Dellen impact melt rock: pyroxenes as a proxy for the melt thermal history and water content

The Dellen impact structure is located in a ca. 20 km wide basin filled by two lakes in the east central Sweden. This complex structure is covered by a thick moraine deposit, and most impact melted material can be found as loose blocks and boulders scattered throughout the moraine. Geophysical measurements show the presence of a coherent impact melt body about 9 km in diameter and 200 to 500 m thick (Henkel, 1992) for which impact ages ranging between 90, 110, to 140 Ma have been determined (see Deutsch et al., 1992; Mark et al., 2014, and references therein).

Here we describe the petrography and mineral chemistry of an impact melt glass (commonly referred to as dellenite) consisting of phenocrysts of subhedral orthopyroxene (average Wo₄En₆₃Fs₃₃), skeletal plagioclase (average An₅₉Ab₃₈Or₃), and euhedral magnetite within a glassy matrix of rhyolitic composition.

Moreover we aimed to use the pyroxene crystals as a petrological tool to define the host rock cooling rate and the water content dissolved in their parental melt during pyroxene growth.

MicroFTIR measurements performed both in the glass matrix and in the euhedral glass blebs within the skeletal plagioclase show the presence of 1.4 wt% water in the rhyolitic glass. Comparing the water content of the glass with the water solubility of a silicate melt with the same composition as determined by the model by Papale, suggests that the studied sample vitrified at a pressure of about 200-300 bar.

Orthopyroxene crystals from the dellenite sample were studied by polarized FTIR spectroscopy, Mössbauer spectroscopy, single crystal X-ray diffraction and electron probe micro analysis. Dellenite orthopyroxenes are iron rich enstatite (Wo₄ En₆₃ Fs₃₃) with a Fe³⁺/Fe_tot ratio of 1.6%, as measured by Mößbauer spectroscopy.

The studied orthopyroxenes have very weak to absent OH vibrational bands in the IR spectra, corresponding to H₂O contents ranging from 0 to 39 ppm H₂O. These variable contents seem to indicate H loss during post-formation processes, which may occur via the relatively fast redox reaction Fe²⁺ + OH^- = Fe³⁺ + O^2- + ½H₂.

In volcanic pyroxenes H loss may be a common process occurring at different moments from crystallization to post-eruption. However, H incorporation in pyroxene is associated with point defects governed by slow kinetics diffusion rates, which are retained in the structure when H is lost. By reversing the redox reaction the original H content of pyroxene can thus be recovered by thermal annealing experiments under reducing conditions (Weis et al., 2016).

In order to restore H that was possibly lost, dellenite pyroxenes were thermally annealed under hydrogen atmosphere (at 1 Atm) in a horizontal glass-tube furnace at the Department of Geosciences (Natural History Museum Stockholm). We performed thermal annealing experiments at 700°C for 17 hours. FTIR spectra were recorded after each heating step. All the samples increased their hydrogen content and the final average water content is 77 ppm.

Cation partition as derived by the single crystal X-ray diffraction and EMPA were used to calculate the orthopyroxene closure temperature (Tc) which is expression of the cooling rate for the cpx-host rock. Preliminary results point to a high Tc (833°C) due to a quite fast host-rock cooling rate.

A tentative model for the evolution of the rhyolitic portion of the Dellen impact melt is under development merging geophysical data with our experimental data on water content of pyroxene and glass, and pyroxene geospeedometry.

 

 

References:

- Deutsch A., Buhl D., Langenhorst F. (1992) on the significance of crater ages: new ages for Dellen (Sweden) and Araguainha (Brazil). Tectonophysics, 216, 205-218.

- Henkel H., (1992) Geophysical aspects of meteorite impact craters in eroded shield environment, with special emphasys on electric resistivity. Tectonophysics, 216, 31-40.

- Mark D.F., Lindgren P., Fallick A.E. (2014) A high precision 40Ar/39Ar age for hydrated impact glass from the Dellen impact, Sweden. Geological Society of London Special Publication 378, 349-366.

- Papale P., (1999) Modelling the solubility of H2O+CO2 fluid in silicate liquids. American Mineralogist, 84, 477-792.

- Weis FA, Skogby H, Stalder R (2016) Experimental hydration of natural volcanic clinopyroxene phenocrysts under hydrothermal pressures (0.5–3 kbar). American Mineralogist 101(10): 2233-2247