Is mid-ocean ridge basalt chemistry a function of melt-rock reaction in the lower oceanic crust?

International audienc

New insights on the origin of troctolites from the breakaway area of the Godzilla Megamullion (Parece Vela back-arc basin) : the role of melt- mantle interaction on the composition of the lower crust

The troctolites and olivine-gabbros from the Dive 6K-1147 represent the most primitive gabbroic rocks collected at the Godzilla Megamullion, a giant oceanic core complex formed at an extinct spreading segment of the Parece Vela back-arc basin (Philippine Sea). Previous investigations have shown that these rocks have textural and major elements mineral compositions consistent with a formation through multistage interaction between mantle-derived melts and a pre-existing ultramafic matrix. New investigations on trace element mineral compositions basically agree with this hypothesis. Clinopyroxenes and plagioclase have incompatible element signatures similar to that of typical-MORB. However, the clinopyroxenes show very high Cr contents (similar to those of mantle clinopyroxene) and rim having sharply higher Zr/REE ratios with respect to the core. These features are in contrast with an evolution constrained by fractional crystallization processes, and suggest that the clinopyroxene compositions are controlled by melt-rock interaction processes. The plagioclase anorthite versus clinopyroxene Mg# [Mg/(Mg + FeTot)] correlation of the Dive 6 K-1147 rocks shows a trend much steeper than those depicted by other oceanic gabbroic sections. Using a thermodynamic model, we show that this trend is reproducible by fractionation of melts assimilating 1 g of mantle peridotite per 1\ub0C of cooling. This model predicts the early crystallization of high Mg# clinopyroxene, consistent with our petrological observation. The melt-peridotite interaction process produces Na-rich melts causing the crystallization of plagioclase with low anorthite component, typically characterizing the evolved gabbros from Godzilla Megamullion

The earliest mantle fabrics formed during subduction zone infancy

http://www.godac.jamstec.go.jp/darwin/cruise/kairei/kr08-07/ehttp://www.godac.jamstec.go.jp/darwin/cruise/kairei/kr09-04/

Serpentinite with and without brucite: A reaction pathway analysis of a natural serpentinite in the Josephine ophiolite, California

A partially serpentinized peridotite from the Josephine ophiolite has been studied in detail in order to characterize the chemical processes of its serpentinization. The original rock was harzburgite, and its olivine and orthopyroxene are partially replaced by veins and patches of lizardite serpentine and magnetite; brucite and talc are completely absent from the serpentinite, regardless of whether the precursor mineral was olivine or pyroxene. Petrographic and mineral-chemical data suggest at least two phases of serpentinization. Incipient serpentinization produced lizardite and magnetite veinlets, from preferential dissolution of orthopyroxene, and/or infiltration of a silica-rich fluid. No talc or brucite was produced, which suggests this serpentinization happened in a chemically open system. Later serpentinization was from a fluid closer to Fe-Mg-Si chemical equilibrium with the harzburgite, which should in theory favor formation of a brucite-bearing serpentinite. Brucite is absent from late serpentine veins, but they have some porosity which could represent former brucite that was dissolved out or was reacted out after serpentinization. Isocon modeling suggests that Si, Fe, and K were added during serpentinization and that Ca was lost; i.e., the serpentinization was not isochemical (except for H2O). Results of petrographic observations, thermodynamic modeling, and mass balance calculations were used to constrain the reactions for global serpentinization of the studied sample. These reactions indicate that water with a concentration of H2 up to two times that of deep sea vent fluids may have been produced during the serpentinization of the Josephine peridotite, which could then have been a potential host for significant biomass

Comparison of bispectral index and composite auditory evoked potential index for monitoring depth of hypnosis in children

BACKGROUND: In pediatric patients, the Bispectral Index (BIS), derived from the electroencephalogram, and the composite A-Line autoregressive index (cAAI), derived from auditory evoked potentials and the electroencephalogram, have been used as measurements of depth of hypnosis during anesthesia. The performance and reliability of BIS and cAAI in distinguishing different hypnotic states in children, as evaluated with the University of Michigan Sedation Scale, were compared. METHODS: Thirty-nine children (aged 2-16 yr) scheduled to undergo elective inguinal hernia surgery were studied. For all patients, standardized anesthesia was used. Prediction probabilities of BIS and cAAI versus the University of Michigan Sedation Scale and sensitivity/specificity were calculated. RESULTS: Prediction probabilities for BIS and cAAI during induction were 0.84 for both and during emergence were 0.75 and 0.74, respectively. At loss of consciousness, the median BIS remained unaltered (94 to 90; not significant), whereas cAAI values decreased (60 to 43; P < 0.001). During emergence, median BIS and cAAI increased from 51 to 74 (P < 0.003) and from 46 to 58 (P < 0.001), respectively. With respect to indicate consciousness or unconsciousness, 100% sensitivity was reached at cutoff values of 17 for BIS and 12 for cAAI. One hundred percent specificity was associated with a BIS of 71 and a cAAI of 60. To ascertain consciousness, BIS values greater than 78 and cAAI values above 52 were required. CONCLUSIONS: BIS