Basalts from Leg 6 of the Deep-Sea Drilling Project

Basalt was successfully cored at Site 54 in the Parace VelaBasin of the Philippine Sea, and at Site 57 on the CarolineRidge as part of Leg 6 of the Deep Sea Drilling Project. Site54 basalts are altered, but selected major and trace elements,particularly REE allow their characterization as high-aluminaolivine tholeiites, with flat REE patterns, no Eu anomalies,and low dispersed trace-element contents. Basalt from Site 57is distinctly different, with higher TiO₂, P₂O₅, Fe/Mg ratio,and dispersed trace elements, low Ni, Mg, and a strongly fractionatedREE pattern. Derivation by fractionation of transitional basaltis suggested. Varying degrees of alteration of Site 54 basalts has causeddepletion in MgO and addition of Rb, Sr, Ba, Ka₂O, and Na₂O.Mineralogically these changes have resulted in alteration ofolivine and pyroxene, but plagioclase and iron-titanium oxidesremain unaffected. Microprobe data are presented for olivine, clinopyroxene, plagioclase,Fe-Ti oxides, and chrome-spinel in all three basalts. In 54–4,54–8 pyroxenes are zoned from salite to ferroaugite, andshow erratic decrease in A1₂O₃ and TiO₂ with increasing ironcontent. In 57–2, pyroxenes are more magnesian than thosein site 54 basalts, and show an increase in A1₂O₃ and TiO₂ withiron-enrichment. The anomalous behaviour of Al₂O₃ and TiO₂ insome clinopyroxenes, and the close textural relationship ofpyroxene and magnesian olivine in 57–2 basalt, suggestthese minerals are xenocrystic, and were incorporated duringmagma ascent. Spinels of picotite composition in early-formed,and possibly xenocrystic, olivine, and plagioclase crystallizedprior to magma eruption, and may have begun crystallizationwithin the mantle. Site 54 basalts add further confirmation that inter-are basaltsclosely resemble ocean ridge tholeiites, although there is apaucity of data from inter-are environments. Site 57 basaltis chemically and mineralogically distinct from ocean ridgebasalts and may be related to similar volcanics erupted on theCaroline Islands.     

Reaction pathways during the progressive deformation of a blueschist metabasite: the role of chemical disequilibrium and restricted range equilibrium

Abstract There are discrete masses of un-deformed metabasite within the blueschist series of the island of Syros. Greece. Around the margins of these masses are zonal sequences through rocks showing intracrystalline deformation but without a geometric fabric, to rocks with discrete and anastomosing shear zones, and finally to penetratively foliated rocks with isolated relics of the original undeformed texture. Textural relics suggest that this spatial sequence is at least qualitatively also a temporal sequence.
This progressive shear zone deformation took place concurrently with a glaucophane-epidote to eclogite reaction. The reaction pathways in the rocks that underwent the shear zone deformation can be compared with those in rocks of a similar composition that suffered a longer deformation history and show no relics of an undeformed parent. Although the final assemblages are in both cases the same, the pathways are different. These differences are in part related to reactions promoted by the change from local to bulk equilibrium on the onset of deformation in the rocks. They are also related to the crystallization and later breakdown during the sequence of progressive equilibration of a metastable phase, in this case an impure glaucophane.     

Evidence of a Temperature-dependent 'Blueschist' to 'Eclogite' Transformation in High-pressure Metamorphism of Metabasic Rocks

Textures in the blueschist metabasic rocks of the island ofSyros, Greece, indicate a reaction forming omphacite and garnetfrom earlier glaucophane and epidote. A balanced ‘wholerock’ reaction can be written using the phase compositionsand modal mineralogy of the products observed. This reactionis continuous in P-T space if Mg and Fe²⁺ are independent components.The available thermodynamic data suggest that the ‘eclogite’(garnet plus omphacite) assemblage is favoured by higher temperatures. It cannot, however, be conclusively shown that the rocks wentthrough an up-temperature interval of metamorphism. The reactionseen could be promoted by reduced silica, or water activity.The details of the reaction textures and mineral zonation patternsshow furthermore that chemical equilibrium was not maintainedthroughout the rock development. The reaction textures differin detail in different samples. In those samples in which thetextures are most consistent with maintained chemical equilibriumhowever, the mineral zonation patterns do imply up-temperaturemetamorphism.     

Kilauea East Rift Zone Magmatism: an Episode 54 Perspective

On January 29–30, 1997, prolonged steady-state effusionof lava from Pu'u'O'o was briefly disrupted by shallow extensionbeneath Napau Crater, 1–4 km uprift of the active Kilaueavent. A 23-h-long eruption (episode 54) ensued from fissuresthat were overlapping or en echelon with eruptive fissures formedduring episode 1 in 1983 and those of earlier rift zone eruptionsin 1963 and 1968. Combined geophysical and petrologic data forthe 1994–1999 eruptive interval, including episode 54,reveal a variety of shallow magmatic conditions that persistin association with prolonged rift zone eruption. Near-ventlava samples document a significant range in composition, temperatureand crystallinity of pre-eruptive magma. As supported by phenocryst–liquidrelations and Kilauea mineral thermometers established herein,the rift zone extension that led to episode 54 resulted in mixtureof near-cotectic magma with discrete magma bodies cooled to