C. Dupuy

Zr/hf fractionation in intraplate basaltic rocks: Carbonate metasomatism in the mantle source

In continental and oceanic intraplate basaltic rocks, the ZrHf ratio is frequently higher than the chondritic value and varies over a large range of 38–87. The ratio increases with the LaSm ratio and, in each volcanic locality, is positively correlated with the degree of silica undersaturation of the basaltic rocks. The variation of the ZrHf ratios probably reflects a heterogeneous mantle source variably metasomatized by carbonate-rich fluids.

Trace element geochemistry of some continental tholeiites

Abstract Continental tholeiites from four regions (Late Cretaceous to Early Eocene Deccan Trap lavas of central India, Early Mesozoic tholeiites from the Atlantic margins of Northwest Africa-Morocco, and northeastern North America-Nova Scotia, Canada and Precambrian Coppermine River basalts from Northwest Territories, Canada) differ from MORB by higher concentrations of K, Rb, Ba and Th and to a lesser degree light REE. Their chondrite-normalized trace element patterns show negative Nb anomalies. The distribution and variation of trace elements indicate that the rocks from all the areas studied were affected by interaction with the continental crust. It is suggested that continental tholeiites have been generated from a similar source as oceanic tholeiites and many of their geochemical differences are related to crustal contamination.

Geochemistry and petrogenesis of Eastern Pyrenean peridotites

Abstract The high-temperature peridotite bodies of the Eastern Pyrenees (France), which are composed of spinel peridotites containing bands of pyroxenites and veins of amphibole-bearing ultrabasic rocks, have gone through a multi-stage evolution. The peridotites underwent partial melting in the stability field of garnet resulting in major variations of Mg, Al, Ca, Na, Ti, Sc, V, Ni and HREE. Then the peridotite residue was invaded by basaltic melts. The pyroxenite bands in the peridotites are high-pressure crystal segregates from these melts. Subsequently, after cooling in subcontinental lithospheric conditions, the peridotites interacted with alkali magma which was probably associated with the Cretaceous alkali magmatism of the Pyrenees. In addition to the crystallization of amphibole-rich ultrabasic rocks in vein-conduits and the re-equilibration of the wall-rock peridotites leading to LREE, Ti and Fe enrichments, this event was accompanied by extensive metasomatic processes. The metasomatism locally affected lherzolites, producing an increase of the modal proportions of clinopyroxene (± amphibole) (Caussou massif). The metasomatism was more widespread in the harzburgites where it produced an enrichment of LREE relative to HREE without a significant change in the modal composition.

Petrogenesis of layered pyroxenites from the Lherz, Freychinéde and Prades ultramafic bodies (Ariége, French Pyrénées)

The pyroxenite bands in spinel Iherzolites from three high-temperature peridotite massifs (Lherz, Freychinede and Prades) in Ariege (French Pyrenees) have geochemical features including distributions of Mg, Ca, K, Ti, Zr and REE typical of cumulates derived by high P−T crystal segregation. However, these features of the whole-rocks are not reflected in the element distribution among the minerals which was partially modified during subsequent recrystallization. In addition, samples from the margins of the pyroxenite bands were affected by secondary processes such as metasomatism or subsolidus re-equilibration. Unlike pyroxenite xenoliths from basalts which are usually assumed to be derived from alkalic basaltic magmas, the pyroxenite layers are related to tholeiitic magmas similar to the parental liquids of the Triassic dolentes encountered throughout the Pyrenees. The data show that high P−T fractionation significantly affected the ascending tholeiitic magma and suggest that the continental tholeiites could have been derived from an upper mantle source with a chondrite shaped REE pattern.

Carbonate metasomatism in the lithospheric mantle: peridotitic xenoliths from a melilititic district of the Sahara basin

The petrological and geochemical study of harzburgitic and dunitic xenoliths from the melilititic district of In Teria (Algerian Sahara) shows that the lighospheric mantle of this region has been affected by a multi-stage metasomatism. The first metasomatic event is related to the injection of alkali silicated (basaltic or kimberlitic) melt and was responsible for the crystallization of phlogopite at depths ranging between 80 and 100 km and the crystallization of amphibole at about 60 km. During this first event, carbonate probably precipitated in the garnet stability field. In a second stage, the spinal peridotites suffered strong mineral changes resulting in an extensive formation of high-Cr endiopside and leading to conversion of harzburgite and dunite into lherzolite and wehrlite. These changes are associated with an enrichment in the most incompatible trace elements including light REE (rare-earth elements), Ta, Th and variable values of ratios such as Th/La and Ta/La. This second event is atributed to the injection (under conditions of decarbonatation and release of CO2) of a carbonatitic melt resulting from incipient melting of the garnet peridotites, which were previously carbonated. This interpretation is corroborated by the calculation of a diffusion-percolation model which reproduces well the observed distribution of incompatible trace elements in the spinel peridotites. Given the proposed sequence of events, it appears that most of the specificities of the In Teria xenoliths can be explained by the successive geochemical modifications induced within the lithospheric mantle during reheating.

Geochemistry of early mesozoic tholeiites from Morocco

Abstract Mesozoic dolerites from two areas of Morocco, the High Atlas fold belt between Marrakech and Demnat and the Anti-Atlas belt in the area of Foum Zquid, are most high-Ti quartz-normative tholeiites whichi in many respects resemble Mesozoic dolerite dikes from eastern North America. The dolerites display a wide range of major and trace element compositions, some of which are due to fractional crystallization. The doleritic sequences from High Atlas also show vertical stratigraphic zonation which is characterized by a progressive depletion of lithophile elements toward the top. This trend together with regularities of trace element ratio variations are indicative of a dynamic melting of an initially homogeneous source. It is suggested that the continental upper mantle source for dolerites of Morocco was enriched in several incompatible elements in comparison with the upper mantle source for ocean floor tholeiites.

Geochemistry and origin of basaltic lavas from Marquesas Archipelago, French Polynesia

The Marquesas Archipelago, a volcanic chain in French Polynesia (south-central Pacific Ocean), is predominantly composed of alkalic, transitional and tholeiitic basalts. The variation trends in these intraplate basaltic rocks imply that the magmas were derived from different upper mantle sources. Model calculations using the total inverse method show that the peridotite source of most Marquesas basalts was enriched in incompatible elements compared to a primordial mantle and had higher than chondritic ratios of several elements such as La/Yb, Ti/V and P/Ce. A metasomatic enrichment event is suggested by the sequence of element enrichment in the source relative to the primordial mantle (Ba>Nb>La>Ce>Sr>Sm>Eu> Zr>Hf>Ti>Y>Yb). On the other hand, some lavas including tholeiites of Ua Pou and alkalic basalts of Hiva Oa, were probably derived from relatively depleted upper mantle. In some islands such as Hatutu, the different types of basalts were generated from sources with rather similar compositions. The residual phases of the Marquesas magmas included garnet. The sources of these magmas were similar in trace element chemistry to the oceanic mantle below Hawaii.

Behaviour of rare earth elements at the freshwater–seawater interface of two acid mine rivers: the Tinto and Odiel (Andalucia, Spain)

The concentrations of dissolved and suspended particulate rare-earth elements (REE) are reported in acid-sulphate waters from the Odiel and Tinto rivers. Shale normalized patterns are typically convex and high REE concentrations (e.g., Ce=0.43–65 μg.l−1) are present in the waters. The REE content of the suspended load is greater by a factor of up to 3000. In the Odiel river, REE patterns of the particulates are essentially convex and sub-parallel to those of the waters; speciation calculations indicate that SO4 complexes play a dominant role in controlling the REE distributions. In the Tinto river, the REE patterns of the suspended load are slightly fractionated and a negative Ce anomaly is apparent in several samples, reflecting the local influence of phosphogypsum deposits. Contrasting with normal estuaries, REE are not intensely removed in the low chlorinity zone. A remobilization in relation to Fe reduction is observed in the Tinto river.

Influence of sorption processes by iron oxides and algae fixation on arsenic and phosphate cycle in an acidic estuary (Tinto river, Spain)

Inorganic arsenic and phosphate distributions have been studied in the acidic mixing zone of the Tinto river in November 1996, June 1997, and April 1998. This mixing zone receives high inputs of As, PO4/3- and Fe in relation with acid mine drainage and run-off from phosphogypsum waste. In the early stage of mixing the dissolution of detritic Fe phases (pyrite and oxides) releases Fe and As to water. This process is counterbalanced by removal due to precipitation of Fe-oxides and Fe-organic complexes and algae fixation. In autumn, the amount of algae is negligible and a release of As, Fe and PO4/3- to the dissolved phase was observed. As a consequence, high As and PO4/3- concentrations are registered in the water (up to 43 nM for As and 330 μM for PO4/3-). In spring, the removal dominates in relation with high algae productivity. As a result As and PO4/3- are depleted in the dissolved phase in spring compared to autumn and high concentrations of As (up to 1530 μg g-1 and 700 μg g-1 in June and in April, respectively) were observed in the suspended particulate matter. In autumn dissolved As is correlated with Fe whilst in spring As behaves in a similar way as PO4/3-

Behaviour of trace elements during magmatic processes in the crust: Application to acidic volcanic rocks of Tuscany (Italy)

Abstract Plio-Quaternary acidic rock suites from three volcanic centres of Tuscany (Italy) — San Vincenzo, Roccastrada and Monte Amiata — are composed predominantly of rhyolitic and rhyodacitic ignimbrites. The rocks are characterized by distinct enrichments, particularly of K and Rb, and display considerable compositional differences among the various volcanic centres. The volcanic rocks of Roccastrada and most of S. Vincenzo were probably generated by partial melting of metapelitic rocks from the Paleozoic basement of Tuscany, leaving a residue composed of biotite, cordierite and quartz and sometimes plagioclase or garnet. The enrichment of K and related elements can be attributed to selective contamination by fluids. The other volcanic rocks including trachytes of Mt. Amiata have also been affected by interaction with an upper-mantle component, probably K-rich basaltic rocks. A similar petrogenetic model can be applied to the granodiorite pluton of Mt. Capanne (Elba) which belongs to the same magmatic cycle. Since the variation trends observed in the Tuscany volcanic rocks are similar to those reported from many acidic plutons, it appears that crustal anatexis accompanied by interaction with upper-mantle components has frequently played an important role in the genesis of the granitic rocks.