Abstract

ABSTRACT Intralava geochemical variations resulting from subtle changes in magma composition are used here to provide insights into the spatial-temporal development of large basalt lava flow fields. Recognition that flood basalt lavas are emplaced by inflation processes, akin to modern pāhoehoe lava, provides a spatial and temporal frame-work, both vertically at single locations and laterally between locations, in which to examine lava flow emplacement and lava flow field development. Assuming the lava inflation model, we combined detailed field mapping with analysis of compositional profiles across a single flow field to determine the internal spatio-temporal development of the Palouse Falls flow field, a lava produced by an individual Columbia River flood basalt eruption. Geochemical analyses of samples from constituent lobes of the Palouse Falls lava field demonstrate that systematic compositional whole-rock variations can be traced throughout the flow field from the area of the vent to the distal limits. Chemical heterogeneity within individual lava lobes (and outcrops) shows an increase from lava crusts to cores, e.g., MgO = 3.24–4.23 wt%, Fe2O3 = 14.71–16.05 wt%, Cr = 29–52 ppm, and TiO2 = 2.83–3.14 wt%. This is accompanied by a decrease in incompatible elements, e.g., Y = 46.1–43.4 ppm, Zr = 207–172 ppm, and V = 397–367 ppm. Systematic compositional variations from the source to distal areas are observed through constituent lobes of the Palouse Falls flow field. However, compositional heterogeneity in any one lobe appears less variable in the middle of the flow field as compared to more proximal and distal margins. Excursions from the general progressive trend from vent to distal limits are also observed and may reflect lateral spread of the flow field during emplacement, resulting in the juxtaposition of lobes of different composition. Transport of magma through connected sheet lobe cores, acting as internal flow pathways to reach the flow front, is interpreted as the method of lava transport. Additionally, this can explain the general paucity of lava tubes within flood basalt provinces. In general, flow field development by a network of lava lobes may account for the occurrence of compositionally similar glasses noted at the proximal and distal ends of some flood basalt lavas.

Highlights

  • Continental flood basalts record some of the largest volcanic eruptions on Earth and require the accumulation of large volumes of eruptible magma

  • Compositional variability greater than analytical error may be problematic for stratigraphic correlations across the Columbia River Basalt Group (CRBG) based on chemistry alone

  • The results of this study reveal that the composition of the Palouse Falls lava lies at the lower end of the compositional range for the Wanapum Basalt for TiO2 and P2O5 with values approaching the “Ti gap”

Read more

Summary

Introduction

Continental flood basalts record some of the largest volcanic eruptions on Earth and require the accumulation of large volumes of eruptible magma These volcanic provinces are constructed of extensive (~ 104 – 105 km2) pāhoehoe lava flow fields (Self et al, 1997; Bondre et al, 2004; Bryan et al, 2010). Structural and morphological evidence for an inflation mechanism of emplacement includes compound lavas with thick crusts, massive cores, and internal vesicular layering (Hon et al, 1994; Self et al, 1998; Thordarson and Self 1998; Bondre et al, 2004). Such evidence results from the endogenous growth of each lava lobe and would have enabled the propagation of lava through insulated pathways to new lobes at an advancing flow front. Several additional lines of evidence supporting emplacement of the majority of lobes in a flood basalt flow field by inflation include: anisotropy of magnetic susceptibility (AMS; CanonTapia and Coe, 2002); quantitative fluid dynamic and thermal constraints

Methods
Results
Discussion
Conclusion

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call

Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.