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Hydroxyl in mantle minerals

by Bell, David Ronald

Abstract (Summary)
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. This thesis is an investigation of the infrared spectroscopic characteristics, abundance, stability and partitioning behavior of hydroxyl (OH) in minerals from the Earth's upper mantle. Using manometric and nuclear reaction analysis techniques to calibrate intensities of OH bands in the mineral IR spectra, the following ranges in OH concentration (expressed as ppm H2O by weight) were determined for natural samples: clinopyroxene 150 - 1080; orthopyroxene 50 - 460; olivine <1 - 245; garnet < 0.1 - 330; kyanite 55 - 222; zircon 28 - 74. Systematics of OH distribution in garnets from various mantle rock types indicate a connection with petrogenetic environment, illustrating qualitative use of OH as an indicator of the activity of hydrous components (e.g., water) in the mantle. Applications to specific mantle assemblages are discussed. Heterogeneous distribution of H in the mantle on local and regional scales is indicated. Trace structural OH in mantle rocks ranges from 30 to 600 ppm H2O in samples examined, implying that nominally anhydrous minerals in peridotite could store all the water (about 100 - 200 ppm) in the depleted upper mantle (MORB source). These minerals also provide a mechanism to recycle water into the mantle in subduction zones. Partitioning behavior of H was investigated in a suite of co-magmatic kimberlite megacryst minerals. Fugacities of water calculated from OH in olivine are compatible with mantle conditions and yield an estimate of ~1 wt.% H2O in the parental megacryst magma. OH concentrations in garnet, pyroxenes and olivine are influenced by water activity, crystal chemistry (particularly Ti substitution) and possibly temperature, resulting in substantial variations in inter-mineral [...] involving garnet, but relatively constant [...] among pyroxenes and olivine and illustrating the complications of quantitative hydrobarometry in nature. Experiments on the thermal stability of OH in garnet and other minerals were conducted at atmospheric, crustal and mantle pressures. These demonstrate slow equilibration of OH in synthetic pyrope. Rapid mobility of H, redox processes and necessity for structural changes in the host crystal are important factors in determining the behavior of H in natural mineral systems. Relatively deuterium-depleted isotopic compositions are indicated for this form of H.
Bibliographical Information:

Advisor:George Robert Rossman

School:California Institute of Technology

School Location:USA - California

Source Type:Master's Thesis

Keywords:geological and planetary sciences

ISBN:

Date of Publication:11/17/1992

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