Trace elements are highly fractionated during large-scale melting associated with
planetary differentiation events. The resulting partition coefficients are used
to constrain a range of geological processes and are known to be influenced
by pressure, temperature, and compositional changes in crystalline structures.
Although recent studies have shown that melt compositional changes affect the
partitioning of trace elements, the degree to which these ratios are influenced
by alterations in the melt structure, especially with increasing pressure, is
poorly constrained due to the difficulty of collecting structural information on
bonding environments in situ. A basic understanding of how these elements
are incorporated in silicate melts is critical to interpreting early planetary
differentiation and crust forming events. This thesis presents results from both
x-ray diffraction and absorption techniques on trace element (Y, Zr, Lu and Nd)
incorporation in silicate melt structures. The structure of two rare Earth element
doped model end member silicate liquids, a highly polymerised haplogranite (Si-
Al-Na-K-O) and a less polymerised anorthite-diopside (Si-Al-Mg-Ca-O), have
been studied. The results are the first to identify trace rare Earth element (REE)
incorporation in silicate melts at high pressure using x-ray diffraction techniques.
The local melt structure around Y and Zr in a highly polymerised haplogranite
has been studied using x-ray absorption spectroscopy up to 8GPa and 1650 K.
Both elements appear to adopt 8-fold coordination within the melt structure
with no variation over the pressure range studied. This was also found for the Lu
bonding environment in the same composition where the coordination number of
Lu-O was found to be 8, with a bond distance rLu-O = 2:36A in the haplogranite
melt. At low pressures, < 5GPa, the bonding environment of Lu-O was found
to be dependent on composition with coordination decreasing to CNLu-O = 6
and rLu-O = 2:29A in the anorthite-diopside melt. This compositional variance
in coordination number at low pressure is consistent with observations made for
Y-O in glasses at ambient conditions and is coincident with a dramatic increase
in the partition coefficients previously observed for rare Earth elements (REE)
with increasing melt polymerisation. However, an abrupt change in both Lu-O
coordination and bond distance is observed at 5GPa in the anorthite-diopside
melt, with CNLu-O increasing from 6 to 8-fold and rLu-O from 2.29 to 2.39A.
This occurs over a similar pressure range where a reduction in the reported heavy
REE partition coefficients is observed.
X-ray diffraction experiments up to 60GPa and 2000K have also been performed
on the incorporation of the larger light REE, Nd, in basaltic-like melts.
The results presented show that incorporation within the anorthite-diopside
composition is dependent on the size of the REE. Nd-O initially shows the
same 6-fold coordination as Lu-O at ambient conditions, although the change
to 8-fold coordination appears to occur at considerably lower pressure between
1-2GPa. Coordination change in both cases can be attributed to collapse of
the silicate network and an increase in the average number of available 'crystal
like' sites in the liquid, with ionic radius of the REE controlling at which
pressure the preference for these sites in the melt occurs. Published mineral-melt
partition coefficients for Nd, with major mineral phases such as garnet, show
very little variation with pressure, in contrast to Lu. The difference in structural
incorporation of Lu and Nd in the melts presented in this thesis could explain
this partitioning behaviour.
Overall this thesis highlights that important structural changes of the trace
element bonding environment in silicate melts occur with both compositional
variation and pressure. Melt structural changes with pressure cannot be
neglected in predictive models of trace element behaviour, and using a single
melt term to normalise the effects of melt on trace element partitioning will not
accurately predict partitioning behaviour at depth during magma formation or
differentiation
