HR: 1340h
AN: V23A-0688    [Abstracts]
TI: Profiles of 87Sr/86Sr and 143Nd/144Nd as Indicators of Magma Dynamics in the Ferrar Dolerite Magmatic System, McMurdo Dry Valleys, Antarctica
AU: Foland, K A
EM: kfoland@geology.ohio-state.edu
AF: Department of Geological Sciences, Ohio State University, 275 Mendenhall Laboratory 125 South Oval Mall, Columbus, OH 43210 United States
AU: * Marsh, B D
EM: bmarsh@jhu.edu
AF: Morton K. Blaustein Department of Earth & Planetary Sciences, Johns Hopkins University, 3400 N. Charles St. Olin Hall, Baltimore, MD 21218 United States
AB: The Ferrar Dolerites of the McMurdo Dry Valleys represent the upper section of a Magmatic Mush Column (MMC). A MMC is an extensive, vertically interconnected stack of sills and chambers, which extends upward through the lithosphere and is capped by a volcanic center. At any instant within the MMC, 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. Solidification fronts sheath all boundaries of the system, and their local rate of advance measures the local level of through-flow magmatic activity. Tracing variations in magma composition upward through the MMC and into associated lavas is key to understanding this overall process. A mush column of this basic nature, which is inferred to exist beneath, for example, Kilauea, Jan Mayen, Reunion, and the ocean ridges, existed in the McMurdo Dry Valleys region of Antarctica. The Dry Valleys system is vertically abbreviated (~3.5 km), but contains all the essential features of a major. The lowermost sill, the Basement Sill, is choked with a regionally systematic concentration of phenocryst-rich ultramafic orthopyroxenite containing up to 20 wt.% MgO with locally abundant lenses and layers of anorthosite. Local ponding of this unit has produced the Dais Layered Intrusion. To understand better the motion of the magma, the detailed processes of crystal sorting, and the overall process of differentiation, we have made a series of Sr and Nd isotopic determinations through two distinctive and well-characterized parts of this system. One profile is through the 600m Dais Layered Intrusion in Wright Valley, which varies at the base from an ultra-mafic orthopyroxenite with intermittent layers of anorthosite to an upper section of tholeiitic materials. Samples here were chosen to investigate the highly distinctive chemical and modal layering revealed in 5 m sampling profiles. The second profile is through the Peneplain Sill near Pandora's Spire at Solitary Rocks. This is the antithesis of the Dais in that it contained no phenocrysts upon emplacement and it texturally uniform from top to bottom; detailed major element analyses show, however, three distinct parts of the sill with marked steps in MgO. As is common in other parts of the Ferrar system, this suite is highly radiogenic and variable in initial calculated ⁸⁷Sr/⁸⁶Sr (~0.71050 to 0.71270) whereas ¹⁴³Nd/¹⁴⁴Nd shows more limited range (~0.51216 to 0.51212, mostly). ⁸⁷Sr/⁸⁶Sr variations in the Peneplain sill show similar variations as the whole rock compositions, indicating that this sill was formed by near simultaneous injections of three isotopically distinct batches of magma. ⁸⁷Sr/⁸⁶Sr variations in the Dais profile show a remarkably detailed correlation with MgO and CaO. The basal ultra-mafic units are by far the most radiogenic (~0.71160) and the upper tholeiitic units are the least radiogenic (0.71060). In the lower section, ⁸⁷Sr/⁸⁶Sr and MgO are intimately anti-correlated and in the upper section they are intimately correlated. Plagioclase content clearly is a major factor in controlling Sr; these grains are small and grew late in the ascending magma and thus characterize the nature of the carrier magma. The large orthopyroxenes, on the other hand, are entrained cumulate crystals that characterize a distinctly different Sr source environment.
DE: 1000 GEOCHEMISTRY
DE: 1036 Magma chamber processes (3618)
DE: 1040 Radiogenic isotope geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005