The global record of trace element and isotopic compositions of ocean island
basalts (OIB) suggest that they are derived from several distinct reservoirs in the mantle.
Major and minor element compositions of OIB also span a relatively large range of
compositions when compared with the more nearly uniform basaltic compositions
erupted at mid-ocean ridges. This may be due to distinct melting lithologies or
conditions of melting beneath different oceanic islands. The upper mantle is widely
believed to be composed primarily of peridotite, which likely contributes to the formation
of OIB. Melting at most OIB localities is confined to depths beneath the rigid oceanic lithosphere (~100 km) at pressures ~3 GPa where garnet is the stable aluminous phase,
and the high incompatible element concentrations in these basalts suggest that melting is
limited to low melt fractions. Thus, identifying the composition of low melt fraction
partial melts of garnet peridotite at 3 GPa is key to understanding the genesis of OIB.
Low melt fraction partial melts of garnet peridotite were produced experimentally
at 3 GPa in a piston cylinder apparatus using a new Re-foil melt trap. Melt traps allow partial melts to be segregated from crystalline residues, preventing changes in the
compositions of small melt pools that can result from crystallization onto nearby mineral
grains while the experiment is quenching. Re-foil melt traps are similar to diamond
aggregate and vitreous carbon sphere traps, but rely upon the wetting properties of Re as
well as pressure gradients to attract the melt phase. Re traps are easier to polish than
diamond traps and, unlike vitreous carbon spheres, are capable of withstanding high
temperatures required for peridotite partial melting at 3 GPa Partial melting experiments were performed in Pt/C capsules containing natural
peridotite powder, KLB-1, or an oxide mixture approaching the composition of KLB-1.
Experiments were performed from 1470-1530 °C and produced melt fractions between
0.059 and 0.235. In contrast with previous peridotite melting studies at 3 GPa, melts in
the lowest melt fraction experiments coexisted with garnet. Resulting melt compositions
are similar to those found by other peridotite partial melting studies for many
components, including: TiO2, Cr2O3, MnO, MgO, and Na2O; however, there are
relatively large differences in FeO* and CaO from the melt compositions of Walter
(1998). FeO* in Re trap experiments varies from 6.6-7.9%, considerably lower than the
Walter (1998) melts which are never lower than 8.8% FeO* at 3 GPa, and CaO varies
from 11.4-14.3%, significantly higher than melts in Walter (1998) that vary from 7.7- 10.9%.
The Re trap method successfully produces large melt pools that can be analyzed
by electron microprobe at melt fractions as low as 0.05, although this may be about the
lower limit of its effectiveness. The disagreements with previous experiments in CaO
and FeO* contents are likely the result of the heterogeneous character of the melt pools
and cast doubt on whether the measured compositions of melts produced in these
experiments are true low-degree peridotite melt compositions.
University of Minnesota M.S. thesis. March 2011. Major: Geology. Advisor: Dr. Marc M. 1 computer file (PDF); v, 33 pages.
Davis, Frederick Arthur.
Determination of small melt fraction peridotite partial melts using re foil melt traps.
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