Journal of Conference Abstracts

Ultrabasic and Gabbroic Cognate Xenoliths from a Tholeiitic Subvolcanic Sill Complex: Implications for Fractional Crystallization and Crustal Contamination Processes

R. Jeremy Preston¹ (j.preston@abdn.ac.uk) & Brian R. Bell² (b.bell@geol.gla.ac.uk)

¹ Department of Geology & Petroleum Geology, Meston Building, King's College, University of Aberdeen, Old Aberdeen AB24 3UE.

² Department of Geology & Applied Geology, University of Glasgow, Glasgow G12 8QQ.

Intruded into the Palaeogene lava field and underlying Moine (Neoproterozoic) crystalline basement rocks around Loch Scridain, Isle of Mull, Scotland, is a suite of high-level, inclined, xenolithic sheets, ranging in composition from basalt, through andesite and dacite, to rhyolite. These sheets, associated with the Mull central volcano, were emplaced post 55Ma. The ⁸⁷Sr/⁸⁶Sr₅₅ of the entire suite ranges from 0.7037-0.7154 in the basalts and basaltic andesites, to over 0.7203 in the rhyolites, with ¹⁴³Nd/¹⁴⁴Nd₅₅ ranging from 0.5129 to 0.5118 over the same compositional spectrum. This greatly extends the range of Sr-Nd isotope values seen in volcanic rocks of the BTIP. The petrogenesis of the host sheets can be shown to entail AFC-type contamination processes involving a tholeiitic basalt parent magma of the Preshal More type, and contaminants derived from the Moine metasediments. As well as numerous crustal xenoliths, the more basic members of the complex contain a diverse suite of ultrabasic and basic xenoliths. Xenolith types include feldspathic peridotite with cumulus olivine and cumulus Cr-spinel, pyroxenite, gabbro with cumulus plagioclase and cumulus clinopyroxene, and pure anorthosite. Olivine in the feldspathic peridotites is unzoned and of a relatively constant composition (Fo_83-85). Cumulus plagioclase in the gabbros and anorthosite is typically highly calcic, and often strongly zoned (An_70-85). Intercumulus and cumulus clinopyroxene is typically a diopsidic augite (~Wo₄₅En₄₁Fs₈), although cumulus pyroxene in the pyroxenites shows a marked Fe-enrichment from core to rim (Wo₅₀En₄₁Fs₉-Wo₃₆En₃₂Fs₃₂). This data, coupled with whole-rock major- and trace-element data from a small number of the xenoliths suggest that the xenoliths represent early-formed cumulates, cognate with their host basalts. Sr and Nd isotope data from the xenoliths confirms the cognate origin, with the values being similar to the less-evolved members of the suite (e.g. ⁸⁷Sr/⁸⁶Sr₅₅ = 0.7081-0.7092, and ¹⁴³Nd/¹⁴⁴Nd₅₅ ~ 0.5119). This suggests that the basalts were still capable of precipitating ultrabasic cumulates, and also shows that the basic magmas suffered crustal contamination at an early stage.

What Drives Displacive Phase Transitions in Silicates?

Alexandra K. A. Pryde¹ (alix@minp.esc.cam.ac.uk), Martin T. Dove¹ (martin@minp.esc.cam.ac.uk), Volker Heine² (vh200@phy.cam.ac.uk) & Kenton D. Hammonds^1,2 (kenton@minp.esc.cam.ac.uk)

¹ Department of Earth Sciences, University of Cambridge, Cambridge, U.K.

² Theory of Condensed Matter, Cavendish Laboratory, University of Cambridge, Cambridge, U.K.

Many framework silicates and aluminosilicates exhibit structural instabilities which give rise to displacive phase transitions. It is commonly found that these transitions occur without causing significant distortion to the constituent polyhedral units (e.g. SiO₄ tetrahedra) which merely pivot around the bridging oxygen atoms in the framework. A phonon which causes such a lattice distortion is hence known as a Rigid Unit Mode (RUM). Since it will require minimal stretching of Si-O bonds and bending of <O-Si-O> angles, a RUM provides a low energy means of distortion for the crystal lattice.

Lattice dynamical calculations have been used to determine the existence and location in reciprocal space of RUMs. They have revealed that there are often many RUMs situated at various locations in reciprocal space. This suggests that there is a choice of RUM distortions available to such a crystal and hence a choice of daughter structures into which the crystal may transform. Naturally this begs the question "What decides which RUM goes soft first and precipitates the displacive phase transition?"

Semi-empirical interatomic potentials have been employed to perform lattice energy minimisation calculations on silicates including quartz and tridymite. From these, we have extracted the lattice energy and the size of the different contributions to the lattice energy (e.g. coulombic, van der Waals) as a function of the extent of progression of the phase transition from a high phase into various lower (often hypothetical) phases. These will form the basis of a study to find answers to the question voiced in the previous paragraph.