FRANCIS ALBAREDE
Centre de Recherches Pétrographiques et Géochimiques et Ecole Nationale Supérieure de Géologie. BP 54501 Vandoeuvre Cedex (France)
Abstract
Albarède, F., 1989. Sm/Nd constraints on the growth rate of continental crust In: L.D Ashwal (Editor), Growth of the Continental Crust Tectonophysics, 161: 299-305.
The time constants are calculated which rule the évolution of Nd isotopes in a model Earth composed of the continental crust, a depleted upper mantle, and a pristine lower mantle, and in which the upper mantle is not constrained to grow or not to grow at the expense of the lower mantle. The growth rate of the continental crust is calculated to be 1.8 ± 0.7 km3/yr, with 2.5 ± 0.5 km3/yr of new crust addition from the mantle and 0.8 ± 0.5 km3/yr of sédiment entrainment at subduction zones. The characteristic times for Nd évolution in the depleted mantie and continental crust imply that the Sm-Nd System evolves far from steady state.
Océan Drilling Program (ODP) Proceedings, Scientific Results, 1989, 111, 41-45.
4. ION MICROPROBE ANALYSES OF THE SULFUR ISOTOPIC COMPOSITION OF SULFIDES IN HYDROTHERMALLY ALTERED ROCKS, DSDP/ODP HOLE 504B1
Jeffrey C. Alt2 and Marc Chaussidon3
ABSTRACT
An ion microprobe was used to perform in-situ measurements of the sulfur isotopic compositions of sulfide miner als in seven samples of hydrothermally altered rocks from DSDP/ODP Hole 504B. The sulfides exhibit a wide range in 634S values, from - 4.1%o to + 8.3%o. Heterogeneities exist among différent sulfide grains in a given sample as well as within individual grains. A transect across a centimeter-size pyrite crystal reveals 534S values that span the range of the entire data set, indicating a complex growth history for this crystal. The positive 634S values are similar to those reported for seafloor sulfide deposits and are interpreted to reflect a combination of sulfur leached from basalts and 34S-rich sul- fide derived from seawater sulfate. Incorporation of seawater-derived sulfur is greatest in a stockworklike sulfide miner- alization in the core, where subsurface mixing occurred between hydrothermal fluids and seawater. Variations in 634S are attributed to (1) incorporation of variable amounts of seawater-derived sulfide, (2) kinetic effects during précipita- tion, (3) réservoir effects during précipitation, (4) and variations in pH and fo,,
Comptes Rendus de l'Académie des Sciences (Paris),1989, t. 308, série II, 39-44.
Détermination expérimentale du fractionnement isotopique
D/H entre tourmaline et eau à 600, 500°C et 3 kbar
Dominique BLAMART, Michel Pichavant, Simon M. F. SHEPPARD
Résumé - La calibration expérimentale du fractionnement isotopique de l'hydrogène entre tourma- line naturelle (schorl-dravite, XFe~0.5) et eau à 600, 500°C et 3 kbar donne un coefficient de fractionnement à l'équilibre de 1,002 et de 0,980 respectivement. Dans cette gamme de température, le facteur de fractionnement (a) entre tourmaline et muscovite peut s'exprimer sous la forme :
1000 In �xlour. _ musc = - 39,3 ( 106 T ~ 2) + 63,4.
Le couple tourmaline-muscovite est le premier géothermomètre minéral, pour les isotopes de l'hydrogène, qui soit applicable aux systèmes de haute température.
Experimental détermination of the D/H isotopic fractionation between tourmaline and water at 600, 500°C and 3 kbar
Abstract - The experimental calibration ofthe D/H isotopic fractionation between natural tourmaline (schorl-dravite with XFe~0.5) and water at 600, 500°C and 3 kbar gives an equilibrium fractionation factor of 1.002 and 0.980 respectively. In this temperature range, the fractionation factor (oc) between tourmaline and muscovite can be expressed as:
1,000 lnaTourmaline_Musc��v,lc= - 39.3 (106T 2) + 63.4.
The couple tourmaline-muscovite is the first hydrogen isotope minerai geothermometer which can be applied to high température Systems
FRANCE-LANORD C, In : Shipboard Scientific Party of Leg 129 (1990). Océan Drilling Program. Ancient Crust on Pacific Plate. Leg 129 shipboard Scientific Party. Nature, 345, p.112.
FRANCE-LANORD C, In : Shipboard Scientific Party of Leg 129 (1990). Océan Drilling Program. Jurassic Oceanic Crust and Sédiments in the Pacifie at last. Geotimes, 35, n° 6, 25-26.
FRANCE-LANORD C, In : Shipboard Scientific Party of Leg 129 (1990). Inorganic Geochemistry. In Y Lancelot, RL Larson et al. , Proc. ODP, Init. Repts., 129: College Station, TX (Océan Drilling Program).
Océan Drilling Program (ODP) Proceedings, Scientific Results, 1990, 116, 43-58.
5. SEDIMENTOLOGY AND ISOTOPIC CHEMISTRY OF THE BENGAL FAN SEDIMENTS: THE DENUDATION OF THE HIMALAYAI,5
Anne Bouquillon,2 Christian France-Lanord,3 Annie Michard,3 and Jean-Jacques Tiercelin4
ABSTRACT
Mineralogical and H, 0, Sr, and Nd isotope compositions hâve been analyzed on a set of représentative samples from the 17-m.y. section in ODP Leg 116 Holes 717C and 718C. Based on the mineralogical composition of the fraction �2 1 £ m together with the lithogenic-biogenic composition of the fraction �63 iim, the whole section can be subdivided intothree major periodsof sédimentation. Between 17.1 and =6 m.y-, and between =0.8 m.y. to présent, the sédiments are characterized by sandy and silty turbiditic inputs with a high proportion of minerais derived from a gneissic source without alteration. In the fraction �2 /xm, illite and chlorite are dominant over smectite and kaolinite. The granulometric fraction �63 tim contains quartz, muscovite, biotite, chlorite, and feldspars. The 6-to 0.8-m.y. period is represented by an alternation of sandy/silty horizons, muds, and calcareous muds rich in smectite, and kaolinite (50% to 85% of the fraction �2 /xm) and bioclastic material. The présence of smectite and kaolinite, as well as the 180/160 and the 87Sr/86Sr ratios of the fraction �2 tim, imply an evolution in a soil environment and exchanges with meteoric ground water.
The ranges of isotopic compositions are limited throughout the section: 6180 quartz = 11.7 to 13.3%" 87Sr/86Sr = 0.733 to 0.760 and iNdo = -17.4 to -13.8. Thèse values are within those of the High Himalaya Crystalline series, and they are considered to reflect this source région. The data imply that, since 17 Ma, this formation has supplied the major part of the eroded material.
Geochimica et Cosmochimica Acta , 1990, 54, 781-795.
Ccrium anomalies in lateritic profiles
JEAN-JACQUES BRAUN,' Maurice Pagei.,2 JEAN-PIERRE PAUL Bilong,4 ANNIE Michard,3 and BERNARD Guilli-t*
'Centre de Pédologie Biologique, CNRS, BP 5, 54501 Votidoeuvrc-lès-Niiicy, Cedex, France 'CREGU and GS CNRS-CREGU, BP 23, 54501 Vand9uvrc-lès-Nancy, Cedex, France 'ORSTOM, Département T.O.A. and Laboratoire de Minéralogie-Cristallographie, UA CNRS 09,
Universités Paris 6 et 7, 4 place Jussieu, 75252 Paris Cedex 05, France
'Département des Sciences de la Terre, Faculté des Sciences de l'Univcrsilé de Yaoundé, BP 812, Yaoundé, Cameroun 'Laboratoire de Géochimie Isotopique, Centre de Recherches Pétrographiques et Géochimiques,
CNRS, 54501 Vanduiuvre-lès-Nnncy Cedex, France
'Laboratoire de Géologie de la Matière Organique, UA CNRS 724, Université d'Orléans, BP 6759, 45067 Orléans Cedex 2, France
(Receivcd May 31, 1989; accepted in revisod form December 15, 1989)
Abstract- The REE geochemistry and mineralogy hâve been studied in four lateritic profiles, one derived from a syenitc (Akongo, SW Cameroon), the olhers beingdevelopcd on a gneissic basement and located along a soil toposequence (Goyoum, E Cameroon). There is a fractionation between LREE and HREE in the lateritic samples during weathering, the weathered residual products being enriched in LREE (from La to Eu) and depleted in HREE (from Gd to Lu); sanipled waters are enriched in HREE in relation to the syenite host-rock. A positive Ce-anomaly has been found systcmatically at the top of the saprolite, beneath a zone of iron oxide accumulation in the laterite. Up to 2000 ppm Ce may be présent. In the Akongo profile, cerianite, Ce02, is présent as very fine coatings in non-ferruginous clayey domains. Primary REE-bcaring accessory minerais are weathered at the bottom of the profile. Specifically, allanite and apatite are transformed into florencite and rhabdophane but thèse phases have no Ce-anomaly. AU the data are interpreted as the resuit of the following proccsses: (1) REE leaching in a reducing environment, (2) oxidation of Ce3+ to Ce4+ in an oxidizing environment, and (3) déposition of cerium as cerianite whereas the other REE remain in solution.