Dellen crater impact melt rock: pyroxenes as a proxy for 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. The 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 wide and 200-500 m thick for which impact ages  between 90, 110, to 140 Ma were determined [1]-[2]. 

The impact melt glass (commonly referred as dellenite) consisting of phenocrysts of subhedral orthopyroxene (Wo₄En₆₃Fs₃₃), skeletal plagioclase (average An₅₉Ab₃₈Or₃), and euhedral magnetite within a glassy matrix of rhyolitic composition. MicroFTIR measurements performed in the glass matrix and euhedral glass blebs within the skeletal plagioclase show the presence of 1.4 wt% water in the rhyolitic glass. We estimated that the dellenite vitrified at ca. P=200-300 bar by comparing the glass water content  with the water solubility of a similar composition silicate melt following  Papale's model [3].

Dellenite orthopyroxene was studied by polarized FTIR spectroscopy, Mössbauer spectroscopy, single crystal X-ray diffraction and EMPA. They are iron rich enstatite (Wo₄ En₆₃ Fs₃₃) with a Fe³⁺/Fe_tot ratio of 1.6%. They have very weak to absent OH vibrational bands in the IR spectra, corresponding to H₂O contents ranging from 0 to 39 ppm H₂O, a variability suggesting 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 [4]-[5].

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 by 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 SC-XRD 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:

[1]Henkel H (1992) Tectonophysics 216, 31-40

[2]Mark DF Lindgren P Fallick AE (2014) Geological Society of London Special Publication 378, 349-366

[3]Papale P (1999) Am Mineral 84, 477-792

[4]Weis FA Skogby H Stalder R (2016)  Am Mineral 101(10): 2233-2247

[5]Nazzareni S Barbarossa V Skogby H Zanon V Petrelli M (2020)  CMP 175:87