An experimental study of lithium partitioning between olivine and diopside at upper mantle conditions

Open Access
Author:
Yakob, Jessica L
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 05, 2011
Committee Members:
  • Maureen Feineman, Thesis Advisor
Keywords:
  • olivine
  • lithium partitioning
  • diopside
Abstract:
The mantle makes up more than 75% of Earth by volume. This vast layer of our planet drives plate tectonics as it redistributes heat and mass with large-scale convection cycles. While analytical geochemistry and numerical modeling have enabled researchers to uncover many properties of Earth’s dynamic mantle, our understanding is not yet complete. Geochemical studies of mantle-derived materials such as basalts and peridotite xenoliths have revealed that the mantle is compositionally heterogeneous on a large scale. These heterogeneities offer insights into the controlling factors in mantle processes. The use of lithium as a geochemical tracer holds promise for mantle studies due to the large variation in Li isotope ratios in natural materials and the structural compatibility of Li in mantle minerals. High lithium diffusion rates coupled with the potential for isotope fractionation during diffusion complicate the interpretation of lithium signals in mantle materials, but create the opportunity to discern the rates of mantle processes. Minerals such as olivine and diopside, found in mantle xenoliths from diverse localities, exhibit unexpectedly large differences in 7Li/6Li ratios, usually with lighter Li found in the diopsides. Because 6Li diffuses faster than 7Li, bulk lithium exchange between two phases could result in dynamic isotopic fractionation, with the receiving phase becoming lighter and the donating phase becoming heavier. Thus if Li becomes more compatible in diopside upon cooling, that is, if DLi(ol/di) is temperature-dependent, the diffusive exchange of Li will generate temporary 6Li enrichment in diopside and depletion in olivine. This thesis explores the hypothesis that a temperature-dependent partition coefficient for lithium between olivine and diopside is responsible for the divergent lithium isotope ratios measured between these two minerals in mantle xenoliths. The conditions causing the different isotope ratios between phases can be inferred by comparing Li concentrations and isotope ratios measured in natural samples to experimentally determined equilibrium distributions between olivine and diopside for various temperatures and pressures. However, the equilibrium elemental distributions and isotope fractionation factors for lithium in mantle minerals at high temperature and pressure are largely unknown. Experiments were conducted using a piston cylinder apparatus at 700 °C, 900 °C, and 1100 °C and 1.5 GPa (1-10 days) to determine DLi(ol/di). San Carlos olivine and De Kalb diopside were finely ground for starting materials, and Milli-Q water with 100 ppm Li and 500 ppm Ba served as the lithium source. Lithium concentrations in olivine and diopside from experiments held for 1, 3, 5, and 10 days were determined by laser ablation inductively coupled plasma mass spectrometry. Partition coefficients (DLi(ol/di)) from experiments conducted at 700-1100 °C are, within error, the same, 2.2 (± 0.3), suggesting that DLi(ol/di) is not temperature dependent. These fall in the lower portion of the range, D = 2-7, of limited previous measurements.