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[SCG42-P01] Mantle processes derived from multivariate analysis of whole rock trace element compositions of Oman mantle peridotite
Keywords:ophiolite, compositional data analysis, independent component analysis, mantle, oman, trace element
The Oman Ophiolite is 500 km long and 80 km wide, and well preserves oceanic lithospheric sequence. Geological and geochemical studies on mantle processes and their spatial distribution in the early island arc have been intensively conducted by studying mantle peridotites of the Oman ophiolite. However, most of these studies have based on the interpretation of some indices, such as mineral compositions and REE patterns, and no multivariate analysis has been conducted. Therefore, this study aims to quantitatively interpret mantle processes and to recognize the spatial distribution of these processes through multivariate analysis.
In this study, we used independent component analysis, which calculates components that are independent of each other (independent components). By geological interpretation of these components, they can be separated into independent mantle processes. The analysis is based on the whole rock compositions (Ga, Rb, Sr, Y, Cs, Ba, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Hf) of 169 peridotites from the Fizh massif in the northern part and the Wadi Tayin massif in the southern part of the Oman Ophiolite. The present analysis extracted four independent components. The chemical composition data exist in a space with one dimension lower than the number of variables. To solve this limitation, the logarithmic ratio transformation method, Additive Log Ratio, was used. The calculations were performed by normalizing the elemental concentrations to SiO2 and taking the natural logarithm.
The components obtained from the analysis are uncorrelated and non-linear, i.e., they are independent components. Each independent component exhibits characteristics indicative of different mantle processes. The first independent component is positively correlated with olivine mode and spinel-Cr# and negatively correlated with pyroxene mode and all trace elements. The second independent component has a weak positive correlation with olivine mode and a weak negative correlation with pyroxene mode. It also has a positive correlation with LILE, HFSE and LREE, and a negative correlation with HREE. The third independent component has a strong negative correlation with Sr and Ba. The fourth independent component strongly correlates positively with LILE for Cs, Rb, Ba, and Th.
Based on these results, the geological significance of the four mantle processes is discussed. The first independent component is partial melting by decompression, the second is flux melting by supplying slab fluid, the third is enrichment in Sr and Ba by serpentinization at low temperatures, and the fourth is metasomatism by slab fluid. Plotting these components on geologic maps and cross-sections determined the spatial distribution of mantle processes. The spatial distribution of mantle processes is examined by plotting these components on geologic maps and cross sections. The first independent component (degree of partial melting by decompression) tends to be low at the base of the mantle and high near Moho. Flux melting from the second independent component occurs unevenly in the mantle, and is stronger in the High Refractory Zone (Kanke and Takazawa, 2014) and near the Moho. The third independent component indicates that Sr and Ba enrichment due to serpentinization was strong at the boundary between the low-temperature deformation region and the medium-temperature deformation region. The fourth independent component indicates that LILE-rich fluids penetrated to a height of 1.7 km above the basement, resulting in metasomatism.
Finally, this study used a statistical method, independent component analysis, to interpret mantle processes in the uppermost mantle of the Oman ophiolite. The geological interpretation of the independent components allows us to explain the chemical variations in the uppermost mantle in terms of four processes. The spatial distribution of each mantle process was also characterized.
1. Kanke, N., Takazawa, E., 2014. A kilometre-scale highly refractory harzburgite zone in the mantle section of the northern Oman Ophiolite (Fizh Block): implications for flux melting of oceanic lithospheric mantle. Geological Society, London, Special Publications 392, 229–246. https://doi.org/10.1144/SP392.12
In this study, we used independent component analysis, which calculates components that are independent of each other (independent components). By geological interpretation of these components, they can be separated into independent mantle processes. The analysis is based on the whole rock compositions (Ga, Rb, Sr, Y, Cs, Ba, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Hf) of 169 peridotites from the Fizh massif in the northern part and the Wadi Tayin massif in the southern part of the Oman Ophiolite. The present analysis extracted four independent components. The chemical composition data exist in a space with one dimension lower than the number of variables. To solve this limitation, the logarithmic ratio transformation method, Additive Log Ratio, was used. The calculations were performed by normalizing the elemental concentrations to SiO2 and taking the natural logarithm.
The components obtained from the analysis are uncorrelated and non-linear, i.e., they are independent components. Each independent component exhibits characteristics indicative of different mantle processes. The first independent component is positively correlated with olivine mode and spinel-Cr# and negatively correlated with pyroxene mode and all trace elements. The second independent component has a weak positive correlation with olivine mode and a weak negative correlation with pyroxene mode. It also has a positive correlation with LILE, HFSE and LREE, and a negative correlation with HREE. The third independent component has a strong negative correlation with Sr and Ba. The fourth independent component strongly correlates positively with LILE for Cs, Rb, Ba, and Th.
Based on these results, the geological significance of the four mantle processes is discussed. The first independent component is partial melting by decompression, the second is flux melting by supplying slab fluid, the third is enrichment in Sr and Ba by serpentinization at low temperatures, and the fourth is metasomatism by slab fluid. Plotting these components on geologic maps and cross-sections determined the spatial distribution of mantle processes. The spatial distribution of mantle processes is examined by plotting these components on geologic maps and cross sections. The first independent component (degree of partial melting by decompression) tends to be low at the base of the mantle and high near Moho. Flux melting from the second independent component occurs unevenly in the mantle, and is stronger in the High Refractory Zone (Kanke and Takazawa, 2014) and near the Moho. The third independent component indicates that Sr and Ba enrichment due to serpentinization was strong at the boundary between the low-temperature deformation region and the medium-temperature deformation region. The fourth independent component indicates that LILE-rich fluids penetrated to a height of 1.7 km above the basement, resulting in metasomatism.
Finally, this study used a statistical method, independent component analysis, to interpret mantle processes in the uppermost mantle of the Oman ophiolite. The geological interpretation of the independent components allows us to explain the chemical variations in the uppermost mantle in terms of four processes. The spatial distribution of each mantle process was also characterized.
1. Kanke, N., Takazawa, E., 2014. A kilometre-scale highly refractory harzburgite zone in the mantle section of the northern Oman Ophiolite (Fizh Block): implications for flux melting of oceanic lithospheric mantle. Geological Society, London, Special Publications 392, 229–246. https://doi.org/10.1144/SP392.12