Manuel Moreira

Helium signature of the subcontinental lithospheric mantle

Peridotites and basalts from continental areas over the world show a helium isotopic ratio that is relatively homogeneous and more radiogenic than the MORB source. The mean ratio is 118 000±15 500 (R/Ra=6.1±0.9), compared to the MORB source ratio of 90 000±10 000 (R/Ra=8±1). This rather constant ratio indicates that the effect is worldwide and can be directly bound to subcontinental mantle geodynamics. A closed system model would imply low uranium content in the subcontinental mantle (<2 ppb), disagreeing with the measured U concentrations in continental mantle peridotites (4–40 ppb). In this study, we evaluate a model where the continental lithospheric mantle is in steady state for helium. Helium residence time in the subcontinental mantle can therefore be estimated and gives values around 100 Ma, compatible with plate tectonic time scales and melt fluxes through the lithosphere. However, this model predicts 3He fluxes between 110 and 1000 mol/yr depending on different U contents and depth of the lithosphere. This appears much higher than the flux observed in continental areas.

Helium and lead isotope geochemistry of the Azores Archipelago

New helium and lead isotopic data for basalts from the Azores archipelago (North Atlantic) show that the Azores have 4He/3He ratios both higher and lower than MORB values. Good covariations of helium and lead isotopes are observed at the scale of the archipelago, and suggest the coexistence of two mantle components in the Azores which are identified by data from Sao Miguel and Terceira. The eastern part of Sao Miguel island displays radiogenic helium (4He/3He > 140,000, R/Ra 100,000) were observed at latitude higher than 40°N and may reflect the influence of the Sao Miguel component at the ridge.

Solar neon in the Icelandic mantle: new evidence for an undegassed lower mantle

New helium and neon data from subglacial Icelandic basalts confirm that the Icelandic mantle source is relatively undegassed. The 3He/4He ratios vary between 14.3 and 25.6 times atmospheric (R/Ra) (4He/3He between 50 500 and 28 000); helium contents vary between 0.3 and 10 μcc STP/g. These values are consistent with the previously published data. The 20Ne contents are highly variable due to atmospheric contamination, and the 20Ne/22Ne ratios vary between atmospheric (9.8) and 12.73±0.04. The most important result comes from the observed 21Ne/22Ne ratios, which are much lower (at a given 20Ne/22Ne) than any other hotspot, despite the relatively low helium isotope ratios. The unique Icelandic helium-neon systematics, as compared to Hawaii, probably reflects different mixing processes related to the location of Iceland on the Mid Atlantic Ridge rather than in a mid-plate setting. If the isotopic variations are caused by mixing, the Icelandic plume end member has a 4He/3He lower than 19 000 (R/Ra>38) and a 21Ne/22Ne ratio lower than 0.035. The 21Ne/22Ne ratio is very close to the solar neon isotope composition and indicates that the plume source has a very low (U+Th)/22Ne ratio that is best explained by a relatively undegassed mantle component.

A primitive plume neon component in MORB: The Shona ridge-anomaly, South Atlantic (51–52°S)

We report on He and Ne isotopes in basaltic glasses from eight dredge stations occupied over the Shona gravity and topographic anomaly high on the Mid-Atlantic Ridge around 51–52°S. The results indicate the presence of a primitive, little degassed, 3He-rich mantle component. 3He4He ratios correlate positively with the bathymetric and gravity anomaly, with values ranging from 12.5 to 6.4 times the atmospheric ratio of 1.38 × 10−6. The highest value is almost identical to that of the Bouvet and Reunion hotspots. Neon isotopic ratios in 20Ne22Ne vs. 21Ne22Ne isotope space indicate recent mixing between a primitive mantle plume component with near-solar Ne and the upper mantle MORB source; and again, the most solar-like Ne found over the Shona ridge-anomaly is similar to that found at the Reunion hotspot. For the first time, a negative trend pointing toward solar values is bridging the commonly observed MORB-air and the Hawaiian L-K-air mixing lines of positive slope. These results suggest that the source of the Shona ridge-anomaly has a similar origin to that of the 3He-rich Bouvet and Reunion plume sources in the deep mantle, which could define a particular noble gas signature in the South Atlantic and the Indian Ocean. It is also evident that the source of the Shona plume is distinct from the 4He-rich source of the Tristan and Gough family of plumes located further north.

Noble gas constraints on degassing processes

Abstract We compare the rare gas elemental ratios measured in glass samples of mid-oceanic ridges (MORs) and oceanic island basalts to current estimates of the same ratios for upper and lower mantle reservoirs. We selected only samples with high 20 Ne/ 22 Ne (>11) in order to minimize atmospheric contamination. The combined use of He, Ne and Ar elemental systematics in these samples allows differentiation between the vesiculation process at mantle plumes, which appears to be in open system, and at normal MORs, where vesiculation occurs in a closed system. Such an open system vesiculation has important consequences for the interpretation of heavy rare gas isotopic ratios in primitive plume-derived materials.

Rare gas systematics on the southernmost Mid‐Atlantic Ridge: Constraints on the lower mantle and the Dupal source

Concentrations and isotopic compositions of He, Ne, Ar, Kr, and Xe have been measured for mid-ocean ridge basalt glasses from the Mid-Atlantic Ridge Discovery section, centered at 47°30′S, thus extending the database for the 50°–53°S Shona section [Moreira et al, 1995]. The 44°–53°S part of the Mid-Atlantic Ridge includes the Discovery and Shona bathymetrie and geochemical ridge anomalies [Douglass et al, 1999], which also appear clearly in the rare gas isotopic record. In addition to air, present at the surface or possibly mantle recycled, three source components are identified, upper mantle, primitive plume, and a Dupal-related component. He and Ne isotopes indicate a very primitive source for both the Discovery and Shona plumes, which must originate in deep, poorly degassed mantle. Ne and Ar, corrected from air based on Ne systematics, reveal very consistent along-strike He, Ne, and Ar isotopic patterns, also consistent with Xe data. These systematics provide evidence that plume argon has low 40Ar/36Ar and plume Xe low isotopic ratios relative to degassed mantle. A segment of the Discovery ridge anomaly shows a Dupal-type, low 206Pb/204Pb component named LOMU (low μ, where μ = 238U/204Pb) by Douglass et al. [1999], and has radiogenic 4He/3He and 21Ne/22Ne, relatively elevated 20Ne/22Ne, mildly radiogenic 40Ar/36Ar, and low Xe isotopic ratios, possibly representing the Dupal rare gas signature. Interpretations of this component as either recycled oceanic crust, or delaminated subcontinental lithosphere are consistent with the rare gas systematics. In the former case, a maximum subduction age of 500 Ma can be calculated. In the latter case, the sublithospheric mantle should have a 40K/36Ar ratio 2–5 times lower than the convective mantle and a 238U/3He ratio 2–3 times higher.

4He/³He dispersion and mantle convection

Histograms of the 4He/³He ratios analysed in MORB glasses from individual ridge segments show a linar correlation between the ridge spreading rate and the standard deviation of the corresponding averaged 4He/³He ratio. We interpret the form of these histograms in terms of stirring time (τstir) of the upper mantle. This stirring time corresponds to the time that perturbations brought in from outside into the upper mantle, in the form of oceanic island source material or slabs, were attenuated by a factor of 1/e through the effect of convection. Using 4He/³He ratios of oceanic basalts, we calculate a stirring time close to 250 Ma, distinct from the residence time of ∼1 Ga. This may indicate the existence of two scale upper mantle convection, rapid convection being responsible for the homogeneisation of helium in the upper mantle, and the consequent uniformity of the 4He/³He ratio, and slower convection being responsible for mantle outgassing and plate tectonic motion.

Pb-Sr-He isotope and trace element geochemistry of the Cape Verde Archipelago

Abstract New lead, strontium and helium isotopic data, together with trace element concentrations, have been determined for basalts from the Cape Verde archipelago (Central Atlantic). Isotopic and chemical variations are observed at the scale of the archipelago and lead to the definition of two distinct groupings, in keeping with earlier studies. The Northern Islands (Santo Antao, Sao Vicente, Sao Nicolau and Sal) present Pb isotopic compositions below the Northern Hemisphere Reference Line (NHRL) (cf. Hart, 1984) , unradiogenic Sr and relatively primitive 4He/3He ratios. In contrast, the Southern Islands (Fogo and Santiago) display Pb isotopes above the NHRL, moderately radiogenic Sr and MORB-like helium signatures. We propose that the dichotomy between the Northern and Southern Islands results from the presence of three isotopically distinct components in the source of the Cape Verde basalts: (1) recycled ∼1.6-Ga oceanic crust (high 206Pb/204Pb, low 87Sr/86Sr and high 4He/3He); (2) lower mantle material (high 3He); and (3) subcontinental lithosphere (low 206Pb/204Pb, high 87Sr/86Sr and moderately radiogenic 4He/3He ratios). The signature of the Northern Islands reflects mixing between recycled oceanic crust and lower mantle, to which small proportions of entrained depleted material from the local upper mantle are added. Basalts from the Southern Islands, however, require the addition of an enriched component thought to be subcontinental lithospheric material instead of depleted mantle. The subcontinental lithosphere may stem from delamination and subsequent incorporation into the Cape Verde plume, or may be remnant from delamination just before the opening of the Central Atlantic. Basalts from Sao Nicolau reflect the interaction with an additional component, which is identified as oceanic crustal material.

Rare gas systematics on Mid Atlantic Ridge (37–40°N)

Rare gas concentrations and isotopic compositions in 19 glasses from the North Atlantic ridge have been analyzed. Sixteen of them are located between 36.8 and 39.9°N on the Azores plateau, three others are located between 21.25 and 30.2°N and may represent normal North Atlantic Mid Ocean Ridge basalts. Helium concentrations decrease by three orders of magnitude between 37° and 38.5°N. These low He concentrations (10−8 cm3 STP/g) are attributed to degassing before eruption. The 4He/3He ratios decrease from 90 000 at 37°N to 75 000 at 38.5°N and then increase to 100 000 at 40.5°N. The low 4He/3He ratio measured on the ridge at 38.5°N indicates a plume–ridge interaction with the Terceira–Pico–Sao Jorge plume, which has a 4He/3He lower than 50 000 (R/Ra>15). More radiogenic 4He/3He ratios (up to 110 000) can also be found in this segment of the ridge and may indicate either in situ production of 4He by radioactive decay or atmospheric contamination of very low helium content samples. The 20Ne/22Ne ratios decrease from a mantle-like value (>12.0) between 20 and 37°N to the atmospheric ratio (9.8) at 38.5°N. The 40Ar/36Ar ratios show the same tendency as the neon isotopic ratios (atmospheric-like at 38.5°N). This may indicate contamination in shallow degassed magma chambers. However, because the maximum measured 20Ne/22Ne ratios are strongly correlated to the 206Pb/204Pb, 87Sr/86Sr and 143Nd/144Nd ratios, an alternative explanation for the atmospheric 20Ne/22Ne is the presence of an atmospheric component in the source. However, due to the large He loss (and therefore Ne and Ar), atmospheric contamination is the more probable explanation.

Subducted oceanic lithosphere and the origin of the ‘high μ’ basalt helium isotopic signature

The isotope geochemistry of ocean island basalts has been used to infer the presence of ancient recycled oceanic crust in the mantle. The helium isotopic ratios of basalts from HIMU sources, having high U/Pb (μ) and low Rb/Sr, are thought to be derived from recycled ocean crust, but are only slightly more radiogenic than average mid-ocean ridge basalt (MORB) values (i.e. 3He/4He of 6–7 R/Ra, compared to ∼8 R/Ra). These values appear to be inconsistent with ancient recycled oceanic crustal sources because the helium should be significantly more radiogenic. We propose a simple model to explain this helium isotopic composition. The basic hypothesis is that the entire oceanic lithosphere is subducted, of which the oceanic crust is only a small fraction. Based on measurements from xenoliths, the residual lithosphere contains ∼5×10−7 ccSTP/g of helium. The relatively low uranium content in the mantle lithosphere (<0.5 ppb) leads to 3He/4He ratios slightly more radiogenic than the upper mantle ratio after 2 Ga, and could be the source of the HIMU helium isotopic composition by mixing with the subducted oceanic crust. Although the helium content of the subducted lithosphere is highly uncertain, existing data suggest that a simple closed system evolution model can explain the helium isotopic composition of the HIMU mantle sources.