufficiently complex that this topic will be modifications to GRB lavas, notably iron depletion, and applied an empirically addressed subsequently elsewhere. derived, minimum FeO threshold to screen samples deemed weathered. It is important that they noted that many flows with a fresh appearance (e.g., dark THE SENTINEL BLUFFS MEMBER gray color, unaltered plagioclase) were nonetheless chemically altered. Their observations corroborated earlier observations (Benson and Teague, 1982) Lavas composing the SB, the uppermost member of the GRB, are the sub- made on samples obtained from deep in the subsurface. Given the indica- ject of this geochemical study because of their widespread distribution (Fig. 1), tions of post-magmatic water-rock interaction in CRBG lavas, it is therefore well-exposed stratigraphic sections of multiple flows throughout much of the important to establish a basis for evaluating whether lava compositions have province, a relatively large range in composition, and large numbers of analy maintained fidelity to their original magmatic compositions. ses available from prior studies noted here. The SB composes a significant The late Pleistocene Missoula floods, which traversed the CRBG, removed part, nearly 5% by volume, of the entire CRBG, and SB lavas span much of the much of the surficially weathered rock in areas where the floods were erosive. extent of the GRB (Reidel et al., 2013; Reidel and Tolan, 2013) (Fig. 1). SB lavas Examination of only flood-scoured exposures of the CRBG can lead to the mis were erupted during the C5Cn.1n polarity chron (Jarboe et al., 2010), which impression that CRBG lavas were somehow resistant to weathering over the according to Hilgen et al. (2012) spanned the time interval 16.27–15.97 Ma. past 15–16 m.y. Surficial alteration (under oxic conditions) of CRBG lavas is Reidel and Tolan (2013) estimated that the SB comprises 11–15 flows or flow widespread across the province and, except in those areas deeply scoured by lobes. Given the estimated SB volume and number of flows, SB eruptions Missoula floods, presents an impediment to sampling unweathered rock. In averaged ~680 km3. the western parts of the province (Coast Range and Willamette Valley), sapro- Despite the accumulation of >1000 chemical analyses of SB lavas, the num- lite commonly is developed in CRBG lavas to depths of 5–10 m and, in places, ber of compositionally distinct flows remains uncertain. In their study of SB to more than 30 m, where lavas may form laterite. In the eastern parts of the flows within the Pasco Basin and vicinity, Landon and Long (1989) recognized province, in the Columbia Basin and vicinity, thick weathering rinds (to 15 cm) three flow packages based on chemical criteria, and a total of 12 flows distrib-

Supplemental File 2: Alteration, mass analysis and magmatic compositions of the Sentinel Bluffs Member, Columbia River flood basalt province, by M. G. Sawlan

Unnormalized Major and Trace element Abundances uted equally among the flow packages. Individual flows within the packages, ment. Sources of intralaboratory bias due to changes in analytical strategy

Chem_ID_fieldChem_ID_lab Section_ID SB_Sec_FlowSB_Series SB_Group Sample_typeSite_contextColl._By Long_NAD83 080721-1914MS 10MS027 Armstrong CynAC-1 39interrindroadcut Sawlan -120.368381 however, were identified from their thicknesses, other physical characteristics, (e.g., background determinations) and grinding media over time are also de- 080721-1807MS 10MS028 Armstrong CynAC-1 39interrindoutcrop Sawlan -120.367873 080721-1701MS 10MS029 Armstrong CynAC-2 39interrindoutcrop Sawlan -120.367195 CB0119 10MS173 BingenBG-01 11interrindoutcrop Sawlan -121.416668 100514-1258MS 10MS174 Bingen BG-0212interseam outcropSawlan-121.413958 100514-1258MS-a 10MS175 Bingen BG-0212hyd. seam outcropSawlan-121.413958 and paleomagnetic inclinations in multiple boreholes from within a relatively scribed in Supplemental File 3 (see footnote 4). 100514-1106MS 10MS178 Bingen BG-0312interseam outcropSawlan-121.413250 100514-1106MS-a 10MS259 Bingen BG-0312hyd. seam outcropSawlan-121.413250 100813-1501MS 10MS176 BingenBG-04 24interseam outcropSawlan-121.412506 100813-1501MS-a 10MS177 Bingen BG-0424hyd. seam outcropSawlan-121.412506 limited area compared to the entire extent of the SB. Among these 12 flows At all sites, a primary sample internal to alteration rinds (inter-rind sam- 100813-1501MS-b 10MS258 BingenBG-04 24inner rind-sec. min. outcropSawlan-121.412506 100512-1730MS 10MS179 Bingen BG-0525interseam roadcutSawlan-121.411408 100512-1624MS 10MS180 Bingen BG-0625interseam roadcutSawlan-121.411090 100512-1624MS-a 10MS260 Bingen BG-0625hyd. seam roadcutSawlan-121.411090 (recognized from contacts), some could possibly correspond to successive ple) was collected from minimally vesicular rock within the dense flow interior, 100512-1349MS 10MS181 BingenBG-07 38interseam roadcutSawlan-121.410362 100512-1349MS-a 10MS261 Bingen BG-0738hyd. seam roadcutSawlan-121.410362 100512-1349MS-b 10MS262 Bingen BG-0738interrind roadcutSawlan-121.410362 100512-1256MS 10MS182 Bingen BG-0838interseam roadcutSawlan-121.409691 100512-1256MS-a 10MS263 Bingen BG-0838hyd. seam roadcutSawlan-121.409691 lobes of one lava flow or to successive lava flows within a flow field (for defi- or flow core (per Self et al., 1996, 1997; Vye-Brown et al., 2013b). Sampling 100512-1051MS 10MS183 BingenBG-09 410interseam roadcutSawlan-121.408681 100512-1051MS-a 10MS264 Bingen BG-09410 hyd. seam roadcutSawlan-121.408681 100512-1051MS-b 10MS265 BingenBG-09 410interrind roadcutSawlan-121.408681 100511-1800MS 10MS184 Bingen BG-10412 interseam roadcut Sawlan -121.407708 nitions of a flow lobe, lava flow, and flow field, see Thordarson and Self, 1998, was carried out to minimize the amounts of macroscopic secondary miner- 100511-1800MS-a 10MS266 Bingen BG-10412 interrindroadcut Sawlan -121.407708 100511-1303MS 10MS185 BingenBG-11 515interseam outcropSawlan-121.406862 100511-1303MS-a 10MS267 Bingen BG-11515 hyd. seam outcropSawlan-121.406862 100511-1401MS 10MS186 Bingen BG-12515 interseam outcropSawlan-121.406806 table 1 therein). als, if present, within the analyzed material. The vesicular parts of lavas were 100511-1620MS 10MS187 BingenBG-13 515interseam outcropSawlan-121.406585 110427-1738MS 10MS147 Butler CynBL-125interseam roadcutSawlan-121.152974 110427-1738MS-a 10MS320 Butler CynBL-125hyd. seam roadcutSawlan-121.152974 100924-1440MS 10MS139 Butler CynBL-226interseam roadcutSawlan-121.150920 100924-1440MS-a 10MS310 Butler CynBL-226hyd. seam roadcutSawlan-121.150920 Based on his observation of chemical differences within some flows, Reidel thus avoided because secondary minerals are typically found within vesicles 100924-1559MS 10MS140 Butler CynBL-337interseam roadcutSawlan-121.149243 100924-1559MS-a 10MS311 Butler CynBL-337hyd. seam roadcutSawlan-121.149243 100924-1559MS-b 10MS312 Butler CynBL-337interrind roadcutSawlan-121.149243 100924-1742MS 10MS141 Butler CynBL-438interseam roadcut Sawlan -121.147706 (2005) suggested that the SB stratigraphy is more complex than that of Landon and crystal-bound voids. This has been shown in samples from surface expo- 100924-1742MS-a 10MS313 Butler CynBL-438hyd. seam roadcutSawlan-121.147706 100925-1125MS 10MS142 Butler CynBL-539interseam roadcutSawlan-121.148144 100925-1125MS-a 10MS314 Butler CynBL-539hyd. seam roadcutSawlan-121.148144 100925-1302MS 10MS143 Butler CynBL-6515 interseam roadcutSawlan-121.147083 100925-1302MS-a 10MS315 Butler CynBL-6515 hyd. seam roadcutSawlan-121.147083 and Long (1989). Relying mostly on TiO2 and P2O5 abundances, he assigned sures (Hearn et al., 1985), as well in samples from deep in the subsurface (see 100930-1545MS 10MS144 Butler CynBL-8515 interseam roadcutSawlan-121.146979 100930-1545MS-a 10MS145 Butler CynBL-8515 inner rind-sec. min. roadcutSawlan-121.146979 flows, or intervals within flows, to one of six compositional types. Reidel’s Zakharova et al., 2012, figure 2 therein). Sampling of the freshest possible flow 3Supplemental File 2. Spreadsheet of XRF analyses (2005) chemical stratigraphy centered on a locally recognized flow within cores restricts the effects of alteration to dissolution of solid phases and trans- of Sentinel Bluffs Member (SB) samples analyzed for the Pasco Basin (informally named the Cohassett flow) that he proposed was port of their soluble constituents, such that any potential change in sample this study. Analyses are presented in three forms— unnormalized, normalized, and mass-normalized. In- formed from the incomplete mixing of four simultaneously erupted, but com- mass will be in one direction, negative. Weathered rock was avoided (except cludes supporting documentation such as stratigraphic positionally distinct, lava flows, each of which was designated a SB composi- in one instance where strongly weathered lava was intentionally sampled), position, geographic coordinates, sample type, and tional type. Multiple SB lavas stratigraphically below the Cohassett flow were thereby limiting alteration (if any) to conditions that were anoxic or nearly so. analysis date. Includes the slope and intercept for 100 MI assigned to one compositional type and others above this flow were assigned As will be shown, these constraints imposed by careful sampling are critical to lines used in calculating the sample mass index and mass-normalized analyses. Please visit http://doi .org to another. Reconciliation of the chemical groups defined here with earlier understanding the chemical effects of alteration. At several sites, unoxidized /10.1130/GES01188.S2 or the full-text article on www versions of SB stratigraphic nomenclature [i.e., informal flow names from the alteration rinds were also analyzed in addition to the primary sample from .gsapubs.org to view Supplemental File 2. BWIP studies (Landon and Long, 1989), and the compositional types of Reidel which alteration rinds were removed. The type of material analyzed is identi- (2005)] is beyond the scope of this report and will be presented elsewhere. fied for each analysis in Supplemental File 2 (see footnote 3). Samples from the Bingen, Butler Canyon, Devils Hole, Sentinel Gap, and Alteration, mass analysis,and magmatic compositions of the Winter Water Creek sections were additionally subsampled in the lab to mini- Sentinel Bluffs Member, Columbia River flood basalt province METHODS AND RESULTS mize pervasive, texturally distinct seams, referred to here as hydration seams Michael G. Sawlan U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 MS-973 Samples of multiple SB lava flows were collected for this study from that are described in Supplemental File 3 (see footnote 4). Subsamples of hy- well-exposed sections on the Columbia Plateau and in the Columbia River dration seams were analyzed at 32 sites to test whether these represented in- SUPPLEMENTAL FILE 3 Gorge, and include SB reference sections (Reidel and Tolan, 2013) at Sentinel cipient alteration. No attempt was made to minimize such seams in samples 1. METHODS Gap (Sentinel Bluffs) and at Devils Hole (Columbia Hills Water Gap section of from the Armstrong Canyon and Patrick Grade sections, and from sites within 1.1 Sampling Protocols Samples were collected usinga protocol aimed at minimizing inclusion of altered rock and Reidel, 2005; Fig. 1). Samples from the Coast Range are typically from iso- the Coast Range and Willamette Valley. Subsample pairs of inter-seam rock secondary minerals in the material to be analyzed. Alteration rinds and secondary minerals lining open space and fractures were the principal materials avoided in field sampling. An effort lated exposures lacking detailed stratigraphic context, usually from quarries or and hydration seams showed minimal chemical differences, if any, between to evaluate the influence of ubiquitous, texturally distinct anastomosing seams, termed “hydration seams”, was made by sub-sampling in the lab. other artificial cuts where only one to several lavas are exposed. them. In part, the lack of clear differences between these pairs might be caused

1.1.1 Field Sampling For this study I analyzed 112 samples from 75 sites. Major and trace ele- by the difficulty in separating inter-seam rock from the seams. Most inter-seam The typical field sampling protocol is illustrated in Figure S2.Several joint-bounded blocks ments were determined by X-ray fluorescence (XRF) using a ThermoARL in- samples were not pure separates, but usually contained 30% or more of the from the denser parts of flows were broken, compared for sample quality, and one was selected for sampling. Such blocks were commonly 20 to 40 cm across (see Figure S2B), but were strument at the Peter Hooper GeoAnalytical Lab, Washington State University, seams. In any case, small chemical differences between hydration-seam and smaller where more closely spaced fractures limited the size of the largest unfractured block. Pullman (herein, the GeoAnalytical Lab). Samples were analyzed from 2009 inter-seam rock are subordinate to the observed changes associated with bulk Samples were trimmed to remove alteration rinds and pockets of secondary minerals. through 2013, except for one sample analyzed in 2007. Analyses, along with rock alteration, and the geochemistry of hydration seams will not be addressed 4Supplemental File 3. Documents sampling proce- supporting documentation, are provided in Supplemental File 23. Field and further in this report. dures, and the precision and accuracy of XRF analyses of Sentinel Bluffs Member (SB) samples performed for laboratory sampling protocols, descriptions of the analytical methods, and the Thus, at most sites one primary sample (±samples of hydration seams and/or this study. Discusses differences between these and accuracy and precision of analyses are provided in Supplemental File 34. alteration rind) was taken from one site within the dense flow core. Five samples earlier (pre-2004) analyses of Sentinel Bluffs Mem- Because chemical differences among SB lavas are small, a clear under- were collected, however, from the core of one SB flow exposed in a small quarry ber (SB) samples. Presents a method for estimating standing of analytical precision is required to distinguish differences in mag- located in the Coast Range along Salmon Creek near the city of Saint Helens, analytical precision, and precision estimates for SB compositions. Describes the supporting documenta- matic compositions from those resulting solely from analytical uncertainty. Oregon (Fig. 1). Among these samples, three were unoxidized rock from which tion provided with analyses in Supplemental File 2. Thus, an assessment of the analytical precision of the GeoAnalytical Lab XRF alteration rinds were removed, one was an alteration rind, and one was severely 100 Explains the use of the MI calculation spreadsheet analyses of SB samples was carried out. This yielded a relationship that quanti- weathered lava from the quarry margin. As shown here, the chemical variations provided as Supplemental File 4. Please visit http:// doi.org /10 .1130/GES01188 .S3 or the full-text article on fies analytical precision as a function of element abundance, thereby enabling among these samples provide important clues to interpreting the effects of alter- www.gsapubs.org to view Supplemental File 3. the precision to be closely estimated at any measured abundance of an ele- ation on lava chemistry, notably on immobile element abundances.

In this report the term lava flow or, more simply, flow, is used to refer to other SB lavas, due mainly to their low Cr values, and these ratios differentiate lava cooling units identified from physical contacts (lower and upper chilled lavas of SB groups 1 and 2 from those of group 3. Series V lavas, the youngest vesicular crusts bounding a denser lava core). In order to distinguish between package of SB flows, are characterized by low Ti/Zr ratios (<65.5) relative to SB lava flows, flow lobes, and flow fields (per Thordarson and Self, 1998), pre- older SB lavas. cise characterizations of magmatic chemical compositions are required, which Three of the four series V chemical groups (groups 13, 15, and 16) have is a key objective of this study. Ti/Zr and Sc/Cr ratios that are indistinguishable, indicating that these lavas are closely related in their petrogenesis. The group 14 lava has even lower Ti/Zr ratios than other series V lavas due to its higher Zr. Series IV lavas have some- MASS ANALYSIS AND SB GEOCHEMISTRY what lower Sc/Cr and Ti/Zr ratios that differentiate these lavas from series II and III, but these ratios cannot distinguish series II from series III lavas. These Characterization of Lava Compositions Using Immobile Element Ratios immobile element ratios, although successful in differentiating chemical series of lavas erupted sequentially, are only partially successful in differentiating On the basis of the XRF analyses presented herein, SB lavas are here cate- individual flow compositions within the SB series. After describing a method gorized into five chemical series, each of which comprises three or four chemi- for determining initial magmatic compositions of both fresh and altered lavas, cal groups (Fig. 3). In concept, the series divisions are similar to the three-level I will return to the topic of differentiating individual SB flow compositions. separation of SB lavas by Landon and Long (1989). Each SB chemical group spans a limited range in composition and includes one or two flows showing Distinguishing Magmatic and Alteration Trends only minor differences in composition. SB chemical series include successive Using Immobile Elements chemical groups having similar immobile element ratios and/or immobile element abundances defining distinct chemical trends. In order to identify magmatic compositions of altered SB lavas, a suite of Primary chemostratigraphic distinctions between SB lavas were made samples is required that includes essentially unaltered samples spanning the using Ti/Zr and Sc/Cr ratios (Fig. 3). Several flow sequences (chemical series), entire range of compositions. Note that a sufficient number of minimally al- and even several individual flows, are distinguished by these ratios. The ear- tered samples were readily obtained for this study using an appropriate sam- liest SB lavas (series I) are characterized by high Sc/Cr ratios compared to all pling protocol (Supplemental File 3; see footnote 4). Selection of an inversely

correlated immobile element pair (e.g., Al2O3, TiO2) is also necessary to differentiate magmatic from alteration-derived trends, the latter forming linear trends with positive slopes in bivariate plots (Fig. 4). 76 The mass of altered rock decreases from dissolution of solid phases (minerals and glass) and transport of mobile (soluble) elements, and increases by 2 1 4-6 the transfer of such elements from aqueous fluids, such as by precipitation of 3 72 secondary minerals. Measured abundances of immobile (insoluble) elements therefore depend on the net change in rock mass, whether by depletion and/or enrichment of mobile elements. The immobile element abundances, how- 7-9 ever, have actually neither decreased nor increased, because low-temperature Ti/Zr 68 10-12 aqueous fluids do not transport these elements. Abundances of immobile ele Series ments vary proportionally to one another in response to rock mass changes,

p I II III IV V 13, 15, 16 thereby preserving the ratios among them. 64 1 4 7 10 13

Grou The mass analysis methodology presented here is based on the premise 2 5 8 11 14 that Al2O3 and TiO2 are enriched in equal proportions due to alteration-gener- 3 6 9 12 15 14 ated mass change, and that Al2O3/TiO2 ratios remain constant throughout the 60 16 n n alteration. Figure 4A presents Al2O3 and TiO2 data (the superscript n herein indi cates values from analyses normalized to 100% volatile free) for five samples of 0.6 0.8 1.0 1.2 1.41.6 1.82.0 2.22.4 the same lava flow exposed in the quarry located in the Coast Range near Saint Sc/Cr Helens, Oregon (Fig. 1). The lava sampled at the quarry margin, with the highest Al On and TiOn, is severely weathered and is now composed largely, if not Figure 3. Variation in Ti/Zr versus Sc/Cr ratios illustrating the discrimination of Sentinel Bluffs 2 3 2 Member (SB) lavas using immobile element ratio pairs. Data are from samples analyzed in this entirely, of clay and other secondary minerals. The other four samples are un- n n study. Symbols are by SB chemical group, as explained in inset. oxidized; two of these are a rind–inter-rind pair. The Al2O3 and TiO2 abundances

n n Figure 4. Variation of Al2O3 versus TiO2 abundances of 28 A 15.4 B samples of Sentinel Bluffs Member group 7 lavas showing the conservation of immobile elements. (A) Data for five samples from one lava flow exposed in a quarry 15.0 near Saint Helens, Oregon (see Fig. 1). Solid line corre- 24 sponds to the average Al2O3/TiO2 ratio (6.955) for group

7 samples having lower Al2O3/TiO2 ratios, the samples 14.6 Al O n in A and those samples shown with magenta and blue 2 3 symbols in B; dotted rectangle is the area of B. (B) All 20 group 7 samples, except for the strongly weathered 14.2 sample having very high Al2O3 and TiO2 abundances shown in A. Symbols: magenta—samples plotting 16 within the dotted rectangle shown in A; blue—samples 13.8 from other locations having Al2O3/TiO2 ratios equivalent to those of the samples shown in A; green—group 7

samples having higher Al2O3/TiO2 ratios. Solid line is 12 13.4 along constant Al O /TiO as in A. Oxide abundances (in 1.6 2.0 2.4 2.83.2 3.6 4.0 1.88 1.94 2.00 2.06 2.12 2.18 2.24 2 3 2 wt%) from analyses normalized to 100% volatile free, as n n TiO2 TiO2 indicated by superscript n.

of all of these samples are strongly correlated (r = 0.99), and plot along a line element (e.g., Ti, Zr) as a proxy for the remaining mass (Nesbitt, 1979; Brimhall

of constant Al2O3/TiO2. As these elements are effectively insoluble in water, the and Dietrich, 1987; Anderson et al., 2002). For an initial unit abundance of an nearly 2× difference between the lowest and highest abundances is attributed immobile element, the change in abundance is by the factor, 1/M s, where M s to a ~50% difference in sample mass. represents the mass fraction of a sample relative to the original unit mass of n n Among the samples having lower Al2O3 and TiO2 abundances, the mag- the parent. In terms of parent and sample compositions, the remaining sample matic composition cannot be identified from these variations alone. Additional mass fraction is described as follows:

criteria are needed to identify the point along the line of constant Al2O3/TiO2 that corresponds to the magmatic composition from which immobile elements s p s MC= i Ci , (1) have increased or decreased. As shown in the following, the sample having the lowest Al On and TiOn corresponds most closely to the original magmatic 2 3 2 where C p and C S are concentrations of immobile element i in the parent (p) composition of the lava flow. i I and sample (s), respectively. Equation 1 applies to any immobile element such An expanded view of the Al-Ti variations for samples from the quarry near that substitution of the ratio of parent to sample abundances of another im Saint Helens, Oregon (exclusive of the severely weathered sample) and for mobile element for M s yields: other samples assigned to chemical group 7 (Fig. 3) is shown in Figure 4B. n These samples define two groups characterized by slightly different Al2O3 p s p s n CCi CCi = i i , (2) and TiO2 : the samples with lower Al2O3/TiO2 ratios define a line of constant 1212

Al2O3/TiO2 (average 6.955) and are clearly from the same lava flow despite n n differences in Al2O3 and TiO2 abundances. Four other samples assigned to where subscripts i1 and i2 refer to the two immobile elements. This relationship

group 7 have higher Al2O3/TiO2 (average 7.128) and likely represent a different, indicates that changes in sample abundances for one immobile element, rela compositionally distinct lava flow within this chemical group. Among the sam- tive to parent abundances, result in identical parent:sample ratios for all im n n ples with lower Al2O3/TiO2, the large variations in Al2O3 and TiO2 at constant mobile elements. Given the differences in mineral-melt partitioning between

Al2O3/TiO2 demonstrate that Al and Ti are immobile and are conserved during elements, proportional changes among all immobile elements would be un- alteration; their range in abundances is attributed to changes in rock mass likely within a magmatic environment.

from changes in the abundances of soluble constituents. In this study, inversely correlated Al2O3 and TiO2 abundances are used to derive a parameter quantifying remaining rock mass, here termed mass in- Quantifying Mass Change from Alteration dex (MI). Al and Ti are typically immobile during low-temperature alteration and weathering, and the immobility of Al and Ti are demonstrated here by Quantifying changes in rock mass is essential to assessing chemical the variations within one lava flow, as shown (Fig. 4). Al-Ti variations for SB changes resulting from alteration. This is routinely achieved in weathering, lavas sampled for this study (except for the one severely weathered sample) alteration, and soil studies using the parent:daughter ratios of an immobile are shown in Figure 5A. Most samples define a prominent inverse correla-

s n n C Figure 5. Variation of Al2O3 versus TiO2 for all Sentinel Bluffs 15.2 A B 94 Member samples illustrating the procedure for detecting and 100 MIs = MI s ·100 quantifying change in sample mass. (A) Symbol shapes differen- 92 CTiO /CTiO 2 2 tiate samples by Sentinel Bluffs Member chemical series (circles 14.8 96 are series I–IV; triangles are series V); solid symbols are sam- s 2 iO 100 s C T ples used to define theMI lines (see text) and having MI = 94 / s 3 99.75–100.25; open symbols are samples with MI < 99.75 or 98 s O n 2 s 100 Al2O3 14.4 C Al MI > 100.25. Black lines indicate the MI lines calculated from = m regressions of data shown with solid symbols. Solid blue and red 96 lines parallel to the MI 100 lines are MI isopleths in increments of MI100 MI 10 C 2 MI units; red—series I–IV; blue—series V. Constant Al2O3/TiO2 14.0 serie 0 s V 98 ratios for the maximum and minimum values are shown by dot- M 1 seriesI 00 ted lines; red—series I–IV; blue—series V. (B) Schematic diagram illustrating calculation of sample mass index, MI s, using TiO as I-IV MI 2 13.6 1 s MI100 100 100 s100 00 coords. for MI _est MI window calculation Diff MI _regr & MI _calc the index element. Points labeled C and C indicate sample TiO2 Al2O3 0.50 1 diff_slope diff_int 1.70014.200 +/- 0.25 0.0000.000 100 2.20013.400 and MI Al2O3 and TiO2 abundances, respectively. Slope, m, of MI 100_est MI s _calc MI 100_regr MI100 s slopeintercept slopeintercept slopeintercept 1.7 1.8 1.9 2.0 2.1 2.2 dashed line is the sample Al2O3/TiO2 ratio. Oxide abundances are -1.600 16.920 -1.629 17.033 -1.629 17.033 C C TiO2 TiO2 n n 100 100 n n Sample ID Unit ID TiO Al O TiO MI Al O _MI s in MI window TiO in MI winAl O in MI win s 2 2 3 2_ 2 3 MI 2 2 3 MI _win Al2O3/TiO2 n n for analyses normalized to 100% volatile free. 10MS1731 2.05213.769 2.0427 13.7049 99.535 6.709 10MS0711 2.07613.650 2.0765 13.6499 100.002OK 2.076513.6495 100.002 6.573 TiO2 TiO2 10MS0791 2.07913.669 2.0763 13.6502 99.862 OK 2.079213.6690 99.862 6.574 10MS1741 1.96613.852 1.9632 13.8345 99.873OK1.965713.8521 99.8737.047 10MS1751 1.95613.846 1.9556 13.8468 100.006OK 1.9555 13.8459100.006 7.080 10MS1781 1.95513.894 1.9493 13.8571 99.733 7.109 10MS2591 1.95513.893 1.9499 13.8562 99.732 7.106 07MS023 12.042 14.465 1.9552 13.8475 95.729 7.083 10MS072 12.024 13.715 2.0268 13.7309 100.119 OK 2.0244 13.7145 100.119 6.775 10MS080 12.019 13.704 2.0235 13.7363 100.239 OK 2.0186 13.7035 100.239 6.789 10MS104 11.980 13.777 1.9835 13.8013 100.179 OK 1.9800 13.7766 100.179 6.958 10MS114 12.001 13.799 1.9978 13.7781 99.851 OK 2.0008 13.7986 99.8516.897 10MS283 11.984 13.758 1.9893 13.7920 100.250 OK 1.9843 13.7576100.250 6.933 08MS0181 1.92814.145 1.8995 13.9383 98.542 7.338 10MS0421 1.94914.440 1.8849 13.9620 96.692 7.407 10MS043 11.923 14.369 1.8718 13.9834 97.315 7.471 n n 08MS019 11.955 14.108 1.9259 13.8952 98.493 7.215 tion baseline; other samples having higher Al O and TiO values are scattered dow changes. The iterative regressions proceed until the input and output 10MS1762 2.04513.750 2.0390 13.7109 99.716 6.724 2 3 2 10MS1772 2.04213.737 2.0380 13.7127 99.825 OK 2.0415 13.7368 99.825 6.729 10MS2582 2.02513.752 2.0230 13.7371 99.890 OK 2.0252 13.7522 99.8906.791 10MS1792 1.97113.899 1.9618 13.8367 99.553 7.053 10MS1802 1.99213.830 1.9871 13.7955 99.749 6.943 above this baseline. As shown herein, the seemingly random scatter in sample lines coincide. These calculations are illustrated in the spreadsheet provided 10MS2602 1.98013.821 1.9785 13.8096 99.919OK 1.9801 13.8207 99.919 6.980 10MS1472 2.03514.002 2.0013 13.7723 98.362 6.882 10MS3202 2.02714.024 1.9928 13.7863 98.306 6.918 5 97SH-X49A2 1.97113.799 1.9734 13.8178 100.135 OK 1.9708 13.7992 100.135 7.002 abundances above the baseline is the result of systematic chemical changes as Supplemental File 4 , and the function of this spreadsheet and the iterative 10MS139 21.926 13.933 1.9215 13.9024 99.781OK 1.9257 13.9329 99.781 7.235 10MS310 21.925 13.922 1.9218 13.9019 99.855OK1.924613.9220 99.8557.234 10MS1403 1.93713.875 1.9374 13.8764 100.011 OK 1.9372 13.8748100.011 7.162 10MS3113 1.94613.852 1.9474 13.8603 100.059 OK 1.9462 13.8521100.059 7.117 produced by varying degrees of alteration. calculation process are described in Supplemental File 3 (see footnote 4). Note that no samples plot significantly below the baseline of Al On and The application of MI 100 lines in calculating sample mass index values, or 5Supplemental File 4. Spreadsheet used to calculate 2 3 n s 100 MI 100 baselines using an iterative regression method. TiO2 abundances. The absence of samples below the baseline arrays indicates MI , is illustrated in Figure 5B. For any sample, the parent or MI abundance Sample data in this spreadsheet are for Sentinel that none of the samples had gained any appreciable mass from precipitation is calculated from the intersection of the MI 100 line and the line (dashed) hav- Bluffs Member series I-IV samples; these data and the of secondary minerals or other secondary processes. As described here and ing a slope equal to the sample Al O /TiO ratio and intersecting the origin. input parameters were used in determining the MI 100 2 3 2 100 p baseline for these samples. Instructions on the use detailed in Supplemental File 3 (see footnote 4), secondary minerals that could Solving for the MI or parent abundance of TiO2 (C TiO2) yields the following of this spreadsheet are given in Supplemental File 3. contribute to mass gain were carefully avoided during sampling. expression, Please visit http://doi.org/10.1130/GES01188.S4 or The Al-Ti baseline is interpreted to be magmatic in origin, as inversely the full-text article on www.gsapubs.org to view 100 100 correlated immobile element abundances cannot be generated by alteration. Cbp =−MI CCs s mMI , (3) Supplemental File 4. TiO2 ()Al23O TiO3 n n Those samples along the baseline have the lowest observed Al2O3 and TiO2 s s n n abundances at a given Al2O3/TiO2 ratio, and these samples are interpreted as where CTiO2 and CAl2O3 refer to sample TiO2 and Al2O3 abundances, repectively having retained essentially all of their original mass. Altered samples having and mMI 100 and bMI 100 are the slope and intercept, respectively, of the MI 100 base- n n s elevated Al2O3 and TiO2 and plotting above the baseline arrays (Fig. 5A) have line. In turn, the sample mass index, MI , is calculated from parent and sample

therefore undergone mass loss due to water-rock interaction. TiO2 abundances as follows, In order to quantify parent (magmatic) compositions, regressions were performed on those samples defining baseline arrays (solid black lines in Fig. MI s = CCp s 100. (4) TiO22TiO ⋅ 5A), and these compositions are defined to have a mass index of 100 MI( 100). It became apparent that series V samples (Fig. 3) do not share a baseline trend The multiplication by 100 converts the sample mass fraction to percent. Note colinear with that of older SB lavas, so separate baseline regressions were that this equation has the same form as Equation 1. made on series I–IV and series V samples. Samples from 14 of the 16 SB groups recognized in this study have MI s The regressions to determine MI 100 baselines were calculated iteratively, values within the MI window. The maximum MI s value among group 3 sam- starting with a visually estimated line at the base of the arrays, and applying ples from this study is 98.5; however, an unpublished data set of SB samples an interval of MI units, or MI window, to constrain samples included in each provided by R. Evarts (2013, personal commun.) includes a high-MI s (99.73) regression. An MI window of 0.5 was used, meaning that MI values for the sample. Among group 11 samples, the maximum MI s value (99.68) is slightly included samples are within 0.25 MI units of MI 100. A regression is calculated lower than the MI window minimum (99.75). This difference in MI s corre- n for samples within the initial MI window, and this regression is used as the sponds to a difference in TiO2 (0.001 wt%) that is within analytical precision input line for a new regression as the sample population within the MI win- (Supplemental File 3; see footnote 4). One sample has slightly higher MI s

(100.36) than the MI window maximum (100.25); however, the difference of and Crmn, respectively) are consistent with, but allow more precise distinctions

0.11 MI units corresponds to a small difference in TiO2 (0.002 wt%) that also is (Fig. 6) than, those made using Ti/Zr and Sc/Cr ratios (Fig. 3). Within four of mn mn within analytical precision. the five chemical series, TiO2 and Cr are inversely correlated, similar to the 100 n n Where MI values are lacking for a chemical group or particular flow Al2O3 –TiO2 variations, but define trends offset from each other, mainly due to within a stratigraphically coherent sequence of lavas, MI s values may be de- differences in Crmn abundances. Mass-normalized abundances (colored sym- termined by assuming that the parent (magmatic) compositions for the lavas bols, Fig. 6) and normalized abundances (black plus symbols, Fig. 6) are also in question are represented along the Al-Ti baseline. This equates, in effect, to compared in Figure 6 for samples having MI s < 98.4. For a given sample, tie an inference that the negatively correlated baselines of compositionally similar lines (blue, dotted, Fig. 6) connect normalized abundances with mass-normal- lavas derive from a common magmatic process. The origin of the inverse Al-Ti ized abundances. These tie lines correspond to residual concentration vectors,

correlation is discussed herein. are along constant TiO2/Cr ratios, and have positive slopes defined by mag-

matic TiO2/Cr ratios. Differences in the tie-line slopes are caused mainly by a Mass Normalization nearly 3× difference in Cr abundances. For immobile elements, mass normalization enables the comparison of The sample mass index, MI s, can be used as a multiplier to normalize abun- magmatic values for samples of fresh and variably altered lava, and prevents dances within an entire chemical analysis (already normalized to 100% volatile the misidentification of samples with lowMI s values. For example, the nor-

free), and this process is here termed mass normalization. Mass-normalized malized TiO2 and Cr abundances of a group 15 sample plot within the group 9 abundances are therefore defined as field (series III), but this lava has a low Ti/Zr ratio characteristic of series V lavas,

which erupted later in the SB sequence (Fig. 6). Its mass-normalized TiO2 and mn ss CCj = j ⋅MI 100, (5) Cr abundances are consistent with the group 15 field. Notably, the mass-nor- 53 malized TiO2 and Cr abundances of a severely weathered (MI ) group 7 sam- where the superscripts mn and s refer to mass-normalized and sample abun- ple plot among those for unaltered group 7 samples. n dances, respectively, and the division by 100 compensates for the expression As shown in Figure 6, mass normalization eliminates variations in TiO2 of MI s in percent (Equation 4). Mass normalization removes the inherent nor- and Crn abundances within a single chemical group (e.g., group 11) that might malization of abundances to 100% due to the analysis of mass-depleted rock as otherwise be interpreted as unique compositions of separate lava flows or as 100 a unit mass. For immobile elements the mass-normalized abundances (MI ) internal variations within a single lava flow. The mass-normalized TiO2 and Cr correspond to their original magmatic values. abundances for group 11 samples are within analytical uncertainty, and these Mass-normalized abundances of immobile elements allow more precise, samples are from locations ~175 km apart, demonstrating the chemical homo distinctions than those made using immobile element ratios. For the SB sam- geneity of this flow over large distances. Similarly, groups 13 and 15 each in- mn ples analyzed in this study, mass-normalized TiO2 and Cr abundances (TiO2 clude samples from locations more than 250 km apart.

GroupSeries 2.2 MI53 16 Figure 6. Plot of TiO2 versus Cr show- 15 ing the discrimination of Sentinel Bluffs V 14 Member (SB) lavas using mass-normal- 2.1 1 13 ized (mn) immobile element abundances. 4 12 Mass-normalized abundances are plotted 11 IV for all samples, and SB chemical groups 2.0 7 5 10 are indicated by colored symbols, as in 2 Figure 3. Normalized (n) abundances (nor- mn, n 9 TiO2 malized only to 100% volatile free) are 6 8 III indicated by black plus symbols and are 1.9 7 s 8 9 shown only for samples with MI < 98.4. 3 6 Tie lines (blue, dotted) connect normal- 10 5 II ized and mass-normalized abundances

1.8 13 4 for a given sample (TiO2 is in wt%, Cr is in 53 12 11 3 ppm). Arrow labeled MI at upper end of 15 2 I tie line (top center) refers to a weathered 14 n n 1.7 16 1 sample whose TiO2 and Cr abundances, 3.71 wt% and 57.9 ppm, respectively, ex- 15 20 25 30 35 40 45 50 ceed the plot limits. Crmn, n

A step-by-step summary of key procedures in mass analysis, including the applicable equations, is presented in Figure 7, and steps mentioned in the fol- 1) De ne the broader chemical series on the basis of immobile element ratios, taking into account lowing refer to the numbered steps in this figure. Initially (step 1), flows are stratigraphic constraints (see Fig. 3). identified that have equivalent immobile element ratios and equivalent stratigraphic positions. These constraints are used to defineMI 100 baseline values 2) Determine the MI100 (magmatic) baseline(s) for inversely correlated immobile elements (e.g., y = (step 2); unaltered samples would ideally span the range of sample Al2O3/TiO2 Al2O3 and x = TiO2)(see Fig. 5A). This line is ratios. Definition of this line is the foundation for calculations of parent TiO2 determined using an iterative regression method (step 3), sample mass index (step 4), and mass-normalized abundances (see Supplemental File 4, explained in (step 5). Characterization of chemostratigraphic units using mass-normalized Supplemental File 3), and has the form y = mx + b. p 100 p 100 immobile element abundances can then be made (step 6). C = mMI • C + bMI Al2O3 TiO2 Figure 7. Stepwise summary of mass analysis as applied to a mag- 3) Using an immobile element (e.g., TiO2) as an DISCUSSION index element, calculate the parent index element matic suite. Elemental abundances, abundances of samples; i.e., abundances at MI100. C ; superscripts: n—sample abun- Magmatic Al-Ti Variations For each sample, these values are the intersection dance from analysis normalized to of lines de ned by the sample immobile element 100% volatile free; mn—mass-nor- 100 malized abundance; s and p—sample ratio (e.g., Al2O3/TiO2) and MI (see Fig. 5B). Sampling was designed to minimize oxidation, textural indications of al- and parent, respectively; subscripts: p MI100 MI100 C = b / C s /–C s m j—any element, or as specified (e.g., teration, and secondary minerals that can potentially contribute elemental ad- TiO2 ( Al2O3 TiO2 ) TiO2, Al2O3). The slope (m) and inter- ditions. By avoiding rock altered under oxic conditions, the samples collected cept (b) of the parent (magmatic) baseline are indicated by the super- were composed primarily of either unaltered rock or rock altered under an- 4) Calculate sample mass index (see Fig. 5B). script MI 100. oxic conditions. The resultant data set of chemical analyses for such samples s p s MI = C / C • 100 mainly shows inverse baseline trends in Al-Ti variations. Earlier I noted that TiO2 TiO2 these trends cannot be generated by alteration, and therefore must be generated by magmatic processes. The origin of the SB Al-Ti baselines is briefly 5) Calculate mass-normalized abundances of all elements j. discussed. mn s s • Stepwise fractional crystallization (FC) experiments on tholeiitic basalt at C j = C j MI / 100 0.7 GPa (Villiger et al., 2007) show that, following initial enrichment of liquids

in Al2O3 and TiO2 (from crystallization and removal of mafic silicates), the 6) Using mass-normalized abundances of withdrawal of subequal amounts of clinopyroxene (Cpx) and plagioclase (Pl), immobile elements, re ne the chemical series along with trace amounts of spinel, generates an inverse Al-Ti trend in liquid identi ed in step (1), while taking into account stratigraphic constraints (see Fig. 6). abundances (Fig. 8). This trend is linear (r = –0.997) over a large range of temperatures (90 °C) and amount of crystallization (84%). The trend’s linearity

indicates that the crystal assemblage maintained a constant Al2O3/TiO2 ratio despite changes in Cpx and Pl compositions with decreasing temperature. The mantle-derived melt, it is clear that the erupted SB magma compositions rep- 0.7 GPa pressure of these experiments corresponds to lower crustal depths in resent only a small fraction of their overall magmatic history. the eastern CRB. It is also comparable to the maximum pressure (0.66 GPa) de- Mass balance calculations relating major element abundances of the least termined from Cpx geobarometry in GRB and other CRBG lavas by Caprarelli and most evolved lavas within four of the SB series (I–III, V) by crystal removal and Reidel (2005), from which they also inferred lower crust crystallization of yield results consistent with the FC experiments discussed here. These rela- CRBG magmas. tively simple mass balance models used SB augite compositions from Ames The SB Al-Ti baseline trends, also shown in Figure 8, are subparallel to (1980), and allowed plagioclase compositions to vary between anorthite and the trend of experimental basaltic liquids. The similarity of the SB and exper- albite. The mass balance models also allowed olivine, pigeonite, and spinel or imental trends, derived from Cpx + Pl crystallization, suggests their genera- magnetite to be part of the assemblage, but these phases did not appear in the tion by the same process. The smaller range of Al-Ti compositions in SB lavas solutions. The model plagioclase compositions are mainly within the range of indicates that it can be generated by a much smaller amount of fractionation. analyzed Pl compositions, but in some models were slightly higher (to ~10%

Offset of the SB trend to lower Al2O3 and/or TiO2 values could potentially result An content) than the analyzed Pl compositions. Models relating group 16 to

from a variety of factors such as differences in the parent magma composition other series V groups, however, yielded anomalously calcic plagioclase (An100). and pressure conditions. In the context of magma evolution from a primary, The anorthite contents of model plagioclase compositions do not warrant pre-

17 The removal of crystal assemblages containing ~50% Cpx, as indicated by both the mass balance models and FC experiments, is consistent with the SB 1210 intraseries Crmn variations (Fig. 6). Within all SB series except series IV, Crmn 16 1180 abundances decrease by ~20%–40% from the least to most evolved compositions. These Crmn variations are consistent with the strong partitioning of Cr 1150 1240 Cr mn 15 into Cpx (DCpx >> 1). The compositional gaps in Cr variations between series 1270 (Fig. 6) suggest that fractionation of SB magmas occurred in batches (series), SC and not by differentiation of a single parent magma. In addition, Al O and TiO n 1120 2 3 2 Al2O3 14 abundances of the SB series partially overlap, rather than occupying separate SB segments of the Al-Ti baseline (Fig. 9). This also indicates that the magmas of 13 each SB series evolved separately, at least in their latest phase of differentiation. Cpx + Pl + Im Although the main SB compositional variations can be explained by rela- Cpx + Pl 1090 tively simple Cpx + Pl fractionation, some exceptions are evident. For example, Cpx + Opx + Pl 12 group 14 compositions have anomalous enrichments in incompatible trace Ol + Cpx 1060 Ol elements (e.g., Zr, K, Ba) relative to other series V lavas; the enrichment in Zr 11 accounts for their distinctly low Ti/Zr ratio (Fig. 3). The similar enrichments in 00.5 1.01.5 2.02.5 3.03.5 both immobile and mobile incompatible elements indicate a magmatic ori- n TiO2 gin and require a more complex petrogenesis. In addition, as shown by Ti-Cr variations (Fig. 6), the small variations among series IV lavas do not define a n n Figure 8. Comparison of Al2O3 versus TiO2 for Sentinel Bluffs Member coherent trend. Because these issues are peripheral to the main focus of this (SB) MI 100 baselines and liquid compositions from stepwise fractional crystallization experiments on tholeiitic magma at 0.7 GPa under dry paper, they will not be examined further. conditions (Villiger et al., 2007). SB series I–IV and series V baselines are Despite their apparently separate evolution in magma batches, composi- shown with thick red and orange lines, respectively, and are labeled SB. tions of SB lavas nonetheless define a linear Al-Ti trend. This is attributed to Experimental liquid compositions are shown with colored circles; run temperatures (°C) are indicated next to data points. Colors of data points indicate coexisting phases as shown in legend. Phase abbreviations: 14.3 Ol—olivine; Opx—orthopyroxene; Cpx—clinopyroxene; Pl—plagioclase; Im—ilmenite. Small amounts of spinel also were reported in all runs. Starting composition is represented by the solid black circle labeled SC. 14.2 Heavy black line is the regression (r = –0.997) of data for the four liquid IV compositions related by Cpx + Pl + Sp removal. Arrows indicate liquid 14.1 evolution with decreasing temperature for other assemblages. III

14.0 V n Al2O3 cise interpretation because these values varied with the Cpx alumina content. 13.9 I For example, plagioclase compositions differed from An72 to An80 for equally Series valid models of series I compositions obtained with Cpx compositions having 13.8 V Al O contents that differed by a factor of 2. IV 2 3 II Successful mass balance models included assemblages of Cpx and Pl in 13.7 III II subequal amounts. The amounts of intraseries fractionation indicated by the I models are minor, and, for example, are 6%–10% for series I–III and V. The 13.6 entire range of series I–IV compositions corresponds to ~15% fractionation if 1.7 1.8 1.9 2.0 2.1 these compositions were generated from one parent magma, but it appears n TiO2 that each series evolved separately. The correspondence of model mineral

n n compositions with those of SB groundmass minerals (except for Pl in series V Figure 9. Al2O3 versus TiO2 for Sentinel Bluffs Member (SB) samples models) is consistent with fractionation at crustal pressures. Although spinel having MI s values from 99.75 to 100.25 (those samples used to de- 100 s was present in small amounts in the Villiger et al. (2007) experiments, the fineMI baselines), except for samples from SB group 3 (MI < 98.6) and group 11 (MI s < 99.7), for which mass-normalized abundances mass balance models indicate that spinel is not required to generate the SB are shown. Symbols are by SB chemical series; brackets indicate the Al-Ti array. compositional range of each series.

the saturation of all SB magmas in Cpx and Pl, such that SB major element depletion of FeO due to alteration. This difference in the sign of changes in compositions are constrained to a boundary curve (cotectic) along which FeO/MgO is sufficient to unambiguously distinguish positive Al-Ti variations liquid compositions vary systematically, as observed in the FC experiments resulting from crystal fractionation from those generated by alteration under discussed above. It appears, therefore, that processes other than Cpx + Pl frac- anoxic conditions. tionation that might have influenced SB compositions to a minor extent did Trace element behavior also differs sharply between magmatic and not cause SB major element abundances to depart from cotectic compositions. groundwater-rock systems. For example, if increased Al and Ti abundances

Inverse Al-Ti variations are not unique to the SB. The Al-Ti arrays of GRB N2 were due to olivine fractionation, Ni abundances should be severely depleted

and R2 members sampled in multiple GRB stratigraphic sections have slopes in the fractionated magmas due to the strong partitioning of Ni into olivine. subparallel to those of the SB lavas, but are offset to higher or lower Ti values. For SB lavas, which have low MgO (~4.4–5.2 wt%) in comparison to primary Ol–liq The compositions of these upper GRB units are more evolved than the SB mantle-derived melts, DNi is likely very high (> 20) given the relationship be- Ol–liq lavas and, as such, would be expected to be saturated in Cpx and Pl. Thus, tween DNi and magma MgO content (Hart and Davis, 1978). In SB lavas, the Cpx + Pl fractionation apparently is a common process in the latter evolution- positive above-baseline Al-Ti trends are not accompanied by strong depletions ary stages of GRB and other CRBG magmas. The protracted Cpx + Pl fraction- in Ni abundances. ation observed at lower crustal pressures (Villiger et al., 2007) likely explains As discussed here, fractionation of Cpx in subequal amounts with plagio the common occurrence of inverse Al-Ti arrays among GRB lavas. clase is inferred from intraseries variations of SB lavas. Removal of Cpx, into Cpx–liq which Cr is partitioned relative to the melt (i.e., DCr >> 1), should significantly reduce Cr abundances. Systematic decreases in Cr of 20%–40% (mass Alteration-Generated Al-Ti Variations normalized abundances, Crmn) are observed within 4 SB series (Fig. 6) and are consistent with Cpx fractionation as part of a Cpx + Pl assemblage. For As discussed herein, positively correlated Al-Ti trends to above-baseline above-baseline samples, normalized Cr abundances (Crn) instead increase due

Al2O3 and TiO2 abundances are attributed to the concentration of immobile to the concentration of insoluble Cr in mass-depleted rock (Fig. 6). elements in altered, mass-depleted rock. Positive Al-Ti trends can also be gen- The increases in Cr abundances are proportional to increases in Al and Ti erated by crystal fractionation in magmatic systems with removal of an as- abundances, and to Zr, Nb, and V abundances. The equivalent sample:parent semblage whose bulk composition lacks or contains only minor amounts of Al enrichments of elements that are both compatible (e.g., Cr, V) and incompat- and Ti. This occurs when the assemblage is composed of mafic silicates (e.g., ible (e.g., Zr, Nb) in terms of mineral-melt partitioning in a magmatic system olivine, clinopyroxene, orthopyroxene), and Al-rich phases, such as plagio are at odds with crystal fractionation of mafic silicates. The similar behavior clase, are absent or compose only a minor amount of the assemblage. The ef- of compatible and incompatible elements is instead attributed to their mutual fect of mafic silicate fractionation on liquid Al and Ti abundances is illustrated insolubility, which produces proportional changes in their abundances due to by the highest-temperature FC experiments of Villiger et al. (2007) discussed alteration-generated changes in rock mass. herein. As shown in Figure 8, removal of such crystal assemblages from the melt prior to the onset of plagioclase crystallization generates a positive Al-Ti trend in their experiments from 1270 to 1210 °C. Magma Homogeneity Considered in isolation, removal of mafic silicates may appear to be a via ble explanation for the positive Al-Ti trends observed among SB composi- The chemical homogeneity of CRBG lavas across hundreds of kilometers tions. The fractionation of mafic silicates would affect the abundances of other has been a common inference of past geochemical studies, which allowed for elements as well. In a magmatic system, mineral-melt partitioning controls some chemical variation within the individual flows (e.g., Wright et al., 1973; element behavior, whereas in a low-temperature water-rock environment, ele Reidel et al., 1989; Hooper, 1984, 1988, 2000; Tolan et al., 2009). For example, ment solubility is the dominant control. These markedly different processes many compositionally distinct Saddle Mountains Basalt flows exhibit ranges

can be clearly distinguished from the changes to lava chemistry. For some in P2O5 and TiO2 abundances that are positively correlated (see Hooper, 2000, elements, the chemical changes are in the opposite sense. figure 3 therein), and the varying slopes of these correlations are similar to the The mafic silicates (olivine, orthopyroxene, and clinopyroxene) prefer- arrays of residual concentration trends due to mass loss (Figs. 4 and 6). min-liq entially incorporate Mg over Fe relative to basaltic liquid (i.e., KD(Fe-Mg) Reports of chemical heterogeneity within a single CRBG lava flow have ≤ 0.3; see Putirka, 2008). Crystallization and removal of any of these silicates been few, and such chemical differences have been attributed to differences in therefore would yield liquids with progressively higher FeO/MgO. Along the magmatic compositions, in combination with lava emplacement mechanisms positive Al-Ti trends in SB lavas, FeO/MgO decreases with increasing Al and (Reidel and Fecht, 1987; Reidel, 1998, 2005; Reidel and Tolan, 2013; Vye-Brown Ti. This change in FeO/MgO is in the opposite sense to that expected from et al., 2013a). These studies, however, did not evaluate the possibility that sec- mafic silicate fractionation. The decrease in FeO/MgO results from the strong ondary processes might be responsible for the chemical differences within

flows, perhaps because of a lack of a framework in which to differentiate the correspond to the values for high-MI (~100) samples (11.1–12.7 wt%) and, with effects of chemical alteration from those of magmatic processes. decreasing mass index, FeOn is progressively depleted. This relationship indi- In this study chemical differences within a flow are observed when ana- cates that alteration occurred under anoxic conditions, thereby enabling trans- lyzed abundances (normalized to 100% volatile free) are considered. Mass-nor- port of reduced iron (Fe2+) from the altered lava. malized immobile element abundances show, however, that such intraflow dif- Because iron can occur as soluble Fe2+ or the nearly insoluble Fe3+, iron ferences are the result of alteration, and not of magmatic processes. With the mobility depends on the groundwater’s redox potential, which is largely a modifications to lava chemistry from chemical alteration taken into account, it function of dissolved oxygen. Dissolved oxygen is consumed by oxidation appears that SB lavas have a high degree of homogeneity that is indistinguish- reactions with redox-active elements such as iron. SB lavas, as well as other able from analytical uncertainty, or nearly so. CRBG lavas, have a large reservoir of reduced iron (>10 wt% FeO) ~85% of The precise SB chemostratigraphy developed here allows the correlation of which (molar basis) is likely ferrous iron in unaltered rock. Consumption of lavas throughout the SB. This study shows that SB flows, which were sampled dissolved oxygen in water from reaction with GRB lava (Winter Water Mem- in different locations and in equivalent stratigraphic position, have composi- ber) has been shown in the experiments of Lane et al. (1984). For example, tions that are within narrow ranges of immobile element ratios and mass-nor- in their experiments performed at 100 °C over 130 days the dissolved oxy- malized immobile element abundances. Samples for which stratigraphic gen content of initially air-saturated, synthetic GRB groundwater (see Jones, context is only broadly constrained (i.e., samples from the Coast Range and 1982) decreased nearly 80%. Comparison of dissolved oxygen consumption Willamette Valley) also have immobile element characteristics that coincide for a GRB lava (Winter Water Member) and several minerals (from elsewhere) with the fields defined by samples from continuously sampled sections of shows similarly high rates of oxygen uptake by basalt and magnetite as com- multiple SB flows in the Columbia Plateau and Columbia River Gorge. pared to the rates for augite and plagioclase (White et al., 1985). This suggests that Fe-Ti oxides, which are abundant in the groundmass of GRB lavas, play an important role in oxygen depletion and, therefore, in influencing the redox Conditions of Alteration conditions of groundwater. To account for rapid oxygen diffusion rates and a lack of Fe on exterior mineral surfaces in laboratory experiments simulating al- Secondary mineral assemblages documented in CRBG lavas indicate that teration, White et al. (1985) further suggested that oxygen diffuses into interior alteration occurred at low temperature (<100 °C) in the presence of ground pore spaces and along the boundaries of grains dominated by Fe-Ti oxides. water, under conditions equivalent to the present-day environment (Benson Marked increases in dissolved Fe2+ coincident with strong decreases in dis- and Teague, 1982). As shown in Figure 10, FeOn abundances of SB lavas cor- solved oxygen and Eh have been observed in the groundwater of aquifers with relate to sample mass index (MI s), indicating that depletion of iron is a signif- distance from recharge areas (e.g., Champ et al., 1979; Edmunds et al., 1987), icant part of the observed mass loss. In this plot, magmatic FeOn abundances as well as within a relatively shallow aquifer (sampled to 60 m depth) in which the flow direction is primarily vertical (White et al., 1990). In these studies, the observed decrease in Eh between oxic and anoxic waters is on the order of 13 400–500 mV, with oxygen-depleted waters commonly having negative Eh values. These studies were performed on aquifers hosted by sedimentary rocks or granite-derived alluvium having relatively low amounts of ferrous iron, but 12 that could contain other reductants such as carbon in sedimentary rocks. Iron mobility and immobility under anoxic and oxic conditions, respectively, have been shown in batch dissolution experiments performed on lava of the Wapshilla Ridge Member of the GRB (Neaman et al., 2005). The compo- Figure 10. FeOn versus sample mass 11 index, MI s, for Sentinel Bluffs Mem- FeOn sition of the sample used in the experiments is broadly similar to SB lavas, but ber samples analyzed in this study. A has ~0.5 wt% less MgO than the most evolved SB lavas. These experiments s severely weathered sample with MI were carried out at a pH of ~6–6.5 (end-experiment pH) near the lower range ~53 and 21.3% FeOn is not shown. 10 of the modern circum-neutral to moderately alkaline GRB-hosted aquifers (pH 6.7–9.4; Steinkamph and Hearn, 1996). These experiments showed significant iron release under anoxic conditions in which redox potentials measured at 9 the end of experiments were reducing (Eh < 100 mV). Iron release under oxic conditions at atmospheric pressure and Eh ~300–450 mV was not observed. 90 92 94 96 98 100 Secondary pyrite lining vesicles and crystal-bound (diktytaxitic) voids in s MI GRB lavas has been observed in GRB lavas in outcrops in several field sec-

tions, and has been reported in fractures and vesicles within CRBG lavas in borehole core samples (Benson and Teague, 1982; McKinley et al., 2000). The 100 pyrite occurrences are consistent with reducing anoxic conditions. Reduced groundwaters (Eh <–200 mV) from deep modern CRBG-hosted aquifers contain anaerobic bacteria (e.g., methanogenic, sulfate reducing; Stevens et al., 1993). Stevens and McKinley (1995) further interpreted keroge 99 nous bacteriomorphs associated with secondary minerals in CRBG lavas as evidence that microbial populations have existed in reducing groundwater of Total CRBG-hosted aquifers in the geologic past. 98 Overall various field and laboratory studies of water-rock interaction support the interpretation that Fe2+ is mobile under anoxic and reducing conditions. The depletion of FeO with increasing mass loss is in agreement with the observations of Wells et al. (2009), who attributed low FeOn (<11 wt%) in 97 GRB lavas to alteration. The strong iron depletion is also in agreement with petrographic observations of borehole samples by Benson and Teague (1982), 90 92 94 96 98 100 who noted that Fe-rich minerals, clinopyroxene and magnetite, are altered to a MIs greater degree than feldspar. The behavior of iron under anoxic conditions contrasts markedly with its Figure 11. Analysis total versus sample mass index, MI s, for Sen- tinel Bluffs Member samples analyzed in this study. Totals are behavior during near-surface weathering, under oxic conditions, in which the sum of abundances (in wt%) of major element oxides and Fe3+ behaves as an immobile element and is concentrated in the weathered trace elements calculated as oxides (from abundances reported rock residue. For example, the severely weathered and mass-depleted (MI 53) in ppm). A severely weathered sample with an analysis total of 74.3 wt% and s ~53 is not shown. group 7 sample analyzed in this study (see Fig. 4A) has FeOn (21.3 wt%) MI that is nearly twice that of unaltered samples having MI s values of ~100. Its mass-normalized iron abundance (11.3 wt%) suggests that only minor Fe loss occurred from alteration. Although total iron is discussed here in terms minerals and glass have been dissolved, and in which negligible amounts of of FeO (Fe2+), the iron in this surficially weathered rock is undoubtedly oxi- hydrous secondary minerals have formed, can therefore yield analyses with dized to Fe3+. totals approaching 100 wt%. The large range in the totals of high-MI s samples (MI s > 99) indicates that volatile acquisition, such as from glass hydration, may occur without accompanying mineral dissolution. Among group 11 samples, Analysis Totals which span a 5% range in mass retention, totals are higher in the low-MI s samples. This suggests a sequence of alteration in which glass was initially Low analysis totals (herein, totals) of the abundances of nonvolatile ele- hydrated and later the hydrated glass was dissolved. In any case, for rock al- ments are commonly interpreted as indicators of the degree of alteration. The tered under anoxic conditions, analysis totals are not a reliable indicator of difference between 100% and the analysis total is attributed mainly to glass the amount of mass loss. The persistence of high totals for rock altered under hydration and/or the presence of hydrous secondary minerals. Totals for SB anoxic conditions does not apply to rock altered under the oxic conditions of lavas do not correlate with MI s, and nearly all samples, including those with surficial weathering. For example, the strongly weathered sample from the MI s values as low as 91, have totals >97 wt% (Fig. 11). This indicates that alter- quarry near Saint Helens, Oregon, has both very low MI s (~53) and a low analy ation resulting from dissolution of minerals ± glass under anoxic conditions sis total (of nonvolatile constituents) of only 74 wt%. is not accompanied by the hydration of glass in proportion to mass loss or by The occurrence of only minimal amounts of hydrous secondary minerals the formation of significant amounts of hydrous secondary minerals. As noted in mass-depleted rock altered under anoxic conditions perhaps results from a herein and described in Supplemental File 3 (see footnote 4), minor amounts lack of available aluminum for clay and zeolite formation due to the preferen-

of secondary minerals that may line the surfaces of or fill vesicles, fractures, tial dissolution of ferromagnesian minerals having low Al2O3 contents. This is and other cavities were avoided with careful sampling. consistent with the petrographic observations of Benson and Teague (1982), Mass-depleted rock that contains minimal amounts of hydrous secondary who noted that clinopyroxene and magnetite appeared to be most affected by minerals and/or hydrated glass can have high totals in part because the de- alteration, with plagioclase less so. This is also in agreement with the obser- pleted residue is analyzed as a unit mass. In effect, the depleted abundances vations of Wells et al. (2009), who characterized plagioclase in rocks showing are inherently normalized to 100 wt% in the process of analysis. Rock in which evidence of chemical alteration as having a fresh appearance.

TiO2-MgO Variations to that observed for mass-normalized Cr abundances (Fig. 6), and supports the interpretation that the MgO abundances of high-MI s samples represent The use of MgO variation diagrams was initially advocated by Wright et al. magmatic abundances. When observations are limited to samples that have

(1973), and TiO2-MgO variations have continued to play an important role in retained nearly all of their original mass (using an MI threshold), the composi- differentiating GRB members (Mangan et al., 1986; Reidel et al., 1989; Reidel tional trends observed for mobile elements are similar to those observed for and Tolan, 2013). Published chemical characterizations of members in terms immobile elements. s of TiO2 and MgO abundances have been imprecise (being only greater or less Due to the mobility of MgO, the low-MI samples are not displaced from s than a given value), however, and the compositions of individual flows within high-MI samples of the same chemical group along lines of constant TiO2-MgO GRB members have not been explicitly defined. Qualitative characterizations ratios, as for immobile elements (Figs. 4–6). Some low-MI s samples have com- of GRB lavas in terms of MgO abundances (as low, intermediate, high, and positions corresponding to those of high-MI s samples of other chemical groups, very high) have since become commonplace. or have compositions not represented by any high-MI s samples, such as the s The TiO2-MgO variations from previously reported SB analyses define a samples with <~4.4 wt% MgO. Overall, the trends of low-MI samples within a seemingly random distribution at higher MgO that separate into two diffuse, chemical group have slightly lower slopes than the magmatic trends, thereby s s inversely correlated trends at lower MgO (Fig. 2). Here I examine TiO2-MgO broadening the combined high-MI and low-MI sample trends, extending them variations in SB lavas from analyses of this study (Fig. 12) using a threshold to lower MgO values, and causing them to merge at higher MgO values. This of sample mass index (MI s = 99) to differentiate samples that have lost <1% of pattern is similar to that observed in the previously published SB data (Fig. 2). their mass (if at all) from those samples that have lost 1%–9% of their mass due to alteration. A lower MI s threshold (98.4) was applied to group 3 samples Previously Published SB Geochemical Analyses because samples with higher MI s are unavailable. The high-MI s samples define two inversely correlated trends, one for The older analyses of SB samples collected during the BWIP and subse- series I–IV samples and the other for series V samples, although some series quent investigations include many samples from deep boreholes, and this IV samples are between these trends (Fig. 12). Within each of series I–III, suc- earlier data set provides an opportunity to examine whether alteration has

cessive chemical groups show an increase in MgO with decreasing TiO2. This generated mass depletion of SB lavas in groundwater environments far from progression from more evolved to less evolved compositions is comparable the influences of near-surface weathering. Mass analysis was applied to these data in the same manner as to the data from this study.

The Al2O3-TiO2 variations of the older SB data set are shown in Figure hi-MIs lo-MIs Group Series 13, in which symbols for data points are given different colors depending on 2.2 16 whether samples were collected from the surface (blue) or the subsurface 15 V (red). Subsurface samples, identified from sample number suffixes indicating 14 depth (Supplemental File 1; see footnote 2), come from drill core or rotary drill 2.1 13 chips collected from boreholes in and around the Pasco Basin. Most borehole 12 11 IV samples are from depths between 540 and 1120 m; the depth of the shallowest 2.0 10 borehole sample is ~120 m. The remaining samples are assumed to have been n collected at the surface. The MI 100 lines calculated for these data were defined TiO2 9 8 III to represent the baselines of the main body of samples, and the relatively few 1.9 7 samples having lower Al2O3 and TiO2 that were disregarded in this process are 6 discussed in the following. As in this study, MI 100 baselines were calculated II 1.8 5 separately for series I–IV and for series V. Assignments of samples to series 4 I–IV or to series V were made mainly on the basis of Ti/Zr ratios (see Fig. 3) 3 (described further in Supplemental File 1; see footnote 2). 2 I 1.7 The Al O -TiO data from these earlier analyses exhibit a pattern similar 1 2 3 2 3.8 4.2 4.6 5.05.4 to that shown by the data of this study (Fig. 5A), i.e., a baseline having nega- MgOn tive slope with scattered abundances plotting above it (Fig. 13). These sample populations differ from those of this study, however, in that scattered samples Figure 12. TiOn versus MgOn for Sentinel Bluffs Member (SB) samples analyzed in this study. 2 also plot below the main populations. The few outlier samples that plot below Data points are given symbols by SB chemical group and sample mass index, MI s: color symbols, samples with MI s ≥ 99 (hi-MI s in legend); black symbols, samples with MI s < 99 (lo-MI s in the main populations were ignored because these samples apparently have legend). gained mass from alteration, as discussed in the following.

16.2 15.4 92 Figure 13. Al On versus TiOn data for previ- AB94 2 3 2 15.8 90 Series I-IV 88 Series V ously reported analyses of Sentinel Bluffs 15.0 Member lavas. (A) Series I–IV. (B) Series V. 92 Blue symbols indicate samples from out- 15.4 96 crop or artificial exposures, and red sym- 94 bols indicate subsurface samples from boreholes. Solid green lines indicate MI 100 15.0 14.6 n 96 98 values and MI (mass index) isopleths in Al2O3 increments of 2 MI units. Green shading 14.6 98 above and below the MI 100 line represents 100 the MI windows (0.3 and 0.5 MI units for 100 14.2 series I–IV and V, respectively), and in- 14.2 cludes the samples used in regression to 102 102 determine MI 100 lines. Dotted green lines are lines of constant Al O /TiO at the 13.8 2 3 2 13.8 maximum and minimum sample values. Data are mainly from the compilation of 13.4 Reidel and Valenta (2000) (Supplemental 1.6 1.7 1.8 1.9 2.0 2.1 2.2 1.6 1.7 1.8 1.9 File 1; see footnote 2). n n TiO2 TiO2

Both surface and subsurface samples show evidence of mass loss, and magmatic in origin. As shown here, TiO2 and other immobile elements are several surface samples have MI s values <92 (Fig. 13). Subsurface samples concentrated in the remaining rock after soluble constituents, such as Fe and s generally have MI values ranging from ~100 to 96, similar to the values for Mg, have been removed by mineral dissolution. Inverse correlations of TiO2

most surface samples. These Al2O3-TiO2 relationships demonstrate that mass and MgO can also result from magmatic processes (Fig. 12), but without appli- loss has occurred in SB lavas buried deep in the subsurface. The decreases in cation of mass analysis the magmatic compositions cannot be distinguished n mass are attributed to mineral ± glass dissolution from alteration. from those modified by alteration. The use of an immobile element (e.g., TiO2 ) A small number of samples from the earlier SB data set that plot below the as a function of a mobile element (e.g., MgOn) therefore has limitations in de- MI 100 baselines indicate minor mass gains (to ~2.6 wt%). The sample with the fining the chemostratigraphy of SB and other GRB lavas. highest MI s has anomalously high CaO, at least 2.5 wt% higher than the CaO The occurrence of alteration under anoxic conditions undoubtedly has con- values of samples with MI s ~100, and presumably contains secondary minerals tributed to the difficulty in recognizing this alteration using physical charac- with abundant calcium, such as calcite. The other analyses with anomalously teristics. The freshest-appearing altered rocks, showing evidence of chemical high MI s may likewise contain significant amounts of secondary minerals. alteration by anoxic groundwater, are typically dark gray, similar to the color The anomalously high MI s values for a minority of the previously published of truly unaltered rock. The abundances of secondary clay and zeolite minerals SB analyses likely represent a net mass change whereby the mass gain from tend to be minimal, and hydration is limited, so that analytical totals (or loss on precipitation of secondary minerals exceeded the mass loss (if any) from min- ignition) are not significantly affected. In a study characterizing the secondary eral dissolution. This underscores the importance of applying (and document- minerals of CRBG lavas obtained from coring to depths >1300 m, Benson and ing) a sampling protocol in which secondary minerals are excluded, thereby Teague (1982) described the disparity between an apparent lack of visible alter- limiting mass change to negative values. If visibly altered samples are ana- ation in hand sample and the pervasive alteration, mainly to clay and zeolite lyzed for purposes other than investigating the magmatic chemistry of CRBG minerals; they concluded that fluids were able to migrate throughout the en- lavas, this should be made clear so that these data are not later misinterpreted tire basalt thickness via fractures, microfractures, and micropore systems. This as magmatic compositions. view is consistent with groundwater studies documenting regional subsurface flow throughout the CRBG lava pile (Burns et al., 2011, 2015; Kahle et al., 2011; Intraflow Chemical Variations Ely et al., 2014). Although visible alteration of CRBG lavas has been reported in thin sec- Prior characterizations of the geochemistry of SB and other GRB strati- tion (Ames, 1980; Benson and Teague, 1982; White et al., 1985; Caprarelli and graphic units have been developed without consideration of the effects of Reidel, 2004), such observations may not detect all chemical alteration. Even low-temperature alteration on lava chemistry, and analyzed abundances of for comparatively young lavas (younger than 200 ka) on Hawai`i, lavas ap- altered samples undoubtedly have often been incorrectly interpreted as being pearing fresh in thin section are nonetheless chemically altered (Lipman et al.,

1990). Ames (1980) also noted that alteration in CRBG lavas, apparent under CONCLUSIONS reflected light, was not apparent in transmitted light petrography. In addition, thin sections of GRB lavas from fine-grained, irregularly fractured parts of the Alteration of CRBG lavas in anoxic groundwater has occurred across the lava flow core typically show nearly opaque, cryptocrystalline masses, from Columbia River flood basalt province. This alteration is subtle in that it is not which little mineralogical information can be ascertained. accompanied by distinct visual cues, such as oxidation and the abundant sec The narrowly defined chemical groups identified in this study greatly ondary minerals that accompany surficial weathering under oxic conditions. The increase the resolution of the SB chemostratigraphy. Due to the lack of rec- groundwater alteration, however, can only be identified by chemical criteria. ognition of the chemical effects of alteration in prior geochemical studies of The primary effect of alteration under anoxic conditions is mineral disso- SB lavas, lava compositions of unaltered and variably altered rock were com- lution and reduction in rock mass. Due to the inherent normalization resulting pared to one another as though all compositions were magmatic in origin. from the analysis of mass-depleted rock as a unit mass, immobile element This undoubtedly resulted in stratigraphic conflicts and seemingly continuous abundances change in inverse proportion to the remaining mass. The magni- variations, and in a generalized SB chemostratigraphy of compositional types tude of these changes can be comparable to or exceed the subtle magmatic (Reidel, 2005). For example, the oldest SB compositional type (McCoy Canyon) variations within SB lavas as well as those within other GRB members. of Reidel (2005) corresponds to groups 1–9 of this study, and his youngest type Mass analysis allows the quantitative assessment of alteration-generated (Museum) corresponds to groups 13–16. mass transfer caused by water-rock interaction. Mass analysis employs sys- The ability to correct for or avoid the effects of chemical alteration reveals tematic immobile element variations (i.e., inverse Al-Ti arrays) among rocks that the SB comprises a greater number of flows and indicates a higher fre- that have retained their original mass (MI 100) to quantify the sample mass (MI s) quency of eruptions within the SB than previously estimated. The 16 chemical remaining after alteration-generated mass loss (if any). Mass normalization, groups recognized in this study indicate that the SB comprises at least this the product of MI s and a sample’s analyzed abundances (normalized to 100%), number of compositionally distinct lava flows. Small differences in immobile yields pre-alteration, magmatic immobile element abundances. element ratios (Fig. 4B), and/or mass-normalized immobile element abun- With high-precision data, mass analysis permits detailed chemical charac- dances, indicate that some SB groups comprise two flows. On this basis, the terizations of individual lava flows from mass-normalized immobile element SB comprises an estimated 21 compositionally distinct flows. abundances, and these abundances are consistent with broader chemical dis- The flows recognized here on the basis of chemical differences are distinctions made using immobile element ratios. Immobile element abundances tinct from flow lobes that are distinguished mainly by cooling surfaces and corrected for mass loss indicate that individual lavas are remarkably homo vesicle size-abundance distributions used to identify flow contacts. In the geneous and can even be identified from rock that has undergone as much as continuous sections sampled, a single, compositionally distinct SB chemical ~50% mass loss. The use of mass-normalized immobile element abundances group may be represented by a vertical succession of two or three physically for chemical characterization of SB lavas eliminates conflicts between chem- defined flows, which may or may not represent flow lobes or flows of the ical characteristics and stratigraphic position that result when only analyzed same eruption. abundances (normalized to 100%) are considered, thereby simplifying the A prerequisite for assessing the chemostratigraphy of SB (and other CRBG) chemostratigraphy. Individual SB flows can be correlated across the entire ex- lavas, therefore, is the application of mass analysis to reverse the influences of tent of the SB, which spans much of the GRB. secondary processes on lava chemistry. To demonstrate heterogeneity along Prior interpretations of intraflow differences in the chemistry of CRBG lavas, a vertical chemical profile within a single lava flow, it is necessary to show mainly between the abundantly vesicular, pervasively fractured flow crusts differences in mass-normalized immobile element abundances, not simply and the dense flow core, have called upon lava emplacement mechanisms differences in analyzed (normalized to 100%) immobile element abundances, to combine magma fed from a heterogeneous magma body or from multiple, which are sensitive to changes in rock mass. simultaneously erupted, chemically distinct but homogeneous magmas into a A magmatic trend defined by unaltered samples, such as the SB inverse single lava flow. Such explanations have spawned from the recognition that Al-Ti baseline, is not essential to the evaluation of magmatic intraflow varia- CRBG lavas are inflated pahoehoe lavas. Given that the more porous and per- tions from vertical sampling through a flow at a given location. In addition, a meable parts of CRBG lavas currently host regional aquifers, there has been completely unaltered sample is not required for mass normalization because ample opportunity for water-rock interaction in these parts of flows in the sub- the objective would be to examine only relative differences within a flow, not surface over much of the ~16 m.y. since these lavas erupted. Commonly, the to identify the original magmatic composition as in this study. Analyses from porous parts of lavas have elevated immobile element abundances compared a vertical profile can be mass normalized to the sample having the lowest to the dense flow core. Such intraflow chemical differences are therefore more abundance of Ti or other immobile element. Tests of intraflow chemical hetero- likely due to alteration of chemically homogeneous lava flows that were fed by geneity therefore can be performed on a lava flow that is not part of a related well-mixed magma reservoirs. Application of mass analysis is thus a prerequi- sequence of compositions such as the SB. site to assessing magmatic intraflow chemical variations in the CRBG.

Major and trace element variations of SB lavas indicate that several batches Burns, E.R., Williams, C.F., Ingebritsen, S.E., Voss, C.I., Spane, F.A., and DeAngelo, J., 2015, (series) of magmas evolved independently, at least in their later stages of dif- Understanding heat and groundwater flow through continental flood basalt provinces: In- sights gained from alternative models of permeability/depth relationships for the Columbia ferentiation. Although SB series evolved separately, their colinear Al-Ti vari- Plateau, USA: Geofluids, v. 15, p. 120–138, doi:10.1111/gfl.12095. ations and overlapping Al and Ti abundances are attributed to the saturation Caprarelli, G., and Reidel, S.P., 2004, Physical evolution of Grande Ronde Basalt magmas, Colum- of all SB magmas in clinopyroxene and plagioclase, reflecting the adherence bia River Basalt Group, north-western USA: Mineralogy and Petrology, v. 80, p. 1–25, doi:10 .1007/s00710-003-0017-1. of liquid compositions to a cotectic. Similar inverse Al-Ti arrays in other GRB Caprarelli, G., and Reidel, S.P., 2005, A clinopyroxene–basalt geothermobarometry perspective members suggest that Cpx + Pl fractionation is a common process that has of Columbia Plateau (NW-USA) Miocene magmatism: Terra Nova, v. 17, p. 265–277, doi:10 generated the chemical variations within GRB members. .1111/j.1365-3121.2005.00611.x. Champ, D.R., Gulans, J., and Jackson, R.E., 1979, Oxidation-reduction sequences in ground Prior interpretations of SB chemostratigraphy and lava heterogeneity have water flow systems: Canadian Journal of Earth Sciences, v. 16, p. 12–23, doi:10 .1139 /e79 -002 . been based on the often incorrect assumption that analyzed abundances are Conlon, T.D., Wozniak, K.C., Woodcock, D., Herrera, N.B., Fisher, B.J., Morgan, D.S., Lee, K.K., and equivalent to magmatic abundances. As a result, earlier interpretations of SB Hinkle, S.R., 2005, Ground-Water Hydrology of the Willamette Basin, Oregon: U.S. Geologi- chemostratigraphy have been generalized because magmatic compositions cal Survey Scientific Investigations Report 2005-5168, 83 p. Dahlem, D.H., 1987, Basalt Waste Isolation Project overview, in Tsang, C.-F., ed., Coupled Pro- could not be distinguished from those modified by alteration. In addition, those cesses Associated with Nuclear Waste Repositories: Orlando, Florida Academic Press, magmatic processes previously thought to have been involved in the genesis p. 105–119, doi:10.1016/b978-0-12-701620-7.50012-X. and emplacement of CRBG lavas warrant reevaluation. With the application of Derkey, R.E., Hamilton, M.M., Stradling, D.F., and Kiver, E.P., 1999, Preliminary geologic maps of the Spokane NE and SE 7.5-minute quadrangles, Spokane County, Washington: Washington mass analysis, future geochemical studies of CRBG lavas have an opportunity Department of Natural Resources Division of Geology and Earth Sciences Open File Report to more precisely refine our understanding of this flood basalt province. 99-6, scale 1:24,000. Derkey, R.E., Hamilton, M.M., and Stradling, D.F., 2004, Geologic map of the Airway Heights ACKNOWLEDGMENTS 7.5-minute quadrangle, Spokane County, Washington: Washington Department of Natural Resources Division of Geology and Earth Sciences Open File Report 2004-1, scale 1:24,000. Jon Hagstrum and Russ Evarts provided invaluable detailed constructive reviews of earlier ver- Edmunds, W.M., Cook, J.M., Darling, W.G., Kinniburgh, D.G., Miles, D.L., Bath, A.H., Morgan- sions of the manuscript. Thoughtful reviews by Marie-Nöelle Guilbaud, Steve Self, and John Wolff, Jones, M., and Andrews, J.N., 1987, Baseline geochemical conditions in the Chalk aquifer, and the editorial work of Julie Roberge and Shan de Silva, contributed to significant improve- Berkshire, U.K.: A basis for groundwater quality management: Applied Geochemistry, v. 2, ments of the manuscript and are appreciated. Rick Conrey generously provided unpublished data p. 251–274, doi:10.1016/0883-2927(87)90042-4. used in determining analytical precision of analyses performed at the Peter Hooper GeoAnalytical Ely, D.M., Burns, E.R., Morgan, D.S., and Vaccaro, J.J., 2014, Numerical simulation of ground Lab, Washington State University (Pullman) as well as information on analytical procedures. I am water flow in the Columbia Plateau Regional Aquifer System, Idaho, Oregon, and Washing- grateful to Russ Evarts, who graciously served as a sounding board in numerous discussions as ton: U.S. Geological Survey Scientific Investigations Report 2014-5127, 90 p. the concepts presented in this paper were developed. Russ Evarts and Ray Wells provided samples Hart, S.R., and Davis, K.E., 1978, Nickel partitioning between olivine and silicate melt: Earth and and/or analyses from locations mainly in the Coast Range and Willamette Valley, and Jon Hag- Planetary Science Letters, v. 40, p. 203–219, doi:10.1016/0012-821X(78)90091-2. strum provided core from paleomagnetic sampling sites in several flows from which geochemical Hearn, P.P., Steinkampf, W.C., Bortleson, G.C., and Drost, B.W., 1985, Geochemical controls on samples were prepared. 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