Alteration, mass analysis, and magmatic compositions of the Sentinel Bluffs Member, Columbia River flood basalt province

Abstract

Accurate characterization of the magmatic compositions of flood basalt lavas is fundamental to interpretations of magma genesis, stratigraphy, and correlation across these extensive provinces. Analysis of the geochemistry of the Sentinel Bluffs Member of the Grande Ronde Basalt, Columbia River Basalt Group (northwestern USA), demonstrates that a mass-based methodology, similar to those routinely used in studies of weathering and soil formation, enables the identification of subtle and previously unrecognized low-temperature alteration, and the determination of primary magmatic geochemical characteristics in rocks modified by secondary processes. This methodology, here termed mass analysis, employs concentrations and ratios of immobile elements, which are not transported by low-temperature alteration processes, to show that alteration has resulted in loss of rock mass due to mineral dissolution in anoxic groundwater. Immobile element abundances corrected for mass loss permit the identification and province-wide correlation of individual flows and flow packages, even for rocks that have undergone nearly 50% mass loss. The methodology developed with Sentinel Bluffs lavas is applicable to other lavas of the Columbia River flood basalt province, and most likely to other volcanic provinces in which lavas have undergone long-term interaction with groundwater.

The three-stage petrochemical evolution of the Steens Basalt (southeast Oregon, USA) compared to large igneous provinces and layered mafic intrusions

“…thesis of Bendaña (2016). There is no evidence for cryptic alteration by anoxic waters, as described for the Sentinel Bluffs Member of the Grande Ronde Basalt (Supplemental File 5 [footnote 1]; Sawlan, 2017).…”

Alteration, mass analysis, and magmatic compositions of the Sentinel Bluffs Member, Columbia River flood basalt province: REPLY

“…The mass analysis methodology introduced by Sawlan (2018) applies a fundamental term, the parent:sample immobile element ratio, which is incorporated in all prior expressions used to quantify low-temperature chemical alteration from water-rock interaction (e.g., Nesbitt, 1979;Brimhall et al, 1992;Anderson et al, 2002). This ratio equals the sample mass fraction remaining after alteration (if any), and its application as a multiplier to element abundances is termed mass-normalization (Sawlan, 2018). Baker et al claim that these expressions are flawed and invalid, but they do not substantiate such claims.…”

“…The two left terms, C C / i p i s , correspond to the parent:sample immobile element ratio, which was defined by Sawlan (2018) as the sample mass fraction (remaining), M s , or, expressed as a percentage, the sample mass index, MI s . Because immobile elements (IE) are not transported in low-temperature waterrock interaction, their abundances in normalized analyses (to 100% volatile-free) of altered rock change only from depletion (or addition) of mobile elements (ME), which causes a change in sample mass.…”

“…The multiplication of normalized element abundances by the sample mass fraction, M s , reverses this inherent normalization and gives element abundances for a sample's mass-depleted state. As explained in Sawlan (2018), for normalized analyses of mass-altered samples IE abundances "change" as the reciprocal of the remaining sample mass fraction, 1/M s , and such changes are predominantly positive due to sample mass loss from mineral dissolution. Substituting M s for the parent:sample IE ratio, C C / i p i s , the expression for tau given in Equation (2) can be rewritten as follows,…”

“…In reply, I will show that both anoxic alteration and oxic surface weathering of CRBG lavas are prevalent both east and west of the Cascade Range, that my methodology is based on quantitative techniques that have been used in the analysis of weathered rocks and soils for decades (Nesbitt, 1979;Brimhall et al, 1992;Anderson et al, 2002), and that my own careful field and laboratory sampling, as documented in Sawlan (2018), can result in an unweathered and relatively unaltered sample collection, whereas those samples previously collected in SB lavas (e.g., Reidel and Valenta, 2000) have not only experienced significant anoxic water-rock interaction with groundwater but have subsequently undergone appreciable oxic weathering at the ground surface.…”