39 research outputs found
Tracing Nitrogen in Volcanic and Geothermal Volatiles from the Nicaraguan Volcanic Front
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 N2/Ar ratios in order to obtain primary magmatic values, composed of contributions
from upper mantle and subducted hemipelagic sediment on the down-going plate. Using a mantle endmember with
d15N= 5&and N2/He = 100 and a subducted sediment component with d15N=+7& and N2/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 Volca´n 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
CO2/N2 exc. ratios (N2 exc. is the N2 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 · 108 mol N/a from 3He flux, 6.9 · 108 mol/a from SO2 flux, and 2.1 · 108 and 1.3 · 109 mol/a from
CO2 fluxes calculated from 3He and SO2, 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 hemipelagic 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
North Atlantic hotspot-ridge interaction near Jan Mayen Island
At slow to ultraslow spreading rates along mid-ocean ridges, thicker lithosphere typically
impedes magma generation and tectonic extension can play a more significant role in crustal
production (Dick et al., 2003). The source of anomalously high magma supply thus remains
unclear along ridges with ultraslow-spreading rates adjacent to Jan Mayen Island in the North
Atlantic (Neumann and Schilling, 1984; Mertz et al., 1991; Haase et al., 1996; Schilling et
al., 1999; Trønnes et al., 1999; Haase et al., 2003; Mertz et al., 2004; Blichert-Toft et al., 2005;
Debaille et al., 2009). Here we show that Jan Mayen volcanism is likely the surface expression
of a small mantle plume, which exerts significant influence on nearby mid-ocean ridge tectonics
and volcanism. Progressive dilution of Jan Mayen geochemical signatures with distance
from the hotspot is observed in lava samples from the immediately adjacent Mohns Ridge,
and morphological indicators of enhanced magma supply are observed on both the Mohns
Ridge and the nearby Kolbeinsey Ridge, which additionally locally overlies a highly heterogeneous,
eclogite-bearing mantle source. These morphological and geochemical influences
underscore the importance of heterogeneous mantle sources in modifying melt supply and
thus the local expression of tectonic boundaries
Estresse ocupacional na administração pública Brasileira: quais os fatores impactantes?
Melt generation beneath Arctic Ridges: Implications from U decay series disequilibria in the Mohns, Knipovich, and Gakkel Ridges
International audienc
Melt generation beneath Arctic Ridges: Implications from U decay series disequilibria in the Mohns, Knipovich, and Gakkel Ridges
(U-234/U-238) and (Th-230/U-238) disequilibria in fresh and altered Kolbeinsey Ridge basalts
Understanding melt generation beneath the slow-spreading Kolbeinsey Ridge using <sup>238</sup>U, <sup>230</sup>Th, and <sup>231</sup>Pa excesses
To examine the petrogenesis and sources of basalts from the Kolbeinsey Ridge, one of the shallowest locations along the global ridge system, we present new measurements of Nd, Sr, Hf, and Pb isotopes and U-series disequilibria on 32 axial basalts. Young Kolbeinsey basalts (full-spreading rate = 1.8 cm/yr; 67°05'–70°26'N) display (230Th/238U) 230Th/238U) > 1 with (230Th/238U) from 0.95 to 1.30 and have low U (11.3–65.6 ppb) and Th (33.0 ppb–2.40 ppm) concentrations. Except for characteristic isotopic enrichment near the Jan Mayen region, the otherwise depleted Kolbeinsey basalts (e.g. 87Sr/86Sr = 0.70272–0.70301, eNd = 8.4–10.5, eHf = 15.4–19.6 (La/Yb)N = 0.28–0.84) encompass a narrow range of (230Th/232Th) (1.20–1.32) over a large range in (238U/232Th) (0.94–1.32), producing a horizontal array on a (230Th/232Th) vs. (238U/232Th) diagram and a large variation in (230Th/238U). However, the (230Th/238U) of the Kolbeinsey Ridge basalts (0.96–1.30) are inversely correlated with (234U/238U) (1.001–1.031). Samples with low (230Th/238U) and elevated (234U/238U) reflect alteration by seawater or seawater-derived materials. The unaltered Kolbeinsey lavas with equilibrium 234U/238U have high (230Th/238U) values (?1.2), which are consistent with melting in the presence of garnet. This is in keeping with the thick crust and anomalously shallow axial depth for the Kolbeinsey Ridge, which is thought to be the product of large degrees of melting in a long melt column. A time-dependent, dynamic melting scenario involving a long, slowly upwelling melting column that initiates well within the garnet peridotite stability zone can, in general, reproduce the (230Th/238U) and (231Pa/235U) ratios in uncontaminated Kolbeinsey lavas, but low (231Pa/235U) ratios in Eggvin Bank samples suggest eclogite involvement in the source for that ridge segment
Understanding melt generation beneath the slow-spreading Kolbeinsey Ridge using U-238, Th-230, and Pa-231 excesses
To examine the petrogenesis and sources of basalts from the Kolbeinsey Ridge, one of the shallowest locations along the global ridge system, we present new measurements of Nd, Sr, Hf, and Pb isotopes and U-series disequilibria on 32 axial basalts. Young Kolbeinsey basalts (full-spreading rate=1.8cm/yr; 67°05'-70°26'N) display (230Th/238U)1 with (230Th/238U) from 0.95 to 1.30 and have low U (11.3-65.6ppb) and Th (33.0ppb-2.40ppm) concentrations. Except for characteristic isotopic enrichment near the Jan Mayen region, the otherwise depleted Kolbeinsey basalts (e.g. 87Sr/86Sr=0.70272-0.70301, εNd=8.4-10.5, εHf=15.4-19.6 (La/Yb)N=0.28-0.84) encompass a narrow range of (230Th/232Th) (1.20-1.32) over a large range in (238U/232Th) (0.94-1.32), producing a horizontal array on a (230Th/232Th) vs. (238U/232Th) diagram and a large variation in (230Th/238U). However, the (230Th/238U) of the Kolbeinsey Ridge basalts (0.96-1.30) are inversely correlated with (234U/238U) (1.001-1.031). Samples with low (230Th/238U) and elevated (234U/238U) reflect alteration by seawater or seawater-derived materials. The unaltered Kolbeinsey lavas with equilibrium 234U/238U have high (230Th/238U) values (≥1.2), which are consistent with melting in the presence of garnet. This is in keeping with the thick crust and anomalously shallow axial depth for the Kolbeinsey Ridge, which is thought to be the product of large degrees of melting in a long melt column. A time-dependent, dynamic melting scenario involving a long, slowly upwelling melting column that initiates well within the garnet peridotite stability zone can, in general, reproduce the (230Th/238U) and (231Pa/235U) ratios in uncontaminated Kolbeinsey lavas, but low (231Pa/235U) ratios in Eggvin Bank samples suggest eclogite involvement in the source for that ridge segment. © 2011 Elsevier Ltd.F.I. 4, 101SCOPUS: ar.jinfo:eu-repo/semantics/publishe