David R. Hilton

Extreme 3He/4He ratios in northwest Iceland: constraining the common component in mantle plumes

Olivine and clinopyroxene phenocrysts contained in late Tertiary basalts from Selardalur, northwest Iceland, carry volatiles with the highest helium isotope ratio yet reported for any mantle plume. 3He/4He ratios measured on three different samples and extracted by stepped crushing in vacuo fall consistently ˜37 RA (RA = air 3He/4He) — significantly higher than previously reported values for Iceland or Loihi Seamount (see K.A. Farley, E. Neroda [Annu. Rev. Earth Planet. Sci. 26 (1998) 189–218]). The Sr, Nd and Pb isotopic composition of the same sample places it towards the center of the mantle tetrahedron of Hart et al. (S.R. Hart, E.H. Hauri, L.A. Oschmann, J.A. Whitehead [Science 256 (1992) 517–520]) — in exactly the region predicted for the common mantle endmember based on the convergence of a number of pseudo-linear arrays of ocean island basalts worldwide (E.H. Hauri, J.A. Whitehead, S.R. Hart [J. Geophys. Res. 99 (1994) 24275–24300]). This observation implies that Selardalur may represent the best estimate available to date of the He–Sr–Nd–Pb isotopic composition of the 5th mantle component common to many mantle plumes.

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).

HELIUM AND ARGON ISOTOPE SYSTEMATICS OF THE CENTRAL LAU BASIN AND VALU FA RIDGE : EVIDENCE OF CRUST/MANTLE INTERACTIONS IN A BACK-ARC BASIN

Abstract We report helium and argon isotope analyses of fresh Lau Basin volcanic glasses from six sites within the central basin (~18°S), six localities along the Valu Fa Ridge (21°–22.4°S), and a single site from the northeastern basin (~ 15°S). Central basin basalts have 3 He 4 He ratios (R) between 8.2–8.5 R A (R A = air 3 He 4 He ), 40 Ar 36 Ar ratios significantly greater than atmosphere (up to 4900), and 4He contents from 3.5–9.4 × 10−6cm3 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 and show evidence of mixing with a component rich in radiogenic helium. There is a clear relationship between the 3 He 4 He ratios of these samples and their chemistry: basaltic andesites have 6.02 R c R a , andesites have lower ratios (2.37−2.65 RA), and a dacite has the lowest value of the entire sample suite (1.19 RA). All lavas have 40 Ar 36 Ar ratios similar to the atmospheric value and low helium concentrations, from 3–11 × 10−8cm3 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., 206 Pb 204 Pb , 87 Sr 86 Sr ) 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 3 He 4 He 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 40 Ar 36 Ar 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 3 He 4 He and 40 Ar 36 Ar 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 3 He 4 He range (6.9RA) of the other basaltic andesites. Interestingly, other helium isotope studies indicate that this part of the basin is characterised by a wide range in 3 He 4 He ratios, from MORB values up to 22RA. The low 3 He 4 He 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.

Helium isotope characteristics of Andean geothermal fluids and lavas

The first comprehensive helium isotope survey of the Andes is reported here. We have sampled geothermal fluids and phyric lava flows from the Southern (svz) and Central (cvz) Volcanic Zones, the volcanically active Pun˜a region and the Precordillera, Salta Basin, Longitudinal Valley and the aseismic region between the two volcanic zones. Although the active areas are characterised by significant differences in crustal age and thickness, the svz, cvz and Pun˜a are characterised by a wide and overlapping range in 3He/4He ratios (for fluids and phenocrysts) from predominantly radiogenic values to close to the MORB ratio. The measured ranges in 3He/4He ratios (R) (reported normalised to the air 3He/4He—RA) are: svz (0.18 < R/RA< 6.9); cvz (0.82 < R/RA< 6.0); and Pun˜a (1.8 < R/RA< 5.4). Modification of magmatic 3He/4He ratios by water/rock interactions (fluids) or post-eruptive grow-in of radiogenic 4He or preferential diffusive loss of 3He (phenocrysts) is considered unlikely; this means that the wide range reflects the helium isotope characteristics of magma bodies in the Andean crust. The mechanism controlling the 3He/4He ratios appears to be a mixing between mantle (MORB-like) helium and a radiogenic helium component derived from radioactive decay within the magma (magma aging) and/or interaction with 4He-rich country rock: a process expected to be influenced by pre-eruptive degassing of the mantle component. Assimilation of lower crust is also capable of modifying 3He/4He ratios, albeit to a much lesser extent. However, it is possible that the highest measured values in each zone were established by the addition of lower crustal radiogenic helium to MORB helium. In this case, the higher ‘base level’ ratios of the svz would reflect the younger crustal structure of this region. In contrast to helium, there is no overlap in the Sr or Pb isotope characteristics of lavas from the active zones: in all areas, therefore, 3He/4He ratios appear to vary independently of Sr and Pb isotope variations. This decoupling between the lithophile tracers and helium reflects the different processes controlling their isotopic characteristics: crust-mantle interactions, alone, for Sr and Pb but for helium the effects of pre-eruptive degassing and possibly magma aging are possibly superimposed. The presence of mantle helium in the Pun˜a region, and to a lesser extent in the Salta Basin, gives an across-arc perspective to the helium isotope distribution and shows mantle melting to occur significantly to the east of the active arc: this is most probably a consequence of lithospheric delamination. The Precordillera hot spring water has the only pure radiogenic helium signal of the entire sample suite and thus marks the western limit of asthenospheric mantle under the Andes.

The helium and carbon isotope systematics of a continental geothermal system: results from monitoring studies at Long Valley caldera (California, U.S.A.)

Abstract This study reports fumarole and hot spring gas chemistry of a 3-year monitoring programme (1986–1988) at the seismically-active Long Valley caldera (LVC) in the Sierra Nevada, eastern California. The focus is on helium and carbon dioxide (isotopes and concentrations) and their variation in response to seismic activity in the region. Within the caldera, both species are predominantly magmatic in origin but their isotopic and elemental characteristics appear to be established prior to shallow-level intrusion and/or are influenced by pre-eruptive degassing. In response to intra-caldera and regional seismicity over the monitoring period, CO2 and helium show markedly different behaviour: the greatest change in various carbon-related parameters (CO2%; δ13C(CO2; CO 2 3 He ) occurred in 1986 and were most likely related to regional seismicity in the nearby Chalfont Valley. Helium did not respond to these events. The largest change (up to ∼ 25%) in 3 He 4 He ratios was seen in 1987 with the occurrence of both increases and decreases relative to the almost constant values observed in 1988. The increases are consistent with magma intrusion occurring within the caldera in 1987 whilst the decreases occurred significantly later (> 6 months) than any seismic activity. It is suggested that decreases in 3 He 4 He are related to the regional seismicity and that the hydrothermal system exerts a (temporal) control on the release of near-surface 4He. At LVC this is related to the timing of the late spring thaw. Results from previous monitoring programmes (when the level of seismicity in the caldera was higher) are evaluated against variations in the present work. There appears to be a convincing link between higher 3 He 4 He values and the level of seismicity in the caldera although factors related to location, magnitude and frequency of seismic events are difficult to quantify. Because of the inferred small isotopic contrast between new magma and presently-degassing magma at LVC, it is anticipated that large variations in 3 He 4 He within the central caldera are unlikely to occur until the magmatic volatile signal wanes as a function of degassing and time. Alternatively, only at those localities situated at significant distances from the region of magma intrusion (i.e. away from the resurgent dome vicinity) are helium and/or carbon likely to respond dramatically to intra-caldera seismicity.

TRACING CRUSTAL AND SLAB CONTRIBUTIONS TO ARC MAGMATISM IN THE LESSER ANTILLES ISLAND ARC USING HELIUM AND CARBON RELATIONSHIPS IN GEOTHERMAL FLUIDS

Abstract We report helium and carbon isotope and CO2/3He ratios from a regional survey of geothermal fluids from the Lesser Antilles island arc, an arc system where there is compelling geochemical evidence for the superimposition of a crustal component onto mantle-derived magmas. A predominant mantle helium isotope signature is observed throughout the arc. The highest ratios coincide with MORB helium (∼8RA where RA = air 3He/4He) and occur towards the centre of the arc (the islands of Martinique, Dominica, Guadeloupe, and Montserrat). In the south and north of the arc (Grenada, St. Vincent, St. Lucia in the south and Nevis and Saba in the north) 3He/4He ratios are lower and lie between 4.9 and 6.8RA. This regional variation is also apparent in the carbon isotope systematics: the central portion of the arc (Martinique to Montserrat) have δ13C(CO2) values between −2‰ and −4‰ (vs. PDB), heavier than the range observed in MORB (−4 to −9‰). The south of the arc (Grenada to St.Lucia) is characterized by MORB-like carbon isotope ratios (centred on −6‰). CO2/3He ratios are significantly higher than the MORB value (∼2 × 109) for the entire arc. The values in the central islands fall close to 1010 whereas the southern volcanoes have higher ratios between 1010−1013. Assuming the Lesser Antilles mantle wedge has a MORB-like helium and carbon composition our data can be explained by mixing of mantle wedge carbon with at least two other carbon components: an isotopically-heavy marine limestone endmember of slab-derivation and an isotopically-lighter component representing either slab-derived organic carbon and/or an upper crustal component with a large fraction of organic carbon. The helium-carbon systematics of the central portion of the arc are consistent with a large slab-derived marine limestone input to the carbon inventory, and we calculate a non-mantle:mantle carbon flux of 5.7:1. MORB-like helium isotope ratios, which are sensitive to perturbation by crustal additions to degassed magmas, imply a relatively minor role for upper crustal contributions in this part of the arc although it could reach a maximum of 20% of the total carbon flux if the light-C component is solely of crustal origin. Higher CO2/3He ratios in the southern islands coupled with lighter δ13C imply (1) an increase in the flux in the non-mantle contribution relative to the central arc, and (2) an enhanced role for an isotopically-light carbon component in this part of the arc. Compared to the central islands, the increase in the non-mantle carbon flux in the southern islands is by a factor of 4.2. Assuming that the isotopically-light carbon has a δ13C of −10‰, then its southern arc flux is a factor of 9.1 times that of the central arc. Lower 3He/4He ratios in the southern arc indicate that the origin of this carbon component is likely to be the uppermost crust: in this case, ∼50% of the total carbon lost via the southern Lesser Antilles would be of crustal-derivation. This conclusion is compatible with the increased availability of carbon both in the southernmost forearc sediments and arc crust. Our approach of combining helium and carbon systematics serves to emphasize the importance of volatile provenance on mass balance considerations of the terrestrial carbon inventory at convergent margins.

Mapping magma sources in the east Sunda-Banda arcs, Indonesia: Constraints from helium isotopes

Abstract We report new helium isotope analyses of olivine and clinopyroxene separates from recent lavas for eleven volcanoes from Flores in the east Sunda arc through the inactive segment between the arcs to Banda Island at the extreme of the contiguous Banda arc. In the east Sunda arc, 3 He 4 He ratios (R) vary between 4.5 RA ( R A = air 3 He 4 He ) for the leucitic Batu Tara volcano to a remarkable low of pure radiogenic helium (0.0075 RA) for Werung at the southern tip of Lomblen Island. Lavas from the inactive zone, which represents the locus of collision of the Australian continent with the arc, have a narrower range in R R A —from 3.9 for Kisu in the straits of Pantar to 1.0 for Romang Island. Our one locality (Banda Island) in the Banda arc gives the highest R R A ratio (3.1) observed along this arc to date. The results are consistent with the involvement of crustal material in magma genesis throughout the east Sunda/Banda arcs, as far west as Iya in central Flores. We combine these helium isotope results with published and on-going strontium isotope studies, and show that the source of the helium in the crustal component is unlikely to be terrigenous sediments derived from the Australian continent; rather, degassing of Australian continental crust appears to be the dominant process controlling the helium budget. The He-Sr isotope systematics also provide a framework to account for the areal pattern of 3 He 4 He in this part of the arcs: the radiogenic crustal component is diluted with mantle helium both in a down-dip direction and with increasing lateral distance from the collision zone. These factors result in an excellent first-order relationship between the 3 He 4 He ratio, degree of He Sr enrichment (relative to the postulated mantle endmember), and alkalinity of the erupted lavas. Such a relationship has a direct bearing on models of the tectonic evolution of the collision zone, and on the observation that helium isotopes are decoupled from strontium and other geochemical signatures along the Banda arc.

Helium isotope variations in Turkey: relationship to tectonics, volcanism and recent seismic activities

The distribution of helium isotope ratios in the various tectonic provinces of Turkey is examined through a synthesis of previously published data and the results of a recent survey along the North Anatolian Fault Zone (NAFZ) following the catastrophic 1999 earthquakes. The R/RA values (where R=sample 3He/4He and RA=air 3He/4He) cover a wide range from 0.05 to 7.87, and the mantle-derived helium is clearly identified in most locations. The mantle-derived component is high (>50% of total He) in (a) regions of central and eastern Anatolia, both of which are associated with historically active volcanoes, and (b) the seismically active west-to-central segment of NAFZ. While the mantle contribution reaches a maximum at Nemrut volcano in eastern Anatolia—a region of dominantly compressional tectonics and with moderate enthalpy geothermal fields, it is relatively low (<50%) in the western Anatolian extensional province even though this region has the highest geothermal potential. The average 3He/enthalpy ratios estimated for the different provinces suggest lithospheric stretching and rise of the geotherm as the major mechanism of high heat flow, yet limited intrusive activity in western Anatolia is suggested as the reason for the comparatively low He additions and consequently low (1.7×106 atoms/J) 3He/enthalpy ratios. A more extensive magmatic activity appears to be responsible for the greater input of both heat and helium in eastern and central Anatolia, with intra-province 3He/enthalpy variations (from 3.1×106 atoms/J in eastern to 0.25×106 atoms/J in central Anatolia) reflecting the ageing of hydro-magmatic systems. The relatively high 3He concentrations in low enthalpy waters of northern Anatolia (2.8×106 atoms/J) are particularly significant since there is no evidence of volcanic activity associated with the strike–slip motion along the seismically active segment of NAFZ. Continuous monitoring of He-isotope compositions along NAFZ should lead to a better understanding of this apparent anomaly, as well as the relationship (if any) between He-isotope variations and seismic activities in this region.

Isotopic constraints on the influence of the Icelandic plume

Abstract Thermally buoyant mantle, in the form of a plume, rises beneath Iceland creating a major topographic anomaly along the Mid-Atlantic Ridge and in the surrounding ocean basin. However, the influence of the Iceland plume on the composition of lavas erupted on adjacent ridges remains a contentious issue. Trace element systematics and radiogenic isotope ratios of Sr, Nd and Pb suggest that the plume influences a region 1200 km in length. In contrast, the3He anomaly associated with Iceland closely corresponds to the 2400 km ridge section affected by thermal uplift. We present evidence that the Sr, Nd and Pb isotope signature of the Iceland plume is in fact as widespread as its thermal and3He anomalies. Results imply that much of the source of North Atlantic ridge basalts has been contaminated by lateral outflow of asthenosphere from the Icelandic plume. Consequently, estimates of the average composition of mid-ocean ridge basalt (MORB) sources are likely to be biased by including data from plume-contaminated regions. True MORB values, and perhaps upper mantle geochemistry, can be constrained only by considering data untainted by plume asthenosphere.

Controls on magmatic degassing along the Reykjanes Ridge with implications for the helium paradox

To consider the 3He characteristics of plume-related lavas, we report a detailed survey of helium isotope (3He/4He) and concentration ([He]) variations along an 800-km transect of the Reykjanes Ridge (RR). 3He/4He ratios vary from 11.0 to 17.6 RA (where RA=air 3He/4He) whereas [He] ranges over three orders of magnitude from >5 cm3 STP/g – in the range of most mid-ocean ridge basalts (MORB) – to lows of 4 ncm3 STP/g. The lowest [He] and intermediate 3He/4He ratios occur along the northern RR (closest to Iceland) where eruption depths are shallow (<1000 m) and water contents of lavas are high (0.3–0.4 wt%). We suggest that low-pressure, pre-eruptive magmatic degassing is extensive in this region with degassed magmas susceptible to addition of radiogenic helium thereby lowering 3He/4He ratios. Along the southern RR, [He] reaches maximum values, and 3He/4He ratios display strong correlations with lead isotopes (206Pb/204Pb) consistent with binary mixing. These correlations indicate that the high-3He/4He plume component has higher absolute abundances of the primordial isotope 3He compared to the source of depleted MORB mantle. This finding implies that the so-called ‘helium paradox’ – the observation that plume-derived oceanic glasses apparently have lower 3He contents than MORB glasses – may be an artifact related to considering lavas (e.g. from Loihi seamount, Hawaii) which have not retained their source volatile inventory as well as those erupted along the southern RR.