On B/Be Systematics of the Mexican Volcanic Belt
Document Type
Article
Publication Date
2-1996
Digital Object Identifier (DOI)
https://doi.org/10.1016/0016-7037%2895%2900415-7
Abstract
Boron and beryllium concentrations were measured in a diverse suite of well-characterized rocks from the Mexican Volcanic Belt (MVB). Low B and high Be result in relatively low B/Be ratios in the MVB, compared to other arcs. Nevertheless, B systematics resemble those of other arcs and provide insights into mantle processes. In the MVB, B enrichment depends first on magma type, and second on edifice type and location. B/Be values are highest (5–15) in andesites and dacites erupted from calcalkaline strato-volcanoes located along the volcanic front, such as Volcan Colima and V. San Juan. Rocks from strato-volcanoes located behind the volcanic front generally have lower values (1–5). B/Be values are also elevated in differentiated members of rock suites that show evidence for significant crustal assimilation. In the westernmost MVB, west of the Michoacán-Guanajuato Volcanic Field (MGVF), cindercone ejecta, including basalts, lamprophyres, and basanites, contain low B/Be values (<5). The lamprophyres and basanites have very low B/Be, despite high Ba/Ce and other common measures of subduction signature. In the MGVF, where cinder cones occur exclusively, B/Be values in primitive calc-alkaline basalts are distinctly higher than those from alkaline basalts (3–8 vs. 1–3), indicating that high B/Be is a mantle-derived feature and not an artifact of crustal assimilation. Comparison among various elemental ratios indicates that Cs and U show enrichment patterns similar to B; all are enriched in calc-alkaline rocks, but not in lamprophyres or basanites. In contrast, Ba, K, and Sr, are enriched in both calc-alkaline rocks and the lamprophyres and basanites.
Multi-stage processes and differing melting mechanisms are inferred to explain the variable characteristics of MVB volcanic rocks. First, slab-derived fluids, rich in fluid-mobile elements including B and Ba, infiltrate the mantle wedge. These fluids cause fluid-fluxed melting that produces calc-alkaline magmas enriched in all fluid-mobile elements. These lavas erupt from large, volcanic-front strato-volcanoes. The slab-derived fluids also metasomatize portions of the mantle wedge, producing phlogopite and/or amphibole. These phases have high partition coefficients for Ba-Sr-K, but may have low partition coefficients for B-Cs-U. Accordingly, subduction-zone metasomatism produces a mantle wedge enriched in Ba-Sr-K, but not necessarily in B-Cs-U. Decompression melting of this type of metasomatized mantle will consume the hydrous phases and produce magmas such as lamprophyres or basanites with high Ba/Ce, Sr/La, and K/La, but low B/Be, Cs/La, and U/La. This interpretation implies two types of subduction-zone signatures: one involving enriched Ba-Sr-K, elements that have longer residence times in the mantle wedge, and another involving enriched B-Cs-U, which all partition so strongly into fluids or melts that they have short residence times in the mantle, and are only enriched in magmas generated by fluid-fluxed melting or that have assimilated crustal material. Assimilation of granites and crustal rocks can also enrich differentiated lavas in B.
Was this content written or created while at USF?
Yes
Citation / Publisher Attribution
Geochimica et Cosmochimica Acta, v. 60, issue 4, p. 613-628
Scholar Commons Citation
Hochstaedter, Alfred G.; Ryan, Jeffrey G.; Luhr, James F.; and Hasenake, Toshiaki, "On B/Be Systematics of the Mexican Volcanic Belt" (1996). Geology Faculty Publications. 145.
https://digitalcommons.usf.edu/gly_facpub/145
Full Text URL
https://doi.org/10.1016/0016-7037%2895%2900415-7
