Mares, Atonal E. (2002): ICP-MS and statistical analysis of major and trace element signatures within the carbonate fraction of marine sediments, Cascadia subduction zone, central Oregon. 217 pp., georefid:2002-081488

The Cascadia Subduction Zone displays widespread evidence of active fluid advection both at and below the sea floor within low porosity intervals. Precipitating carbonates within these intervals have anomalous compositions compared with the in situ pore fluids. These anomalies suggest a warm, deep fluid migrating to shallower depths within the accretionary wedge. These anomalies are preserved in cores recovered by the Ocean Drilling Project (ODP) from the Cascadia subduction zone, Leg 146, Sites 891 and 892, and in additional samples collected at the seafloor by the Alvin submersible from 2 nearby locations. By comparing the geochemistry of minerals precipitating at various stratigraphic intervals may provide clues to the sequence and timing of faults within the accretionary wedge. The purpose of this study is to extract the geochemical signatures of carbonate fraction within selected samples and statistically analyze the trace element compositions. A weak buffered acetic acid solution was used for selective leaching of the carbonate fraction and concentrations were measured by ICP-MS. Carbonates vary from pure calcite to dolomite and represent from 3 to 100 weight % of samples. Three statistical methods, R- and O-mode factor analysis, Q-mode principal coordinate analysis, and Q-mode cosine theta coefficient analysis, were used to analyze 28 elements in 53 samples. Statistical analysis divided the sample population into 9 sample groups revealing element and sample associations, and relationships to additional physical and chemical data. Strong correlation of data appears only among sample groups related to prominent fault intervals. Although all other sample groups display discreet geochemical properties, they do not correlate to stratigraphy, pore fluid chemistry or to the physical properties of the sediments. Alvin samples generally reflect high carbonate content and extremely low trace element input. ODP samples vary in trace element concentration but generally display low carbonate content. Element associations suggest 2 major reservoirs. One source correlates with fault related samples and displays high Mn, Y, and REE with enrichment of La, but low concentrations of U and low Mg/Ca ratios. This source associates with isotopic data that suggests migration of deep fluids to shallow levels. The remaining source shows higher concentrations of Ba, K, Na, Rb, Ga, REE, and metals (Cd, Zn, Fe, Co, and Ni). This composition may characterize the sediment matrix of the upper stratigraphic levels most likely influenced by the lithic fraction of continental turbidites and marine sediments.
West: -125.1934 East: -125.0705 North: 44.4032 South: 44.3838
Expedition: 146
Site: 146-891
Site: 146-892
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