Petrology and Geochemistry of Basalts from the Red Sea Axial Rift at 18? North     

EISSEN  JEAN-PHILIPPE; JUTEAU  THIERRY; JORON  JEAN-LOUIS; DUPRE  BERNARD; HUMLER  ERIC; AL'MUKHAMEDOV  ALEXANDER 《Journal of Petrology》1989,30(4):791-839

Detailed studies by submersible were carried out in the axialzone of the Red Sea Rift near 18?N during the Soviet Red Seaexpedition of the Oceanological Institute of the Academy ofSciences (December 1979–March 1980). The initial bathymetric,magnetic and seismic surveys established the general organizationof the symmetric tectonic steps (1–3) descending towardsthe axial rift. The 4–5 km wide inner floor of the riftwas explored during 21 dives. It is occupied by 100–300m high, young pillowed volcanoes, isolated or grouped to formelongated hills, frequently cut by open fissures except in thezone of most recent extrusion. The 42 samples collected are typical plagioclase ? olivine ?clinopyroxene ? spinel, more or less porphyritic mid-ocean ridgebasalts whose compositions were mainly controlled by polybaricfractionation of plagioclase, olivine and minor clinopyroxene.They have been separated into porphyritic and sub-aphyric groupsusing modes and mineralogical criteria. Mineral-liquid equilibria,crystal zonation, and modal proportions indicate some magmamixing but probably only of closely related magma batches withineach described group, as can occur inside a single magma chamber.Crystal accumulation is believed to have played a significantrole in only a few porphyritic samples. Three sub-groups (from less to more evolved; (a) FeO^*/MgO<1?22;(b) 1?16<FeO^*/MgO < 1?48; and (c)FeO^*/MgO>1?49) weredistinguished on the basis of glass and whole-rock major elementchemistry. Glass compositions follow the multisaturated cotectic-likecurve for MORB-type basalts and show a general evolution verycomparable to what is observed on the Mid-Atlantic Ridge near36?N, but arc less diverse than in the FAMOUS area. ⁸⁷Sr/⁸⁶Sr,¹⁴³Nd/¹⁴⁴Nd, and ²⁰⁶Pb/²⁰⁴Pb data for 4 samples show strongsimilarities to those from the Mid-Atlantic and East PacificRidges, and indicate no continental contamination despite thefact that they have been produced during recent continentalbreak-up and ocean opening. ²⁰⁶Pb/²⁰⁴Pb values, Th/Ta vs. Th/Tbcorrelations, and rare earth element patterns allow recognitionof three different groups of samples, indicating that the RedSea Rift near 18?N is fed by a heterogeneous mantle source.The chondrite-normalized LREE.  相似文献   

Isotopic Heterogeneities in the Granitic Intrusion of Monte Capanne (Elba Island, Italy) and Dating Concepts   总被引：1，自引：1，他引：1  

JUTEAU  MARTINE; MICHARD  ANNIE; ZIMMERMANN  JEAN-LOUIS; ALBAREDE  FRANCIS 《Journal of Petrology》1984,25(2):532-545

The Monte Capanne granodiorite-rhyolite-aplite association exhibitsK-Ar ages which cluster around 7.9 m.y. Rb-Sr mineral ages andU-Pb dating on zircons, however, give 6.2 m.y. which is thepreferred age of emplacement. Evidence of subsequent eventsrelated to either dyke emplacement, or conductive/convectivecooling is provided by ages as young as 5.4 m.y. A significantheterogeneity of the whole-rock ⁸⁷Sr/⁸⁶Sr initial ratios, whichfall in the range 0.711–0.715, is interpreted as beingdue in part to exchanges with the low ⁸⁷Sr⁸⁶Sr country rocksat the deuteric stage via fluid interaction, and in part topossible different magma sources. In the geological future,these heterogeneities should be progressively smeared out bythe radioactive decay of ⁸⁷Rb, but even if the system remainsperfectly closed, a residual bias in excess of 100 m.y. shouldpermanently affect the theoretical isochron ages. It is inferredthat Rb-Sr isochron ages determined on samples of Precambrianage must be regarded with caution: a residual bias may somewhatalter the information obtained even from statistically acceptableisochrons. In this respect, when great accuracy is needed forplutonic emplacement ages (to within few m.y.), U-Pb datingappears to be less sensitive to inherited components and providesmore reliable chronometric information.  相似文献