Annie Michard

Rare earth element systematics in hydrothermal fluids

Abstract Rare earth element concentrations have been measured in hydrothermal solutions from geothermal fields in Italy, Dominica, Valles Caldera, Salton Sea and the Mid-Atlantic Ridge. The measured abundances show that hydrothermal activity is not expected to affect the REE balance of either continental or oceanic rocks. The REE enrichment of the solutions increases when the pH decreases. High-temperature solutions (>230°C) percolating through different rock types may show similar REE patterns.

Crustal growth in West Africa at 2.1 Ga

Birimian (∼2.1 Ga) terranes in the West African craton are a mixture of highly metamorphosed volcanic, sedimentary and plutonic rocks and low grade metavolcanics and metasediments. The volcanic units contain thick, commonly pillowed, tholeiitic basalts overlain by pelagic sediments cherts and black shales; The sedimentary units are characterized by an abundance of clastic turbiditic sediments. Andesites and calc-alkaline felsic volcanics occur at uppermost stratigraphic levels and as dykes. Field relationships between the volcanic and sedimentary units remain a matter of debate. Calc-alkaline and local alkaline granites, which intruded in distinct pulses and occasionally are related to transcurrent tectonics, represent almost half of the Birimian terranes. New isotopic work on the highly metamorphosed units greatly improved the chronology for the Birimian crust. The age of the early Dabakalian event is precisely defined by a U-Pb zircon age at 2186 ± 19 Ma, while Rb-Sr and Sm-Nd methods give ages of 2162 ± 19 Ma and 2141 ± 24 Ma, respectively. A Sm-Nd garnet-whole rock age of 2153 ± 13 Ma suggests that metamorphism culminated at about the same time. In contrast, the most precise zircon U-Pb and Sm-Nd data for the more widespread Birimian terranes (sensu stricto), from this study and from the literature, cluster between 2.12 and 2.07 Ga. The major evolution of the Birimian crust apparently lasted less than 50 Ma. Isotopic evidence indicates that Birimian granitoids contain a negligible component of Archean crust: eNd(2.1-Ga) values are positive and similar to those of Birimian basalts, crustal residence times are shorter than 200 Ma, U-Pb ages for detrital zircons from clastic sediments range from 2098 ±11 Ma to 2125 ± 17 Ma, while granite chemistry and Nd isotopic characteristics are unrelated. Only very locally in Guinea is there isotopic evidence of interaction between Birimian felsic magmas and the Archean rocks from the Man craton. In accord with Abouchami et al.s (1990) suggestion that Birimian basalts represent oceanic plateaus, the present data argue that the protolith of much of the West African continent was created around 2.1 Ga in an environment remote from Archean crust. Intrusion of calc-alkaline magmas into the tholeiitic units suggests that island arcs formed on top of the assumed oceanic plateaus which then collided with the Man Archean craton. Taking the Birimian formations from the Guyana shield into account, the minimum crustal growth rate at 2.1 Ga is about 1.6 km3/a, some ∼60% higher than the present growth rate. Birimian crust growth at 2.1 Ga is reminiscent of Archean processes but contrasts with 1.7 – 1.9 Ga crust formation in the North Atlantic continent which generally involved significantly more interaction with older continental crust. A comparison of the Birimian crustal growth rate with the average crustal growth rate over the Earth history implies that a large part of the Birimian crust has been recycled into the mantle or incorporated into younger orogenic segments. This apparent deficit in the crustal budget is even more dramatic for the Archean crust.

The REE content of some hydrothermal fluids

REE concentrations were determined in solutions from submarine hydrothermal vents on the East Pacific Rise and, for the first time, from continental geothermal fields in Tibet and Bulgaria. They indicate that, in hydrothermal systems, REE are immobile at temperatures below 350°C and for water/rock ratios of <105. ϵNd-values of submarine solutions and anhydrites (+4.7 to +6.2) suggests recharge on the sedimentcovered flanks of the ridge. Carbonate anions, as measured by alkalinity, appear to increase the solubility of heavy REE somewhat, but not to the extent that substantial amounts of REE could be driven out from rocks by hydrothermal activity.

Nd isotopes in French Phanerozoic shales: external vs. internal aspects of crustal evolution☆

Nd isotopic composition has been determined on shales of Paleozoic (Brittany and Languedoc) and Mesozoic (Lorraine) age. ϵNd(T) values range from −6 to −12 while Nd crustal residence ages are typically in excess over their stratigraphic ages by some 1.4 Ga. Exceptions to this rule are the sediments coeval with Hercynian, Caledonian and Cadomian orogenic events (ca. 300, 440 and 560 m.y.), the ϵNd(T) values of which suggest additional of mantle material to the sediment in the form of volcanoclastic detritus. In Brittany, this is confirmed by the local zircon chronology which provides upper intercepts of Concordia up to 800 m.y. in excess of Nd crustal residence ages. n nComparison of crustal residence ages with stratigraphie ages through geologic time suggests a three stage history: n1. na) for rocks older than 2 Ga, stratigraphie and crustal residence ages coincide, n n2. nb) from 2 to I Ga, crustal residence ages level off at ca. 2 Ga and then n n3. nc) decreases down to 1.4 Ga in recent sediments. n n nTwo extreme models can account for the observed variations: an internally driven model, in which variable quantities of mantle material are added to the crust, and an externally driven model, in which the rate of crustal recycling is low in the Archean but increases rapidly at the onset of the atmospheric oxygen buildup and development of soil supported life. n nA quantitative model is calculated which enables the balance of both processes to be taken into account.

Chemistry of solutions from the 13°N East Pacific Rise hydrothermal site

Abstract Ten samples were recovered by the submersible “Cyana” submersible from two groups of hydrothermal vents located 2600 m deep along the East Pacific Rise at 13°N. The maximum measured temperature was 317°C and minimum pH 3.8. A systematic determination of major and trace elements has been carried out and mixing lines between a high-temperature component (HTC) and seawater are observed. The water chemistry of the HTC slightly differs for several elements at the two sites. This HTC is deprived of SO 4 and Mg and is greatly enriched in most other species. Maximum concentrations are (in units per kg): Cl = 0.72mol; Br = 1.1mmol; Na = 0.55mol; K = 29mmol; Rb = 14 μmol; Ca = 52mmol; Sr = 170 μmol; Mn = 750 μmol; Fe = 1mmol; Al = 15 μmol; Si = 21mmol . For many elements, the magnitude of the anomaly relative to seawater does not compare with the results obtained from the Galapagos or East Pacific Rise 21°N. The enrichment of cations relative to seawater is likely related to the huge Cl excess through charge balance. The Br/Cl ratio is close to that for seawater. However, it is not clear whether the Cl excess is due to gas release or basalt hydration (formation of amphibole chlorite or epidote). P-T dependence of SiO 2 solubility suggests that water-rock interaction last occurred at a depth in excess of 1 km below the sea floor. A mixing line of 87 Sr/ 86 Sr vs. Mg/Sr demonstrates that the HTCs have a nearly identical 87 Sr/ 86 Sr ratio of 0.7041 for both sites. A water/rock ratio of about 5 is inferred, which differs from the 1.5 value obtained at 21°N.

The assessment of REE patterns and 143Nd/144Nd ratios in fish remains

Abstract The REE content and isotopic composition of Sr and Nd have been determined in fish teeth ranging in age from the Trias to the present and from various localities mostly around the Atlantic. These measurements have been carried out on Selachian and Teleost remains from the same locality in Togo and show no appreciable difference, which suggests, with the help of a mass balance calculation of the Ce anomaly, that diagenetic effects are not responsible for the REE enrichment of biogenic phosphates. One group of fossil teeth has about 3 times the REE abundances of shale and a shale-normalized pattern with a minimum at Sm: it is thought to reflect deposition in the open-sea environment. A second group has REE concentration about 10 times higher than the first group with either a regular light REE enrichment or, more frequently, a maximum in the middle REE, both being probably indicative of deposition in estuarine or near-shore conditions. The shape of the REE spectra and the size of the Ce anomaly can be used semi-quantitatively to determine the depth of deposition. The results presented here on Late Cretaceous/Eocene fish teeth samples from Morocco reflect an increasing influx of deep waters with a lowLa/Yb ratio and strong negative Ce anomaly, which agrees well with the evolution of sediment chemistry and microfauna associations. In contrast, ɛNd is typical of the water mass in which the fish debris decayed. Examples of nearly isolated basins identified with Nd isotopes include the South Atlantic prior to the Lutetian (ɛNd ≈ −13.5), the Miocene Persian Gulf (ɛNd = −3.1), and Bolivia during the Late Cretaceous (ɛNd = −12.8). Togo and Guinea-Bissau results suggest that, in the South Atlantic, the meridional oceanic circulation had not started before 45 Ma ago. Combination of REE andɛNd data suggests that the assignment of Jurassic-Cretaceous samples measured so far to open-sea water masses is still ambiguous.

87Sr/86Sr ratios in hydrothermal waters and deposits from the East Pacific Rise at 21°N

Abstract 87 Sr/ 86 Sr ratios of three hydrothermal waters collected on the East Pacific Rise at 21°N define a mixing line between seawater and a hydrothermal end-member at 0.7030 which is derived by seawater-basalt interaction at ca. 350°C and water/rock ratio of about 1.5. Sr concentrations are not affected in the process while Mg uptake from seawater is almost complete. Up to2/3 of this hydrothermal component is involved in anhydrite precipitation while the Sr isotopic ratio in sulfides (chalcopyrite + sphalerite) cannot be distinguished from that of sulfate. It is estimated that ca. 1 × 10 10 moles of strontium are yearly cycled in the hydrothermal systems of mid-oceanic ridges, thereby affecting the 87 Sr/ 86 Sr budget of seawater. Mass balance between river runoff, limestone precipitation and ridge basalt alteration suggests that the 87 Sr/ 86 Sr ratios of the river runoff are in the range 0.7097–0.7113, and are largely dominated by limestone alteration.

Major and trace element abundances, and strontium isotopes in the Nyong basin rivers (Cameroon): constraints on chemical weathering processes and elements transport mechanisms in humid tropical environments

This paper aims to improve our understanding of chemical weathering processes and element transport mechanisms in the humid tropical environments. We studied the Nyong River basin (27,800 km2) located on the northwestern part (Ntem Complex) of the Congo craton (central Africa). The dissolved concentrations (i.e., <0.20 μm) of major and trace elements, dissolved organic carbon (DOC) and the 87Sr/86Sr ratios have been measured in more than 20 rivers draining watersheds with various surface areas (∼1 to 28,000 km2). All these rivers exhibit low major cations concentrations (i.e., Na, Mg, K, Ca) but high concentrations of some trace elements (Al, Fe, Th, Zr, Y, REE), silica, and DOC. The total dissolved load (TDS) is low (∼20 mg l−1) and dominated by silica and organic matter. The comparison of different watersheds shows us that chemical weathering is more efficient in the small unit-watersheds. All the sampled rivers exhibit a wide range of 87Sr/86Sr ratios and high Ca/Na ratio that can be explained by the heterogeneity of the crystalline rock constituting the cratonic basement. n nFour selected rivers (Mengong, Awout, Soo, and Nyong) having different drainage areas and hydrological parameters were sampled over a 2-year period. Collected data show that all these rivers present the same monthly seasonal variations, with higher concentrations during rainy season and lower concentrations during dry season. This implies that the weathering and transport mechanisms of small watersheds can be extended to the whole Nyong basin. In the small unit-watersheds, chemical weathering mainly occurs in swamp zones where mineral dissolution is enhanced by humic substances. These swamp zones constitute a pool of organic-rich water, which can be quickly mobilized during rain seasons. In these waters, DOC and insoluble element concentrations (e.g., Al, Fe, and Th) were strongly correlated which show the key role of organic colloidal matter in the transport of some insoluble elements. Some other relationships (Al vs. Fe, REE vs. Al) were also examined in order to get information about chemical weathering or element transport. n nBased on these data, it has been concluded that the chemical composition of these river waters is controlled by geomorphic and historic factors (e.g., thick cation-poor soil). In contrast, the present day climatic parameters (high rainfall and temperature) play a minor role in water chemistry regulation even though they are likely to enhance mineral dissolution. Even if organic matter favors mineral dissolution, chemical weathering in this area is low compared to other world regions, which suggest, on a global scale, a relatively small effect of these environments on the CO2 consumption.

The transition from alkali basalts to kimberlites: Isotope and trace element evidence from melilitites

Eighteen Cenozoic melilitite samples from Spain, France, West Germany and Czechoslovakia have been analyzed for major and trace elements (including REE) together with their Sr and Nd isotopic compositions. Leaching experiments produced significant shifts of their87Sr/86Sr ratio indicative of a contamination by a crustal component. Most samples fall within the Sr-Nd mantle array with ɛNd values in the 1.5–6 range. These values are considered as minimum for the melilitite mantle source hence demonstrating its time integrated LRE depletion. The Ni and Cr contents of the samples are typical of primary magmas and exclude extensive crystallization of olivine and pyroxene in a closed system. However, the chemical relationships suggest that dilution of the liquids by mafic minerals of the conduits during their ascent has been important. The REE patterns show some variations which are interpreted by this dilution effect. Once normalized to Yb they are closely similar and perfectly distinguishable from those of alkali basalts and kimberlites. All of these rocks have Ce/Yb ratios which are high but distinctive for each rock type: 40 to 200 times the chondritic ratio for kimberlites, 20 to 30 for melilitites, 8 to 15 for alkali basalts. As contamination is likely to have modified somewhat the isotopic characteristics of most of these rocks, there is no overwhelming evidence that their source is chemically different. The Ba and Rb contents together with the REE patterns of the melilitites would constrain the degree of melting to be very small (<0.2%).The calculation of batch melting and steady zone refining models suggests that kimberlites, melilitites and alkali basalts may have been derived by equilibration of deep melts with different upper mantle levels characterized by decreasing garnet/clinopyroxene ratios.The strongly incompatible elements are enriched in the melt during its ascent by leaching of the wall rocks. For the steady zone refining model, the degree of melting concept loses its significance and the difficult requirement of extracting small liquid fractions from a molten source disappears.Within the frame of this model, the preenrichment of the kimberlite, melilitite and alkali basalts source in incompatible elements by metasomatic fluids is no longer necessary.

Isotope and trace element geochemistry of Colorado Plateau volcanics

Abstract Basalts from the San Francisco Peaks and North Rim of Grand Canyon, nephelinites from the Hopi Buttes and Navajo minettes (Colorado Plateau) have been analyzed for trace element contents and Sr, Nd, Pb isotope compositions. The ages increase eastward from the Quaternary (basalt) to 5 Ma (nephelinite) and 30 Ma (minette) as does the depth of melt generation inferred from xenolith mineralogy and major element geochemistry. The three rock types present an enrichment of incompatible elements (although minettes present negative concentration spikes for Nb, Zr, Ti, Ba, Sr) relative to other magma types. The chondrite-normalized Ce/Yb ratio changes from 8–22 (basalt) to 25–30 (nephelinite) and 33–60 (minette) and reflects small degrees of partial melting of a mantle source with a garnet/clinopyroxene ratio increasing with depth. The negative Eu anomaly present in minette, the low Sr/Nd and high Pb/Ce suggest the presence of a recycled continental crust component in their mantle source. The 87Sr/86Sr ratio varies from 0.7032-0.7045 (basalt and nephelinite) to 0.7052-0.7071 (minette), while eNd is remarkably more constant at +0.8 to +3.7 (nephelinite) and −2.6 to +2.2 (basalt and minette). Good linear correlations are observed in both 207Pb/204Pb and 208Pb/204Pb vs. 206Pb/204Pb diagrams with basalt being the least and nephelinite the most radiogenic and indicate a 2.3 ±0.1 Ga age and a Th/U of 3.4. Three lithospheric source components are indicated: a) an OIB-type depleted mantle source, b) an end-member with unradiogenic Sr, Nd and Pb for basalt and nephelinite and c) a recycled crustal component for minette.