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Source versus process-driven geochemical variations of mafic lavas from different tectonic regimes


Understanding the geochemical evolution between the source and resultant magmas is an important step in refining our understanding of plate tectonics. Primitive, mafic lavas are generally considered the most likely to preserve mantle source characteristics as these melts have been the least modified by geochemical effects of magmatic and secondary processes. Such processes (e.g. partial melting, fractional crystallization and contamination through assimilation of crustal material) exert geochemical controls on the composition and evolution of such magmas.

The primary aim of this thesis is to study geochemical source signatures of magmas from a variety of tectonic settings and the processes that have affected such magmas. Major chapters of this thesis explore the generation and transfer of the geochemical signature of Pacific altered oceanic crust (AOC) subducting into the Izu-Bonin trench, the effects of Pacific oceanic plateau formation on the upper mantle source of the Pacific-Izanagi mid-ocean ridge basalts during the Mid-Cretaceous and the generation of compositionally similar alkalic lavas on both continental and oceanic crust in the West Antarctic Rift System (WARS). I used a variety of geochemical measurements, including major and trace element and Sr-Nd-Pb-Hf-Os isotope analyses to analyze two unique sets of samples to illuminate the geochemical mantle source-primary melt-erupted volcanic evolution at a convergent margin, an established mid-ocean ridge setting and a rift zone.

The samples studied in Chapters 2 and 3 were dredged AOC from an along-arc transect. They have a Pacific-type Pb-isotope signature and show a progressive trace element and radiogenic isotope enrichment from older crust in the south to younger, northern crust. This observation indicates that the previously documented Indian-type Pb-isotope signature of Izu-Bonin arc lavas is not directly sourced from the subducting material and that the Indian-type mantle wedge has a greater than anticipated role in the generation of the Pb-isotopic signature of the associated arc lavas. Chapter 3 focuses on this same suite of transect samples but addresses the unanticipated geochemical enrichment trend from the older northern crust to the younger southern crust. This compositional trend likely results from contamination of the Pacific upper mantle due to the eruption of the Ontong-Java, Manihiki and Hikurangi plateaus in a massive magmatic pulse centered around 125 Ma. Chapter 4, on the other hand, uses an ocean-continent transect of alkalic lavas from the West Antarctic rift system (WARS) to assess the effects and degree of crustal contamination in WARS lavas. My findings indicate a similar mantle source for these lavas with variable degrees of crustal contamination that are most evident in the continental samples. This result emphasizes the pervasive nature of crustal contamination, especially in continental settings.

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