Trace element sector zoning in clinopyroxene as a function of undercooling: An experimental evaluation in trachybasaltic magmas

MacDonald, Alice1, Ubide, Teresa1, Masotta, Matteo2, Mollo, Silvio3, Pontesilli, Alessio4

1University Of Queensland, Brisbane, Australia, 2University of Pisa, Pisa, Italy, 3Sapienza University of Rome, Rome, Italy, 4University of Otago, Dunedin, New Zealand

Clinopyroxene chemistry is increasingly being utilised to investigate magmatic processes, due to its ability to record an extensive history of physiochemical changes in the host magma. However, clinopyroxene chemistry is not only influenced by pressure, temperature, and magma composition, but also by kinetic effects that may generate compositional zoning, such as sector zoning. Previous experimental work has highlighted the role of undercooling (ΔT = T­liquidus – Tsystem) on crystal morphology and major element chemistry of sector-zoned clinopyroxene. In this regard, the spatial distribution of trace elements in clinopyroxene remains relatively underexplored.
Here we present trace element data collected by laser ablation ICP-MS mapping of experimental sector-zoned clinopyroxenes crystallised from a trachybasalt melt representative of one of the most primitive magmas ever erupted at Mt Etna volcano (Italy). Experiments were conducted an isobaric pressure of 400 MPa, and a range of temperatures (1050 – 1200°C) and H2O contents (0-4 wt.%). Undercooling was attained by cooling the experiments from a starting temperature (1300°C) above the clinopyroxene liquidus at a rate of 80°C/min to a resting temperature (Tsystem)  resulting in a range of ΔT (23-173°C).
Our results indicate that clinopyroxene crystals show different styles of zoning across the entire range of ΔT, where zones enriched in Al are also enriched in HFSEs and REEs. At ΔT < 40 °C, clinopyroxene is sector-zoned with distinct Al-poor hourglass and Al-rich prism sectors. At ΔT = 75 – 123°C, skeletal morphologies dominate as crystal growth transitions from interface controlled to diffusion limited. These crystals are comprised of Al-rich skeletons and Al-poor overgrowths. At ΔT = 123-173°C, clinopyroxene is primarily dendritic, with subtle Al zoning.
The overall correlation between Al and trace element composition is attributed to charge-balancing mechanisms. Highly charged cations are favourably incorporated into the M1 and M2 sites with increasing Al, to compensate for the substitution of Si for Al in the tetrahedral site. 
Thermodynamic modelling of lattice strain parameters for 3+ cations in the M2 site (REEs + Y) illustrates that the strain-free partition coefficient (D0) is strongly correlated with ΔT. Conversely, the optimum radius (r0) and Young’s modulus (E), remain constant across our dataset.
The application of trace element calibrations to natural samples from Mt Etna supports the growing conception that sector zoning in clinopyroxene is related to low degrees of ΔT, whereas microlites crystallised at moderate degrees of ΔT. Our new experimental data could bring crucial new insights into magmatic processes which occur under polythermal and polybaric crystallisation regimes, in the lead up to volcanic eruptions. 


Biography

I am a current PhD candidate at the University of Queensland, supervised by Dr. Teresa Ubide. My research is focused on investigating sector-zoned clinopyroxenes from volcanic settings, through the analysis of natural and experimental samples using high-resolution techniques.

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