Absarokites from the Western Mexican Volcanic Belt : constraints on mantle wedge conditions
We have investigated the near liquidus phase relations of a primitive absarokite from the Mascota region in Western Mexico. Sample M.102 was chosen because it has high MgO contents, a high Mg# and F090 olivine phenocrysts, indicating it is primitive mantle melt. High-pressure experiments on a synthetic analogue of the absarokite composition with a H2O content of either -1 7 wt% or -5.1 wt% were cared out in a piston cylinder apparatus. The composition with -1.7 wt% H2O is multiply saturated with olivine and orthopyroxene as liquidus phases at 1.6 GPa and 1400°C. At the same pressure clinopyroxene appears 30°C below the liquidus. With a H2O content of -5.1 wt% composition M.102 is multiply saturated with olivine and orthopyroxene on the liquidus at 1.7 GPa and 1300°C. Assuming batch melting, we suggest that absarokite M.102 segregated from a depleted lherzolite or harburgite residue at depth -50 km depth in the mantle wedge. Unlike most lavas in the region, the absarokite has not ponded and fractionated at the crust mantle interface (-35-40 km), and the temperatures of multiple saturation indicate that the mantle wedge beneath the Jalisco block is hotter than previously thought. The low degree batch melting of an original metasomatised harzburgite source, can produce the observed trace element abundances. The liquidus phase relations are not consistent with the presence of non-peridotitic veins at the depth of last equilibration. Therefore, we propose that the Mascota absarokites segregated at an apparent melt fraction of less than 5% from a depleted peridotitic source. They initially formed by a small degree of melting of a metasomatised original source at greater depth.
Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 2002
Showing items related by title, author, creator and subject.
Helium and lead isotope geochemistry of oceanic volcanic rocks from the East Pacific and South Atlantic Graham, David W. (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1987-09)The isotopic evolution of helium and lead in the Earth is coupled by virtue of their common radioactive parents uranium and thorium. The isotopic signatures in oceanic volcanic rocks provide constraints on the temporal ...
Temporal and petrogenetic constraints on volcanic accretionary processes at 9-10 degrees north East Pacific Rise Waters, Christopher L. (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2010-06)Volcanic accretion at the fast-spreading East Pacific Rise (EPR) occurs over a ~2-4 km wide neo-volcanic zone on either side of the axial summit trough (AST). Eruption ages are critical for understanding the distribution ...
Helium isotope geochemistry of oceanic volcanic rocks : implications for mantle heterogeneity and degassing Kurz, Mark D. (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1982-06)The concentrations and isotopic compositions of helium have been measured in a number of mantle derived oceanic basalts. The goal of this research is to use the helium isotopic systematics to constrain the nature and ...