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[1] Geochronologic and geological data define a 600 ka age for the current volcanic front in Costa Rica. In Nicaragua, this age is less constrained but is likely within the range 600 ka to 330 ka. In Costa Rica, the new geochronologic data significantly improve estimates of the volumes of the volcanoes because they define the contact between the active volcanoes and the previous volcanic front, which is substantially older (2.2 to 1.0 Ma). In addition, the contrast in extrusive volcanic flux between western Nicaragua (1.3 Â 10 10 kg/m/Myr) and central Costa Rica (2.4 Â 10 10 kg/m/Myr) is greatly reduced from previous estimates and now within the range of error estimates. We estimate the subducted component of flux for Cs, Rb, Ba, Th, U, K, La, Pb, and Sr by subtracting estimated mantle-derived contributions from the total element flux. An incompatible element-rich OIB source for the Cordillera Central segment in Costa Rica makes the subducted element flux there highly sensitive to small changes in the modeled mantle-derived contribution. For the other three segments studied, the estimated errors in concentrations of highly enriched, subductionderived elements (Cs, Ba, K, and Pb) are less than 26%. Averaged over the time of the current episode of volcanism, the subduction-derived fluxes of Cs, Ba, K, Pb, and Sr are not significantly different among the four segments of the Central American volcanic front in Nicaragua and Costa Rica. The subductionderived fluxes of Th and La appear to increase to the SE across Nicaragua and Costa Rica, but the estimated errors in their subduction-derived concentrations are very high, making this variation questionable. The lack of change in the fluxes of Cs, Ba, K, Pb, and Sr argues that the well-defined regional variation in Ba/La is the result of changes in the mode or mechanics of fluid delivery into the mantle wedge, not the total amounts of fluids released from the slab. Concentrated or focused fluids in Nicaragua lead to high degrees of melting. Diffuse fluids in Costa Rica cause lower degrees of melting.

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[1] New geochemical analyses of volcanic rocks in El Salvador add to existing data from Nicaragua and Costa Rica to create a comprehensive set of geochemical data for Central American volcanics. These data coupled with previously published 40 Ar/ 39Ar ages covering the past 30 Ma show that Costa Rica and Nicaragua had similar U/Th and Ba/La values until 10 Ma when the region developed the distinctive along arc variations that made this margin famous. U/Th values increased in Nicaragua since the Miocene, while remaining unchanged along the rest of the volcanic front. This coincides temporally with the Carbonate Crash, which caused a transition in Cocos plate sediments from low-U carbonates to high-U, organic rich hemipelagic muds. Increases in uranium are not observed in Costa Rica because its lower slab dip produces a more diffuse zone of partial melting and because of the contribution from Galapagosderived tracks dilutes this signal. Ba/La has been used as a geochemical proxy for contributions from the subducting slab; however, our analyses indicate that the Ba concentrations do not vary significantly along strike either in the subducting sediment or the volcanic front. Along-arc variation is controlled by changes in La, an indicator of the degree of partial melting or source enrichment. Trace element models of five segments of the volcanic front suggest that a subducting sediment component is more important to magmas produced in El Salvador and Nicaragua than in Costa Rica, where the geochemistry is controlled by recent (<10 Ma) recycling of Galapagos tracks.

Soil and geologic evidence has been examined in the FBI Laboratory since 1939, and long admitted into trials, both in the US and abroad. However, to the best our knowledge soil evidence did not undergo a formal admissibility challenge within the US court systems until 29 th January 2016. Forensic soil analysis is typically a comparison between two or more samples to see whether they originated from different sources. When soil samples are indistinguishable, the possibility that they originated from a single source cannot be eliminated. The challenge in State of Kansas v. Kyle Flack, 13CR104 (2016), involved the admissibility of soil comparisons at the trial, as well as the qualifications of the forensic geologist who conducted the examinations. The views expressed are those of the authors and do not necessarily reflect the official policy or position of the FBI. Names of commercial manufacturers are provided for identification purposes only, and inclusion does not imply endorsement of the manufacturer or its products or services by the FBI.