Date of Award

2010

Document Type

Honors Thesis (On Campus Access Only)

Department

Colby College. Geology Dept.

Advisor(s)

Valerie S. Reynolds

Second Advisor

Robert A. Gastaldo

Third Advisor

D. Whitney King

Abstract

Oxygen (O) and lithium (Li) are fluid-mobile elements used as geochemical tracers of a crustal component in the mantle. The isotopes of each element fractionate according to the degree and temperature of hydrothermal alteration. Previous studies demonstrate that Li and O isotopic trends are coupled within the upper crust near mid-ocean ridges but are decoupled in subduction-related volcanism. To better understand the average crustal signature of these systems, it is necessary to characterize these systems in the presently poorly understood lower oceanic crust. This study uses lower crustal samples from IODP Expedition 305 Hole U1309D exhumed at the Atlantis Massif, 30°N. Oxygen isotope values range from 0.985 to 5.045 ‰ whereas lithium isotope values range from -2.7 to 2.6 ‰. These values are coupled with depth for most samples and are significantly lower than the unaltered MORB values of 5.5 ‰ for oxygen and 4.0 ‰ for lithium. This indicates that high-temperature alteration affected both lithium and oxygen within the core complex. Heavy lithium isotope outlier values of, 6.1 ‰ and 7.1 ‰, are interpreted to have been affected by fluids with evolved compositions. Lithium concentration data range from 1.32 to 12.37 ppm and exhibit no systematic trend or correlation with lithium isotopes, indicating that lithium concentration is independent of hydrothermal alteration. When compared to other deep-sea drill core samples of the lower crust, trends in oxygen isotopes with depth in Expedition 305 drill core vary significantly. The heterogeneity in the geochemical tracer signatures of the lower crust could significantly affect interpretations of crustal recycling into the mantle or continental crust at subduction zones. To obtain a better understanding of this heterogeneity and obtain a better lower-crustal average for geochemical tracers, it is imperative to study the isotopic signatures in lower oceanic crust at spreading centers in various ocean basins.

Comments

Full-text download restricted to Colby College campus only.

Keywords

oxygen isotopes, lithium isotopes, lithium concentrations, Atlantis Massif, lower oceanic crust, geochemical signature

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