J.-G. Schilling

Azores mantle blob: Rare-earth evidence

Abstract Rare earths (RE) in basalts erupted within the rift of the Mid-Atlantic Ridge show a progressive change from light-RE enriched to depleted patterns from the Azores Platform (40°N) down to 33°30′N. South, the pattern remains light-RE depleted as along other “normal ridge” segments. A progressive increase in chemical variability of the basalts towards the Azores is also noted. The latitudinal RE profile and corresponding ΣFeO/ΣFeO + MgO variations, together, indicate that the origin of these basalts cannot be accounted for simply by considering variable extents of partial melting of a single mantle source and subsequent fractional crystallization during the ascent of the magmas. These two processes produce only second-order effects on the RE patterns. The data requires the presence of a distinct, light-RE richer, mantle source beneath the Azores Platform relative to that of south of 33°30′N and an intermediate zone where both mantle types mix. The relative contribution of the Azores mantle source to the mix appears to decrease fairly regularly southward along the ridge and becomes negligible at 33°30′N. Increasing chemical variability of the basalts towards the Azores is probably caused by correspondingly larger extent of fractional crystallization at shallow depth, and/or greater variability in the extent of partial melting, apparently subsequent to, and superimposed on the mixing of the two mantle sources. The combined morphological, geophysical and RE evidence along the profile are consistent with a model suggesting upwelling of a major blob (plume) under the Azores Plateau; and reveal the present extent of the blobs overflow and mixing with the asthenosphere depleted in large ionic lithophile trace elements. The influence of the Azores blob is geochemically detectable up to 1000 km southwestward beneath the ridge axis.

Helium isotopic variations in the mantle beneath the central North Atlantic Ocean

We have determined the helium concentrations and helium isotopic ratios for a set of 35 basaltic glass samples dredged from the Mid-Atlantic Ridge between 28° and 53°N latitude. Total helium concentrations vary between< 1 × 10−8cm3STP/g and1.4 × 10−5cm3STP/g. Most of this variation can be accounted for by gas loss on the sea floor via vesiculation, since the samples with the lowest concentrations have extremely high vesicularities. Analysis of the plagioclase phenocrysts from one of the samples suggests that helium behaves as an incompatible element (KD ≪ 0.01). Results of in vacuo crushing and melting experiments confirm that there is no helium isotopic fractionation during melt-vesicle or melt-phenocryst partitioning. The variations in3He/4He ratio along the ridge therefore must be interpreted to be a result of heterogeneity within the oceanic mantle. The3He/4He ratios for this set of samples range from 6.5 to 11.1 times the atmospheric ratio. These data, coupled with strontium isotopic analyses performed on the same samples by White and Schilling (1978), show the existence of two distinct geochemical provinces: the regions between 50–52.5°N and 27–33°N, which are characterized by a trend toward high3He/4He ratios and high87Sr/86Sr ratios, and the region between 33° and 50°N, characterized by a trend toward low3He/4He ratios and high87Sr/86Sr ratios. Since primitive mantle should retain more primordial3He, the low3He/4He trend requires the presence of a mantle reservoir with lower3He/(Th + U) and higher Rb/Sr. The generation of this component requires separation between He and Th + U, either by selective loss of He (degassing), or the addition of Th + U. We believe that the most reasonable source for this component is subducted oceanic crust that is recycled back into the mantle.

Helium and hydrogen isotopes in ocean-ridge basalts north and south of Iceland

The concentrolion and isotopic composi1ion of helium and hydrogen have been measured in submarine basalt glasses from the Mid-Atlantic Ridge between 52° and 73°N and in recent volcanic rocks from Iceland. Observations or “plume-type” 3 He/ 4 He ratios ( R greater than the mean MORB value of 8 R A ) show that the effect of the Icelandic mantle plume on helium isotope rn1ios is observable along the entire length of the Reykjanes Ridge ( R = 11−16.1 R A ) and to 70°N on the Kolbeinsey Ridge ( R = 10.3−12 R A ). These two ridges are unique among all mid-ocean ridge segments so for studied in possessing high 3 He/ 4 He ra1ios along their entire length. Northof 70°N, basalts associated with the Jan Mayen province (based on major element chemistry) have 3 He/ 4 He ratios somewhat lower than the MORB average ( R = 6.8−8.0 R A ) and similar to other alkali-enriched provinces such as the Azores Platform. In detail, however, the 3 He/ 4 He distribution does not correspond to trace element variations. A maximum ratio of 16.1 R A is present near 60°N on the Reykjanes Ridge: north of this point, toward Iceland, the ratio decreases to a minimum of 12.7 R A at 63°N. The 3 He/ 4 He ratios increase again on Iceland, reaching a maximum in south-central Iceland. In the area between 60° and 63°N, the 3 He/ 4 He ratios tend to be inversely correlated with La/Sm and 87 Sr/ 86 Sr, which suggests that the isotopic heterogeneities beneath Iceland and the Reykjanes Ridge are long lived (> 10 8 years). The concentration and hydrogen isotopic composition of the water in this suite of basalts display a good positive correlation with trace clement data (e.g. La/Sm). Between 61°N and 63°N on the Reykjanes Ridge, the water concentration in the basalt is high (0.22–0.41 wt.%) and its isotopic composition is enriched in deuterium (δD = −61 to −74‰; with one extreme value to −50‰), relative to MORB levels (0.2 wt.% and −77‰). South of 61 °N, both the concentration and isotopic composition of water are normal MORB values (0.11–0.25 wt.% and δD = −71 to −91‰). The Mohns Ridge basalts have elevated water concentrations (0.37–1.23 wt.%) and an isotopic composition (−44 to −60‰) which is unique among all MOR basalts so far studied. It remains to be seen if thesehigh δ D values represent a “primary” mantle water, distinct from the MORB source, or “secondary” water, the result of recycling during subduction.

Spinels in Mid-Atlantic Ridge basalts: Chemistry and occurrence

Abstract Three groups of spinels have been identified in dredged basalts from the Mid-Atlantic Ridge in the Azores region (30–40°N): (1) magnesiochromites with 0.4–0.5 Cr/(Cr + Al) are most common and characteristic of olivine tholeiites of the region; (2) titaniferous magnesiochromites are found in an olivine basalt with alkali affinities, of local occurrence and evolved in relatively high fugacity of oxygen; (3) chromian spinels with 0.23 Cr/(Cr + Al) occur in unusual high-Al picrites of local occurrence and possible high-pressure origin. Spinels are restricted in occurrence to the least fractionated lavas, with FeO * /FeO * + MgO ratio less than 0.575 and with Cr content greater than 350 ppm. A close relationship between Al content of liquidus spinel and Al content of magma has been observed for basaltic types. High-Al spinels deviating from this relationship, such as those found in picritic lavas from the Mid-Atlantic Ridge, may have crystallized at high pressure. The use of spinels as geobarometers in magmas of a restricted compositional range seems a promising prospect. There is no evidence of systematic variation in spinel chemistry of occurrence along the Mid-Atlantic Ridge, such as could be related to different mantle sources of the basalts, plume versus non-plume or binary mantle mixing.

Dispersion of the Jan Mayen and Iceland mantle plumes in the Arctic: A He‐Pb‐Nd‐Sr isotope tracer study of basalts from the Kolbeinsey, Mohns, and Knipovich Ridges

He-Pb-Nd-Sr isotope systematics in basalts from the Mid-Atlantic Ridge (MAR) from 65°N to 78°N are reported for mapping the zone of influence of the Jan Mayen and Iceland mantle plumes in the Arctic. The geographical variation and the two distinct trends observed in the He-Pb-Nd-Sr isotope space clearly indicate that the boundary between the zone of influence of the “low 3He/4He” Jan Mayen plume and the “high 3He/4He” Iceland mantle plume is in the vicinity of the Spar FZ. Modeling indicates that the dispersion of the Jan Mayen plume is not purely radial, but extends preferentially northward, probably because of decreasing spreading rate and the cascading of the buoyant mantle plume across the Jan Mayen fracture zone (FZ) due to the large denivellation (change of level) of the base of the lithosphere caused by the large age offset (∼20 Myr). The incompatible parent and daughter (PD) element concentrations and their ratios for the basalt population from the Jan Mayen plume are highly coherent with the Pb-Nd-Sr isotope ratios and show essentially the same geographical pattern in spite of large variations in the mean degree of fusion and extent of fractional crystallization. In contrast, over the southern Kolbeinsey Ridge, unusual decouplings are observed between He-Pb-Nd-Sr isotopic systematics and incompatible element variations, as well as inferred melting conditions. These decouplings are best explained by a modified version of the dispersion model of the Iceland plume by Mertz et al. [1991] which was based on more limited isotopic data, and the fluid dynamic models of Ito et al. [1999]. In addition to binary mixing of the Iceland plume with the depleted asthenosphere a combination of (1) defluidization of the Iceland mantle plume occurring deeper than the dominant zone of dry decompression melting for enhancing He and heat transport along the southern Kolbeinsey Ridge and (2) fractional melting accompanying the northward dispersion and decompression of the Iceland mantle plume is required in order to explain the difference in wavelength between the He gradient and the Pb-Nd-Sr isotope gradients observed in mapping the dispersion of the Iceland plume along the MAR. Note that the Tjornes TZ does not act as a dam against the northward dispersion of the Iceland plume, contrary to what the rare earth element variation, by itself, previously suggested. Finally, over the Knipovich Ridge the large scatter and lack of any systematics between the Pb-Nd-Sr isotope and related parent/daughter ratios along this immature, discontinuous, and shear-dominated ridge, running parallel and close to the Svalbard continental break (74°–79°N), suggest the involvement in the melting of randomly distributed continental mantle lithosphere schlierens present in the depleted upper mantle source of the Knipovich Ridge basalts.

Faeroe-Iceland plume: Rare-earth evidence

Abstract Rare-earth abundances through the 3000-m thick Faeroes Plateau basalt monitor the Faeroe-Iceland plume activity with time. An abrupt change from light Re-enriched to depleted patterns is observed naer the boundary of the middle and upper series of the Faeroes Plateau basalt. The discontinuity seems to reflect a change of volcanic regime from plume-derived to more akin to oceanic type. The change also coincides with field evidence for beginning of subsidence of the plateau. Using existing spreading rate history for the North Atlantic during the 50–60-m.y. B.P. period, which is one of deceleration, the change of volcanic regime suggests a decline of intensity of the Faeroe-Iceland plume during the late part of the period. Rising blobs, rather than a continuous plume, appears preferable for modeling the temporal plume activity.

Mantle heterogeneity from trace elements: MAR triple junction near 14°N

Abstract The trace element composition of basalts dredged at the axis of the Mid-Atlantic Ridge between 12°N and 17°N by the R/V “Akademik Boris Petrov” demonstrates the presence of a high-amplitude geochemical anomaly, centered around 14°N and extending at least 300 km along the strike of the Rift Valley. The anomaly does not fit easily into any of the models that have been proposed: it may reflect the upwelling of an embryonic mantle plume or of a passive mantle domain responsible for or associated with a triple junction, possibly marking a recent shift into the area of the South American/North American plate boundary.

Mantle heterogeneity beneath the southern Mid-Atlantic Ridge: trace element evidence for contamination of ambient asthenospheric mantle

le Roux, P. J., le Roex, A. P., Schilling, J. G., Schimizu, N., Perkins, W. W., Pearce, N. J. G. (2002). Mantle heterogeneity beneath the southern Mid-Atlantic Ridge: trace element evidence for contamination of ambient asthenospheric mantle. Earth and Planetary Science Letters, 203 (1), 479-498. Article Number: PII S0012-821X(02)00832-4.

Helium isotope ratios in Easter microplate basalts

Abstract 3 He/ 4 He ratios in Easter Microplate basalt glasses show clear evidence of the effects of a mantle plume. The East Rift of the microplate between 26 and 28°S, identified by La/Sm, Sr and Pb isotopes and ridge crest elevation as the region of maximum plume influence, has 3 He/ 4 He ratios spanning the entire range from 7.5 to 11.7 R A . The Easter Microplate is the only section of the entire East Pacific Rise that is associated with a known ‘hotspot’ track (mantle plume) and has elevated 3 He/ 4 He ratios. Although most of the West Rift basalts contain MORB helium (8.0–8.7 R A ), the basalt closest to the East Rift has an elevated 3 He/ 4 He ratio (11.3 R A ), consistent with a significant plume component. The diversity in isotopic signatures also indicates that homogenization of isotopic anomalies does not occur, even in this region of ‘super-fast’ spreading. The overall 3 He/ 4 He/ 206 Pb/ 204 Pb and 3 He/ 4 He 87 Sr/ 86 Sr trends have positive correlations, although the high 3 He/ 4 He, 87 Sr/ 86 Sr and 206 Pb/ 204 Pb component is not a single value but rather a broad band. The apparent ‘decoupling’ between the He and Sr isotope distribution is modeled in the context of a plume source-migrating ridge sink. During channeling of the plume toward the ridge, helium is preferentially lost from the center of the channeled plume, resulting in lower He/Pb and He/Sr concentration ratios in the high 3 He/ 4 He component. Mixing trajectories in He Sr isotopic space between a LILE depleted asthenosphere and a variably degassed plume component provide a reasonably good fit to the data and may explain the isotope systematics of plume-ridge interactions in the context of modern theories of plume dynamics.

Evolution of the Iceland hotspot

Mantle geochemical enrichment beneath the Iceland–Faeroes hotspot has been episodic, suggesting that blobs rather than a continuous plume may be rising in the region. Recurring rift propagation southward from central Iceland causing progressive melting of the lower crust followed by the upper mantle, best explains the geochemical variations found along the south-west and south-east neovolcanic zones and their degree of thermal maturity.