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Vargas, JA, Hilton DR, Ramirez C, Molina J.  2018.  Metals in bivalve mollusks from the Jaco Scar seep, Pacific, Costa Rica. Revista De Biologia Tropical. 66:S269-S279. AbstractWebsite

Deep sea-research has made significant discoveries thanks to the availability of high resolution bathymetric mapping and vehicles able to reach hydrothermal vents and cold seeps. The Pacific continental margin of Costa Rica includes cold seeps that are inhabited by vesicomyid clams, which are expected to accumulate metals. Data on metals from cold seep clams are scarce. Thus, the objective of this study is to present the concentrations of Al, Cd, Cu, Fe, Pb, Mn, Ni, Sb and Zn in samples from seven clams, a mussel, sediment, and a rock, together with clam morphometric data. The bivalves (Archivesica sp.?) were collected in 2005 at a depth of 1888 m on the Jaco Scar (09 degrees 06' N - 84 degrees 50' W) during DSRV Alvin dive 4129. Metals were analyzed by Flame Atomic Absorption Spectrometry (FAAS) and Graphite Furnace (GFAAS). Concentrations are in mu g/g dry weight. The order of decreasing maximm concentrations and range in tissues of seven clams, were: Zn (43.4 - 266.3) > Fe (27.2 - 100.0) > Al (5.0 - 69.9) > Cd (0.1 - 12.2) > Sn (2.8 - 9.5) > Cu (4.0 - 7.3) > Mn (1.1 - 2.2) > Pb (0.2 - 0.8) > Ni (0.19 - 0.58 ). The gills had the maximum concentrations of Fe and Al. Maximum concentrations in the only mussel specimen collected, were: Zn (80.4 - gills), Fe (70.6 - gills). Cu (31.0 - gills). Al (26.6 - gills), Sn (4.8 - mantle), Mn (1.7 - mantle). Ni (0.97 - muscle), Pb (0.7 - muscle), Cd (0.57 - gills). The sediment sample had: Al (40 800), Fe (26 500), Mn (72.0), Zn (64.7), Cu (29.4), Ni (19.3), Sn (15.5). Pb (2.98), Cd (0.16). A rock fragment had: Fe (15 650), Al (9 240), Mn (170), Sn (99.5), Zn (36.5), Ni (20.4), Cu (13.4), Pb (1.6), Cd (traces). Clam gills concentrated metals such as Fe and Al. Fe was below the range reported for hydrothermal vent clams. while concentrations of other metals were near the lower range. Fe, Cd, Mn, and Pb in mussel tissues were lower than those in mussels from hydrothermal vent sites, while Cu and Zn were within the range. Metals in the sediment and rock samples appeared very rich in certain metals like Al and Fe and very poor in others, such as Cd. There is a paucity of information on metals and pollutants in clams and other macrofaunal species from Costa Rican cold seeps. Data presented herein must be complemented with future studies conducted jointly on the geochemistry, biology. and management of these deep-sea systems.

Macpherson, CG, Hilton DR, Hammerschmidt K.  2010.  No slab-derived CO2 in Mariana Trough back-arc basalts: Implications for carbon subduction and for temporary storage of CO2 beneath slow spreading ridges. Geochemistry Geophysics Geosystems. 11   10.1029/2010gc003293   AbstractWebsite

The Southern Mariana Trough is particularly well suited to study mass balance in subduction zones because the flux of material recycled from the subducted slab has been shown to diminish to negligible levels in the southernmost part of the area. We present new He and Ar concentration and isotopic data for 16 back-arc basaltic glasses and combine these with previously published CO2 and H2O concentration and delta C-13 data to explore the recycling of carbon and light noble gases in the Mariana back arc. Degassing has affected all samples and is particularly extensive in more water-rich samples, i.e., those containing the largest recycled component. The degassing history features three stages: (1) deep degassing which commenced when the melt reached saturation of CO2 and noble gases in the mantle, (2) preeruptive degassing during storage in the crust-mantle transition zone which involved addition of extraneous CO2 to the vapor phase, and (3) eruption. CO2 released during stage 1 was, at least partially, incorporated into wall rock and subsequently remobilized during stage 2 degassing of later magma batches. Reconstructed parental values for He-3/He-4, delta C-13, CO2/He-3, and CO2/Ar-40* are indistinguishable from those of mid-ocean ridge basalt. This implies that there is negligible recycling of subducted carbon, helium, or argon into the source of Mariana Trough basalt.

Tian, L, Castillo PR, Hawkins JW, Hilton DR, Hannan BB, Pietruszka AJ.  2008.  Major and trace element and Sr-Nd isotope signatures of lavas from the Central Lau Basin: Implications for the nature and influence of subduction components in the back-arc mantle. Journal of Volcanology and Geothermal Research. 178:657-670.   10.1016/j.jvolgeores.2008.06.039   AbstractWebsite

New major and trace element and Sr-Nd isotope data are presented for basaltic glasses from active spreading centers (Central Lau Spreading Center (CLSC), Relay Zone (RZ) and Eastern Lau Spreading Center (ELSC)) in the Central Lau Basin, SW Pacific. Basaltic lavas from the Central Lau Basin are mainly tholeiitic and are broadly similar in composition to mid-ocean ridge basalts (MORB). Their generally high (87)Sr/(86)Sr ratios, combined with relatively low (143)Nd/(144)Nd ratios are more akin to MORB from the Indian rather than Pacific Ocean. In detail, the CLSC, RZ and ELSC lavas are generally more enriched in large ion lithophile elements (Rb, Ba, Sr, and K) than average normal-MORB, which suggests that the mantle beneath the Central Lau Basin was modified by subducted slab-derived components. Fluid mobile/immobile trace element and Sr - Nd isotope ratios suggest that the subduction components were essentially transferred into the mantle via hydrous fluids derived from the subducted oceanic crust; contributions coming from the subducted sediments are minor. Compared to CLSC lavas, ELSC and RZ lavas show greater enrichment in fluid mobile elements and depletion in high field strength elements, especially Nb. Thus, with increasing distance away from the arc, the influence of subduction components in the mantle source of Lau Basin lavas diminishes. The amount of hydrous fluids also influences the degree of partial melting of the mantle beneath the Central Lau Basin, and hence the degree of melting also decreases with increasing distance from the arc. (C) 2008 Elsevier B.V. All rights reserved.

Elkins, LJ, Fischer TP, Hilton DR, Sharp ZD, McKnight S, Walker J.  2006.  Tracing nitrogen in volcanic and geothermal volatiles from the Nicaraguan volcanic front. Geochimica Et Cosmochimica Acta. 70:5215-5235.   10.1016/j.gca.2006.07.024   AbstractWebsite

We report new chemical and isotopic data from 26 volcanic and geothermal gases, vapor condensates, and thermal water samples, collected along the Nicaraguan volcanic front. The samples were analyzed for chemical abundances and stable isotope compositions, with a focus on nitrogen abundances and isotope ratios. These data are used to evaluate samples for volatile contributions from magma, air, air-saturated water, and the crust. Samples devoid of crustal contamination (based upon He isotope composition) but slightly contaminated by air or air-saturated water are corrected using N(2)/Ar ratios in order to obtain primary magmatic values, composed of contributions from upper mantle and subducted hernipelagic sediment on the down-going plate. Using a mantle endmember with delta(15)N = -5 parts per thousand and N(2)/He = 100 and a subducted sediment component with delta(15)N = +7 parts per thousand and N(2)/He = 10,500, the average sediment contribution to Nicaraguan volcanic and geothermal gases was determined to be 71%. Most of the gases were dominated by sediment-derived nitrogen, but gas from Volcan Mombacho, the southernmost sampling location, had a mantle signature (46% from subducted sediment, or 54% from the mantle) and an affinity with mantle-dominated gases discharging from Costa Rica localities to the south. High CO(2)/N(2exc.) ratios (N(2 exc.) is the N(2) abundance corrected for contributions from air) in the south are similar to those in Costa Rica, and reflect the predominant mantle wedge input, whereas low ratios in the north indicate contribution by altered oceanic crust and/or preferential release of nitrogen over carbon from the subducting slab. Sediment-derived nitrogen fluxes at the Nicaraguan volcanic front, estimated by three methods, are 7.8 x 10(8) mol N/a from (3)He flux, 6.9 x 10(8) mol/a from SO(2) flux, and 2.1 x 10(8) and 1.3 x 10(9) mol/a from CO(2) fluxes calculated from (3)He and SO(2), respectively. These flux results are higher than previous estimates for Central America, reflecting the high sediment-derived volatile contribution and the high nitrogen content of geothermal and volcanic gases in Nicaragua. The fluxes are also similar to but higher than estimated hernipelagic nitrogen inputs at the trench, suggesting addition of N from altered oceanic basement is needed to satisfy these flux estimates. The similarity of the calculated input of N via the trench to our calculated outputs suggests that little or none of the subducted nitrogen is being recycled into the deeper mantle, and that it is, instead, returned to the surface via arc volcanism. (c) 2006 Elsevier Inc. All rights reserved.

Shaw, AM, Hilton DR, Fischer TP, Walker JA, de Leeuw GAM.  2006.  Helium isotope variations in mineral separates from Costa Rica and Nicaragua: Assessing crustal contributions, timescale variations and diffusion-related mechanisms. Chemical Geology. 230:124-139.   10.1016/j.chemgeo.2005.12.003   AbstractWebsite

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 4.6R(A) to 7.5R(A), with no evidence for crustal thickness influencing measured (3)He/(4)He 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 (3)He/(4)He values. (3)He/(4)He 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 (3)He/(4)He ratios (4.6-7.0R(A)) than the olivines ((3)He/(4)He=6.1-7.5R(A)) 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 (4)He additions, preferentially influencing pyroxene grains. In the first case, we reject diffusion-related firactionation of He isotopes since lower (3)He/(4)He 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 (3)He/(4)He 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 (3)He/(4)He source during ascent, presumably the surrounding crust. (c) 2005 Elsevier B.V. All rights reserved.

Jaffe, LA, Hilton DR, Fischer TP, Hartono U.  2004.  Tracing magma sources in an arc-arc collision zone: Helium and carbon isotope and relative abundance systematics of the Sangihe Arc, Indonesia. Geochemistry Geophysics Geosystems. 5   10.1029/2003gc000660   AbstractWebsite

[1] The Sangihe Arc is presently colliding with the Halmahera Arc in northeastern Indonesia, forming the world's only extant example of an arc-arc collision zone. We report the first helium and carbon isotopic and relative abundance data from the Sangihe Arc volcanoes as a means to trace magma origins in this complicated tectonic region. Results of this study define a north-south trend in He-3/He-4, CO2/He-3, and delta(13)C, suggesting that there are variations in primary magma source characteristics along the strike of the arc. The northernmost volcanoes (Awu and Karangetang) have higher CO2/He-3 and delta(13)C (up to 179 x 10(9) and -0.4parts per thousand, respectively) and lower He-3/He-4 (similar to5.4 R-A) than the southernmost volcanoes ( Ruang, Lokon, and Mahawu). Resolving the arc CO2 into component structures (mantle-derived, plus slab-derived organic and carbonate CO2), the northern volcanoes contain an unusually high (> 90%) contribution of CO2 derived from isotopically heavy carbonate associated with the subducting slab ( sediment and altered oceanic basement). Furthermore, the overall slab contribution (CO2 of carbonate and organic origin) relative to carbon of mantle wedge origin is significantly enhanced in the northern segment of the arc. These observations may be caused by greater volumes of sediment subduction in the northern arc, along-strike variability in subducted sediment composition, or enhanced slab-derived fluid/melt production resulting from the superheating of the slab as collision progresses southward.

Macpherson, CG, Hilton DR, Mattey DP, Sinton JM.  2000.  Evidence for an O-18-depleted mantle plume from contrasting O-18/O-16 ratios of back-arc lavas from the Manus Basin and Mariana Trough. Earth and Planetary Science Letters. 176:171-183.   10.1016/s0012-821x(00)00002-9   AbstractWebsite

Back-are basin glasses from the Mariana Trough and Manus Basin display contrasting oxygen isotope characteristics that require differences in their mantle sources. In both basins, the lavas that are most depleted in high field strength elements possess delta(18)O values of around 6.0 parts per thousand, that are elevated with respect to mid-ocean ridge basalt (MORB). This characteristic is consistent with a mantle source that has been infiltrated by fluids released from subducted oceanic lithosphere. The nature of the more fertile mantle component differs between the two basins, The lowest delta(18)O values in the Mariana Trough are similar to MORB and suggest that the ambient upper mantle interacts with a subduction-modified mantle to produce Mariana Trough back-arc basin basalts. Oxygen isotope ratios of basaltic glasses from the Manus Basin display a negative correlation with helium isotope ratios. The subduction-modified component is associated with He-3/He-4 ratios typical of the upper mantle, Glasses with He-3/He-4 ratios greater than average MORB, characteristic of a deep mantle plume, have delta(18)O values that are lower than expected for upper mantle melts. This suggests that the Manus Basin plume taps a reservoir that is O-18-depleted relative to the source of MORB. Two mechanisms are identified that might generate this reservoir. Deep recycling of oceanic crust that has been hydrothermally altered at high temperature may provide large O-18-depleted domains in the deep mantle. Alternatively, we propose that interactions between silicate and iron alloy during the segregation of the Earth's core may have the potential to generate such reservoirs. Resolution between these mechanisms:requires further experimental investigation of oxygen partitioning between silicates and iron alloy. Each of these mechanisms has distinct implications for the origins and dynamics of the Manus Basin plume. (C) 2000 Elsevier Science; B.V, All rights reserved.

Gasparon, M, Hilton DR, Varne R.  1994.  Crustal Contamination Processes Traced by Helium-Isotoped - Examples from the Sunda Arc, Indonesia. Earth and Planetary Science Letters. 126:15-22.   10.1016/0012-821x(94)90239-9   AbstractWebsite

Helium isotope data have been obtained on well-characterised olivine and clinopyroxene phenocrysts and xenocrysts from thirteen volcanic centres located between central Sumatra and Sumbawa in the Sunda arc of Indonesia. Olivine crystals in mantle xenoliths (lherzolite) from Bukit Telor basalts are primitive (Mg# = 90), and their He-3/He-4 value (R/R(A) = 8.8) indicates that the Sumatran mantle wedge is MORB-like in helium isotope composition. All other samples have lower He-3/He-4 ratios ranging from 8.5R(A) to 4.5R(A), with most (thirteen out of eighteen) following a trend of more radiogenic He-3/He-4 values with decreasing Mg#. The only exceptions to this trend are phenocrysts from Batur, Agung and Kerinci, which have MORB-like He-3/He-4 values but relatively low Mg# (Mg# = 70-71), and two highly inclusion-rich clinopyroxenes which have He-3/He-4 values lower than other samples of similar Mg#. The results indicate that crustal contamination unrelated to subduction in the Sunda arc is clearly recorded in the He-3/He-4 characteristics of mafic phenocrysts of subaerial volcanics, and that addition of radiogenic helium is related to low-pressure differentiation processes affecting the melts prior to eruption. These conclusions may have widespread applicability and indicate that helium isotope variations can act as an extremely sensitive tracer of upper crustal contamination.

Hilton, DR, Hammerschmidt K, Loock G, Friedrichsen H.  1993.  Helium and Argon Isotoope Systematics of the Central Lau Basin and Valu Fa Ridge - Evidence of Cruse Mantle Interactions in a Back-Arc Basin. Geochimica Et Cosmochimica Acta. 57:2819-2841.   10.1016/0016-7037(93)90392-a   AbstractWebsite

We report helium and argon isotope analyses of fresh Lau Basin volcanic glasses from six sites within the central basin (approximately 18-degrees-S), six localities along the Valu Fa Ridge (21-degrees-22.4-degrees-S), and a single site from the northeastern basin (approximately 15-degrees-S). Central basin basalts have He-3/He-4 ratios (R) between 8.2-8.5 R(A) (R(A) = air He-3/He-4), Ar-40/Ar-36 ratios significantly greater than atmosphere (up to 4900), and He-4 contents from 3.5-9.4 X 10(-6) cm3 STP/g, similar to N-MORBs worldwide. These results are consistent with trace element and other (radiogenic) isotope data on these samples which indicate derivation from a depleted mantle source region. In contrast, evolved lavas from the Valu Fa Ridge all have R/R(A) < MORB and show evidence of mixing with a component rich in radiogenic helium. There is a clear relationship between the He-3/He-4 ratios of these samples and their chemistry: basaltic andesites have 6.02 < R(C)/R(A) < 7.65, andesites have lower ratios (2.37-2.65 R(A)), and a dacite has the lowest value of the entire sample suite (1.19 R(A)). All lavas have Ar-40/Ar-36 ratios similar to the atmospheric value and low helium concentrations, from 3-11 x 10(-8) cm3 STP/g, or between 30 and 300 times less than the central basin basalts. Although the helium isotope results of the Valu Fa lavas mirror the shift to more radiogenic values of other isotope systems (e.g., Pb-206/Pb-204, Sr-87/Sr-86) which indicate addition of subducted sediment to these magma sources, we find no evidence that the radiogenic helium has a mantle or slab derivation or is in any way coupled to these other tracers. Instead, the most plausible mechanism to explain its incorporation into the Valu Fa lavas is by assimilation of old Lau crust in the near-surface environment by previously degassed magma. We argue that this mechanism has general applicability and can explain a number of hitherto apparently paradoxical geochemical features of some back-arc and ocean ridge lavas such as their high volatile and LIL element contents with low rare gas concentrations, and their mantle He-3/He-4 ratios with (hydrated oceanic) crustal D/H values. The realisation that the helium and argon systematics of the Valu Fa lavas are controlled by crust/mantle interactions has important implications for distinguishing between a number of models proposed for the formation of such evolved lavas, and we show that fractional crystallisation processes can most readily account for the low concentrations of, and systematic trends in, the mantle-derived helium and argon component of these lavas. In addition, because pre-existing crust in the Lau Basin must be old and/or altered enough to supply the radiogenic helium and atmospheric-like Ar-40/Ar-36 component to the Valu Fa lavas, the occurrence of crust/mantle interactions implies that old (forearc) crust may have been trapped within the Lau Basin: such a scenario has a clear bearing on ideas of the tectonic development of the basin. Finally, because of the potential of crust/mantle interactions to modify He-3/He-4 and Ar-40/Ar-36 ratios of mantle-derived melts, we assess the implications for using He and Ar tracers to characterise mantle sources in arcs, back-arcs, and spreading ridges, and consider the consequences for the combined use of rare gases with other (radiogenic) isotopic tracers of magma provenance at such settings. The basaltic andesite from the northeastern basin may also be influenced by the same kind of crustal interaction as the Valu Fa lavas as it falls within the He-3/He-4 range (6.9 R(A)) of the other basaltic andesites. Interestingly, other helium isotope studies indicate that this part of the basin is characterised by a wide range in He-3/He-4 ratios, from MORB values up to 22 R(A). The low He-3/He-4 ratio of the basaltic andesite, therefore, serves to illustrate the possible effects of magma chamber processes on the rare gas and other volatile characteristics of hotspot lavas: an observation which is important not only for this part of the Lau Basin but for other localities worldwide.