Role of Li and B bearing fluids in crystallization and alteration of granitic pegmatites, San Diego, California

The San Diego County pegmatites occur in the Mesozoic Peninsular Ranges Batholith that stretches from southwestern California to Baja California, Mexico. These zoned, lithium-cesium-tantalum type pegmatite dikes intruded between 2 and 3 kbar and thus contain miaroles, or "pockets," that may contain gem-quality minerals including euhedral quartz, elbaite tourmaline, and spodumene. Feldspars in pockets are usually altered to kaolinite. This study investigates the role that fluid-soluble elements, Li, B, Cl, and F, have in controlling the mineralogy of pegmatites and subsolidus alteration includes two of these pegmatites: the gem-bearing San Diego and the more barren La Posta pegmatite suites. The fluxing properties of Li and B in melt, in particular, may be responsible for increasing water solubility in pegmatite melts, and therefore the development of large pockets that form by accumulation of aqueous fluids near centers of dikes. Pressure-corrected microthermometric homogenization temperatures >350 ??C of primary fluid inclusions in intermediate morphologic zones support sub-thermodynamic solidus crystallization of the pegmatites. Chemical analysis of fluid inclusions by laserablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and ion chromatography (IC) reveals that Li, B, Na, Cl, and F are all components in both pegmatite systems. Li concentrations in the San Diego pegmatite range between 0-51 atom % (relative to all detected cations) and B ranges from 10-65 at.%. In the La Posta pegmatite, inclusions contain significantly less Li, but B is a large component (up to 50 at.%). Significant Ca (up to 10 at.%) is also present in primary fluid inclusions. In both pegmatites, secondary inclusions, thought to represent fluids that equilibrated with feldspars after crystallization, are dominated by Na. The fluid inclusion data shows that in the San Diego pegmatite Li was highly concentrated in residual liquids and separated aqueous fluids, which ultimately led to crystallization of Li-rich minerals i

Name thesis:degree_name

M.S.

Level thesis:degree_level

Masters

Discipline thesis:degree_discipline

Geological sciences (MU)

Grantor dc:publisher

University of Missouri--Columbia

Year dc:date.issued

2014

Author dc:creator

• Gammel, Elizabeth M.

Advisor dc:contributor.advisor

• Nabelek, Peter Igor

• Author supplied: pegmatite, boron, fluid inclusions, lithium, San Diego, CA

Language dc:language.iso

eng, English

DOI

https://doi.org/10.32469/10355/44268

OAI identifier oai:identifier

oai:mospace.umsystem.edu:10355/44268