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Compositional Variations in Late Quaternary Basaltic and Andesitic Scoria Cones

This study explores the variations in major and trace element concentrations within individual eruptions of scoria cones. It compares the compositions of scoria and lava, and investigates the location of scoria near Mt. Lassen. The study suggests that basaltic melt batches are heterogeneous and should be regarded as the rule rather than the exception.

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Compositional Variations in Late Quaternary Basaltic and Andesitic Scoria Cones

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  1. Compositional variations within individual late Quaternary basaltic to andesitic scoria cones. Variations in major and trace element concentrations within single eruptions Scoria vs. Lava

  2. Location of Scoria Study Near Mt. Lassen at southern end of Cascade Volcanic Arc. East of Mendicino Triple Junction, above subducting Gorda plate. Quarrying exposed top 75% of cinder cones. Study group consisted of 7 cinder cones. X-ray influorescence and inductively coupled mass spectrometry at Washington State University

  3. Chemical Comparisons ACE: Mantle Normalized incompatable trace element diagrams. Model 1: best fit for REE Model 2: best fit for High Field Strength Elements Model 1 and 2 show that it’s impossible to derive scoria and lava flow from same source by simple melting models. BDF: Rare Earth Element (REE) plots.

  4. A: Radiometric Isotopes from 2 cones compared to regional Quaternary rocks. LKOT= low-potassium olivine theolite B: Variations of Large Ion Lithophile/ High Strength Field Elements within scoria and between scoria and lava Note: Day Pitt and Round Barn Scoria become progressively rich in LILE/HFSE ratios moving up in sequence. C: Horr’s Corner Scoria becomes progressively enriched in light rare earth elements. b=early stage scoria, e=later stage scoria BSE: Bulk Silicate Earth

  5. What’s Going On Here? Fractional Crystallization? May have occurred at each of the cones but did not contribute significantly to chemical variations. Variable Slab Components and Partial Melting? Higher amounts of fluid mobile components result in higher degrees of partial melting and lower HSFE concentrations. Decreasing degree of partial melting results in increasing light REE/heavy REE ratio with progressive eruptive activity. Nb/Zr ratios from cones suggest differential partial melting does play a role in bimodal eruptive products. Crustal Contamination? Not a viable process based on eruptive chemical components as the andesite has higher Mg, Ni, and Cr components and a more primative basalt than that which would result from silicic component contamination. Tapping of Different Mantle Sources? HFSE, REE and radiogenic isotopes suggest that different magma batches had different mantle sources, one with low HFSE/heavy REE ratios (similar to MORBs) and one with higher HFSE (similar to IOB)

  6. Conclusion Magmas may rise through dike like systems as a diffuse entity rather than being stored with a chance to homogenize until future accent. As small volumes of basalt are thermally challenged in accent it is suggested that they can only reach the surface when they can follow the preheated path of recent melt ascent. Authors suggest that the heterogeneity of basaltic melt batches should now be regarded as the rule rather than the exception.

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