It is important to note that even in areas like the Portland Basin (Oregon) where the basalt has undergone extreme alteration, good samples can still be obtained with care in collecting and preparation. However, Reidel (2005) and Reidel and Tolan (2013) applied an additional important step where chips were washed in an ultrasonic cleaner to remove smaller particles that might otherwise escape notice and impact the analysis. Sawlan (2018) appears to have applied a similar procedure. Hand samples are typically broken into smaller chips and inspected for signs of alteration, with altered or amygdule-rich chips removed prior to crushing. This involves an initial inspection for (1) signs of alteration and weathering, (2) significant fractures, and (3) vesicles that characterize flow tops and bases as well as vesicle cylinders and lenses in the more massive parts of flows. Workers with decades of experience on the CRBG have established a standard procedure for sample collection to avoid alteration (e.g., Swanson and Wright, 1981). ESTABLISHED METHODOLOGY OF SAMPLE COLLECTION, THIN-SECTION ANALYSIS, AND XRF GEOCHEMISTRY As a contribution to this debate, we (1) examine long-established procedures and sampling techniques that have allowed workers to avoid significant alteration when collecting and analyzing CRBG flows (2) describe data collection of the Sentinel Bluffs Member and chemical variations that have been attributed to both magmatic processes and secondary alteration and (3) discuss the concerns we have on the rationale and analytical methodology that form the basis for Sawlan’s (2018) conclusions that compositional variations in the Sentinel Bluff Member are the result of chemical alteration from basalt-water interaction. It is our contention that careful sample collection of the CRBG has generated largely unaltered chemical compositions, and that the alteration hypothesis of Sawlan (2018) is incorrect due to flawed methodology.
We therefore embrace debate on the potential role of these aquifers in modifying primary magmatic compositions. Such data could be queried as problematic or invalid in a variety of studies that focus, for example, on chemostratigraphic correlations (e.g., Reidel, 2005 Conrey and Wolff, 2010 Vye-Brown et al., 2013 Martin et al., 2013 Wells et al., 2009) or on petrogenesis and evolution of the basalts (e.g., Davis et al., 2017 Blake et al., 2010 Martin, 1989 Wolff et al., 2008 Wolff and Ramos, 2013 Ramos et al., 2005, 2009, 2013 Tollstrup et al., 2002 Hooper, 1984 Hooper and Hawkesworth, 1993 Rodriguez and Sen, 2013 Thordarson and Self, 1998), as well as in paleomagnetic studies where the results are extremely sensitive to alteration especially in the case of iron minerals (e.g., magnetite, hematite) and rely on compositional data for flow identification (e.g., Wells et al., 1989 Hagstrum et al., 2010).Ĭonsideration and scrutiny of the Sawlan (2018) study is both timely and relevant to workers on the CRBG, but it is also pertinent to workers on other basalt provinces where aquifers are present (e.g., Hawaii, the Snake River Plain, and the Oregon High Lava Plains). The ramifications of widespread, hitherto undocumented alteration are compelling, in part because of the ambiguity such alteration would place on previous interpretations that rely on high-quality chemical data. The fact that nearly all CRBG lava flows are in contact with the confined aquifer system, even those in the anticlinal ridges, suggests that such alteration, if it existed, could be widespread. Sawlan (2018) argues instead that flow heterogeneity in the Sentinel Bluffs Member is largely the result of previously unrecognized chemical alteration of basalt in contact with aquifers, accompanied by significant mass loss (up to 50%). The compositional heterogeneity of individual flows, although small, has been interpreted by many workers as being the result of magma chamber processes (e.g., Wright et al., 1973 Wright and Swanson, 1980 Mangan et al., 1986 Hooper, 2000 Reidel, 2005) combined in places with surficial mixing as the flows are emplaced (e.g., Reidel and Tolan, 2013). Department of Energy’s (USDOE) Hanford Site (southeastern Washington State), and borehole investigations focused in part on petroleum exploration in the Columbia Basin. These flows have been studied for over 50 years, resulting in numerous scientific publications from academic institutions, federal and state regional mapping projects, more detailed mapping associated with the U.S. The Sentinel Bluffs Member is the youngest unit of the Grande Ronde Basalt, having typical physical characteristics of most large-volume Columbia River Basalt Group (CRBG) sheet flows.
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