John E. Lupton

Primordial neon, helium, and hydrogen in oceanic basalts

A primordial neon component in neon from Kilauea Volcano and deep-sea tholeiite glass has been identified by the presence of excess20Ne; relative to atmospheric neon the20Ne enrichments are 5.4% in Kilauea neon and about 2.5% in the basalts. The20Ne anomalies are associated with high3He/4He ratios; the ratio in Kilauea helium is 15 times the atmospheric ratio, while mid-ocean ridge basalts from the Atlantic, Pacific, and Red Sea have uniform ratios about 10 times atmospheric. Mantle neon and helium are quite different in isotopic composition from crustal gases, which are highly enriched in radiogenic21Ne and4He. The21Ne/4He ratios in crustal gases are consistent with calculated values based on G. Wetherills18O (α,n) reaction; the lack of20Ne enrichment in these gases shows that the mantle20Ne anomalies are not radiogenic.21Ne enrichments in Kilauea neon and “high-3He” Pacific tholeiites are much less than in crustal neon, about 2 ± 2% vs. present atmospheric neon, as expected from the much lower4He/Ne ratios. Neon concentrations in two Atlantic tholeiites were found to be only 1–2% of the values obtained by Dymond and Hogan; helium concentrations are slightly greater and our He/Ne ratios are greater by a factor of 150. The large Ne excess relative to solar wind and meteoritic gases is thus not confirmed. Pacific and Atlantic basalts appear to be quite different in He/Ne ratios however, and He and Ne may be inversely correlated. He concentration variations due to diffusive loss can be distinguished from variations due to two-phase partitioning or mantle heterogeneity by the effects on3He/4He ratios. The He isotopic and concentration measurements on “low-3He” basalts are consistent with diffusive loss and dilution of the 3/4 ratio by in-situ radiogenic4He, and may provide a method for dating basalt glasses. Deuterium/hydrogen ratios in Atlantic and Pacific tholeiite glasses are 77% lower than the ratio in seawater. The inverse correlation between deuterium and water content observed by Friedman in erupting Kilauea basalts is consistent with a Rayleigh separation process in which magmatic water is separated from an initial melt with the same D/H ratio as observed in deep-sea tholeiites. The consistency of the D/H ratios in tholeiites containing primordial He and Ne components indicates that these ratios are probably characteristic of primordial or juvenile hydrogen in the mantle.

Abiogenic Hydrocarbon Production at Lost City Hydrothermal Field

Low-molecular-weight hydrocarbons in natural hydrothermal fluids have been attributed to abiogenic production by Fischer-Tropsch type (FTT) reactions, although clear evidence for such a process has been elusive. Here, we present concentration, and stable and radiocarbon isotope, data from hydrocarbons dissolved in hydrogen-rich fluids venting at the ultramafic-hosted Lost City Hydrothermal Field. A distinct “inverse” trend in the stable carbon and hydrogen isotopic composition of C1 to C4 hydrocarbons is compatible with FTT genesis. Radiocarbon evidence rules out seawater bicarbonate as the carbon source for FTT reactions, suggesting that a mantle-derived inorganic carbon source is leached from the host rocks. Our findings illustrate that the abiotic synthesis of hydrocarbons in nature may occur in the presence of ultramafic rocks, water, and moderate amounts of heat.

Gradients in the composition of hydrothermal fluids from the Endeavour segment vent field: Phase separation and brine loss

Hydrothermal fluid samples collected in 1984, 1987, and 1988 from a large vent field near 47°57′N on the Endeavour segment of the Juan de Fuca Ridge (JFR) have been analyzed for major and minor elements and gases. There are of the order of 100 individual smoker vents on ∼10 large sulfide structures, which are localized along faults and fault intersections across the vent field. Each sulfide structure has a characteristic fluid composition, which varies very little from one vent orifice to the next, or from year to year, on a given structure. However, there are large gradients in fluid composition across the vent field, with endmember chlorinity increasing from ∼255 mmol/kg in the SW to 505 mmol/kg in the NE. End-member concentrations of major elements are well correlated with chlorinity, and endmember volatile concentrations in the lowest chlorinity fluids are approximately twice as high as in the highest chlorinity fluids. The gradients in composition across the vent field and measured vent fluid temperatures >400°C are consistent with supercritical phase separation and loss of brine phase below the seafloor. The factor-of-2 variation in CO2 (and H2S) is larger than expected for loss of a very high-chlorinity brine. Concentrations of iron and manganese are not positively correlated with chlorinity, suggesting that temperature and pH are more important in controlling metal solubility. Elevated ammonia and bromide/chloride ratios indicate that there has been subseafloor interaction between the hydrothermal fluids and organic matter, and high boron concentrations point to a sedimentary source.

Excess 3He in oceanic basalts: Evidence for terrestrial primordial helium

Abstract Helium trapped in the chilled glass rims of Pacific Ocean basalts is highly enriched in 3 He; the 3 He/ 4 He and 3 He/Ne ratios are respectively 10 and 1000 times the atmospheric ratios. We interpret these large enrichments as further evidence that primordial 3 He is still present in the interior of the earth. The 3 He/ 4 He ratio in basalt glass is the same as the isotope ratio of the “excess helium” in Pacific Ocean deep water, supporting the theory that the atmospheric escape rate of 3 He is balanced by a flux of primordial 3 He from the mantle.

Magmatic events can produce rapid changes in hydrothermal vent chemistry

The Endeavour segment of the Juan de Fuca ridge is host to one of the most vigorous hydrothermal areas found on the global mid-ocean-ridge system, with five separate vent fields located within 15 km along the top of the ridge segment. Over the past decade, the largest of these vent fields, the ‘Main Endeavour Field’, has exhibited a constant spatial gradient in temperature and chloride concentration in its vent fluids, apparently driven by differences in the nature and extent of subsurface phase separation. This stable situation was disturbed on 8 June 1999 by an earthquake swarm. Owing to the nature of the seismic signals and the lack of new lava flows observed in the area during subsequent dives of the Alvin and Jason submersibles (August–September 1999), the event was interpreted to be tectonic in nature. Here we show that chemical data from hydrothermal fluid samples collected in September 1999 and June 2000 strongly suggest that the event was instead volcanic in origin. Volatile data from this event and an earlier one at 9° N on the East Pacific Rise show that such magmatic events can have profound and rapid effects on fluid–mineral equilibria, phase separation, 3He/heat ratios and fluxes of volatiles from submarine hydrothermal systems.

Intra-oceanic subduction-related hydrothermal venting, Kermadec volcanic arc, New Zealand

Intra-oceanic volcanic arcs mark the boundaries between converging lithospheric plates where subduction produces volcanic and tectonic activity that ensures a steady supply of magmatic heat and hydrothermal fluids to the seafloor. Here we report on the first broad and systematic survey of hydrothermal emissions generated along a submarine arc front. More than half (seven of 13) of the volcanoes surveyed along 260 km of the southern Kermadec arc, NE of New Zealand, are hydrothermally active. Our results indicate that volcanic arcs represent a previously unheeded but potentially extensive source of shallow (<2 km water depth) vent fields expelling fluids of a unique and heterogeneous composition into the oceans.

Mantle plume helium in submarine basalts from the galapagos platform.

Helium-3/helium-4 ratios in submarine basalt glasses from the Galapagos Archipelago range up to 23 times the atmospheric ratio in the west and southwest. These results indicate the presence of a relatively undegassed mantle plume at the Gal�pagos hot spot and place Gal�pagos alongside Hawaii, Iceland, and Samoa as the only localities known to have such high helium-3/helium-4 ratios. Lower ratios across the rest of the Gal�pagos Archipelago reflect systematic variations in the degree of dilution of the plume by entrainment of depleted material from the asthenosphere. These spatial variations reveal the dynamics of the underlying mantle plume and its interaction with the nearby Gal�pagos Spreading Center.

Hydrothermal helium plumes in the Pacific Ocean

Hydrothermal activity along the global mid-ocean ridge system and at active seamounts introduces a 3 He-rich signal into the deep ocean basins, which can be used to trace patterns of ocean circulation and mixing. This hydrothermal helium signal is especially strong in the Pacific Ocean, where the spreading rate of the ridges is the greatest. In several areas of the Pacific the hydrothermal activity is of sufficient strength to produce intense helium plumes which clearly define the regional circulation. Among these are (1) a pair of helium plumes at 2500-m depth, which extend westward from the East Pacific Rise at 10°N and 15°S into the interior of the Pacific basin; (2) a helium plume at 2000-m depth extending southwest from sources on the Juan de Fuca Ridge in the northeast Pacific; and (3) a plume at 1100-m depth emanating from Loihi Seamount at 20°N, which extends eastward from Hawaii to the coast of Mexico. The flow field implied by the helium distribution appears to be in reasonable agreement with Reids [1997] steric height analysis for the Pacific.

Helium-3 and manganese at the 21°N East Pacific Rise hydrothermal site

Abstract Water samples collected at the 21°N hydrothermal site on the East Pacific Rise crest, including Deep-Tow and hydrocast samples collected in 1977 and three hot vent water samples collected recently with the submersible “Alvin”, contain significant additions of 3 He, 4 He, and Mn. Although the vent water collections were at least 50-fold diluted with ambient seawater, they are up to 53 times enriched in 3 He and 7.4 times enriched in 4 He relative to saturated seawater, with concentrations of total dissolvable manganese (TDM) up to 310 μg/kg. 3 He and 4 He covary in the vent samples, with 3 He/ 4 He about 8 times the atmospheric ratio, reflecting a mantle helium source. In contrast to the helium isotopes the Mn/ 3 He ratio in the vent samples is variable, ranging from 4.3 × 10 4 up to 1.0 × 10 5 g/cm 3 . Profiles of 3 He/ 4 He and TDM in the water column at 21°N show a sharp maximum of δ( 3 He) = 47%and TDM= 0.69 μg/kg , much higher than the average values of 34% and 0.2 μg/kg for the deep water in this region. This spike in 3 He and Mn occurs at 2400 m depth, 200 m above the level of the 21°N vents, and 100 m higher than any local bathymetry, evidence for upward transport of the hydrothermal discharge via rising plumes of hot vent water. Two of the 21°N Deep-Tow samples associated with small ( ⩽0.010°C ) temperature anomalies had δ( 3 He) = 38%and TDM= 0.28 and 0.58 μg/kg , also slightly elevated relative to background. The Deep-Tow and hydrocast samples have lower Mn/ 3 He ratios than average vent samples due to Mn removal by scavenging. Comparison of vent samples and water column measurements at 21°N indicate that the pure vent water could be detected using 3 He and Mn even when diluted ∼10 5 times with seawater, confirming that these two tracers are extremely sensitive indicators of submarine hydrothermal activity.

Long-term eruptive activity at a submarine arc volcano

Three-quarters of the Earths volcanic activity is submarine, located mostly along the mid-ocean ridges, with the remainder along intraoceanic arcs and hotspots at depths varying from greater than 4,000 m to near the sea surface. Most observations and sampling of submarine eruptions have been indirect, made from surface vessels or made after the fact. We describe here direct observations and sampling of an eruption at a submarine arc volcano named NW Rota-1, located 60 km northwest of the island of Rota (Commonwealth of the Northern Mariana Islands). We observed a pulsating plume permeated with droplets of molten sulphur disgorging volcanic ash and lapilli from a 15-m diameter pit in March 2004 and again in October 2005 near the summit of the volcano at a water depth of 555 m (depth in 2004). A turbid layer found on the flanks of the volcano (in 2004) at depths from 700 m to more than 1,400 m was probably formed by mass-wasting events related to the eruption. Long-term eruptive activity has produced an unusual chemical environment and a very unstable benthic habitat exploited by only a few mobile decapod species. Such conditions are perhaps distinctive of active arc and hotspot volcanoes.