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 are
a 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 a
s Volcan Colima and V. San Juan. Rocks from strato-volcanoes located b
ehind the volcanic front generally have lower values (1-5). B/Be value
s are also elevated in differentiated members of rock suites that show
evidence for significant crustal assimilation. In the westernmost MVB
, west of the Michoacan-Guanajuato Volcanic Field (MGVF), cinder-cone
ejecta, including basalts, lamprophyres, and basanites, contain low B/
Be values (<5). The lamprophyres and basanites have very low B/Be, des
pite high Ba/Ce and other common measures of subduction signature. In
the MGVF, where cinder cones occur exclusively, B/Be values in primiti
ve calc-alkaline basalts are distinctly higher than those from alkalin
e 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 patt
erns similar to B; all are enriched in calc-alkaline rocks, but not in
lamprophyres or basanites. In contrast, Ba, K, and Sr, are enriched i
n both calc-alkaline rocks and the lamprophyres and basanites. Multi-s
tage processes and differing melting mechanisms are inferred to explai
n the variable characteristics of MVB volcanic rocks. First, slab-deri
ved fluids, rich in fluid-mobile elements including B and Ba, infiltra
te the mantle wedge. These fluids cause fluid-fluxed melting that prod
uces calc-alkaline magmas enriched in all fluid-mobile elements. These
lavas erupt from large, volcanic-front strato-volcanoes. The slab-der
ived fluids also metasomatize portions of the mantle wedge, producing
phlogopite and/or amphibole. These phases have high partition coeffici
ents for Ba-Sr-K, but may have low partition coefficients for B-Cs-U.
Accordingly, subduction-zone metasomatism produces a mantle wedge enri
ched in Ba-Sr-K, but not necessarily in B-Cs-U. Decompression melting
of this type of metasomatized mantle will consume the hydrous phases a
nd produce magmas such as lamprophyres or basanites with high Ba/Ce, S
r/La, and K/La, but low B/Be, Cs/La, and U/La. This interpretation imp
lies two types of subduction-zone signatures: one involving enriched B
a-Sr-K, elements that have longer residence times in the mantle wedge,
and another involving enriched B-Cs-U, which all partition so strongl
y into fluids or melts that they have short residence times in the man
tle, and are only enriched in magmas generated by fluid-fluxed melting
or that have assimilated crustal material. Assimilation of granites a
nd crustal rocks can also enrich differentiated lavas in B.