ABSTRACT

Spilitic alteration increases the ammonium content of basaltic rocks. A series of spilitic rocks from southwest England was found to contain an average of 54 ppm of ammonium ion. Most of this ammonium is fixed as an isomorphous constituent in feldspars and micas, but a small proportion (averaging 5 ppm) is held in an exchangeable form. Subduction of oceanic crust containing ammonium-enriched igneous rocks may be a mechanism for recycling atmospheric nitrogen back into the mantle.