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Browsing by Author "Aaltonen, Milla"

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  • Aaltonen, Milla (2019)
    Jurassic (182 Ma) Karoo flood basalt province shows great variety in geochemistry. The complexity is thought to be inherited from distinct mantle sources. Luenha River exposure in northern parts of Mozambique includes primitive picrites possibly representing the still undefined parental magma type for the North Karoo Lavas. The previously determined whole-rock data revealed chondritic to very radiogenic 87Sr/86Sr ratios and nearly chondritic eNd values. The diverse 87Sr/86Sr ratios can result from processes such as e.g. subsolidus alteration, contamination, magma mixing or source heterogeneities, which complicates assessment of petrogenetic processes. To make a contribution to this, plagioclase phenocrysts from six Luenha samples were used as tracers of magma chamber processes. In situ studies on plagioclase growth zones were performed using the CIS methods (crystal isotope stratigraphy). Cold-cathode cathodolumenescence microscopy (CL) was used to visually reveal zonation, the electron microprobe (EMPA) was utilized for major element content (core-to-rim), and laser ablation-multicollector- inductively coupled plasma- mass spectrometry (LA-MC-ICP-MS) was used for in situ (87Sr/86S)i ratio measurements. The anorthite content of plagioclase cores (n = 65) is An65 ̶ 90 and core to rim variations alternate between normal oscillatory to reverse zoning. Is situ isotope examination revealed isotopic disequilibrium in (87Sr/86Sr)i between phenocrysts (cores 0.70511–0.70671, n = 10; rims 0.70539–0.70709, n = 11) and bulk groundmass (0.70660– 0.71061, n = 12). Plagioclase cores are always less radiogenic compared to whole rock (0.70690–0.71019), but internal variation within and between lava flows exists. Core-to-rim microsampling revealed four different (87Sr/86Sr)i evolution paths reflecting heterogeneous crystallization conditions. An, open complex magma plumbing system with progressing contamination is the likely scenario. The relatively radiogenic plagioclase cores compared with the uncontaminated plume-like sample (87Sr/86Sr 0.70410) indicate that contamination was ongoing prior to plagioclase crystallization and continued until eruption. Phenocryst migration between compositionally and thermally distinct reservoirs at crustal depths could explain the heterogeneous plagioclase (An and (87Sr/86Sr)i) of Luenha picrites.