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Title: Phase relations in subducted lithologies at lower mantle conditions
Author: Ross, Jennifer Anne
ISNI:       0000 0004 5922 4236
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2014
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Water and alkalis such as K and Na can be delivered to the lower mantle in subducted slabs and the fate of these components in lower mantle are investigated here. A series of experiments on the MgO-Si02-H20 (MSH) and nepheline-spinel-kalsilite (NSK) ternaries have been completed using a Laser-Heated Diamond Anvil Cell and analysed using X-ray diffraction (XRD), FEO (Field Emission Gun)-probe and EDS-SEM (Energy Dispersive Spectroscopy-Secondary Electron Microscope) techniques. Phase relations in the MSH ternary appear complex. Complete melting of experiments at mantle adiabatic temperatures indicates that hydrous lithologies would melt at ambient lower mantle conditions. The stability of phase D has been confirmed at lower mantle conditions equivalent to subducted geotherms, suggesting that phase D remains stable in cold subducted slabs only. The [101] peak for this phase varies systematically as a function of pressure, thought to be related to variations in composition. Phase D was not found in high pressure experiments (>~45 GPa). A possible new phase, phase H, previously predicted to exist by Tsuchiya (2013) was observed in some experiments. This new phase may be important in storing and delivering water to the lower mantle. The NSK system, was systematically investigated at lower mantle conditions. At pressures <50 GPa and mantle adiabatic temperatures, melting occurs. Above 50 GPa, CF (Calcium-ferrite type phase) was stable in all compositions. NAL (New Aluminous phase), was found in potassium rich compositions at this pressure and is thought to be stabilised to higher pressures by potassium. This work implies that CF and NAL are both stable in sediments and MORB, below 45 GPa, but as pressure increases to >50 GPa CF alone is stable. Differentiating between NAL and CF found in diamond inclusions using potassium content may not be valid, since CF is able to contain potassium in its structure at high pressure.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available