NSF Forms - Ridge 2000 Program

the EPR and JdF ridges. While the project description focuses on testing our diffusion based
hypothesis, interpretations of the data will, of course, involve multiple hypothesis assessment and
application of other proposed models for seamount geochemical variations.
a. Major element modeling
A major reason for studying seamount is the high proportion of “primitive” high-MgO
glasses found within these suites relative to those on the ridge axis. Major element analyses of all
glasses have already occurred. However, detailed modeling of compositional changes during
possible melt rock reaction has not occurred. In addition, our study will allow us to address the
question, is there a relationship between 238 U excess and Mg#? (e.g. Lundstrom et al., 2000)
Many of the high-MgO lavas are highly depleted in incompatible elements and have low silica,
low K 2 O and high Al 2 O 3 . Allan et al. (1989) discussed the observation that many of these high-MgO lavas
hover around compositions that are multiply saturated with a mantle assemblage near 8-10 kbars;
however, this interpretation is less favored these days with polybaric fractional melts being equally able to
match such compositions (Asimow and Longhi, 2004). If a thermal gradient were present from dunite
(hot) to surrounding peridotite (cold), melts accumulating in the dunite might be expected to be high-
MgO and aluminous. The difference between
seamounts and the 9-10°N EPR axial glasses is
readily apparent (Fig. 3). The question is what do the
wide variations in seamount lava composition tell us:
major element heterogeneity in the source? Melt-rock
reaction processes during dunite formation? Diffusive
interactions around an existing dunite conduit? By
combining the major element data with a
comprehensive analysis of other geochemical criteria,
we will be able to quantitatively assess these different
possibilities. One thing is clear from Fig. 3; any lava
that passes through the AMC is fundamentally
changed in composition to fall on a very tight and
narrow LLD (forced equilibration with a mush?).
Thus information about the mantle and its plumbing
system is mostly lost.
This comparison leads to many petrologic
questions that have not yet been addressed. Why do
LLDs of seamount suites differ from adjacent ridge
axes? How do seamounts fit into current melting
parameterizations for ridges (e.g. Langmuir et al.
1992)? If larger temperature gradients around conduits relative to that at the ridge drive greater input of
mafic components from peridotite into conduit melt, is there a signature in the seamount LLD compared
to the axial lavas? We are now at a point in time where large amounts of major element data exist to do
this comparison; we assert that with some additional geochemical data (isotopes, traces), these questions
can now be answered.
One global based reason for integrating major element compositions with U-series analyses
reflects the need to confirm or refute an interpretation currently based on scant data. Is there a
relationship that exists between high Mg# MORB and 238 U excess (Tepley et al. 2004; see Fig. 9 of
Lundstrom et al., 2000)? The conclusion of Presnall and Hoover (1986) that few MORB are primitive has
stood up well with time. The current Smithsonian MORB glass database shows 0.3% of 11,786 MORB
glasses having Mg# ! 70. PetDB indicates 0.7% of 8855. As summarized in Lundstrom et al. (2000), out
of 73 U-series MORB analyses, 15% had ( 230 Th)/( 238 U) < 1.1. If there were no relationship between Mg#
and ( 230 Th)/( 238 U),