Spinel lherzolite xenoliths from Xalapasco de La Joya, San Luis Potosi, Mexico, are divided into two distinct groups according to their major element and trace element characteristics. Group Ia xenoliths are characterized by light rare earth element (LREE) depletion ((La/Lu)N=0.10-0.77 in clinopyroxene) and linear major and compatible trace element relationships. Group Ib xenoliths are characterized by FeO and Na2O enrichment and higher (La/Lu)N ratios (0.80-4.1 in clinopyroxene) and complex major element relationships. These samples, which have a range of equilibrium temperatures of 910¿-1070 ¿C, exhibit protogranular textures and typical orthopyroxene+clinopyroxene+spinel clusters. Modal abundances and chemical compositions of the group Ia xenoliths vary from primitive (15.2% clinopyroxene, 38.5% MgO, 1824 ppm Ni) to moderately depleted (6.4-8.7% clinopyroxene, 43.8-44.1% MgO, 2192 ppm Ni). Systematic variations of major elements and compatible trace elements in the group Ia xenoliths are interpreted to result from various degrees (<25%) of partial melting and melt extraction, followed by subsolidus equilibration and annealing. The extracted melts have a range of compositions similar to picritic basalts. Abundances of moderately incompatible trace elements, Sc and Cr, in the group Ia minerals have been substantially redistributed during subsolidus equilibration. In a few of these xenoliths there appears to be vestiges of incipient metasomatism, but metasomatism has not substantially influenced the group as a whole. Group Ib xenoliths have been substantially influenced by metasomatic processes.

The &Sgr;FeO and Na2O contents of the cores of clinopyroxenes in group Ib xenoliths are higher than clinopyroxenes in group Ia samples. The higher La contents and La/Lu ratios in group Ib clinopyroxenes (compared to group Ia), together with this FeO and Na2 enrichment, suggest that equilibration of basanites with residual mantle has been a major process in the evolution of group Ib peridotites. Clinopyroxenes in the group Ib xenoliths are reversely zoned with higher Mg ♯ (100¿Mg/(Mg+&Sgr;Fe)), CaO, and TiO2 and with lower Na2O and Al2O3 in their rims than in their cores. Bulk rock Al2O3 and SiO2 contents of the group Ib xenoliths tend to be lower than those of group Ia at the same MgO contents. The Al2O3 and SiO2 depletion in bulk rocks and Al2O3 and Na2O depletion in the rims of the group Ib minerals result from reaction with an alkaline-element-enriched, H2O dominated fluid. The history of group Ib xenoliths appear to be dominated by an early phase of melt extraction followed by interaction with a basanitic melt to produce the FeO, Na2O, and LREE enrichment in clinopyroxenes. These peridotites then appear to have been affected by a H2O-rich fluid that depleted Al2O3 and Na2O from the rims of clinopyroxenes and produced incipient melt veins in the xenoliths. ¿ American Geophysical Union 1990