Two detailed cross-arc transects of the Central American subduction zone have been completed across southeastern Guatemala and Honduras. The two transects display both contrasting and similar cross-arc geochemical variations. Across southeastern Guatemala, ratios of fluid mobile to relatively fluid immobile incompatible elements, such as Cs/La and Ba/Nb, are typically high at the volcanic front, low behind the front, and, most importantly, unchanging once behind the front. Across Honduras, by contrast, such incompatible element ratios often systematically decline from the elevated values characterizing the volcanic front. Across both transects, many incompatible element contents and ratios of highly incompatible to moderately incompatible elements, such as the La/Yb ratio, are higher behind the front. The similarities between the two transects may be caused by (1) declining total slab inputs away from the volcanic front, (2) more contributions from subducting carbonate sediments behind the volcanic front, (3) declining degrees of melting away from the volcanic front, (4) more decompression-induced melting away from the volcanic front, and (5) more melting of enriched domains in the mantle wedge away from the volcanic front. The distinctions between the two transects are attributed to varying proximity to the North American--Caribbean transform boundary. Motions along this boundary have caused greater extension close behind the front in southeastern Guatemala, serving to abruptly cut off or reduce chemical transfer from the subducting Cocos plate. Behind the front in Honduras, by contrast, the chemical umbilical to the Cocos plate may be only progressively cut or, alternatively, may also be abruptly cut, but at a much greater distance from the volcanic front. In both transects, however, certain ratios of fluid mobile to relatively fluid immobile incompatible elements, such as Ba/La and Pb/Ce, remain considerably above mid-ocean ridge basalt or oceanic island basalt values some 100 km behind the front. This common slab signature may represent fossil slab modification of the Central American mantle wedge. ¿ 2000 American Geophysical Union