U-series Disequilibria in Guatemalan Lavas, Crustal Contamination, and Implications for Magma Genesis Along the Central American Subduction Zone

New U-series results indicate that Guatemalan volcanic rocks display both 238U and 230Th excesses. 230Th excess is restricted to volcanoes in central Guatemala, both along and behind the front. 230Th excess correlates with a number of incompatible element ratios, such as Th/Nb and Ba/Th. It also shows a negative correlation with MgO. Guatemalan volcanic rocks have (230Th/232Th) ratios that overlap those of Costa Rican volcanics and are therefore considerably lower than the unusually high ratios characterizing volcanic rocks from Nicaragua. Along-arc variations in (230Th/232Th) therefore mirror those of a number of diagnostic geochemical parameters, such as Ba/La, which are symmetrical about a peak in west central Nicaragua. The one siliceous lava analyzed, from the Cerro Quemado dome complex, has a recognizable crustal imprint, distinguished, for instance, by high Th/Nb and low Ba/Th. In mafic samples, 238U excess is attributed to addition of a U-enriched fluid component from the subducting Cocos plate. Our preferred explanation for 230Th excess in Guatemalan mafic samples, on the other hand, is crustal contamination, consistent with the relatively high Th/Nb and low Ba/Th ratios in these samples. We suspect, however, that crustal contamination only exerts a sizable control over the U-series disequilibrium of mafic magmas in Guatemala, and not elsewhere along the Central American volcanic front. This agrees with previously published trace element and isotopic evidence that throughout Central America, with the exception of Guatemala, mafic magmas are largely uncontaminated by crustal material.The work was supported by NSF grant OCE-0405666

The impact of city-wide deployment of Wolbachia-carrying mosquitoes on arboviral disease incidence in Medellín and Bello, Colombia: study protocol for an interrupted time-series analysis and a test-negative design study.

Background: Dengue, chikungunya and Zika are viral infections transmitted by Aedes aegypti mosquitoes, and present major public health challenges in tropical regions. Traditional vector control methods have been ineffective at halting disease transmission. The World Mosquito Program has developed a novel approach to arbovirus control using Ae. aegypti stably transfected with the Wolbachia bacterium, which have significantly reduced ability to transmit dengue, Zika and chikungunya in laboratory experiments. Field releases in eight countries have demonstrated Wolbachia establishment in local Ae. aegypti populations. Methods: We describe a pragmatic approach to measuring the epidemiological impact of city-wide Wolbachia deployments in Bello and Medellín, Colombia. First, an interrupted time-series analysis will compare the incidence of dengue, chikungunya and Zika case notifications before and after Wolbachia releases, across the two municipalities. Second, a prospective case-control study using a test-negative design will be conducted in one quadrant of Medellín. Three of the six contiguous release zones in the case-control area were allocated to receive the first Wolbachia deployments in the city and three to be treated last, approximating a parallel two-arm trial for the >12-month period during which Wolbachia exposure remains discordant. Allocation, although non-random, aimed to maximise balance between arms in historical dengue incidence and demographics. Arboviral disease cases and arbovirus-negative controls will be enrolled concurrently from febrile patients presenting to primary care, with case/control status classified retrospectively following laboratory diagnostic testing. Intervention effect is estimated from an aggregate odds ratio comparing Wolbachia-exposure odds among test-positive cases versus test-negative controls. Discussion: The study findings will add to an accumulating body of evidence from global field sites on the efficacy of the Wolbachia method in reducing arboviral disease incidence, and can inform decisions on wider public health implementation of this intervention in the Americas and beyond. Trial registration: ClinicalTrials.gov: NCT03631719. Registered on 15 August 2018

Early Arc ,agmatism: geochemical characteristics of volcanic clasts from Punta Sámara, Costa Rica

The geochemical characteristics of volcaniclasts from Punta Sámara are used to infer magmaticprocesses that generated Late Cretaceous – Middle Paleocene arc lavas in the western margin of the Caribbeanplate. The Ba/La ratios of the volcaniclasts are higher than the ratios for any of the modern lavas from Costa Rica, and the La/Yb ratios are lower; suggesting that the lavas from the primitive arc were generated by greatercontribution of slab material than the modern arc lavas. Furthermore, the lower La/Yb ratios imply that thedegree of melting was significantly higher and/or that the mantle source was more depleted than in modernCosta Rica. The low Zr/Nb ratios for the modern lavas from central Costa Rica imply a more enriched mantlesource (OIB-like), whereas the higher ratios for the primitive arc volcaniclasts imply a more depleted mantlesource (MORB-like). We use the geochemical information to infer the conditions of subduction during the earlystages of arc magmatism in the southern part of the Central American arc. We infer that the primitive arc wasassociated with a subducting slab that dip at a steeper angle than the present. The steeper angle would imply thatthe subducting Farallon plate was older, colder and denser than the present Cocos plate.Se usan las características geoquímicas de los volcaniclastos de Punta Sámara, para inferir los procesos magmáticos que generaron las lavas de arco en el Cretácico tardío – Paleoceno Medio. Las relaciones Ba/La sonmás altas que las de las lavas jóvenes de Costa Rica y las de La/Yb son más bajas, lo que sugiere que las lavas delarco primitivo se generaron por una mayor contribución del material de la placa subducida que las lavas del arcomoderno. Además, las relaciones más bajas de La/Yb implican que el grado de fusión era significativamente más al-to y/o la fuente mantélica estaba más empobrecida que en la actualidad. La baja razón de Zr/Nb para las lavas jóve-nes de Costa Rica central implica una fuente mantélica más enriquecida (tipo OIB), mientras que las razones mayo-res para los volcaniclastos del arco primitivo implican una fuente mantélica más empobrecida (tipo MORB). Usa-mos la información geoquímica para inferir las condiciones de la subducción durante las etapas tempranas del mag-matismo en la parte sur del arco de América Central. Inferimos que el arco primitivo estaba asociado con una placaque se subducía a un ángulo mayor que el presente, lo que implicaría que la subducida placa Farallón era más vieja, fría y densa que la actual placa del Coco. 

Evaluating Complex Magma Mixing via Polytopic Vector Analysis (PVA) in the Papagayo Tuff, Northern Costa Rica: Processes that Form Continental Crust

geoscience

Calcic cores of plagioclase phenocrysts in andesite from Karymsky volcano: Evidence for rapid introduction by basaltic replenishment

Calcic cores in plagioclase of Karymsky andesite of the 1996–2000 eruptive cycle texturally and compositionally (both trace and major elements) mimic the plagioclase phenocrysts of basalt erupted 6 km away at the onset of the cycle. These observations support the view that simultaneous eruption of andesite and basalt at Karymsky in the beginning of the cycle represents an example of replenishment and eruption triggering of an andesitic reservoir. Homogeneity of andesitic output occurred within two months. This suggests to us that blending of injected basalt into reservoir magma was thorough and rapid

Origin of silicic magmas along the Central American volcanic front: Genetic relationship to mafic melts

Silicic pyroclastic flows and related deposits are abundant along the Central American volcanic front. These silicic magmas erupted through both the non-continental Chorotega block to the southeast and the Paleozoic continental Chortis block to the northwest. The along-arc variations of the silicic deposits with respect to diagnostic trace element ratios (Ba/La, U/Th, Ce/Pb), oxygen isotopes, Nd and Sr isotope ratios mimic the along-arc variation in the basaltic and andesitic lavas. This variation in the lavas has been interpreted to indicate relative contributions from the slab and asthenosphere to the basaltic magmas [Carr, M.J., Feigenson, M.D., Bennett, E.A., 1990. Incompatible element and isotopic evidence for tectonic control of source mixing and melt extraction along the Central American arc. Contributions to Mineralogy and Petrology, 105, 369–380.; Patino, L.C., Carr, M.J. and Feigenson, M.D., 2000. Local and regional variations in Central American arc lavas controlled by variations in subducted sediment input. Contributions to Mineralogy and Petrology, 138 (3), 265–283.]. With respect to along-arc trends in basaltic lavas the largest contribution of slab fluids is in Nicaragua and the smallest input from the slab is in central Costa Rica — similar trends are observed in the silicic pyroclastic deposits. Data from melting experiments of primitive basalts and basaltic andesites demonstrate that it is difficult to produce high K2O/Na2O silicic magmas by fractional crystallization or partial melting of low-K2O/Na2O sources. However fractional crystallization or partial melting of medium- to high-K basalts can produce these silicic magmas. We interpret that the high-silica magmas associated Central America volcanic front are partial melts of penecontemporaneous, mantle-derived, evolved magmas that have ponded and crystallized in the mid-crust — or are melts extracted from these nearly completely crystallized magmas

Resumen de Ponencia: Mineralogía de las rocas volcánicas (Cretácico al Presente) de Costa Rica: Un avance de investigación.

ponencia -- Universidad de Costa Rica. Escuela Centroamericana de Geología, 2008. Resumen de la ponencia.Desde hace varias décadas, se tienen estudios mineralógicos detallados con microsonda electrónica de rocas volcánicas de Costa Rica de diversa composición petrográfica y ambiente geotectónico. Sin embargo, no se ha realizado una integración de todos estos datos geoquímicos para establecer luces sobre patrones, similitudes y diferencias en el tiempo y en el espacio. El presente trabajo es un primer avance en dicho sentido, en el cual se han compilado en forma digital hasta el momento unos 2125 análisis químicos de microsonda electrónica. Los feldespatos son los minerales más abundantes y prácticamente omnipresentes, predominando las plagioclasas del tipo oligoclasa hasta anortita (An28-95), tanto como fenocristales (fn: 0-50%) con inclusiones de cpx, ol, mt, apat, y vidrio. Suelen tener zonación normal, inversa y oscilatoria. Las maclas son polisintéticas (leyes de albita, albita-Carlsbad y menos frecuente albita-periclina), otras debido a esfuerzos mecánicos, o bien como uniones tipo Banat. La sanidina (fn: 0-4%) y anortoclasa (fn: 0-0,5%) son casuales y se les encuentra en traquitas de la isla del Coco o en la ignimbrita Tiribi, por ejemplo. Algunas anortitas son xenocristales en lavas del Arenal y del Poas. Le siguen los clinopiroxenos (fn: 0-24%) que suelen ser de tipo augita (Wo19-87 En10-54 Fs1-30), algunas ferriferas, otras frecuentemente con zonación en reloj de arena pero con contenidos bajos en Ti (TiO2 3%) y augitas egirinicas en rocas alcalinas del Caribe. Los ortopiroxenos (fn: 0-12,5%) son poco zonados, del tipo ferrohipersteno hasta eulita (En80-60), abundantes en facies subalcalinas. El olivino (fn: 0-54%), suele ser del tipo hialosiderita hasta fosterita (Fo91-53), en ocasiones con inclusiones de picotita o con borde de clinopiroxeno. Suele estar presente en rocas con contenido de SiO2 inferior a 59%, y pueden llegar a ser muy abundante en basaltos picriticos, meimequitas, picritas y picritas komatiiticas del Cenozoico y Neogeno. Los anfiboles (0-6%), son abundantes en dacitas y riolitas y son del tipo hornblendas magnesiferas y pargasiticas, mientras que en algunas lavas antiguas de la Península Santa Elena puede presentarse además como ferrohornblendas y hornblenda actinoliticas, o en las rocas alcalinas como del tipo kaersutita. En rocas subalcalinas, los anfíboles suelen aparecer a partir de un contenido de SiO2 mayor a 59%. La biotita (fn: 0-20%) y el cuarzo (fn: 0-10%) son abundantes en flujos piroclásticos Pleistocenos en Guanacaste, y raramente en lavas. Dentro de los feldespatoides se tienen la nefelina (fn: 0-5%), hayna y sodalita, que se presentan en lavas alcalinas. Los minerales opacos (fn: 0-5,5%) son ubicuos en la matriz, particularmente magnetita (algunas ricas en Ti o Cr) y mas raramente espinela cromifera, entre otros. Minerales subordinados y hasta casuales son el apatito (fn: 0-0,5%) y zircón (fn: 0-0,2%). La matriz suele contener las mismas fases de los fenocristales, ocasionalmente analcima, tridimita y cristobalita intersticial, y raramente flogopita en facies alcalinas Contacto del expositor del trabajo grupal : [email protected] de Costa Rica, Escuela Centroamericana de Geología.UCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela Centroamericana de Geologí