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Mid-ocean Ridge Basalt (mid-ocean + ridge_basalt)
Selected AbstractsDeep Fractionation of Clinopyroxene in the East Pacific Rise 13°N: Evidence from High MgO MORB and Melt InclusionsACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 2 2009Guoliang ZHANG Abstract: Mid-ocean ridge basalts (MORBs) from East Pacific Rise (EPR) 13°N are analysed for major and trace elements, both of which show a continuous evolving trend. Positive MgO,Al2O3 and negative MgO,Sc relationships manifest the cotectic crystallization of plagioclase and olivine, which exist with the presence of plagioclase and olivine phenocrysts and the absence of clinopyroxene phenocrysts. However, the fractionation of clinopyroxene is proven by the positive correlation of MgO and CaO. Thus, MORB samples are believed to show a "clinopyroxene paradox". The highest magnesium-bearing MORB sample E13,3B (MgO = 9.52%) is modelled for isobaric crystallization with COMAGMAT at different pressures. Observed CaO/Al2O3 ratios can be derived from E13,3B only by fractional crystallization at pressure >4 ±1 kbar, which necessitates clinopyroxene crystallization and is not consistent with cotectic crystallization of olivine plus plagioclase in the magma chamber (at pressure ,1 kbar). The initial compositions of the melt inclusions, which could represent potential parental magmas, are reconstructed by correcting for post-entrapment crystallization (PEC). The simulated crystallization of initial melt inclusions also produce observed CaO/Al2O3 ratios only at >4±1 kbar, in which clinopyroxene takes part in crystallization. It is suggested that MORB magmas have experienced clinopyroxene fractionation in the lower crust, in and below the Moho transition zone. The MORB magmas have experienced transition from clinopyroxene+plagioclase+olivine crystallization at >4±1 kbar to mainly olivine+plagioclase crystallization at <1 kbar, which contributes to the explanation of the "clinopyroxene paradox". [source] Origin of metamorphic soles and their post-kinematic mafic dyke swarms in the Antalya and Lycian ophiolites, SW TurkeyGEOLOGICAL JOURNAL, Issue 3-4 2003Ö. Faruk Çeli Abstract The Antalya and Lycian ophiolites are situated in the western part of the Tauride belt (SW Turkey). Ophiolite-related metamorphic sole rocks in the Tauride belt are observed either at the base of the tectonites or in mélange units. Geochemical observations from the metamorphic sole rocks of Köyce,iz ophiolite indicate three different geochemical affinities: mid-ocean ridge basalt (MORB), island-arc tholeiite (IAT) and within-plate basalt (WPB) or seamount are present at the base of the Lycian ophiolites. The sole rocks of the ophiolite are made up of amphibolite, comprising mainly amphibole, pyroxene and plagioclase. Below the amphibolites are epidote-bearing rocks and, at the base, micaschists. The metamorphic sole below ophiolites exhibits an inverted metamorphic zonation. Very strong deformation within kyanite-garnet-bearing micaschists located far from the peridotites was observed, whereas the upper part of the metamorphic sole (near the contact with the peridotites) present relatively less deformation than the lower part. The metamorphic sole rocks of the Lycian ophiolite are cross-cut by some doleritic dykes with a typical greenschist facies mineral assemblage. However, while the metamorphic sole rocks exhibit well-developed lineation and foliation; the dykes lack such structures. Copyright © 2003 John Wiley & Sons, Ltd. [source] Rare Earth, Major and Trace Elements in the Kunimiyama Ferromanganese Deposit in the Northern Chichibu Belt, Central Shikoku, JapanRESOURCE GEOLOGY, Issue 4 2005Yasuhiro Kato Abstract. Rare earth, major and trace element geochemistry is reported for the Kunimiyama stratiform ferromanganese deposit in the Northern Chichibu Belt, central Shikoku, Japan. The deposit immediately overlies greenstones of mid-ocean ridge basalt (MORB) origin and underlies red chert. The ferromanganese ores exhibit remarkable enrichments in Fe, Mn, P, V, Co, Ni, Zn, Y and rare earth elements (excepting Ce) relative to continental crustal abundance. These enriched elements/ Fe ratios and Post-Archean Average Australian Shale-normalized REE patterns of the ferromanganese ores are generally analogous to those of modern hydrothermal ferromanganese plume fall-out precipitates deposited on MOR flanks. However in more detail, Mn and Ti enrichments in the ferromanganese ores are more striking than the modern counterpart, suggesting a significant contribution of hydrogenetic component in the Kunimiyama ores. Our results are consistent with the interpretation that the Kunimiyama ores were umber deposits that primarily formed by hydrothermal plume fall-out precipitation in the Panthalassa Ocean during the Early Permian and then accreted onto the proto-Japanese island arc during the Middle Jurassic. The presence of strong negative Ce anomaly in the Kunimiyama ores may indicate that the Early Permian Panthalassa seawater had a more striking negative Ce anomaly due to a more oxidizing oceanic condition than today. [source] Mantle heterogeneity beneath the Antarctic,Phoenix Ridge off Antarctic PeninsulaISLAND ARC, Issue 1 2008Sung-Hi Choi Abstract We determined the Sr, Nd and Pb isotopic compositions of basalts recovered from the Antarctic,Phoenix Ridge (APR), a fossil spreading center in the Drake Passage, Antarctic Ocean, in order to understand the nature of the subridge mantle source. There are no known hotspots in close proximity to the site. We observe that small-scale isotopic heterogeneity exists at a shallow level in the subaxial mantle of the APR. Enriched (E-type) mid-ocean ridge basalts (MORB) coexist with normal (N-type) MORB in this region. The E-type basalts are: (i) relatively young compared to the N-type samples; (ii) were erupted after the extinction of the APR; and (iii) have been generated by low-degree partial melting of an enriched mantle source. Extinction of the APR likely caused the extent of partial melting in this region to decrease. We interpret that the geochemically enriched materials dispersed in the ambient depleted mantle were the first fraction to melt to form the E-type MORB. [source] | ||||||||||