Geology paper on rapakivi zircon

We just got a manuscript accepted in Geology.

The paper (A priori evidence for zircon antecryst entrainment in megacrystic Proterozoic granites) presents a first-order U-Pb and trace element comparison of suspected antecryst zircon populations that are included within the megacrystic rapakivi texture -forming alkali feldspar ovoids to their respective groundmass zircon populations.

We utilized a simple (but also rather tedious) microdrilling approach to extract zircon from within the rapakivi ovoids and were able to show that the populations show dissimilar geochronological and compositional characteristics.

The paper will appear in the March 2016 issue...


Abstract
Entrainment of antecrystic zircon in early saturated main silicate phases has been recognized as a complicating factor in high-precision U-Pb geochronology of Phanerozoic granitic systems, but has not been demonstrated for Precambrian rocks. We report U-Pb ages (secondary ion mass spectrometry) and trace element (laser ablation–inductively coupled plasma–mass spectrometry) compositions of zircon from three main ferroan granite types of the late Paleoproterozoic Wiborg rapakivi granite batholith of southeastern Finland and vicinity. Zircon was extracted from the interior parts of megacrystic (diameters as large as 15 cm) rapakivi-textured alkali feldspar ovoids using a microdrilling technique. Compared to zircon in ovoid groundmass, zircon in the ovoids has higher incompatible element values (e.g., uranium and rare earth elements) and, for two of the three samples examined, slightly older (by ∼1 m.y.) average U-Pb ages. These observations suggest that the ovoid material of the rapakivi granites entrained zircon from earlier magmas that were compositionally different than the final magmatic host of the ovoids. Groundmass zircon implies similar average U-Pb ages for all the studied samples, ca. 1628 Ma; this is regarded as the final crystallization age of the granites. These observations show that it is viable to measure statistically valid age differences between antecrysts and groundmass of Proterozoic granites. Overall, our sampling technique provides enhanced textural control of antecrystic zircon in a wide range of sample materials.