Albert Jambon a and Jean Louis Zimmermann b
a
Laboratoire MAGIE, Université Pierre et Marie Curie, Paris VI, 4 place Jussieu, 75252 Paris Cedex 05, France b
C.R.P.G., B. P. 20, 54501 Vandoeuvre Lès Nancy, France Received October 9, 1989; revised version accepted June 19, 1990
ABSTRACT
Analyses of water in MORB glasses hâve been performed using mass spectrometry techniques on spécimens selected from worldwide localities (Mid Atlantic Ridge, East Pacific Rise, Red Sea, Lau Basin). Spécimens belong to both normal and enriched types. Some OIB glasses hâve also been analyzed. The amounts of water found. comparable to those previously reported, are within the 1700-6000 ppm range. Positive corrélations are observed with other trace éléments, particularly K20. Samples may be divided into two groups: (1) N-MORB (La/Sm � 1) with a constant K20/H20 ratio of 0.25 ± 0.05; (2) enriched glasses (La/Sm � 1) with K20/H20 varying from 0.4 up to 1. The corrélation found for N-MORB may be explained by variable degrees of partial melting (possibly complicated by fractional crystallization), water and potassium behaving as incompatible éléments. The worldwide validity of the corrélation demonstrates that K20/H20 is constant in the source even though that source may hâve been depleted to a variable extent. The enriched samples though cannot be derived from the same source as the N-MORB through simple processes such as partial melting and fractional crystallization.
The respective importance of source composition and degree of partial melting may be assessed using additional data available in the literature. For sources with H20 contents between 70 and 550 ppm a 5-15% partial melting may be proposed. The positive corrélation between (La/Sm) and K20/H20 requires the existence of (at least) two mantle components that may be variously depleted or enriched. One is the depleted MORB source with low La/Sm and K20/H20, while the other displays both high La/Sm and K20/H20 ratios. The enriched source cannot be a primitive one because addition of an external composition (crust + océan) with a K20/H20 of 0.14 to a depleted MORB source (K20/H20 = 0.25) cannot produce a source with an overall ratio close to 1. If a model of oceanic crust recycling is considered, then the recycled crust must be significantly dehydrated before mixing with the mantle source. This serves to explain why water is more abundant than K20 in the external réservoir, even though its incompatible behaviour is undistinguishable from that of K upon melting. An estimate of bulk earth water abundance is poorly constrained and strongly model dépendent, and the proposed 1300 ppm is spéculative. A value in the 550-1900 ppm range is however very plausible.
Chemical Geology, 1990, 89, 87-115.
Elemental fluxes during hydrothermal altération of the
Trinity ophiolite (California, U.S.A.) by seawater
C. Lécuyer", M. Brouxelba and F. Albarèdeb
'Laboratoire de Géochimie, CAESS-CNRS, Université de Rennes. Campus de Beaulieu, F.35042 Rennes, France "Centre de Recherches Pétrographiques et Géochimiques, B.P. 20. F-54501 Vandoeuvre-les-Nancy, France
'École Nationale Supérieure de Géologie, F-54501 Vandoeuvre Cedex, France (ReceivedDecember 7, 1989; revised and accepted luly 10, 1990) ABSTRACT
Lécuyer, C., Brouxel. M. and Albarède, F., 1990. Elemental fluxes during hydre-thermal altération of the Trinity ophiolite (California, U.S.A.) by seawater. Chem. Geol., 89: 87-115. 5.
The Trinity ophiolite has undergone hydrothermal altération in greenschist- to transitional amphibolite-facies condi- tions. The mineralogical assemblages and Sr isotopic results reveal a widespread seawater circulation which has principally affected the upper part of the oceanic crust. The interface between sheeted dikes and isotropic gabbros has reacted with the largestamountofheated seawater with water/rock ratios (W/R) upto 16.
W/R ratios and température are the most important parameters in delermining the direction and amplitude of Chemical changes during seawater-rock interaction. In addition, calculation of elemental fluxes by mass balance on a crustal section reveals the imponant role of magma chemistry and related variations ofprimary model compositions. The large chemical fluxes calculated in Trinity rocks agrée with their strong degree of altération. The W/R of - 8 in the Trinity basalts are larger than those from Hole 504B and East Pacific Rise ( - 1.5). This is a possible conséquence of a longer life-time convective hydrothermal System in relation with a slow spreading center generating a thin crust. Contrasting with Hole 504B, this hydrothermal pattem is likely able to equilibrate the chemical budget for Mg, Na and partly for K between seawater and oceanic crust.
The strong degree of alteration recorded by ophiolites corresponds to the time-integrated effects ofboth initial and off- axis hydrothermal activity. Ophiolites certainly represent valuable probes in tracking the chemical budget of the oceanic réservoir.
Comptes Rendus de l'Académie des Sciences (Paris),1989, t. 308, série II, 225-230. rMise en évidence d'un domaine océanique au Paléozoïque
dans la Central Belt de la Sierra Nevada. Nord Californie
Christophe Lecuver. Olivier Rouer. Henriette LAPIERRE et Jean-Louis Zimmermann
Résumé � La « Central Belt » du Nord-Est de la Sierra Ne\ada comprend une puissante série sédiiiientaire - la « Chert-Argillite Unit » d'âge permo-triasique. Elle renferme des niveaux volcani- ques basaltiques et des écailles plutoniques uitrabasique^ basiques dont les caractères pétrographiques et les variations géochimiques peuvent être expliqués par un modèle de mélange de magmas de type N-MORB et P-MORB. Ces magmas transitionnels anorogéniques ont pu être émis à l'aplomb d'une ride qui aurait fonctionné dans un bassin marginal au Paléozoïque terminal.
Remuants of an oceanic domain during Paleozoic times in the Central Belt, northern California
Ahstract - The Central Bell in northeastern Sierra Nevada comprises a thick sedimeniary séquence, rite Permo-Triassic "Chert Argillite Unit". Il contains pillowed basaltic flows and plut onic slices. The pétrographie features and geochemical variations of the basalts and ultrabasic-basic cumulâtes can be explained by a mixing model between N-MORB and P-MORB magmas. Thèse anorogenic magmas may hâve been produced in a marginal basin during Laie Paleozoic limes.
Cosmochimica Acta , 1989, 53, 745-750.
Rare earth élément systematics in hydrothermal fluids
ANNIE MICHARD
Centre de Recherches Pétrographiques et Géochimiques �Ecole Nationale Supérieure de Géologie de Nancy, B.P. 20, 54501 Vandoeuvre-lès-Nancy Cedex, France
(Received November 22, 1988; accepted in revisedform January 31, 1989)
Abstract-Rare earth élément concentrations hâve 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 hydrothermai 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 similar
Comptes Rendus de l'Académie des Sciences (Paris), 1990, t. 310, série II, 1063-1068.
Apparente hydrophobie du talc et affinité de l'azote pour ce minéral
Laurent Michot, Jacques YvoN, Jean Maurice CASES, Jean- Louis Zimmermann et Richard BAEZA
Résumé - En utilisant des résultats de microcalorimétrie d'immersion dans l'eau, de volumétrie d'adsorption-désorption de l'azote à 77 K, et d'analyse des produits désorbés par spectrométrie de masse, on met en évidence une forte affinité de l'azote pour la surface du talc. On attribue à ce phénomène l'apparente hydrophobie du minéral dont la structure laisserait prévoir un comportement hydrophile.
Apparent hydrophobicity of talc and affinity of nitrogen for this minerai Abstract Making use of results from immersion microcalorintetry in water, gas adsorption-desor ption volumetry of nitrogen at 77 K, and mass speclrometry of evolved substances, a strong affmily of nitrogen for the talc surface is shown. This phenomenon is considered as the origin of the apparent hydrophobicity of talc, structure which would suggest hydrophilic properties.
Comptes Rendus de l'Académie des Sciences (Paris), 1990, t. 310, série II, 47-51.
Reconstitution d'un arc insulaire intra-océanique au Mexique central : la séquence volcano-plutonique
de Guanajuato (Crétacé inférieur)
Olivier MONOD, Henriette LAPIERRE, Michel CHIODI, Juventino Martinez, Philippe CALVET, Enrique Ortiz et Jean-Louis ZIMMERMANN
Résumé - Au Mexique central, l'identification de six unités tectoniques superposées permet de reconstruire la succession des termes magmatiques d'un arc insulaire d'âge Crétacé inférieur. La pétrographie et la géochimie attestent le caractère co-génétique des termes plutoniques et volcaniques, qui s'apparentent aux tholéiites d'arc insulaire. L'arc de Guanajuato est un premier exemple d'arc intra-océanique complet.
Restoration of an intra-oceanic island arc in central Mexico: the Guanajuato pluto-volcanic séries (Early Cretaceous)
Abstract � In Central Mexico, the récognition ofsix super posed teclonic units, defined in the Sierra de Guanajuato, allows the reconstruction of the magmatic succession of an imra-oceanic island arc of Early Cretaceous age. Petrographical and geochemical data show that the pluto-volcanic basic association is co-magmatic and belongs to the island-arc tholeiitic series. Tlte Guanajuato island arc is a ftrst example of a complète intra-oceanic arc.
Journal of Volcanology and Geothermal Research, 1990, 43, 333-352.