SPINEL-HOSTED INCLUSIONS IN PLAGIOCLASE-DUNITES: CLUES TO MELT-PERIDOTITE REACTION

More than 80% of the Earth’s volcanism occurs at mid-ocean ridges. Magma is produced at depth in the mantle and migrates upwards due to its buoyancy. Prior to eruption as basalt, it interacts with the shallow-most part of the mantle and previously formed crust. Of particular interest is the transition from mantle to crust, as a number of important processes are taking place that determine the final composition of the erupted basalts. In addition to earlier models with a sharp transition from mantle to crust, hybrid rocks with both mantle and crustal character have recently been recognized. We analyzed rocks from the mantle- crust transition containing typically 80% olivine, 16% plagioclase, and 2% each of clinopyroxene and spinel from a fertile peridotite massif in Bosnia-Herzegovina. A window into the melt composition is provided by inclusions in spinel in these rocks, which frequently serve as a container that prevent late, low temperature alteration of the original minerals. The inclusions therefore can potentially provide information about the composition of the melt migrating through the system at the time of spinel growth. Knowing the melt composition will help constrain the processes at the crust-mantle transition and differentiation of the original melt, and place constraints on temperature and pressure in mantle transition zone. Samples from different parts of the massif show a variation in olivine grain size and abundance and location of spinel grains with inclusions. The spinel inclusions usually contain pargasite and Na-phlogopite. Finer-grained plagioclase-dunite contain abundant spinels with inclusions located both within olivine grains and interstitially. Compositionally, those spinels are more Mg-rich, less Cr-rich and TiO2-rich than spinels in the more coarse-grained sample. In the more coarse-grained samples, spinels with inclusions are themselves inclusions in olivine grains, and overall have fewer inclusions. Compositions of spinels from these samples show a trend that may suggest the progress of melt-peridotite reaction.