Re-equilibration of clinopyroxene-titanomagnetite clusters: the effect of isothermal annealing time and melt water content

Clustering of clinopyroxene (Cpx) and titanomagnetite (Tmt) is commonly observed in magmatic products and crystallisation experiments. The existence of crystallographic orientation relationships (CORs) between Cpx and Tmt is thought to indicate their formation by heterogeneous nucleation. Heterogeneous nucleation is promoted by high degrees of undercooling, and thus associated with disequilibrium microstructures and compositions. We studied the effect of isothermal annealing on Cpx-Tmt clusters exhibiting CORs, in order to examine whether information about cluster formation is preserved during re-equilibration at depth in a magmatic system.

We analysed samples synthesized in experiments of Pontesilli et al. (2019), which aimed to reproduce the crystallisation behaviour of an Etnean trachybasalt, under nominally anhydrous (0 wt.% H₂O) and hydrous (2 wt.% H₂O) conditions, at mid-crustal storage conditions (400 MPa, 1100°C, NNO+1 oxygen buffer), corresponding to a degree of undercooling of 120°C and 80°C, respectively. After superheating at 1300°C for 30 minutes, samples were cooled at 80°C/min to 1100°C and annealed for dwell times ranging from 0.5h to 8h.

We employed electron backscatter diffraction (EBSD) analysis to characterise microstructures and detect CORs. In hydrous samples, phase fraction, maximum crystal size, and perimeter/area ratio are unaffected by dwell time. In contrast, anhydrous samples exhibit decreasing crystal fraction with increasing dwell time. Although crystallinity falls overall in anhydrous samples, area fraction of Tmt increases slightly up to 2h dwell time. The increase in Tmt area correlates with an increase in maximum Tmt size and a decrease in Tmt perimeter/area ratio.

Tmt exhibits two closely related CORs to Cpx, COR1 ([-110]_tmt//[010]_cpx, [111]_tmt//(100)*_cpx, [-1-12]_tmt//[001]_cpx) and  COR 2 ([-110]_tmt//[010]_cpx, [-1-11]_tmt//(-101)*_cpx, [112]_tmt//[101]_cpx). The fraction of the total length of Cpx-Tmt boundaries that follow one of the two CORs (F_COR1+2) exceeds 60% in all samples. However, the relative frequencies of the two CORs vary. In hydrous samples with dwell times of 4h and below, F_COR2 (~55%) exceeds F_COR1 (~10%). However, at 8h dwell time, the frequency of both CORs is ~30%. In anhydrous samples at dwell times of 1h and below, the pattern is reversed, with F_COR1 (~40%) exceeding F_COR2 (~20%). The frequency of both CORs is once again ~30% for dwell times of 2h and above. The normalised abundance (total length/map area) of boundaries with a COR does not change in the hydrous samples, and only decreases slightly in anhydrous samples. After 8h, the total abundance of boundaries with a COR is similar, regardless of water content.

The different COR frequencies observed at short dwell times in hydrous and anhydrous samples imply that the Cpx-Tmt clustering mechanism is affected by degree of undercooling. Re-equilibration of COR frequencies progresses faster in the anhydrous samples, correlating with the greater intensity of microstructural re-equilibration observed. In rapidly cooled systems, relative frequencies of different Cpx-Tmt CORs could potentially be used to estimate degree of undercooling. Total abundance of boundaries associated with a COR remains constant, suggesting that Cpx-Tmt CORs preserve some information about heterogeneous nucleation on longer timescales.

Pontesilli et al. (2019), Chem Geol 510:113-129. 10.1016/j.chemgeo.2019.02.015

Funded by the Austrian Science Fund (FWF): P 33227-N