Evidence from fluid inclusions for two generations of apatite at the Duluth Complex, Minnesota

Fluid inclusion studies of hydrously altered gabbroic and troctolitic rocks from the SW footwall of the Duluth Complex, Minnesota, indicate two distinct environments of apatite formation. Type 1 apatites are associated with sericitized plagioclase, amphibole and chlorite. They contain few one-phase inclusions and abundant 2- and 3-phase aqueous inclusions. Fluid inclusion analysis by microthermometry indicates a highly saline, CaCl[sub 2] solution. Many contain unidentified daughter minerals with very high birefringence, halite daughters are absent, and clathrate is often present. The aqueous inclusions homogenize to liquid at temperatures ranging from 320[degree] to over 400[degree], and homogenization to vapor occurs at 320[degree]. Type 2 apatites are much smaller and occur only within interstitial quartz. Some have dark cores of undetermined composition, while others display lower interference colors in the central, thicker portion of the grain, indicating isotropic or hollow interiors. They contain no fluid inclusions of workable size, but the host quartz is rich in one-phase gas inclusions of CH[sub 4]+N[sub 2][+-]CO[sub 2]. Fluid inclusion analysis by microthermometry and Raman microsampling spectroscopy indicate a wide range of compositions and salinities. Daughters include halite and calcite; clathrate is sometimes present. The aqueous inclusions homogenize to liquid at temperatures ranging from 100[degree] to overmore » 400[degree], and homogenization to vapor occurs above 430[degree]. These observations suggest: (1) crystallization and fluid trapping of Type 1 apatites occurred at high temperatures, with very saline, high-calcic and low-sodic aqueous fluids; (2) crystallization of Type 2 apatites occurred in conjunction with interstitial quartz which is related to a wide range of temperatures and fluid compositions.« less