Oxygen and hydrogen isotope geochemistry of zeolites

Abstract

Oxygen and hydrogen isotope ratios were obtained for natural samples of the zeolites, analcime, chabazite, clinoptilolite, laumontite, mordenite, and natrolite. Zeolites were dehydrated by heating under high vacuum to 450°C to remove all the channel water. In most cases isotopic ratios were measured both for the zeolite framework ( δ 18 O f ) and the channel water ( δ 18 O cw , δD). Most of the data were collected on analcime—49 samples from 34 world-wide locations and a variety of geological settings and ages. Dehydrated analcime has a range in δ 18 O f that extends from 4.3.to 26.6%., which is unusually wide for a single mineral species. There is a rough correlation between δ 18 O f and the inferred formation temperature such that “igneous” analcimes have low values (8.7 to 14.3%.) and sedimentary analcimes have high values (16.6 to 24.5%.). It is, however, impossible to prove on the basis of oxygen isotope data whether an analcime is a primary igneous mineral since δ 18 O f values for hydrothermal analcimes (4.3 to 26.6%.) overlap with those of “igneous” analcimes. The oxygen isotope systematics of associated hydrothermal analcime-calcite pairs and two well-constrained samples from Surtsey Island and DSDP site 417A suggest that the analcime-water fractionation is similar to that of calcitewater. Fewer data were obtained for the other zeolites. High to intermediate δ 18 O f values for five chabazite (20.6 to 25.8%.), two soil clinoptilolites (31%.), one natrolite (18.2%.), and one mordenite (17%.) may indicate low-temperature origin for these samples. Laumontite (6%.), however, may have formed at higher temperatures than the above zeolites. The oxygen isotope relationship between laumontite and calcite suggests that the laumontite has not preserved its original oxygen isotope ratio and exchanges more readily than calcite. Oxygen and hydrogen isotopic ratios for the channel waters indicate that the channel waters in analcime and natrolite are related to meteoric waters at the sample site. Analcime channel waters ( δ 18 O cw : −5 to −24%.; δD: −49 to −167%.) fall near the meteoric water line and vary systematically with the latitude of the sample site, suggesting that the isotopic signature was inherited from the sample locality. Natrolite channel water may also reflect that of the sample site. In contrast, isotopic composition of chabazite, clinoptilolite, laumontite, and mordenite channel waters are derived from the ambient water vapor where the zeolites were last stored.