Publication Date

5-2019

Date of Final Oral Examination (Defense)

3-22-2019

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geoscience

Department

Geosciences

Major Advisor

Dorsey V. Wanless, Ph.D.

Advisor

Mark Schmitz, Ph.D.

Advisor

Brittany D. Brand, Ph.D.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

Abstract

Lavas erupted at off-axis seamounts provide a window into mantle heterogeneity and melting systematics that are not easily observed on-axis at fast spreading mid-ocean ridges (MORs). This is because axial melts are efficiently mixed and homogenized within shallow axial magma chambers prior to eruption. To understand the deeper magmatic processes contributing to oceanic crustal formation, we examine the compositions of lavas erupted along a chain of off-axis seamounts, adjacent to the East Pacific Rise (EPR). Essential questions include: (1) What is the range of compositional variability erupted off-axis and how does that compare to lavas erupted on-axis? (2) Is there a systematic spatial or morphological distribution of compositional variations off-axis? (3) What petrologic processes form the basalts erupted off-axis?

To investigate off-axis magmatism, we systematically mapped the 8˚20’ N seamount chain in November of 2016 on R/V Atlantis using shipboard EM122 multibeam system and AUV Sentry. This ~200 km-long chain of off-axis seamounts and volcanic ridges is oriented perpendicular to the ridge axis, west of the EPR and north of the Siqueiros Fracture Zone. The high-resolution shipboard and AUV-based multibeam and sidescan sonar maps are combined with geochemical analyses of ~300 basalts, collected using HOV Alvin and dredging, to evaluate the magmatic plumbing system feeding these off-axis eruptions. Major and trace element contents and radiogenic isotope ratios are used to assess extents of fractional crystallization, magma mixing, melting systematics, and mantle source variability.

A complex distribution of highly variable lava compositions is observed along the seamount chain, reflecting a mantle and magmatic plumbing system that produce heterogeneity on small spatial scales. Major and trace element concentrations as well as radiogenic isotopes reveal a continuum of depleted, normal, and enriched basalt compositions across the seamount chain and within individual seamounts. For example, MgO contents vary from 10.25 to 4.56 wt. % across the seamount chain and by as much as 3.61 wt. % from features sampled at an individual seamount. (La/Sm)N contents span the entire range of known values for basalts from the northern EPR (0.45—2.76), as do radiogenic isotope ratios (i.e., ƐNd = 6.26—11.18). In some cases, isotope ratios are correlated with trace element ratios – while others are not – indicating complex relationships exist between mantle sources, melting, and mixing processes on both regional and local scales off-axis. We combine the geochemical results and bathymetric maps with petrologic models to evaluate extents and depths of fractional crystallization, source variability, source mixing, and mantle melting in the off-axis environment.

DOI

10.18122/td/1509/boisestate

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