Crystal Cargo Chemistry from the Alarcon Rise mid-ocean ridge evolved magma suite: evidence for mixing and mingling?

Generation of highly silicic magmas on the deep marine mid-ocean ridge (MOR) is extremely rare, with only one such occurrence of rhyolite being discovered at the northernmost tip of the East Pacific Rise (EPR). Models for the development of such highly evolved rocks on the MOR have invoked processes of fractional crystallization, crustal assimilation, magma-mixing and liquid immiscibility. This study will investigate the application of such models, namely magma-mixing, to better understand the unusual range of chemical compositions found within the Alarcon Rise spreading segment adjacent to Baja, Mexico.

Supervisors

Main supervisor: Ryan Portner, GEO-UiB
Co-supervisor: Håvard Stubseid, GEO-UiB

Project description

Interestingly, an evolved lava suite was discovered at the northern end of Alarcon Rise where the North American/Pacific plate boundary transitions from seafloor spreading of the EPR to transtension of the Gulf of California. Geochemical trends from the lava suite have been interpreted to have formed through fractional crystallization, with little to no input from assimilated oceanic or continental crust. However, major element chemistry reveals the presence of several “hybrid” lavas that may preserve evidence for magma mixing or other processes. This project will focus on these hybrid lavas and interpret their petrogenetic origin by collecting and interpreting trace and major element geochemistry from host glass and phenocrysts.

Map of the Alarcon Rise — Alarcon Rise location map. Photo: Ryan Portner / UiB

Chemical zonation in crystals preserve changes in host magma composition related to petrogenetic processes. By analyzing this zonation, and looking at textural relationships of core-rim boundaries we can determine the nature and evolution of crystal growth during its host magma evolution. This project will characterize the trace and major element geochemistry of phenocrysts (plagioclase, pyroxene, olivine) and their felsic, intermediate and mafic host glasses in the Alarcon Rise MOR eruptive suite. Thermobarometry from select crystal phases will also be used to outline the relative depths of magmatic perturbations (i.e. mixing) during its ascent to the surface.

Field-, lab- and analysis work

Student will conduct detailed LA-ICPMS trace and major element analysis of host silicate glasses and phenocryst populations from mafic through felsic lava samples. These data will be paired with SEM observations of internal crystal growth textures. Computer modelling using MELTS will be employed to help characterize sample petrogenesis.

Proposed course plan

GEOV242 Petrology (10 ECTS),
GEOV342 Geochemical toolbox (10ECTS)
GEOV302 Data analysis in Earth Science (10 ECTS)
GEOV252 Field Course in Geological Mapping (10 ECTS),
GEOV231 Marine Geological Field- and Laboratory course (10 ECTS),
GEOV341 Thermochronology and Tectonics (5 ECTS),
GEOV345 Regional Geologic Excursion to Western Norway (5 ECTS).

Last updated: 23.06.2026