Alison M. Shaw

Contrasting He–C relationships in Nicaragua and Costa Rica: insights into C cycling through subduction zones ☆

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).

Nucleogenic neon in high 3He/4He lavas from the Manus back-arc basin: a new perspective on He–Ne decoupling

Abstract We report new neon isotope data obtained for well-characterised basaltic glasses from the Manus back-arc basin where helium studies have identified a mantle plume component (mean 3He/4He∼12 RA). In three-isotope neon space, seven of the Manus samples lie along a trajectory between air and an endmember more nucleogenic than MORB i.e., compared to typical MORB, samples have a higher 21Ne/22Ne ratio for a given 20Ne/22Ne ratio. Thus the slope of the Manus Basin line is less than that of the MORB line [Sarda et al., Earth Planet. Sci. Lett. 91 (1988) 73–88]. This is the first observation of lavas with high 3He/4He ratios having nucleogenic neon isotope systematics, indicating a unique decoupling of He from Ne. We evaluate five possible explanations for the observed trend. We discount: (1) crustal contamination, (2) devolatisation of the subducting Solomon Sea plate and (3) addition of neon from an ancient recycled slab component – based upon mass balance considerations of the availability of nucleogenic Ne. Two possibilities remain – both of which must produce an elevated He/Ne ratio in the Manus Basin source region to account for the nucleogenic neon: (4) a previous degassing event which would leave a Ne-depleted residual reservoir, or (5) a deep mantle source heterogeneity preserving a unique signature inherited from Earth’s accretion. We find that isolation times as short as 10 Ma for a previously degassed source are sufficient to grow in the nucleogenic Ne without significantly altering the plume-like 3He/4He ratios. Alternatively, solubility-controlled outgassing/ingassing of a magma ocean in contact with a proto-atmosphere may have produced the requisite high He/Ne ratio, although an open-system style of equilibration is necessary. At present, insufficient evidence is available to discriminate between these alternatives.

Insight into volatile behavior at Nyamuragira volcano (D.R. Congo, Africa) through olivine-hosted melt inclusions

We present new olivine-hosted melt inclusion volatile (H2O, CO2, S, Cl, F) and major element data from five historic eruptions of Nyamuragira volcano (1912, 1938, 1948, 1986, 2006). Host-olivine Mg#s range from 71 to 84, with the exception of the 1912 sample (Mg# = 90). Inclusion compositions extend from alkali basalts to basanite-tephrites. Our results indicate inclusion entrapment over depths ranging from 3 to 5 km, which agree with independent estimates of magma storage depths (3–7 km) based on geophysical methods. Melt compositions derived from the 1986 and 2006 Nyamuragira tephra samples best represent pre-eruptive volatile compositions because these samples contain naturally glassy inclusions that underwent less post-entrapment modification than crystallized inclusions. Volatile concentrations of the 1986 and 2006 samples are as follows: H2O ranged from 0.6 to 1.4 wt %, CO2 from 350 to 1900 ppm, S from 1300 to 2400 ppm, Cl from 720 to 990 ppm, and F from 1500 to 2200 ppm. Based on FeOT and S data, we suggest that Nyamuragira magmas have higher fO2 (>NNO) than MORB. We estimate the total amount of sulfur dioxide (SO2) released from the 1986 (0.04 Mt) and 2006 (0.06 Mt) Nyamuragira eruptions using the petrologic method, whereby S contents in melt inclusions are scaled to erupted lava volumes. These amounts are significantly less than satellite-based SO2 emissions for the same eruptions (1986 = ∼1 Mt; 2006 = ∼2 Mt). Potential explanations for this observation are: (1) accumulation of a vapor phase within the magmatic system that is only released during eruptions, and/or (2) syn-eruptive gas release from unerupted magma.

Flux measurements of explosive degassing using a yearlong hydroacoustic record at an erupting submarine volcano

at NW Rota-1 are primarily H2O, SO2, and CO2. Instantaneous fluxes varied by a factor of � 100 over the deployment. Using melt inclusion information to estimate the concentration of CO2 in the explosive gases as 6.9 � 0.7 wt %, we calculate an annual CO2 eruption flux of 0.4 � 0.1 Tg a � 1 . This result is within the range of measured CO2 fluxes at continuously erupting subaerial volcanoes, and represents � 0.2–0.6% of the annual estimated output of CO2 from all subaerial arc volcanoes, and � 0.4–0.6% of the mid-ocean ridge flux. The multiyear eruptive history of NW Rota-1 demonstrates that submarine volcanoes can be significant and sustained sources of CO2 to the shallow ocean.