Major and trace-element whole rock and mineral variations in composite hornblendite-peridotite xenolith Ba-2-1, from Dish Hill, CA, are due to a single event of metasomatism in the mantle. The hornblendite is the crystallized selvage of a dike conduit charged with incompatible-element-enriched hydrous mafic magma. The magma infiltrated the refractory peridotite wallrock, reacted with its constituent minerals, and simultaneously deposited amphibole. The systematic data from this study show considerable variation in isotopic values and trace elements. These data provide insight into a mantle process that was defined previously from samples without context, lacking evidence about the number or source of metasomatic events.

In the contact zone of Ba-2-1, peridotite is enriched in Fe, Ti, CO₂) and H₂O; clinopyroxene and amphibole also are enriched in Fe and Ti, but clinopyroxene appears slightly depleted in CaO. Compared to chondrites, peridotite, clinopyroxene, and probably amphibole are enriched in light rare earth (LREE_cn) and other incompatible trace elements. Values of ⁸⁷Sr⁸⁶Sr and ¹⁴³Nd¹⁴⁴Nd in the contact zone are close to isotopic equilibrium with the dike. Whole rock and constituent clinopyroxene compositions change to those of refractory peridotite with distance from the contact.

These compositional variations were modelled using Gresens' equation for whole-rock major and minor elements, and calculations for isotopic ratios and REEs, which emulate the effects of Chromatographic fractionation. The choice of endmembers was restricted to compositions actually present in mantle samples from Dish Hill.

Model results indicate that:

• 1. the variations can be explained as the result of a single metasomatic event, probably a single pulse of previously fractionated liquid;
• 2. the ratio of total interacting liquid to peridotite was at least 1:3 by weight in the contact zone; and
• 3. the composition of the metasomatic liquid changed progressively as it infiltrated beyond that zone. The small distance over which variations occur is due to the small amount of liquid that infiltrated. Only in the contact zone was peridotite wallrock saturated by a liquid composition similar to the dike.

Comparison of the Ba-2-1 data with those of another xenolith from Dish Hill suggests that the compositional variations of mantle metasomatism result from both the compositional contrast between the metasomatizing liquid and wallrock and the relative abundances of each. Compositional and volumetric variations of mantle partial melts and their fractionates, and repeated events of melting and reaction in contiguous mantle, can create broad ranges of metasomatic “signatures” from the same process.