We report helium and carbon isotope and relative abundance data of fumaroles, hot springs, water springs, mud-pots and geothermal wells from El Salvador and Honduras to investigate both along and across-arc controls on the release of CO2 from the subducted slab. El Salvador localities show typical volcanic front volcanic gas signatures, with He-3/He-4 ratios of 5.2-7.6 R-A, delta C-13 values of -3.6% to -1.3% and CO2/He-3 ratios of 8-25 x 10(9). In Honduras, we find similar values only for volatiles collected in the Sula Graben region located similar to 200 km behind the volcanic front. All other areas in Honduras show significantly lower He-3/He-4 ratios (0.7-3.5 R-A), lower delta C-13 values (<-7.3%) and more variable CO2/He-3 ratios (6.2 x 10(7)-2.0 x 10(11)): characteristics consistent with degassing-induced fractionation of CO2 and He and/or interaction with crustal rocks.The provenance of CO2 released along the volcanic front is dominated by subducted marine carbonates (L=76 +/- 4%) and organic sediments (S = 14 +/- 3%), with the mantle wedge (M) contributing 10 +/- 3% to the total carbon flux. The L/S ratio of the El Salvador volatiles (average = 5.6) is comparable to volcanic front localities in Costa Rica and Nicaragua [A.M. Shaw, D.R. Hilton, T.P. Fischer, L.A. Walker, G.E. Alvarado, Contrasting He-C relationships in Nicaragua and Costa Rica: insights into C cycling through subduction zones. Earth Planet. Sci. Lett. 214 (2003) 499-513] but is approximately one-half the input value of sediments at the trench (L. Li, G.E. Bebout, Carbon and nitrogen geochemistry of sediments in the Central American convergent margin: Insights regarding subduction input fluxes, diagenesis, and paleoproductivity, J. Geophys. Res. 110 (2005), doi: 10.1029/2004JB003276). We use the L/S ratio of El Salvador geothermal fluids, together with estimates of the CO2 Output flux from the arc, to constrain the amount and composition of subducted sediments involved in the Supply of CO2 to the arc. For the El Salvador segment of the volcanic front, a similar to 180 m continuous section of the incoming sedimentary pile - with the uppermost similar to 42 in removed by under-plating, is required. Significantly, there is no need for oceanic basaltic basement to Supply CO2 to El Salvador or any other part of the volcanic front. This new approach, combining provenance characteristics of CO2 from the slab (L/S ratio) and CO2 flux estimates of the volcanic output, allows a more realistic estimate of the recycling efficiency of slab-derived sedimentary CO2 through the Central American Volcanic Arc to the atmosphere. Furthermore, the low L/S ratio (4.8) of Sula Graben samples from behind the front in Honduras is inconsistent with continued supply of slab-derived sedimentary CO2 following volatile loss at sub-are depths, thereby pointing to ancient enrichment and/or lateral entrainment processes controlling CO2 in the mantle wedge below Honduras. (C) 2007 Elsevier B.V All rights reserved.

We report new He abundance and isotope measurements of phenocryst phases in volcanic tephra and lavas from the Nicaragua-Costa Rica section of the Central American arc, where significant variations in crustal thickness have been inferred. Helium isotope values range from 4.6RA to 7.5RA, with no evidence for crustal thickness influencing measured 3He/4He ratios. A comparison of He abundances and isotopes measured in mafic phenocrysts from tephra vs. lavas from two separate eruptions at Cerro Negro show that both sampling media preserve phenocrysts with high 3He/4He values. 3He/4He ratios measured in phenocryst phases show good agreement with He isotope values of geothermal fluids from the same volcanoes. However, we note that the pyroxenes tend to have lower 3He/4He ratios (4.6–7.0RA) than the olivines (3He/4He=6.1–7.5RA) over a range of concentration values and are consistently lower in cogenetic phenocryst pairs at all locations sampled. In order to assess how this difference arises, we explore two alternative mechanisms: (1) diffusion-related isotopic fractionation, and (2) late-stage radiogenic 4He additions, preferentially influencing pyroxene grains. In the first case, we reject diffusion-related fractionation of He isotopes since lower 3He/4He ratios are not accompanied by a decrease in He concentration values. The second scenario is evaluated on the basis of Mg numbers in cogenetic phenocryst pairs and by petrological modeling of the crystallization sequence. Mg numbers and modeling results at low pressure conditions (=1 kbar) suggest that olivine crystallization preceded pyroxene crystallization. However, since lavas do not show evidence for extensive crustal contamination, we suggest that the best explanation for the lower 3He/4He ratios in pyroxenes is related to the closure temperatures of the phenocryst phases. Given its lower closure temperatures and higher He diffusion rates, we suggest that pyroxenes would be more susceptible to late-stage He exchange with a low 3He/4He source during ascent, presumably the surrounding crust.

In the Central American arc, southeastern Guatemala hosts the most diverse volcanism. Large stratovolcanoes at the volcanic front (VF) form as a result of subduction of the oceanic Cocos plate beneath the continental Caribbean plate. Behind the volcanic front (BVF) volcanism, however, has undergone a fundamental change in eruptive style during the Quaternary from older, polygenetic central volcanism to younger, monogenetic cinder cone volcanism. Magmas that traverse the 40-45-km-thick crust in southeastern Guatemala are highly susceptible to crustal contamination. Consequently, mineral chemical data, whole-rock oxygen isotope, and light element geochemistry are used to investigate the relationship between edifice type and the magnitude of crustal contamination.The lack of systematic variation between compositions of phenocryst phases and host rocks strongly suggests that open system processes were operating. Moreover, phenocryst core compositions are generally out of equilibrium with host rock compositions. Olivine from BVF cinder cones deviate only slightly from the equilibrium line in comparison to the older behind the volcanic front (OBVF) central volcanoes and VF stratovolcanoes, suggesting less assimilation of crustal lithologies. Steep arrays on the delta(18)O-SiO(2) diagram cannot be explained by crystal fractionation and favor the incorporation of (18)O-enriched crustal rocks. Higher delta(18)O values in the OBVF central volcanoes and VF stratovolcanoes support the idea that larger, shallow magma bodies experienced greater amounts of crustal contamination. Regional extension in the Ipala Graben of southeastern Guatemala likely promoted short residence times in crustal reservoirs and small degrees of crustal assimilation for the BVF cinder cone magmas.

Volcanic gases are a powerful tool for assessing magmatic processes in subduction zones. We report gas chemistry and nitrogen isotope compositions of fumaroles, bubbling springs, and geothermal wells from the Costa Rican segment of the Central American volcanic segment (CAVS), and new correlation spectroscopy (COSPEC) SO2 flux measurements of Poás and Arenal volcanoes. N2/He ratios (100–8,250) and nitrogen isotope compositions (δ15N = −3.02 to +1.69‰) of Costa Rica volatiles are consistent with sources ranging from typical arc‐type end‐members, with nitrogen addition from the subducting slab, to MORB end‐member, having experienced no slab modification. Overall, nitrogen‐helium chemistry of Costa Rican material indicates a diminished slab contribution versus other locations along the arc (e.g., Nicaragua and Guatemala). We use SO2 flux measurements of Poás and Arenal (1.80*105 ± 4.00*104 and 8.30*103 ± 4.00*103 kg/day, respectively, or 1.30*105 ± 6.25*104 and 2.81*106 ± 6.25*105 mol/day, respectively) to extrapolate a SO2 flux for the Costa Rica segment of 1.09*109 mol/day. Using CO2/St (St = total sulfur) of 2.7 and 5.9, we calculate CO2 fluxes of 1.88*108 and 4.11*108 kg/yr, respectively (2.94*109 and 6.42*109 mol/yr, respectively). Other volatile fluxes (N2, He, H2, Ar, HCl, and H2O) are calculated using CO2/St and regional gas chemistry. For Costa Rica, the output/input ratios of nitrogen are less than unity (0.03 to 0.06 for CO2/St of 2.7 and 5.9, respectively), suggesting more N is subducted than released in the subarc, possibly resulting from sediment offscraping, forearc devolatilization, limited fluid availability in the subarc, or subduction past the subarc.

The major and trace element geochemistry of lavas erupted from four volcanic front (VF) stratovolcanoes in southeastern Guatemala show differences in the relative importance of flux and decompression melting in a continental are setting. The VF stratovolcanoes exhibit a wide compositional range from basalt to dacite, although modern Pacaya erupts basaltic lavas. The VF basalts have relatively low MgO contents and plot outside the field of primary arc magmas defined by melting experiments on hydrous peridotite. After subtracting the effects of the fractionation, assimilation, and alteration of some VF lavas, separate partial melting and mixing trends were identified for Agua-Pacaya and Tecuamburro-Moyuta.The distinct chemical signatures of the hemipelagic and carbonate sediments subducted off Guatemala provide constraints on material transfer processes that occurred between the slab and mantle wedge. Model fluids and melts from the subducted slab were calculated using recently published mineral-aqueous fluid partition coefficients. Wide separation of the model fluid and melt compositions on a U/La versus Ba/Th diagram creates diagnostic mixing curves with an enriched mid-ocean ridge basalt source. Fluid from mature ocean crust has high U/La, fluid from carbonate sediment has high Ba/Th, and fluid and melt from hemipelagic sediments have both high U/La and Ba/Th. In a simple single-stage model, a mantle metasomatized by fluid originating largely from the oceanic crust with only minor sediment fluid contributions best explains the overall large ion lithophile element composition of the VF lavas. (Th/Rb)(N) ratios of similar to 1 in the VF lavas from southeastern Guatemala require a component of sediment melting. Therefore, a more realistic two-stage model to describe the Guatemalan arc data involves an initial hemipelagic sediment melt input to the wedge followed by minor fluid additions from the oceanic crust or sediments. Correlation between measures of slab input and extent of melting in the older VF lavas from Tecuamburro and Moyuta favors flux-dominated melting near the base of the mantle wedge. In sharp contrast, the lack of a relationship between slab additions and melting in younger lavas from Agua and Pacaya volcanoes implies a significant role for decompression melting closer to the top of the wedge. In this melting scenario, the rate of crustal extension determines the extent of melting. (C) 2002 Elsevier Science B.V. All rights reserved.

We report 3He/4He ratios, relative He, Ne, and CO2 abundances as well as δ13C values for volatiles from the volcanic output along the Costa Rica and Nicaragua segments of the Central American arc utilising fumaroles, geothermal wells, water springs and bubbling hot springs. CO2/3He ratios are relatively constant throughout Costa Rica (av. 2.1×1010) and Nicaragua (av. 2.5×1010) and similar to arcs worldwide (∼1.5×1010). δ13C values range from −6.8‰ (MORB-like) to −0.1‰ (similar to marine carbonate (0‰)). 3He/4He ratios are essentially MORB-like (8±1 RA) with some samples showing evidence of crustal He additions – water spring samples are particularly susceptible to modification. The He–CO2 relationships are consistent with an enhanced input of slab-derived C to magma sources in Nicaragua ((L+S)/M=16; where L, M and S represent the fraction of CO2 derived from limestone and/or marine carbonate (L), the mantle (M) and sedimentary organic C (S) sources) relative to Costa Rica ((L+S)/M=10). This is consistent with prior studies showing a higher sedimentary flux to the arc volcanics in Nicaragua (as traced by Ba/La, 10Be and La/Yb). Possible explanations include: (1) offscraping of the uppermost sediments in the Costa Rica forearc, and (2) a cooler thermal regime in the Nicaragua subduction zone, preserving a higher proportion of melt-inducing fluids to subarc depths, leading to a higher degree of sediment transfer to the subarc mantle. The absolute flux of CO2 from the Central American arc as determined by correlation spectrometry methods (5.8×1010 mol/yr) and CO2/3He ratios (7.1×1010 mol/yr) represents approximately 14–18% of the amount of CO2 input at the trench from the various slab contributors (carbonate sediments, organic C, and altered oceanic crust). Although the absolute flux is comparable to other arcs, the efficiency of CO2 recycling through the Central American arc is surprisingly low (14–18% vs. a global average of ∼50%). This may be attributed to either significant C loss in the forearc region, or incomplete decarbonation of carbonate sediments at subarc depths. The implication of the latter case is that a large fraction of C (up to 86%) may be transferred to the deep mantle (depths beyond the source of arc magmas).

Most Central American volcanoes occur in an impressive volcanic front that trends parallel to the strike of the subducting Cocos Plate. The volcanic front is a chain, made of right-stepping, linear segments, 100 to 300 Km in length. Volcanoes cluster into centers, whose spacing is random but averages about 27 Km. These closely spaced, easily accessible volcanic centers allow mapping of geochemical variations along the volcanic front. Abundant back-arc volcanoes in southeast Guatemala and central Honduras allow two cross-arc transects. Several element and isotope ratios (e.g. Ba/La, U/Th, B/La, 10 Be/ 9 Be, 87 Sr/ 86 Sr) that are thought to signal subducted marine sediments or altered MORB consistently define a chevron pattern along the arc, with its maximum in Nicaragua. Ba/La, a particularly sensitive signal, is 130 at the maximum in Nicaragua but decreases out on the limbs to 40 in Guatemala and 20 in Costa Rica, which is just above the nominal mantle value of 15. This high amplitude regional variation, roughly symmetrical about Nicaragua, contrasts with the near constancy, or small gradient, in several plate tectonic parameters such as convergence rate, age of the subducting Cocos Plate, and thickness and type of subducted sediment. The large geochemical changes over relatively short distances make Central America an important margin for seeking the tectonic causes of geochemical variations; the regional variation has both a high amplitude and structure, including flat areas and gradients. The geochemical database continues to improve and is already adequate to compare to tectonic models with length scales of 100 Km or longer.

We report N and He isotopic and relative abundance characteristics of volatiles emitted from two segments of the Central American volcanic arc. In Guatemala, delta(15)N values are positive (i.e., greater than air) and N-2/He ratios are high (up to 25,000). In contrast, Costa Rican N-2/He ratios are low ( maximum 1483) and delta(15)N values are negative ( minimum -3.0 per mil). The results identify shallow hemipelagic sediments, subducted into the Guatemalan mantle, as the transport medium for the heavy N. Mass balance arguments indicate that the subducted N is efficiently cycled to the atmosphere by arc volcanism. Therefore, the subduction zone acts as a barrier to input of sedimentary N to the deeper mantle.

Mafic lavas from the central Nicaraguan portion of the Central American volcanic front exhibit considerable variability in the magnitude of high-field-strength element (HFSE) depletions. This variability cannot be attributed to variable magmatic differentiation, or more significantly, to variable depletion originating in the mantle wedge. Instead the HFSE depletions are thought to be the product of variable contributions from the subducting Cocos plate. Both subducted hemipelagic sediment and subducted oceanic crust are identifiable contributors to the overall signal. The latter could exert important control over HFSE depletions, but only if the dehydrating crust is rutile-saturated. Slab control over HFSE depletions is possible throughout the Central American subduction zone, even where slab contributions to magma generation are thought to be minimal, as in central Costa Rica. In some other subduction zones, slab control over HFSE depletions is apparent, but in others, such as the Marianas and the Aleutians, HFSE depletions are probably wedge-based.

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.

Lavas erupted behind the volcanic front in southeastern Guatemala have many important distinctions from lavas erupted on the volcanic front. These include: generally higher MgO, Nb, Sr, TiO2, and rare earth element concentrations; higher La/Yb and Nb/Y ratios; and lower Ba/La, La/Nb, Ba/Zr and Zr/Nb ratios. These major and trace element distinctions are caused by reduced fractionation during ascent and storage in the crust, lower degrees of melting in the source, and greatly reduced contributions from the subducted Cocos plate in the source. In addition, because all of these important distinctions are even borne in lavas erupted within 20 km of the front, there is little apparent petrogenetic continuity between front and behind-the-front magmas. What little geochemical continuity exists is in radiogenic isotopes: 143Nd/144Nd falls across the arc, Pb isotopic ratios (except 206Pb/204Pb) rise across the arc, and 87Sr/86Sr rise across the arc after an initial discontinuity within 20 km of the front. These continuous across-arc changes in radiogenic isotopes are caused by increased contamination with older, more isotopically disparate rocks, away from the front. Once the effects of crustal contamination are removed, the remaining isotopic variability behind the front is non-systematic and reflects the inherent isotopic heterogeneity of the source, the mantle wedge. Geochemical disconnection in southeastern Guatemala suggests that behind-the-front magmas are produced by decompression melting near the top of the wedge, not by flux-dominated melting near the base of the wedge.

Lavas and pyroclastic material from the Masaya Caldera Complex have a number of distinctive geochemical features: relative compositional homogeneity, low Al2O3 and high FeO contents, a tholeitic differentiation trend, and elevated, large-ion-lithophile (LIL)-element concentrations (e.g., Ba approximately 800 ppm). On CMAS projections their compositions always fall on or near low-pressure cotectics. In addition, the basalts of Masaya have unusually high Sr-87/Sr-86 and Be-10. Masaya has exhibited medium-term compositional cycles, best exhibited by the sawtoothed changes in TiO2 and FeO*/MgO. There are also a number of longer-term compositional changes which are abrupt and generally coincide with caldera formation.Many of the geochemical characteristics of Masaya, coupled with a number of volcanological observations, indicate Masaya is underlain by a large, shallow, open-system magma chamber, perhaps on the order of 10 km3 in size. Although fractional crystallization is a significant magmatic process in Masaya's open-system chamber, magma mixing/contamination is equally important. Magma mixing is necessary to explain the discontinuous stratigraphic changes in magma composition observed at Masaya, and crustal contamination is necessary to explain their generally elevated Sr-87/Sr-86 and LIL-element concentrations. Two components, therefore, have been admixed into the magma chamber of Masaya: a LIL-poor basaltic component such has been erupted from the nearby Nejapa and Granada cinder cones; and a LIL-rich acidic component such has been erupted from the nearby calderas of Apoyo and Apoyeque. Admixtures of the former have dominated. Admixtures of the latter ended with caldera formation. Ironically, open-system behavior has exerted fundamental control on the maintenance of relative compositional homogeneity.

Journal Article The Petrogenetic Significance of Interstratified High- and Low-Ti Basalts in Central Nicaragua Get access JAMES A. WALKER, JAMES A. WALKER 1Department of Geology, Northern Illinois UniversityDeKalb, Illinois 60115 Search for other works by this author on: Oxford Academic Google Scholar MICHAEL J. CARR, MICHAEL J. CARR 2Department of Geological Sciences, Rutgers UniversityNew Brunswick, New Jersey 08903 Search for other works by this author on: Oxford Academic Google Scholar MARK D. FEIGENSON, MARK D. FEIGENSON 2Department of Geological Sciences, Rutgers UniversityNew Brunswick, New Jersey 08903 Search for other works by this author on: Oxford Academic Google Scholar RUTH I. KALAMARIDES RUTH I. KALAMARIDES 1Department of Geology, Northern Illinois UniversityDeKalb, Illinois 60115 Search for other works by this author on: Oxford Academic Google Scholar Journal of Petrology, Volume 31, Issue 5, October 1990, Pages 1141–1164, https://doi.org/10.1093/petrology/31.5.1141 Published: 01 October 1990 Article history Received: 28 September 1989 Accepted: 15 February 1990 Published: 01 October 1990

Tephras provide stratigraphic control that allows documentation of changes in magma composition during eruptions. Stratigraphic sections of five recent mafic tephra deposits show a variety of different changes in composition, but three patterns stand out: elements abundant in plagioclase, Al, Sr and to a lesser extent Ca, are sometimes concentrated in the earliest eruptive products; elements common in ferromagnesian minerals, Mg, Fe, Cr and Ni, are usually concentrated in the latest eruptive products; the incompatible elements and Si are highest during the early phases of the eruption, although the very first erupted material will not have the highest incompatible element and Si contents if a high proportion of plagioclase crystals are present.

Cerro Negro volcano in Nicaragua first erupted in 1850. It has erupted at least ten more times, the last in February 1971. It is therefore a polygenetic, parasitic cinder cone that may eventually evolve into a composite cone. Lavas produced during the five important eruptions since 1923 have been sampled. These recent lavas from Cerro Negro are unusually phenocryst-rich basalts (29%–43% phenocrysts by volume) with phenocrysts of plagioclase (An 96−85 ), olivine (Fo 81−72 ), clinopyroxene (En 45 Wo 38 Fs 16 - En 41 Wo 45 Fs 14 ), and subordinate titanian magnetites (Usp 16–20 ). All of the basalts have relatively high Al 2 O 3 and low K 2 O and other incompatible element contents. There have been gradual but significant changes in lava composition with time. All multiply sampled flows show significant intra-lava inhomogeneity. The Fe-Mg contents of analyzed olivines indicate disequilibrium between the olivines and host lavas. All of the inter-lava compositional variation, most of the mineral-lava disequilibrium, and most of the intra-lava inhomogeneity are the result of differential olivine and clinopyroxene accumulation during magma ascent. The magma originated from an essentially homogeneous magma chamber. Phenocrysts in the basalts could have experienced much of their growth during ascent. Ascent velocities were probably on the order of 10 1 –10 −5 cm/sec. Increasing efficiency of crystal settling with time suggests that a narrow feeder, probably a north-south oriented dike, may be widening beneath the cone.

A large peak in the crestal mountains of the Mid-Atlantic Ridge, about 16 km west of the AMAR rift valley at 36°25′N, was sampled for basalt with a submersible electric rock core drill on a comparable surficial scale as the FAMOUS area. Twenty-eight basalt samples from seven drilling stations have been analyzed for major and trace elements. Many of the samples come from flows lying under a cover of carbonate rocks and therefore could not have been sampled by a submersible or a dredge.Through comparisons with published compositional data, it appears that, unlike "FAMOUS-generated" basalts, "AMAR-generated" basalts are, on average, more evolved and are always LREE enriched. Most of the in- and between-hole compositional variation can be accounted for by low-temperature alteration, accumulation of phenocrysts, and low-pressure, relatively low-temperature fractional crystallization. A source heterogeneous in trace elements or undergoing variable degrees of partial melting is necessary to explain the remaining compositional variation. If the large peak can be interpreted as a single volcano, it may be that lavas become progressively more differentiated with time at mid-ocean ridge volcanoes as they commonly do at subduction zone volcanoes.

Most of the lavas at the nine volcanic centers along the volcanic front of El Salvador are basalts, basaltic andesites and andesites. The compositional variation within and among these centers can be explained by fractionation processes within the crust. Cognate gabbroic inclusions found in the lavas have appropriate mineralogy (plagioclase, olivine, magnetite and augite) to be cumulates formed by fractional crystallization. Two main variation trends occur, depending on the proportion of plagioclase removal. The more common, or normal, trend has a high (> 55%) proportion of plagioclase being removed. A less common, Al-rich, trend has a low (40%) proportion of plagioclase being removed. The Al-rich trend is found only at volcanoes that lack large negative Bouguer gravity anomalies. These volcanoes are unlikely to have large shallow magma chambers and fractionation probably occurs deeper in the crust where plagioclase removal is inhibited.