MASS SPECTROMETRIC IDENTIFICATION OF BORIC ACID IN FLUID INCLUSIONS IN PEGMATITE MINERALS

Abstract

Direct, on-line mass spectrometric analyses were performed on volatiles released from microscopic fluid inclusions in quartz, feldspar, and tourmaline from the miarolitic Belo Horizonte #1 pegmatite in the San Jacinto district, and Himalaya pegmatite dike system in the Mesa Grande district of southern California. These analyses are the first inclusion volatile studies to indicate the presence of significant and variable concentrations of B compounds in pegmatite formation fluids. Boron appears as boric acid B(OH)3, which is found at levels ranging from less than detection limit (< 10-7 mole fraction) to as high as 10-4 mole fraction. High B concentrations are seen in inclusion fluids from miarolite filling quartz, cleavelandite variety albite feldspar, and schorl tourmaline from the Belo Horizonte #1, while negligible amounts appear in late-stage green/pink-zoned gem elbaite tourmalines from that mine. Fluid inclusions in quartz, as well as grey and pink tourmaline from the miarolites in the Himalaya mine, have undetectable levels of B compounds. In addition to confirming the presence of very high boric acid concentrations in some pegmatite forming solutions, observations of large variations in abundance may provide new constraints on fluid chemical evolution trends during the genesis of these regionally and paragenetically complex gem deposits.