Abstract

Major‐element compositions, cell constants, and oxygen‐ and hydrogen‐isotope compositions are presented for six analcimes from differing geologic environments, including proposed primary (P‐type) analcime microphenocrysts from a late Quaternary minette lava near Colima, Mexico. The Colima analcimeh as XSi,tet: 0.684, a0 = 13.712Å, and one of the lowest δ18O values yet recorded for analcime (+9.28m). The major difficulty in identifying primary (P‐type) igrreous analcime is distinguishing it from analcime formed by ion‐exchange conversion of leucite (L‐type analcime) or other precursor minerals. Petrographic criteria are shown to be unreliable in discriminating P and L analcimes, although the higher K2O and Rb contents and δ18O values of L‐type crystals may be diagnostic. P‐and L‐type analcimes can be distinguished from classic hydrothermal varieties (H‐type) by the lower Fe contents of the latter. H‐type analcimes, however, can have δ18O values as low as 8.9Vm and cannot be distinguished from P‐type analcimes by this criterion. Analcimes formed from volcanic glass or zeolite precursors in saline, alkaline lakes (Stype) or metamorphic sequences (M‐type) can be distinguished from other varieties by their higher silica contents, smaller cell constants, and higher δ18O values of > + 17.7Vm. For all types of analcime, δ18O of the channel water does not correlate with 6180 of the framework oxygen. With the exception of the P‐type Colima sample, analcime channel waters have δ18O; and 6D values that fall on the meteoric water line, but differ from modern meteoric water at the individual sample sites. The channel waters may reflect fluids that entered the analcime shortly after the mineral formedand have not been replaced by more modern waters. The Colima analcime has δ18O slightly higher than expected for magmaticanalcime, based on exchange partitioning with mafic minerals in the Colima minettes. This enrichment in framework δ18O and the distinct isotopic composition of the channel water in the Colima analcime indicates that exchange between channel water and framework oxygen has occurred.

In previous discussions of primary igneous analcime, most attention has focused on blairmorites and analcimites containing centimeter‐sized euhedral phenocrysts of analcime in the absence of other primary hydrous minerals. We consider these rocks to be unlikely hosts for P‐type analcime, which is far more likely to occur as microphenocrysts and ground mass microlites in mica ‐or hornblende‐ bearing lamprophyres, magmas characterized by high water contents and silica undersaturation. The Quaternary minettes from Colima are the youngest and freshest lamprophyres yet described and appear to represent the strongest case for primary igneous analcime.

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