Christophe Hémond

The heterogeneous Iceland plume: Nd‐Sr‐O isotopes and trace element constraints

We present a comprehensive set of Sr, Nd, and O isotope data and trace element concentrations from tholeiitic and alkaline lavas of the neovolcanic zones of Iceland (picrites, olivine and quartz tholeiites, transitional and alkali basalts, differentiated rocks). Variations in the oxygen isotope results allow us to distinguish two groups. The first, which comprises quartz tholeiites and more differentiated rocks usually associated with central volcanoes, has low δ18O values (+5 to +1‰) resulting from interaction with the hydrothermally altered Icelandic crust. The second group, which contains picrites, olivine tholeiites, and alkali basalts, has normal mantle oxygen isotopic compositions (δ18O = +5 to +6‰) which are thought to represent those of the mantle source. Nd isotopic compositions vary greatly, from 143Nd/144Nd = 0.51314 in picrites to 0.51295 in alkali basalts. To produce such a variation for rocks with the chemical compositions of Icelandic volcanics (147Sm/144Nd = 0.12=0.28) requires >200 m.y., a period that greatly exceeds the maximum age of Icelandic crust. Previous models, in which the Sr isotopic variations were explained in terms of evolution of crustal reservoirs, are invalidated, and mantle reservoirs with different Nd and Sr isotopic compositions are indicated. The Iceland data define a linear array in the Sr-Nd isotope diagram which overlaps both mid-ocean ridge basalt and oceanic island basalt fields and indicates mixing between depleted and enriched end-members. Alkali basalts come preferentially from an isotopically and chemically enriched component of the Iceland plume, and picrites come from a more refractory, more depleted portion. Positive Sr, Rb, and Ba anomalies are present in picrites and other lavas with low trace element contents. These anomalies are not correlated with isotopic differences but are nevertheless believed to result from interaction between the parent magmas of these rocks and altered Icelandic crust. This indicates that even the most primitive Icelandic lavas have been contaminated with some crustal material.

A Neogene back-arc origin for the Banda Sea basins: geochemical and geochronological constraints from the Banda ridges (East Indonesia)

Abstract Dredgings conducted during the French–Indonesian cruises Banda Sea II and III collected volcanic rocks from several ridges of the Banda Sea area (Tukang Besi ridge, site 218; Lucipara ridge, sites 214 and 305; Nieuwerkerk–Emperor of China, sites 219 and 220). With the exception of one 46-Ma-old N-MORB type basalt, thought to belong to an ophiolitic complex, K–Ar and Ar–Ar datings indicate that all the dredged volcanics are Neogene. They range in age from ca. 10 Ma (Tukang Besi back-arc basalts) to 8–7 Ma (Nieuwerkerk–Emperor of China calc-alkaline andesites) and to 7–3 Ma (Lucipara OIB-type transitional basalts and cordierite-bearing andesites). Radiogenic isotopic signatures of andesites are consistent with an AFC (Assimilation coupled with Fractional Crystallization) process involving assimilation of continental crust. 8–3-Ma-old calc-alkaline volcanic activity is also recorded on the Wetar segment, an inactive part of the East Sunda arc, and corresponding isotopic compositions are also consistent with an AFC process involving continental crust. These features suggest that Lucipara–Nieuwerkerk–Emperor of China ridges and the Wetar segment were representing a single volcanic arc 8–7 Ma ago. The corresponding calc-alkaline activity was related to the subduction of the Indian oceanic lithosphere beneath continental blocks of Australian origin. Back-arc opening processes occurred from 6 to 3 Ma as a multi-rift opening for the Wetar basin and as a single-rift opening for the Damar basin while subduction-related magmatism was still active in the Wetar segment. Volcanic activity stopped at 7 Ma in the Nieuwerkerk–Emperor of China ridge. On Lucipara ridge, 6–3 Ma volcanic activity emplaced concomitantly transitional basalts and cordierite-bearing andesites. The mineralogical and chemical features of the latter are consistent with an AFC process involving assimilation of continental crust by mantle-derived basaltic magmas. The end of magmatic activity on both volcanic segments at 3 Ma is thought to result from the collision of Timor with the Wetar segment of the Sunda arc.

Origin of MORB enrichment and relative trace element compatibilities along the Mid‐Atlantic Ridge between 10° and 24°N

The origin of chemically and isotopically enriched basalts along mid-ocean ridges (E-MORB) has been the subject of recent interest, particularly in cases where they are not related to a specific mantle plume. We present new isotope dilution analyses for Th, U, La, Ce, and Pb in 26 mostly glassy tholeiite samples dredged from the Mid-Atlantic Ridge (MAR) between 10° and 24°N. We interpret these data in combination with other previously published trace element data and isotope ratios for the same samples. We focus on the region at 14°–15°N, where incompatible trace element abundances are enriched by as much as factors of 40, relative to the depleted basalts found to the north and the south of this region. We invoke simple recycling of alkali basalts found on oceanic islands and seamounts as an alternative E-MORB origin to a recently published metasomatic enrichment model. A small amount of recycled alkali basalt is added to the local depleted mantle peridotite. Melting of such mixed sources can produce the observed E-MORB at 14°–15°N MAR and elsewhere. The bulk partition coefficients of Nb, Ta, K, and Pb (relative to other trace elements) are geochemically important because these elements are widely used as tracers of source chemistry. We evaluate their partitioning using simple log-log plots, and we compare the 10°–24° MAR basalts with a similar suite from the CIR 18°–20°S. In both suites, Nb (bulk) partitioning is close to Th, and Pb is close to Pr. Tantalum partitioning is also very close to Nb in the MAR setting but between U and K in the CIR setting. Potassium is slightly more compatible than U in the MAR suite but less compatible in the CIR setting. Thus, although the bulk partition coefficients of these elements do vary slightly in different regions, their overall consistency is remarkable considering that the partitioning of the individual elements is governed by different mineral phases.

Intraplate versus ridge volcanism on the Pacific‐Antarctic Ridge near 37°S–111°W

Exploration of the Foundation Volcanic Chain (33°S–131°W; 37°S–111°W) revealed the existence of different magmatic provinces with relation to their geological settings. (1) The Pacific-Antarctic Ridge (PAR) is made up of several en echelon segments where both glassy midocean ridge basalts (MORBs) with low incompatible elements (K2O 150 ppm) and Ce (>48 ppm)) at about 306–1300 km from the PAR axis, (4) The Old Pacific Seamounts built on a crust older than 23 m. y. located west of longitude 124°W (>1300 km from the PAR axis) consist of T and EMORB. On the PAR axis, extensive crystal fractionation (>65%) produced the silicic lavas. On the basis of Pacific plate reconstruction using a half spreading rate of about 50 mm/yr and integrating the observed compositional changes with respect to the structural settings, it is inferred that the last volcanic events giving rise to the FS took place at about 110 km from the PAR axis about 5 m. y. ago. The Oblique Ridges built between 5 m. y. and 23 m. y.) with MORB volcanics comparable to those of the the Oblique Ridge-PAR provinces, could also have been formed by an interaction between the Foundation Seamount (dredge site 28) hotspot magmatism and that of an ancient accreting ridge magmatism precursor of the PAR.

The provenance of sediments in the Gulf of Lions, western Mediterranean Sea

In this study, we undertook a reconnaissance study of sediments provenance in the Gulf of Lions focusing over the last 16 ka. We used geochemical and isotopic tracers to determine the source of sediments and give insight into the weathering conditions prevailing. Sediments samples were selected both onshore and offshore from the western, eastern, and central part of the Gulf of Lions. We analyzed bulk sediments, coarse and fine silt, and clay fractions. Elemental and Nd isotope compositions appeared to differ from one grain size fraction to another one. These are interpreted in terms of zircon addition in the coarse silt fraction for the elemental concentrations and variable sources influences for the Nd isotope compositions. Our results indicate that sediments in the Gulf of Lions mainly originated from the Rhone River watershed although a contribution of Saharan dust is seen in one sample. Influence of Pyrenean small rivers is minor in these samples. Some Sr isotope compositions shifts are interpreted as reflecting variable amounts of chemical weathering that are consistent with published paleoclimatic reconstructions.

Helium isotopic textures in Earth’s upper mantle

We report 3He/4He for 150 mid-ocean ridge basalt (MORB) glasses from the Southeast Indian Ridge (SEIR). Between 81°E and 101°E 3He/4He varies from 7.5 to 10.2 RA, encompassing more than half the MORB range away from ocean island hot spots. Abrupt transitions are present and in one case the full range occurs over ∼10 km. Melting of lithologically heterogeneous mantle containing a few percent garnet pyroxenite or eclogite leads to lower 3He/4He, while 3He/4He above ∼9 RA likely indicates melting of pyroxenite-free or eclogite-free mantle. Patterns in the length scales of variability represent a description of helium isotopic texture. We utilize four complementary methods of spectral analysis to evaluate this texture, including periodogram, redfit, multitaper method, and continuous wavelet transform. Long-wavelength lobes with prominent power at 1000 and 500 km are present in all treatments, similar to hot spot-type spectra in Atlantic periodograms. The densely sampled region of the SEIR considered separately shows significant power at ∼100 and ∼30-40 km, the latter scale resembling heterogeneity in the bimodal distribution of Hf and Pb isotopes in the same sample suite. Wavelet transform coherence reveals that 3He/4He varies in-phase with axial depth along the SEIR at ∼1000 km length scale, suggesting a coupling between melt production, 3He/4He and regional variations in mantle temperature. Collectively, our results show that the length scales of MORB 3He/4He variability are dominantly controlled by folding and stretching of heterogeneities during regional (∼1000 km) and mesoscale (∼100 km) mantle flow, and by sampling during the partial melting process (∼30 km).

OIB/seamount recycling as a possible process for E-MORB genesis

[1] This paper deals with the origin of enriched MORB independent from any hot spot activity. Indeed, MORB enrichment was readily attributed to a ridge/hot spot interaction and in absence of identified neighboring hot spot, to more questionable processes (e.g., incipient plume or plume activity residue). More recently, the existence of enriched MORB away from any identifiable hot spot was attributed to different origins (i.e., recycled oceanic crust and/or enriched mantle after subduction metasomatism). Within this frame, we present here a new set of geochemical analyses of major and trace elements and Sr, Nd and Pb isotopes on samples collected by submersible on both intersections of the 15 20′N fracture zone with the spreading axis of the Mid-Atlantic Ridge. This area is characterized by an increasing enrichment of the lava compositions from north to south through the fracture zone. Results show that the geochemical enrichment observed with a different intensity on both sides of the fracture zone is linked to the 14 N topo-graphic and geochemical anomaly. Our modeling shows that both trace element and isotopic compositions are consistent with a binary mixing between the regional depleted MORB mantle source and a recycled OIB/seamount, as previously proposed to explain the observed enrichment at 14 N. This model can also account for other enriched MORB i.e., the 18 –20 S region of the Central Indian Ridge, illustrating that it does not represent an isolated and local process. On the basis of our results and on the DMM isotopic evolution, the age of the recycled OIB/seamount is estimated to be

Building of the Amsterdam-Saint Paul plateau: A 10 Myr history of a ridge-hot spot interaction and variations in the strength of the hot spot source

250 Ma, suggesting a recycling within the upper mantle. Considering the huge number of ocean islands and seamounts upon the ocean floor, their recycling into the upper mantle is a plausible process to produce enriched MORB.

The Amsterdam-St. Paul Plateau: A complex hot spot/DUPAL-flavored MORB interaction

The Amsterdam-Saint Paul plateau results from a 10 Myr interaction between the South East Indian Ridge and the Amsterdam-Saint Paul hot spot. During this period of time, the structure of the plateau changed as a consequence of changes in both the ridge-hot spot relative distance and in the strength of the hot spot source. The joint analysis of gravity-derived crust thickness and bathymetry reveals that the plateau started to form at similar to 10 Ma by an increase of the crustal production at the ridge axis, due to the nearby hot spot. This phase, which lasted 3-4 Myr, corresponds to a period of a strong hot spot source, maybe due to a high temperature or material flux, and decreasing ridge-hot spot distance. A second phase, between similar to 6 and similar to 3 Ma, corresponds to a decrease in the ridge crustal production. During this period, the hot spot center was close to the ridge axis and this reduced magmatic activity suggests a weak hot spot source. At similar to 3 Ma, the ridge was located approximately above the hot spot center. An increase in the hot spot source strength then resulted in the building of the shallower part of the plateau. The variations of the melt production at the ridge axis through time resulted in variations in crustal thickness but also in changes in the ridge morphology. The two periods of increased melt production correspond to smooth ridge morphology, characterized by axial highs, while the intermediate period corresponds to a rougher, rift-valley morphology. These variations reveal changes in axial thermal structure due to higher melting production rates and temperatures.

Correlated patterns in hydrothermal plume distribution and apparent magmatic budget along 2500 km of the Southeast Indian Ridge

The Amsterdam-St Paul (ASP) oceanic plateau results from the interaction between the ASP hot spot and the Southeast Indian ridge. A volcanic chain, named the Chain of the Dead Poets (CDP), lies to its northward tip and is related to the hot spot intraplate activity. The ASP plateau and CDP study reveals that ASP plume composition is inherited from oceanic crust and pelagic sediments recycled in the mantle through a 1.5 Ga subduction process. The ASP plateau lavas have a composition (major and trace elements and Sr-Nd-Pb-Hf isotopes) reflecting the interaction between ASP plume and the Indian MORB mantle, including some clear DUPAL input. The Indian upper mantle below ASP plateau is heterogeneous and made of a depleted mantle with lower continental crust (LCC) fragments probably delaminated during the Gondwana break-up. The lower continental crust is one of the possible reservoirs for the DUPAL anomaly origin that our data support. The range of magnitude of each end-member required in ASP plateau samples is (1) 45% to 75% of ASP plume and (2) 25% to 55% of Indian DM within 0% to a maximum of 6% of LCC layers included within. The three end-members involved (plume, upper mantle and lower continental crust) and their mixing in different proportions enhances the geochemical variability in the plateau lavas. Consequently, the apparent composition homogeneity of Amsterdam Island, an aerial summit of the plateau, may result from the presence of intermediate magmatic chambers into the plateau structure.