Metasomatism and the crystallization of zircon megacrysts in Archaean peridotites from the Lewisian complex, NW Scotland.

Zircon megacrysts are locally abundant in 1-40 cm-thick orthopyroxenite veins within peridotite host rocks in the Archaean Lewisian gneiss complex from NW Scotland. The veins formed by metasomatic interaction between the ultramafic host and Si-rich melts are derived from partial melting of the adjacent granulite-facies orthogneisses. The interaction produced abundant orthopyroxene and, within the thicker veins, phlogopite, pargasite and feldspathic bearing assemblages. Two generations of zircon are present with up to 1 cm megacrystic zircon and a later smaller equant population located around the megacryst margins. Patterns of zoning, rare earth element abundance and oxygen isotopic compositions indicate that the megacrysts crystallized from crustal melts, whereas the equant zircon represents new neocryst growth and partial replacement of the megacryst zircon within the ultramafic host. Both zircon types have U-Pb ages of ca. 2464 Ma, broadly contemporaneous with granulite-facies events in the adjacent gneisses. Zircon megacrysts locally form > 10% of the assemblage and may be associated to zones of localized nucleation or physically concentrated during movement of the siliceous melts. Their unusual size is linked to the suppression of zircon nucleation and increased Zr solubility in the Si-undersaturated melts. The metasomatism between crustal melts and peridotite may represent an analog for processes in the mantle wedge above subducting slabs. As such, the crystallization of abundant zircon in ultramafic host rocks has implications for geochemistry of melts generated in the mantle and the widely reported depletion of high field strength elements in arc magmas.

Careful counsel: Management of unintended pregnancy.

PURPOSE: To provide a guideline for the care of women with unintended pregnancies, highlight the magnitude of unintended pregnancy, identify the demographic population at highest risk, offer assessment and diagnostic information, and describe the importance and practice of comprehensive, nondirective pregnancy options counseling, including information on abortion, adoption, and parenthood. DATA SOURCES: Extensive literature review of government publications, professional clinical, nursing, and medical journals and textbooks. CONCLUSIONS: The practice of counseling women and couples in a nondirective manner and providing comprehensive overview of options is paramount and should be the standard practice for all clinicians, especially within organizations funded by Title X funds. IMPLICATIONS FOR PRACTICE: As we strive toward diligent patient advocacy, it is essential that NPs implement and endorse the ethical provision of options counseling through promoting values clarification and continuing education.