| Lavas from southeastern Nicaragua were geochemically analyzed, including several from a volcano never before sampled, in order to better understand the relationship between the chemistry of the lavas and the physical parameters of the Central American arc, which include depth to the slab, distance from the trench, slab dip and the age of the downgoing slab. The analyses were done by x-ray fluorescence, by high resolution inductively coupled plasma mass spectrometry, and by thermal ionization mass spectrometry. The results show that the abundance of high-field strength elements in the lavas is particularly influenced by the depth to the slab underlying a volcano and the distance of the volcano from the trench. Additionally, the results show that decompression melting is an important process in the generation of these lavas.The chemistry of mantle xenoliths from Cerro Mercedes, Costa Rica, were studied to elucidate the compositional and physical state of the mantle wedge underlying Central American. The composition of the mantle wedge is clarified through analyses of the whole rock information about the physical state of the mantle is obtained from analyses of the minerals that constitute the xenoliths. The analytical methods used for the whole rock chemistry were x-ray fluorescence and high resolution inductively coupled plasma mass spectrometry. Electron probe microscopy and laser ablation inductively coupled plasma mass spectroscopy were used to analyze the minerals. Results show that the mantle wedge composition is predominantly peridotite (&sim65% modal olivine) that has experienced variable degrees of modification by melt- and fluid-rock reactions, with pockets of cumulate pyroxenite. Although the majority of the peridotites are multi-phase residues, one sample, CM52, may be a first-stage residue. The xenoliths are spinel facies rocks, which correspond to pressures between 1--2.5 GPa, and have equilibrium temperatures ranging from 860°C--1030°C at 1 GPa, which place them between 30km and 85km within the mantle. The potential temperature of the system, based on whole rock major oxide modeling, is 1580°C. Estimated values for the oxidation state of the mantle range from Delta fo2FMQ +0.9 -- Deltafo 2FMQ +3.25 for the peridotites and up to Delta fo2FMQ +4.05 for the pyroxenites. |
