Oxygen Isotopes of Orthopyroxene and Plagioclase from the Dais Layered Intrusion of the Ferrar Dolerite Magmatic System; McMurdo Dry Valleys, Antarctica

The Dais Layered Intrusion of Wright Valley was formed by ponding of the massive and extensive Basement Sill. The Basement Sill carries a pervasive ultra-mafic (20% MgO) tongue of entrained phenocrysts of large (~5 mm) orthopyroxene (and subordinate cpx) and small fresh plagioclase. The opx tongue is almost 300 m thick in Bull Pass and thins outward in all directions from the central point of filling for over 50 km. The opx phenocrysts, although commonly euhedral, show strong signs of wear and tear and often form clumps, which evidently reflects extraction from a long-standing cumulate bed where textural equilibrium was approached. The plag crystals are generally much smaller (< 0.5 mm) and are fresh and sugary reflecting nucleation and growth during ascent and emplacement. Due to the large difference in crystal size between plag and opx, the plag has commonly sieved between the opx during shear to form stringers of anorthosite throughout the opx tongue. This sorting culminates in the Dais Intrusion where anorthosite layers up to ~0.5 m are common along with many other types of plag layering and residual brows of massive orthopyroxenite. The Dais Intrusion thus varies from massive orthopyroxenite at the base alternating with thin anorthosites into a much more tholeiitic upper section with smaller swings in CaO and MgO. Initial 87Sr/86Sr isotopes on the same samples (Foland & Marsh this mtng) are strongly radiogenic throughout and show a strong negative correlation with MgO in the basal section, which is the most radiogenic, and a strong positive correlation with MgO in the upper section. Oxygen isotopic (δ 18O) determinations performed on separates of opx and plag show 6 +/- 0.5 per mil variations for opx (0.1.per mil uncertainty relative to SMOW) and slightly higher values for plag (6.25 +/- 0.5). There is no clear correlation between oxygen values for plag and opx, nor is there any clear correlation between oxygen and initial 87Sr/86Sr in these same rocks, which may eliminate serious hydrothermal effects. Moderate positive correlation exists between opx oxygen and whole rock initial 143Nd/144Nd, but none between plag and Nd-143/144. Oxygen fractionations between plag and opx are in the range -1.38 to 0.87, which is unusual. Whole rock oxygen is also elevated beyond the usual 5.7-5.8 per mil values of MORB. Overall, the strongly radiogenic 87Sr/86Sr of these rocks coupled with elevated oxygen suggests magma either variably and extensively contaminated with continental crust, which has often been suggested for Ferrar Dolerites, or that this magmatic mush has been formed from a more deliberate mixture of older sub-continental orthopyroxenite and rift-relate dolerite magma. The late plag clearly reflects growth from a magma isotopically distinct from that which gave rise to the massive opx phenocrysts. This is consistent with the Ferrar Dolerites representing the upper section of a Magmatic Mush Column, which is an extensive, vertically interconnected stack of sills and chambers, extending upward through the lithosphere. At any instant both fractionated and primitive melts are present as pools of nearly crystal-free or crystal-rich magma, thick beds of cumulates, and open or congested (by cognate and wall debris) conduits. The chemical nature of the erupted product is dependent on the strength of the erupted flux; the stronger the flux, the more crystal-laden and primitive is the eruptive material.