RESULTS FOR A NEW COOLING RATE PROBE APPLIED TO MORB FROM THE GALAPAGOS SEAFLOOR

Thursday, 2 February 2017
Marina/Gretel (Hobart Function and Conference Centre)
Andrew B. Palm1, Leo Pure1,2, Penelope Lineton King1, Charles Le Losq1, Alice Colman3, John M Sinton3, Michael S Ramsey4 and Rachel Lee5, (1)Australian National University, Research School of Earth Sciences, Canberra, ACT, Australia, (2)Victoria University of Wellington, Wellington, New Zealand, (3)University of Hawaii at Manoa, Honolulu, HI, United States, (4)University of Pittsburgh, Pittsburgh, PA, United States, (5)SUNY College at Oswego, Oswego, NY, United States
Abstract:
Understanding the processes that form seafloor basalts is critical for predicting their eruptive behaviour and the genesis of some ore deposits. Numerous models exist for calculating the temperature of sub-seafloor basaltic magma chambers, but there are few methods for calculating cooling rates of basalts post-eruption. This is a surprising gap in our knowledge given the role of the cooling rate on basalt morphology and eruptive behavior over time.

Our aim is to determine whether reflectance micro-Fourier-Transform Infrared (R-μFTIR) spectroscopy records the cooling rate of basaltic glasses. Industrial silicate glasses show changes in the position of the R-μFTIR band at ~800-1100 cm-1related to the average Si-O bonding, which may be a function of the thermal history (if composition remains constant).

A Galapagos MORB starting material with ~50 wt. % SiO2was heated using a differential scanning calorimeter to above the glass transition temperature (1200°C) and then cooled at different rates to 850°C (10, 25, 50, 60, 70 and 100 °C/min). Each sample was quenched in water and analyzed using R-μFTIR.

The cooling rate of the glass is negatively correlated with the R-μFTIR band position (in cm-1). Glasses formed with slower cooling rates have a band position at higher wavenumbers than glasses cooled at faster rates. This provides a good indication that polymerization and structural relaxation in glasses is heavily influenced by cooling rate: the average bond length is smaller in slowly cooled glasses resulting in a band position at higher wavenumbers. This technique provides a new method for probing the complete thermal history of natural basaltic glasses.

We compared our experimental results to a naturally quenched Galapagos MORB glass of similar composition. Our calibration indicates that the natural glass cooled at ~60-70 °C/min, consistent with expectations in a submarine setting.

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