MAGMATIC IDDINGSITE: CHANGES IN H2O IN MAGMA CHAMBERS PRIOR TO ERUPTION

Iddingsite is a mixture of orthopyroxene, maghemite, hematite, cristobalite, and amorphous silica that is formed from olivine by weathering or alteration at magmatic temperatures. In this study, iddingsite from seven basaltic and andesitic lava flows (50 to 58% SiO₂) of the Oligocene Uvas volcanic field of southern NM were analyzed by electron microprobe. Iddingsite exhibits four textures: 1) red-brown iddingsite zones around olivine phenocryst cores that are themselves jacketed by Fe-rich olivine. Olivine rims show that this iddingsite is clearly magmatic; 2) red-brown iddingsite rims on olivine phenocrysts, similar to the first texture but lacking olivine overgrowths. We interpret these to also be magmatic, but to have grown in magmas that were too evolved to crystallize the olivine rim; 3) irregularly-shaped cores of red-brown iddingsite that are jacketed by thick, euhedral crystals of orthopyroxene, reminiscent of opx jackets on corroded olivine crystals. We interpret this texture also as magmatic; the glassy groundmass shows that the rock is not extensively weathered; 4) red-brown iddingsite along fractures in olivine phenocrysts, the only texture that could have a weathering origin.

Iddingsite has a very large compositional range (SiO₂=23.0-47.8%; MgO=7.2-38.2%: FeO=13.4-39.5%). The four textural varieties have overlapping compositional ranges, supporting a magmatic origin for all. In addition, iddingsite contains elements that are not present in olivine, such as K₂O (0-2.7%), TiO₂(0-0.9%), and volatiles (calculated by difference from 100%, 0.06-19.3%). Furthermore, iddingsite compositions are weakly correlated to the whole-rock compositions. Iddingsite with the highest MgO (>40%) and lowest K₂O (<0.2%) are found in lavas with 6.1-8.1% MgO and 0.6-1.2% K₂O; iddingsite with the lowest MgO (<15%) and highest K₂O (>2.0%) are found in more evolved magmas (5.0-4.2% MgO and 2.0-2.75% K₂O).

These data suggest that the iddingsite-forming reaction is magma + olivine + water=iddingsite. We concur with Goff (1996, JVGR, 71:167-185), who showed that the presence of hematite in magmatic iddingsite requires an incursion to high fO₂ and H₂O contents, probably due to the infiltration of meteoric water, followed by a return to normal magmatic conditions as the meteoric water is consumed.