Light Elements and Li Isotopes Across the Northern Portion of the Central American Subduction Zone

Quaternary mafic rocks have been erupted from volcanoes both on and behind the volcanic front in southeastern Guatemala and western El Salvador. Behind-the-front volcanism extends almost continuously some 110 km from the front. B and Li contents, as well as B/Nb and Li/Yb ratios, are generally higher in the mafic volcanic rocks of the volcanic front and lower in those behind the front. Beryllium concentrations show the opposite relationships, with generally higher values behind the front. In addition, many behind-the-front samples have lower δ7Li (2–3‰ versus 4–6‰ for the volcanic front). Guatemalan volcanic rocks, in general, have lower δ7Li than those found along the remainder of the Central American margin. The light element variations across northern Central America clearly indicate that the volcanic front receives a much more pronounced fluid contribution from dehydrating, subducting Cocos lithosphere than the behind-the-front region. This dehydration pulse occurs at slab depths between 85 and 105 km, and selective enrichments of B, Cs, and Cl in volcanic rocks of the volcanic front suggest that the source of this pulse is dehydrating serpentinite. In addition, the dehydration pulse from subducted serpentinite is likely superimposed on a continuous dehydration signal provided by subducting oceanic crust which continues behind the volcanic front to slab depths \u3e200 km. The enhanced fluid contributions at the front produce higher degrees of wedge melting, thus explaining the lower Be contents of front samples. There are two possible explanations of the lower δ7Li of many behind-the-front volcanic rocks from southeastern Guatemala/western El Salvador: selective loss of 7Li during progressive dehydration of subducting Cocos lithosphere or diffusive fractionation accompanying late stage, selective crustal contamination. Since the slightly lower δ7Li of Guatemalan volcanic rocks in general is consistent with a minor crustal overprint, the second explanation is favored and could be relevant to the few subduction zones where lower Li isotopic values characterize back-arc regions. Light element variation across southeastern Guatemala and western El Salvador is consistent with the operation of decompression melting behind the volcanic front. Be/Zr ratios across the northern portion of the Central American subduction zone may indicate some mobility of Be