Serge Fourcade

Origin of anomalous rare-earth element and yttrium enrichments in subaerially exposed basalts: Evidence from French Polynesia

Abstract Basalts from French Polynesian islands occasionally display extremely high abundances and anomalous distributions of rare-earth elements (REE) and yttrium, whereas other incompatible element concentrations and O, Sr, Nd and Pb isotopic ratios do not differ from those of “normal” basalts from the same area. The REE- and Y-enriched basalts contribute up to 15% of the sample set, suggesting that this feature is more widespread than previously thought. REE-Y enrichment is related to the presence of rhabdophane-type REE-Y-phosphate identified through electron microprobe analyses in the most enriched sample and inferred from leaching experiments in the others. This phenomenon is confined to subaerially exposed basaltic sequences, indicating a close relationship to supergene processes. This is supported by negative Ce anomalies in these basalts, since decoupling of Ce from the other REE is restricted to oxidizing, low-temperature, aqueous environments. Similar Nd isotopic ratios for enriched and normal basalts allow us to exclude the possibility that additional REE and Y are derived from marine sediments or guano, but rather suggest an origin from the local basalts. Moreover, light REE enrichment in the REE-Y-phosphates suggests short migration distances of the fluids, supporting the conclusion that additional REE and Y were mobilized from weathered basalts and transported by descending meteoric waters.

THE INFLUENCE OF ALTERATION ON THE TRACE-ELEMENT AND ND ISOTOPIC COMPOSITIONS OF KOMATIITES

Abstract To investigate the effects of hydrothermal alteration and metamorphism on the chemical and isotopic compositions of komatiites, we studied samples from the Alexo and Texmont regions in the 2.7-Ga Abitibi belt of Canada and from the Weltevreden, Mendon and Komati Formations of the 3.2–3.5-Ga Barberton belt of South Africa. Particular emphasis was placed on multiple samples from individual layered spinifex-olivine cumulate flows, the argument being that if these flows showed variations in the ratios of elements incompatible with olivine, then these variations were most likely due to chemical mobility during alteration. Data for rare-earth (REE) and high-field-strength elements (HFSE) reveal anomalies (non-chondritic HFSE/REE ratios), particularly in the most altered samples. After taking into account the limits of analytical precision, these anomalies are attributed to element mobility, not to fractionation of high-pressure minerals, as suggested by other authors for Abitibi komatiites. In Barberton komatiites three separate processes produced HFSE anomalies: element mobility during alteration, crust assimilation in some samples, and majorite fractionation. The effects of the latter process were recognized from systematic relationships between HFSE/REE and Al/Ti that coincided with calculated majorite fractionation trends. Initial Nd isotopic compositions of the komatiites were determined by analysis of magmatic pyroxenes. The initial ϵNd of pyroxene from the 2.7-Ga Alexo komatiite is +3.8, slightly higher than in whole rocks ( +0.6 to +3.5, average ∼ +2.5). In the carbonatized Texmont samples, the range in initial ϵNd is far greater ( −6.2 to +8.8). The variable initial ϵNd-values in the rocks are attributed to isotopic exchange of Nd with surrounding rocks during early alteration, and fractionation of Sm/Nd during later events. Pyroxene from 3.3-Ga Barberton komatiitic basalt had initial ϵNd of + 2.3.

Post-collisional transition from calc-alkaline to alkaline volcanism during the Neogene in Oranie (Algeria): magmatic expression of a slab breakoff

Abstract During the Neogene, a magmatic change from calc-alkaline to alkaline types occurred in all the regions surrounding the western Mediterranean. This change has been studied in Oranie (western Algeria). In this area, potassic to shoshonitic calc-alkaline andesites (with La/Nb ratios in the range 4–6) were mainly erupted between 12 and 9 Ma. They were followed (between 10 and 7 Ma) by basalts displaying geochemical features which are transitional between calc-alkaline and alkaline lavas (La/Nb=1–1.7). After a ca. 3-Ma quiescence period, volcanic activity resumed, with the eruption of OIB-type alkaline basalts (La/Nb=0.5–0.6), from 4 to 0.8 Ma. A combined geochemical approach, using incompatible elements and Sr, Nd and O isotopes, allows us to conclude that the transitional basalts derived from the melting of a heterogeneous mantle source, at the boundary between lithosphere and asthenosphere. We propose that melting of a previously subduction-modified lithospheric mantle occurred between 12 and 10 Ma, in response to the upwelling of hot asthenosphere flowing up into an opening gap above a detached sinking slab. As a result, calc-alkaline magmas were formed. From 10 to 7 Ma, the transitional basalts were generated through melting of the boundary mantle zone between the lithosphere and the upwelling asthenosphere. During that stage, the contribution of the lithospheric source was still predominant. Then, as sinking of the oceanic slab progressed, the increasing uprise of the asthenosphere led to the formation and emplacement (from 4 to 0.8 Ma) of typical within-plate alkaline basalts derived from a plume-modified asthenospheric mantle.

A detailed Th, Sr and O isotope study of Hekla: differentiation processes in an Icelandic Volcano

Abstract238U−230Th disequilibria and Sr and O isotope ratios have been measured in a suite of samples from most of the known prehistoric and historic eruptions of Hekla volcano, Iceland. They cover the compositional range from basaltic andesite to rhyolite. Recent basalts erupted in the vicinity of the volcano and a few Pleistocene basalts have also been studied. Geochemical data indicate that the best tracers of magmatic processes in Hekla are the (230Th/232Th) and Th/U ratios. Whereas most geochemical parameters, including Sr, Nd and O isotopes, could be compatible with crystal fractionation, (230Th/232Th) and Th/U ratios differ in the basalts and basaltic andesites (1.05 and 3.2, respectively) and in the silicic rocks, dacites and rhyolites (0.98 and 3.4–3.7, respectively). This observation precludes fractional crystallization as the main differentiation process in Hekla. On the basis of these results, the following model is proposed: basaltic magmas rise in the Icelandic crust and cause partial melting of metabasic rocks, leading to the formation of a dacitic melt. The basaltic magma itself evolves by crystal fractionation and produces a basaltic andesite magma. The latter can mix with the dacitic liquid to form andesites. At higher levels in the magma chamber, the dacitic melt sometimes undergoes further differentiation by crystal fractionation and produces subordinate volumes of rhyolites. Together all these processes lead to a zoned magma chamber. However, complete zoning is achieved only when the repose time between eruptions is long enough to allow the production of significant volumes of dacitic magma by crustal melting. This situation corresponds to the large plinian eruptions. Between these eruptions, the so-called intra-cyclic activity is characterized by the eruption of andesites and basaltic andesites, with little crustal melting. The magmatic system beneath Hekla most probably was established during the Holocene. The shape and the size of the magma chamber may be inferred from the relationships between the composition of the lavas and the location of the eruption sites. In a cross-section perpendicular to Heklas ridge, a bell-shaped reservoir 5 km wide and 7 km deep appears the most likely; its top could be at depth of 8 km according to geophysical data.

Origin of silicic magma in Iceland revealed by Th isotopes

Th, Sr, Nd, and O isotopes have been determined in a suite of volcanic rocks from Hekla and in a few samples from Askja and Krafla volcanic centers in Iceland. Although {sup 87}Sr/{sup 86}Sr and {sup 143}Nd/{sup 144}Nd ratios are nearly the same for all compositions at Hekla, the ({sup 230}Th/{sup 232}Th) ratios differ and thus clearly show that the silicic rocks cannot be derived from fractional crystallization of a more primitive magma. Similar results are obtained for the Krafla and Askja volcanic centers, where the {delta}{sup 18}O values are much lower in the silicic magma than in the mafic magma. These data suggest that large volumes of silicic rocks in central volcanoes of the neovolcanic zones in Iceland are produced by partial melting of the underlying crust.

New geochemical constraints for the origin of ridge-subduction-related plutonic and volcanic suites from the Chile triple junction (Taitao peninsula and Site 862, LEG ODP141 on the Taitao ridge)

Abstract Several features of Neogene and Quaternary magmatism in the region south of the present-day Chile Triple Junction (CTJ) at 46°12′S are directly related to the migration of the triple junction. Due to the obliquity of the ridge orientation with respect to the subduction front, the triple junction migrated from South to North during the last 14 Ma. The Taitao Peninsula — the westernmost promontory of the Chile coast — and the Taitao Ridge — a submarine promontory north of the Taitao Peninsula — provide the most complete collection of ridge subduction-related magmatic products in the region. The emplacement of near-trench volcanics, the intrusion of a variety of plutonic rocks and the related hydrothermal activity at these two sites have been interpreted as resulting from magma interactions between subducted ridge segments of the Chile spreading centre and the continental crust. We present new field observations and geochemical data that help to better constrain the problem of the sources and evolution of the Taitao magmas. The new geochemical data were obtained on samples collected from the Taitao Peninsula during a field expedition in 1995, and from samples of the Taitao Ridge during Leg ODP141, Site 862, which have been re-sampled in 1996 by one of us. Selected major- and trace-element compositions of 20 volcanic rocks from the Taitao Ridge are discussed together with 53 analyses from different rock types from the Taitao Peninsula including 24 unpublished analyses. Nd and Sr isotopic compositions were obtained from 5 whole rocks and separated minerals of the Taitao Peninsula together with the oxygen isotope composition of four separated clinopyroxenes. Six main magmatic types are identified: (1) N-type MORB; (2) E-type MORB; (3) LREE-depleted MORB showing some trace-element features typical of arc basalts; (4) moderately Nb-depleted E-MORB; (5) calc-alkaline andesites, dacites and rhyolites; and (6) andesites and dacites with adakitic signature. Chemical similarities exist between some forearc magmas of the Taitao Ridge and the Taitao Peninsula and magmas emplaced at the Chile active spreading ridge. One important result, based on isotope data, is that the lavas emplaced over the continental crust (Taitao Peninsula) did not originate from melting of continental crust nor from extensive assimilation of such a crust by mantle-derived magmas. The likely source of these basalts could be the hot convective oceanic mantle of the southern Chile spreading ridge buried at moderate depth (10–30 km).

Mantle and crustal contribution in the genesis of Recent basalts from off-rift zones in Iceland: Constraints from Th, Sr and O isotopes

Along the two volcanic off-rift zones in Iceland, the Sn˦fellsnes volcanic zone (SNVZ) and the South Iceland volcanic zone (SIVZ), geochemical parameters vary regularly along the strike towards the centre of the island. Recent basalts from the SNVZ change from alkali basalts to tholeiites where the volcanic zone reaches the active rift axis, and their87Sr/86Sr andTh/U ratios decrease in the same direction. These variations are interpreted as the result of mixing between mantle melts from two distinct reservoirs below Sn˦fellsnes. The mantle melt would be more depleted in incompatible elements, but witha higher3He/4He ratio (R/Ra≈ 20) beneath the centre of Iceland than at the tip of the Sn˦fellsnes volcanic zone (R/Ra≈ 7.5). From southwest to northeast along the SIVZ, the basalts change from alkali basalts to FeTi basalts and quartz-normative tholeiites. TheTh/U ratio of the Recent basalts increases and both (230Th/232Th) andδ18O values decrease in the same direction. This reflects an important crustal contamination of the FeTi-rich basalts and the quartz tholeiites. The two types of basalts could be produced through assimilation and fractional crystallization in which primary alkali basaltic and olivine tholeiitic melts ‘erode’ and assimilate the base of the crust. The increasingly tholeiitic character of the basalts towards the centre of Iceland, which reflects a higher degree of partial melting, is qualitatively consistent with increasing geothermal gradient and negative gravity anomaly. The highest Sr isotope ratio in Recent basalts from Iceland is observed inOr˦fajokull volcano, which has a3He/4He ratio (R/Ra≈ 7.8) close to the MORB value, and this might represent a mantle source similar to that of Mauna Loa in Hawaii.

Mixing of metamorphic and surficial fluids during the uplift of the Hercynian upper crust: consequences for gold deposition

A detailed geochemical study of fluids from representative quartz-sealed faults hosting late Hercynian gold concentrations shows that fluids percolating the mineralised faults had two main distinct reservoirs: one was a quite shallow and the other rather deep-seated. Both fluids have lost a great part of their original geochemical signature through interactions with host metamorphic formations. Early fluids, present during the primary sealing of the faults by quartz, are considered to have effectively equilibrated with the metamorphic pile and then predominantly flowed upwards along the faults. They are characterised by CH4/CO2/H2O ratios rather typical of fluids equilibrated with graphite, and moderate to medium chlorinities with a high Br/Cl ratio. The striking feature of the gold-bearing quartz is that gold is not synchronous within any quartz deposition, and appears located in late microfractures and associated with Pb–Bi–Sb sulphosalts and sulphides. These late stages are characterised by fluids whose salinities decrease to very low values indicating their progressive dilution by waters of more surficial origin in the fault system. The long-lived activity of the fault favoured the connection between two distinct fluid reservoirs at a critical time during the basement uplift. The fluids evolved through two main driving mechanisms which were responsible for the Au deposition: (i) decrease in temperature accompanying decompression from supra-lithostatic to hydrostatic conditions which yielded, in some instances, volatile unmixing in the faulted systems, (ii) mixing of the resulting fluids with waters entering the hydrological systems from shallower reservoirs. In addition to dilution and fluid mixing which are favourable factors for decreasing the gold solubility, the presence of microfractured sulphides could have enhanced gold precipitation through electrochemical processes. D 2002 Elsevier Science B.V. All rights reserved.

Loss of isotopic (Nd, O) and chemical (REE) memory during metamorphism of komatiites: new evidence from eastern Finland

Komatiites are often considered to depict the chemical and isotopic composition of their source rocks in the Archean mantle. However, a weakness of these rocks in tracking the initial compositional heterogeneity of the Earths mantle is the ubiquitous presence of metamorphic recrystallization, which casts some doubt about the preservation of primary chemical and isotopic characteristics. Two spinifex-textured komatiite flows from the 2.75 Ga old Kuhmo greenstone belt (Siivikkovaara area) of eastern Finland document this weakness. Both flows have experienced low to medium grade metamorphism (T=450±50°C), and now consist entirely of secondary metamorphic assemblages of amphibole±chlorite±plagioclase, with minor proportions of magnetite and ilmenite. MgO contents range from 25 to 8%, which suggests that low pressure differentiation was likely controlled by olivine and clinopyroxene fractional crystallization. However, neither major nor trace elements fall on olivine and/or clinopyroxene control lines. This is particularly well illustrated by the REE as there is an overall 60% variation of (Ce/Sm)N ratios (0.38 to 0.91), which far exceeds that expected from olivine and clinopyroxene fractionation alone. In fact, careful evaluation of petrographic (including mineral composition data) and chemical characteristics shows that most elements of geological interest (including the reputedly immobile REE) were mobile on a whole-rock scale during metamorphic recrystallization of these two flows. This view is fully supported by Sm-Nd isotopic data since both whole-rock and mineral (amphibole and plagioclase) samples lie on a single isochron relationship at T≈ 1800 Ma, an age which corresponds to the time of regional metamorphism. Thus, the meta-komatiite flows from Siivikkovaara document a case of komatiite flow units in which metamorphism has induced whole-rock scale resetting of primary REE patterns and Sm-Nd isotope systematics. As regards the nature of the mechanism responsible for this resetting, it is suggested here that the secondary mineralogy played an important role, as there are correlations between whole-rock 147Sm/144Nd ratios, major and trace element chemistry, oxygen isotopic ratios and modal proportions of secondary minerals. Consideration of oxygen isotopic data, as well as previous results from a komatiite flow from the nearby Tipasjärvi belt, further enable us to propose that the REE carrier was likely a CO2-rich fluid. The process of secondary REE redistribution prevents estimation of the true initial Nd isotopic compositions of the two flows. Taken as a whole, the results presented in this paper show that great care should be be exercised in the use of meta-komatiites as probes of Archean mantle composition.

The origin of fluids and the effects of metamorphism on the primary chemical compositions of Barberton komatiites: New evidence from geochemical (REE) and isotopic (Nd, O, H, 39Ar40Ar) data

Numerous greenstone relics, all containing the two lowermost formations of the Onverwacht Group, occur in the Archean trondhjemitic/tonalitic gneiss terrains south of the Barberton Greenstone Belt. In this study, we report detailed petrological, geochemical and isotopic (Nd, O, H, 40Ar39Ar) data obtained on komatiites from the Schapenburg Greenstone Remnant (SGR), the largest and best-preserved greenstone relic. The main goals are 1. (1) to date the metamorphism affecting the SGR using the 40Ar39Ar dating method on amphiboles, 2. (2) to evaluate the effect of metamorphism on the preservation of primary isotopic and chemical signatures, 3. (3) to estimate the temperature and water/rock ratios that prevailed during metamorphic recrystallization in order to constrain the composition and origin of the reacting fluid phase. 40Ar39Ar ages of 2.9 Ga obtained on two amphibole separates from the Schapenburg metavolcanics reveal the existence of a metamorphic event younger than the emplacement age (3.5 Ga). This metamorphic event belongs to a series of discrete periods of thermal activity from 3.4 to 2 Ga, each of which coincides with a major episode of magmatic activity. The ultrabasic lava flows acquired their δ18O values (from +3.2 to +5%.) at high temperature (≈450°C) under high water/rock ratios. The reacting water had initial isotopic values typical of metamorphic fluids (δ18O = +5 to +7%.; δD = −65 to −50%.). REE patterns were not disturbed by metamorphic recrystallization. Despite the long time interval between emplacement and metamorphism (≈600 Ma), ϵNd(T) values are uniform throughout the whole magmatic suite, indicating that the Sm-Nd system was closed on the sample scale during metamorphism. The mantle source of these greenstones was depleted in LREE as evidence by ϵNd(T) ≈ +2.5. Chemical fluxes during metamorphism were calculated for elements unfractionated by olivine removal (e.g. Na, Ca, Ti, Al, and Sr), by normalizing to Nd. They suggest a significant mobility of most major elements in the cumulate zones of the lava flows. By contrast, spinifex zones appear to have preserved most of their primary chemical signatures during metamorphic recrystallization. Their CaOAl2O3 and Al2O3TiO2 ratios can be used with confidence to determine the PT conditions of melting in the mantle source. A general model of water-rock interactions applied to the sedimentary and magmatic rocks of the Onverwacht Group is also presented. The model involves conditions of metamorphism deduced from this study and available data from the literature. In addition to metavolcanic rocks, most of the oxygen isotope compositions of carbonates and cherts (except the highest values) can be explained by reequilibration with metamorphic fluids under greenschist-amphibolite facies conditions.