McLeod, Graham William
Titanite zoning and magma mixing.
PhD thesis, University of Glasgow.
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Titanite is a calcium titanium silicate accessory mineral that serves as a sink for trace elements, especially the rare-earth elements and the high-field strength elements (Cerny and Rivadisa.L 1972; Groat, Carter et al. 1985; Enami, Suzuki et al. 1993; Perseil and Smith 1995; Della Ventura, Bellatreccia et al. 1999; Piccoli, Candela et al. 2000). It is also characterised by very sluggish diffusion of these elements, and as such it has the ability to resist most sub-solidus alteration. These characteristics make titanite a prime candidate for the ability to preserve a record of the petrogenetic processes which formed the igneous rocks in which it is found.
In order to assess titanite’s ability to serve as a petrogenetic tool, a textural and geochemical study of titanite from different igneous environments was undertaken. The Ross of Mull Granite is an igneous complex located on the south west coast of the Isle of Mull, Scotland. It is a Caledonian age pluton that displays evidence of magma-mixing processes at depth and also contains notable amounts of titanite. Samples of different host granite and diorite enclave lithologies and relationships were documented and collected in order to asses the ability of titanite to retain evidence of magma-mixing processes within its compositional zoning.
Titanite from the Ross of Mull Granite displays a large variety of textural features, both with respect to crystal shape and compositional zoning. The variation of the trace elements observed in titanite is a direct reflection of the compositional zoning pattern. Melts that are enriched in particular trace elements pass on that signature to the titanites. If melts of significantly contrasting chemistries mix or mingle, then titanite may record any significant instance of trace element diffusion between the melts. More importantly, there doesn’t need to be a transfer of trace elements between mingling/mixing melts for titanite to record evidence of the interaction. If the melts are of significantly different oxygen fugacities, then titanite may become destabilised and partially dissolve. This dissolution may only be temporary, but it may be difficult to tell if it was a product of increasing heat of the melt of if it was due to a change in oxygen fugacity. Titanite however, may record evidence of the cause of dissolution based on the nature of re-growth. As shown by some of the titanites within the Ross of Mull Granite, re-growth following a period of dissolution may be attributed to a change in oxygen fugacity if the new titanite growth is characterised by unusually low REE content. This is related to the nature of trace element incorporation in the titanite structure; REE are taken up due to coupled substitution mechanisms involving tetravalant Al and Fe. If a melt becomes reduced then the activity of ferric Fe will also be reduced, thus inhibiting the ability of titanite to take up REE by affecting the activity of the coupled substitution (Wones 1989; Piccoli, Candela et al. 2000).
Titanite from the ROMG is largely characterised by compositional zoning that reflects the trace element chemistry of one of the end-member lithologies (host granite or dioritic enclaves), or has compositional zoning that reflects the changing conditions of the magmatic environment, namely: changes in melt composition, changes in temperature and pressure and changes in oxidation of the magma. In the case of the ROMG, all of these changes were wholly, or in part, brought about by magma mixing processes. Another way in which titanite reflects processes of petrogenesis is that it has been found to reflect the degree of homogenisation of the melt from which it has crystallised.
To further asses the ability of titanite serve as a petrogenetic tool, a study was made of the characteristics of titanite from the volcanic environment. The Fish Canyon Tuff, from the San Juan volcanic field, Colorado, USA, was chosen as a suitable candidate for study due to its well documented nature; it is the largest known pyroclastic eruption to be documented. The Fish Canyon tuff also contains numerous phenocrysts of titanite.
Eruption of the Fish Canyon Tuff is thought to have been brought about by the thermal rejuvenation of a batholith-sized magma chamber which had previously cooled to a rigid crystal mush (Bachmann and Bergantz 2003). The main conclusion from the study of titanites from the Fish Canyon Tuff and is that evidence for the thermal rejuvenation of the magma chamber by a mafic melt prior to eruption is preserved in the textural and chemical signature preserved in the compositional zoning of the titanites as dissolution and re-growth textures.
This study illustrates that titanite is able to not only preserve evidence of magma mixing processes occurring at depth, but it is also able to preserve evidence of similar processes all the way through to the volcanic environment.
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