The Baja California Peninsula represents an area of anomalous heat flow evidenced by an abundance of subaerial geothermal manifestations and submarine hydrothermal vents. We report helium and carbon isotopic and relative abundance data from 13 geothermal springs throughout northern Baja California. Results of this study reveal 3He/4He values ranging from 0.11 to 1.74 RA (where RA = air 3He/4He) and concentrations of 4He, corrected for air contamination, vary from 0.284 to 1207 (×10-6) cm3 STP/gH2O. Carbon isotopes (δ13CO2) vary between -19.39 to +9.08‰ (vs. PDB) and CO2/3He values vary over several orders of magnitude (2.02 × 105 to 1.06 × 1013). 3He/4He values are attributed to mixing between mantle-derived helium and a radiogenic component derived from the crust. The highest 3He/4He values lie toward the east in the Gulf Extensional Province (GEP), are proximal to spreading centers, and are in agreement with low mantle velocity zones observed in recent tomography studies. Variable δ13CO2 and CO2/3He values at these localities are consistent with phase separation and/or calcite precipitation in shallow-level hydrothermal systems. Both processes result in CO2 loss which exacerbates the effects of contamination by crustal gases. The value of the majority of samples in the present study lies with discerning the potentially complicating effects of degassing and/or crustal contamination on the resulting CO2 record. Ultimately, the Puertecitos and Punta Estrella regions can be considered a promising geothermal prospect with a potential to satisfy the increasing energy demands of the Baja California Peninsula.

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.

The origin of the Juan Fernandez Islands in the South Pacific is of geologic importance due to high-3He/4He ratios that span a considerable range (7.8-18.0 Ra). To constrain the petrogenesis and mantle source of the islands, bulk trace element abundances and Pb isotopic compositions were obtained for mafic lavas from islands Robinson Crusoe and Alexander Selkirk. Trace element data confirm grouping where on Robinson Crusoe, group I represents the shield-building phase and group II represents post-shield building. Group III represents shield-building on Alexander Selkirk. Sub-parallel incompatible trace elements patterns among all samples suggest a common mantle origin. Shield-building groups I and III have nearly identical trace element concentrations, and lower Nb/Zr, Ba/Zr and La/YbN than the more enriched group II. Incompatible trace element modeling indicate group III was produced by the highest degree of partial melting, while low degrees of partial melting plus fractional crystallization account for group II. Robinson Crusoe lavas exhibit more radiogenic Pb isotopes (206Pb/204Pb: 19.163-19.292) than Alexander Selkirk (206Pb/204Pb: 18.939-19.228). The range of Sr-Nd-Pb and He isotopes result from variable degrees of partial melting of a slightly heterogeneous mantle plume source. The source contains an EM1 mantle component, and lies near the focus zone (FOZO). Group I represent young HIMU in the source of ocean island basalts while group III represent EM1. Juan Fernandez is unlike other high-3He/4He volcanic chains for lack of binary mixing between its low and high-3He/4He sources, showing no correlation between Sr-Nd-Pb and He isotopes.

Volcanism in Toro Ankole and Virunga provinces at the northernmost tip of the Western Branch of East African Rift system (EARS) is associated with the earliest stages of rifting. Although the provinces are spatially close, Toro Ankole lavas are relatively more silica-undersaturated, potassic, carbonatitic, and geochemically enriched than Virunga lavas. New petrographic, major-trace element and Sr-Pb isotopic data indicate such compositional differences are due to the different stages of tectonic rifting and concomitant magmatism in the region. Its underlying continental lithospheric mantle (CLM) is being metasomatized by volatile-rich small degree partial melts from the African Superplume. With decreasing pressure, carbonatitic-rich volatiles exsolve ahead of the upwelling partial melts and accumulate in the uppermost CLM layers beneath the cold, strong crust prior to rifting. Initial rifting in Toro Ankole generates explosive volcanism drawn from the uppermost carbonatitic metasomatized CLM layers. As rifting progresses in Virunga, less explosive volcanism emanates from the deeper silicate metasomatized CLM. More advanced rifting in the nearby South Kivu province additionally involves the asthenospheric mantle. The Ethiopian rifts in the Eastern Branch of EARS are most likely advanced stages of the same dynamic tectono-magmatic evolutionary process. Additionally, however, the Western Branch is underlain by the relatively more stable Paleoproterozoic to Archean basement whereas the Eastern Branch in Ethiopia is underlain by the Neoproterozoic Mozambique Belt. Thus, rifting and magmatism in the entire EARS is due to the dynamic interaction between the African Superplume and local CLM.