The abundance of ammonium in the granites of central Spain, and the behaviour of the ammonium ion during anatexis and fractional crystallization

The granites of the Sistema Central Espanol are richer in ammonium than those of most other regions, and have a mean NH4+ content of 84 ppm (range = 1-243 ppm). Among the possible causes for the high level of ammonium, a high proportion of organic-rich pelitic protolith and reducing conditions during anatexis are considered to be the most significant. The behaviour of the ammonium ion during magmatic differentiation is discussed by reference to its distribution in the Pedrobernardo layered intrusion: ammonium is depleted in the final liquid fraction, but there is no relative fractionation of NH4+ and K+. The depletion of the melt in NH4+ during crystallization is attributed to its removal by biotite and to a lesser extent by K-feldspar. The behaviour of the ammonium ion during anatexis is discussed with reference to the Pefia Negra migmatite complex. It is shown that large amounts of NH4+ are present in these high grade metamorphic rocks, and that NH4+ is preferentially partitioned into the restite fraction during partial melting. These relationships are attributed to the preferential incorporation of NH4+ into potassic host minerals in the order: biotite > muscovite > K-feldspar.