Analcime phenocrysts in igneous rocks: Primary or secondary?

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 (δ¹⁸O_f) and the channel water (δ¹⁸O_cw, 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 δ¹⁸O_f 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.