Philippe Vidal

HIMU−EM : the French Polynesian connection

Abstract himu, em i and em ii are three of the main geochemical mantle components that give rise to oceanic island basalts [1]. They represent the end members that produce the extreme isotopic compositions measured on intraplate volcanics. In French Polynesia, all three mantle components are represented in volcanic rocks. The characteristic himu signature is found in Tubuai, Mangaia and Rurutu, em i is present in the source of Rarotonga and Pitcairn volcanics and em ii dominates the composition of most Society Islands. Intermediate values between the three end members are found on most islands. We suggest that the three components are not independent but are physically related in the mantle. The himu component is thought to be recycled oceanic crust that lost part of its Pb through hydrothermal processes prior to and during subduction. em i and em ii are believed to acquire their isotopic and trace element characteristics through entrainment of sediments that were subducted together with the oceanic crust. The trace element pattern and the isotopic composition of himu lavas can be quantitatively modelled using a mixture of ∼ 25% old recycled morb crust and 75% mantle peridotite. The extreme Pb composition is modelled assuming that Pb was lost from oceanic crust when hydrothermal alteration at the ridge leached Pb from the basalt to redeposit it as sulphides on top of and throughout the crust, followed by preferential dissolution of sulphides during dehydration in the subduction zone. These processes led to a drastic increase of the U/Pb ratio of the subducted material which evolved over 2 Ga to very radiogenic Pb isotopic compositions. Pb isotopic compositions similar to those of em i and em ii are modelled assuming that sediments with average crustal Pb isotopic compositions were subducted and recycled into the mantle together with the underlying morb oceanic crust. Pelagic sediments ( μ ∼ 5 and κ ∼ 6 ) account for the Pb isotopic composition of em i whereas terrigenous sediments ( μ ∼ 10 and κ ∼ 4.5 ) evolve towards the em ii end member. A few percent of sediment in the recycled crust-sediment mixture will destroy the characteristic Pb isotopic signature of the himu component. This, together with the low probability of isolating oceanic crust in the mantle for ⩾ 2 Ga, explains why the extreme himu composition, as seen on Tubuai and St Helena, is sampled so rarely by oceanic volcanism.

Isotope and trace element geochemistry of sediments from the Barbados Ridge-Demerara Plain region, Atlantic Ocean

Abstract Twenty-four piston core sediment samples and 13 sediments and 3 basalts from DSDP Leg 78 Site 543 were analyzed for Sr, Nd and Pb isotopic compositions. The results show sediment with highly radiogenic Pb 206 Pb 204 Pb up to 19.8) and rather radiogenic Sr and unradiogenic Nd has been deposited in the region since the Cretaceous. The source of this sediment is probably the Archean Guiana Highland, which is drained by the Orinoco River. Pb and Sr isotopic compositions and sediment thickness decrease and 143 Nd 144 Nd increases northward due to a decrease in turbiditic component. This decrease is partly due to the damming action of basement ridges. Rare earth concentrations in the sediments are somewhat low, due to the abundance of detrital and biogenic components in the sediment and rapid sedimentation rates. Both positive and negative Ce anomalies occur in the surface sediments, but only positive Ce anomalies occur in the Site 543 sediments. It is unlikely that sediment subducted to the source region of Lesser Antilles arc magmas could be the cause of negative Ce anomalies in those magmas. Isotopic compositions of Site 543 basalts show some effect of contamination by seawater-basalt reaction products and sediments. Beyond this, however, they are typical of “normal” depleted MORB.

Multi-chronometric ages and origin of Archaean tonalitic gneisses in Finnish Lapland: A case for long crustal residence time

The Tojottamanselkä gneisses of the Koitelainen region, northern Finland, have been dated by the Sm-Nd and the common Pb methods. The Sm-Nd data of seven samples from a small area (100 m × 100 m) define an isochron ofT=3.06±0.12 (2σ) Ga, with correspondingINd=0.50848±9 (2σ), orεNd(T)=−3.7±1.8. This age is in good agreement with the zircon U-Pb discordia age (3.1 Ga) reported by Kröner et al. (1981) and is interpreted as the time of magmatic emplacement. The distinctly negativeεNd(T) value is found for the first time for Archean tonalitic gneisses and implies derivation of these magmas by remelting of continental material with a long (200–500 Ma) crustal residence time. A few samples, on the other hand, possessεNd(T) values close to zero, hence they are thought to be derived by partial melting of basaltic sources with nearchondritic REE distribution patterns.Common Pb isotopic data yield an isochron age of 2.64±0.24 (2σ) Ga which is in agreement, within error limit, with the published Rb-Sr isochron age of 2.73±0.24 Ga (Kröner et al. 1981). The age of ca. 2.7 Ga is interpreted as the time of regional metamorphism during which both Pb and Sr isotopes were rehomogenised.The tonalitic gneisses have highly fractionated REE patterns with (La/Yb)N ratios varying from 9 to 43. Like most Archean gneisses of TTG composition (tonalite-trondhjemite-granodiorite), they could be derived by partial melting of crustal sources of basaltic to granodioritic compositions. Direct derivation by melting of mantle peridotites is excluded.The present geochemical study indicates that the Tojottamanselkä gneisses have had a very complex history that involved multi-stage development. Together with the published age data for the basement gneisses and greenstone belts of eastern central Finland (Vidal et al. 1980; Martin et al. 1983a), we conclude that the Archean crustal development in Finland started at least 3.5 Ga ago and passed through a series of magmatic and metamorphic events at 3.1, 2.85, 2.65 and 2.5 Ga before the final intrusions of K-rich granites about 2.4 Ga ago.

Basalts from Mururoa, Fangataufa and Gambier islands (French Polynesia): Geochemical dependence on the age of the lithosphere

Abstract Basaltic volcanics from Mururoa, Fangataufa and Gambier islands, which belong to the Pitcairn island chain, have been studied for trace elements and Sr, Nd and Pb isotopic composition. While the trace element ratios remain constant, the 87 Sr/ 86 Sr ratios range from 0.70292 to 0.70367, 143 Nd/ 144 Nd ranges from 0.51303 to 0.51288 and the 206 Pb/ 204 Pb isotopic composition ranges from 18.99 to 19.62. These values, which are clearly distinct from those of Pitcairn basalts, vary according to distance from the present hot spot and are correlated with the age of the underlying lithosphere. It is suggested that the basalts studied were produced by variable degrees of partial melting from an oceanic lithospheric plate metasomatized by prior passage over the Easter hot spot.

Precadomian relicts in the Armorican Massif: Their age and role in the evolution of the western and central European Cadomian-Hercynian belt

Abstract Well-dated Precambrian is mostly developed in the north of the Armorican Massif, in which area voluminous Cadomian magmatism is dated at between 650 and 550 Ma. Much older relicts occur at Cap de la Hague, in Guernsey and in the Tregor, and are also found in the northern continental margin of the Iberian Peninsula. In all these occurrences, whole-rock systems have been opened, so that the true ages cannot be determined by the Rb-Sr whole rock isochron method. Four U-Pb zircon ages are between 1.8 and 2 Ga (in Guernsey: Icart orthogneisses; in Tregor: Port Beni, Trebeurden, Morguignen orthogneisses). There is no evidence from strontium isotopes that these isolated and scattered relicts have a wide extension or that such ancient continental crust played an important role in magma genesis from 650 Ma to 270 Ma ago. On the contrary, the evolution of initial 87Sr/86Sr ratios with time shows that the observed mid- and west European continental crust is probably not older than 700 Ma. The increase of the initial 87Sr/86Sr ratios of the magmas with time suggests that, after its formation in Cadomian times, this segment of continental crust evolved virtually as a closed system and Hercynian magmatism arose principally from re-melting of relatively young sialic components.

Differential behaviour of the Rb-Sr and K-Ar systems of spilitic flows and interbedded metasediments: the spilite group of Erquy (Brittany, France). Paleomagnetic implications

The Erquy series (Côtes du Nord, France) consists, in its upper part, of spilitic pillow lavas with some interbedded volcano-sedimentary horizons.The Rb-Sr system of the pillows allowed the construction of a whole-rock isochron at 482±10 M.a. with an initial87Sr/86Sr ratio of 0.7055±0.0002. These rocks and the associated keratophyres give, on the other hand, K-Ar ages of 285±16 M.a. interpreted as the consequence of late-hercynian tectonism.A volcano-sedimentary horizon interbedded with such pillow flows has been studied from petrographic, mineralogical, geochemical and isotopic (Rb-Sr and K-Ar) points of view. The sequence keeps a sedimentary “memory”. Its clay fractions <2 μm and corresponding whole-rocks fit an isochron which is identical to that of the volcanic rocks: 494±11 M.a. with an initial87Sr/86Sr ratio of 0.7052±0.0005. The clay fractions give K-Ar data at about 450 M.a., but those which contain important amounts of volcanic glass, at the top of the horizon, have K-Ar values as low as 400 M.a., and those which contain almost no glass have a K-Ar age close to the Rb-Sr age at 480 M.a.This study emphasizes the possibility of a complete reset of the K-Ar system of spilitic rocks by a tectonic event without notice-able temperature increase. This result may have important implications on combined paleomagnetic and K-Ar studies: it seems that a least for spilites and keratophyres, the Curie point and Ar blocking temperature can be very different.