The origin of analcime phenocrysts in volcanic rocks is problematic. Are they primary minerals crystallized directly from melts or are they secondary minerals formed from preexisting igneous minerals such as leucite? To address this question, we have obtained stable isotope (H, N, O), electron microprobe, and ion microprobe data for analcime-bearing samples from the Crowsnest Formation in Canada and the Colima volcanic complex in Mexico. Isotopic ratios were obtained for the framework (δ18Of) and the channel water (δ18Ocw, δD) for two Crowsnest samples and one Colima sample. Both O and H isotopic ratios of channel water in all three samples fall on the meteoric water line, reflecting local meteoric water, and are clearly not magmatic. The δ18Of values for Crowsnest (13.6 and 14.2‰) and Colima (8.7‰) indicate that these analcime samples have either exchanged with external fluids at subsolidus temperatures or have formed from a preexisting igneous mineral such as leucite. Elevated δ18O values of sanidine phenocrysts (8.2 and 10.9‰) demonstrate O isotopic exchange with an external reservoir in two Crowsnest trachytes. Evidence of low-temperature water-rock interaction is also found in the Colima minette SAY-104. Its whole-rock δ18O value (7.6‰) is significantly higher than those of associated basanites and leucite basanites (5.2-6.1‰), and the hlgh N content (6 ppm) and low δ15N value (3.4‰) imply interaction with water either during magma genesis, transport, or postextrusion. The glass matrix in the Colima minette is also unusually H2O rich (4 wt%) suggesting posteruption, glass-fluid interaction. Collectively our data for the Crowsnest and Colima samples favor a secondary origin for analcime in these rocks.

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