On B/Be ratios in the Mexican Volcanic Belt

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.