In particular, I have been studying gabbroic and ultramafic xenoliths erupted during the postshield stage of volcanism on Mauna Kea volcano, Hawaii. This is largely a microprobe and petrography endeavor, but has also included whole-rock analyses for major and trace elements, and some Sr, Nd, and Pb isotopes. Other Hawaiian studies involve the lavas and gabbroic xenoliths of Kahoolawe Island, Kilauea volcano, and Mauna Loa.
The Mauna Loa research is an outgrowth of grad student Renee McCarter's thesis project -- a petrologic study of gabbroic lithic fragments 'blown' out of the summit caldera (see pictures below) Her work (year 2001-02) has shown that these gabbroic 'xenoliths' represent a prehistoric lava lake that crystallized at the summit, and it gives us good insight to crystallization dynamics and compositional evolution that occur in magma reservoirs. Particularly interesting is that mineral compositions in this kind of closed-system evolve much more than the compositions we typically observe in lavas -- for example, olivine Fo to 60 mol% and clinopyroxene Mg#s to <70 -- and glassy segregation veins cross-cutting the gabbroic xenoliths (picture below) are really low in MgO, <4 wt.%, and high in SiO2, 54-57 wt.%. There are some really MgO-rich xenoliths, too -- such as 18 to 21 wt.%, probably reflecting a cumulate environment in that lava lake.
Studies of continental alkalic basalts in recent years are for Tertiary provinces along the Hungary-Slovakia border (e.g., Nograd province) and in northeastern Brazil. These studies have involved whole-rock major- and trace- element compositions, isotope compositions, and microprobe analyses for mineral compositions.
Mantle xenoliths studies are largely based on those from Tertiary alkalic basalt centers in northeastern Brazil. These are spinel lherzolites and harzburgites that manifest a large range of equilibration temperatures, ~1200oC to 850oC.
Recent studies that concentrate on compositions of some not-so-common minerals address hollandite in Hawaiian basalt, rhabdophane (REE-phosphate) in Hawaiian basalt, and rhoenite in an ankaramite from Columbia seamount (offshore Brazil). I am also examining Ca-phosphate/glaucony nodules from offshore North Carolina.