Shock-recovery experiment in dunite: shock features and melting hotspots from controlled imperfections.

Introduction: Shock-recovery experiments are standard practice for geoscientists who desire to understand and reproduce shock features in planetary rocks [1]. When carefully designed [2,3], such experiments can induce pressures from a few GPa (impedance technique) to 70 GPa (reverberation technique). In this study, we describe the results of a shock-recovery experiment using the reverberation technique, which surpassed initial expectations by producing localized melting within a dense dunite host. The study was initially designed to investigate shock melting of troilite and its potential migration into fractures [4]. For this reason, troilite powder was placed into drilled cavities (DCs) within a dunite disk before shock loading.

Fig. 1. Setup of the cylindrical shock-recovery experiment with a) the dunite sample disk with drilled cavities (DCs), and b) the cylindrical assemblage. A tantalum foil wrapped the sample.

 

Methods: A dunite [5] sample disk hosted nine DCs of ~1 mm diameter and a few hundred μm in depth. Three DCs were filled with troilite (FeS [6], Fig. 1a). The sample disk was wrapped in tantalum (Ta) foil, placed into an ARMCO-iron (i.e., soft iron, >99.85% Fe) sleeve, and tightly sealed with a piston-like ARMCO-iron element (Fig. 1b). The setup was impacted by propelling a flyer plate onto an ARMCO-iron driver plate positioned ahead of the sample disk.

After shock loading, the sample was analysed at three locations: (i) the bottom surface of the sample adhering to the driver plate, (ii) the surface of the piston-like iron element, and (iii) transverse cross-sections of the disk and driver plate. Computer tomography was used to locate the DCs before preparing thin sections. Semi-quantitative chemical analyses (energy-dispersive spectrometry, EDS) were conducted to characterize the melts. The experiment was modeled with the iSALE shock physics code [7], simulating the sample with a FeS-filled DC, including Ta foil and an iron driver plate. 

Fig. 2. SEM-BSE images depicting a) the heavily shocked central DC initially filled with FeS, inducing Ta, silicate, and sulfide melts, b) an initially empty DC with shock-induced Ta melts and partially melted olivine, c) an EDS map showing intrusions of Ta, silicate, and troilite melts in fractures of the dunite disk, and d) partial melts of olivine in contact with Ta melts.

 

Results: The sample was shock-loaded to a pressure of ~60 GPa after reverberations. The shock caused fracturing of the sample disk, inducing large tensile fractures and parallel fractures in olivine grains. Substantial crushing of olivine grains at DC locations triggered the formation of larger shock-induced cavities. The experimental pressure was sufficient to induce collapse of the DCs and partial melting of troilite within the FeS-filled DCs. However, the presence of DCs caused secondary pressure reverberations with localized melting of metallic phase components (iron, Ta foil) and silicates alongside troilite.

The resulting melts intruded into fractures, forming melt veins and melt breccia within and around the former DCs (Fig. 2). Partial melting of olivine with Ta is observed at the DCs (Fig. 2d). In the FeS-filled DC (Fig. 2a,c), the intruding melts contain FeS-rich particles embedded in a matrix of Ta- and silicate-rich melt. These FeS particles developed elongated morphologies (Fig. 2c). At initially empty DCs, Fe- and Ta-rich particles included a matrix of Ta- and silicate-rich melts (Fig. 2b). Some FeS-rich melts remained as dense clusters, exhibiting partial melting. Notably, melts at the site of initially empty DC cemented large SICs that had developed through the entire thickness of the samples beneath the DCs (Fig. 2 b). Some melts entrained unaffected olivine fragments.

 

Discussion and conclusions: All observations on the sample disk (melting, fracturing, deformations) generally aligned with the numerical models, except for the melting of Ta and iron  (Table 1); iSALE does not simulate fracturing and melt migration. Although troilite was not the dominant contributor to melt intrusion into fractures, the resulting melt veins resemble melt breccias [8], which typically form when melts entrain grains while propagating into a less shocked environment (here, away from the DCs).

According to numerical models, these melts occurred from DCs that caused pressure concentration points (hotspots) where pressures exceeded the nominal shock pressure of ~60 GPa by at least 10 GPa. Considering that Ta appeared to shock-melt more intensively than the silicate and troilite components, the sequence of shock melting needs further explanation. Tantalum melts at 3000 K at ambient pressure or at shock pressures over 150 GPa. In contrast, troilite shock-melts at pressures >60 GPa, and dunite at >100 GPa. Therefore, the shock melting of Ta likely resulted from several compressive and tensile events, which allowed for cumulative heat buildup.

We hypothesize that Ta foil, initially bent, was first shocked by the collapsing iron driver plate and then re-shocked by the impact of its shock-heated fragments projected into the DC walls during the collapse (spalling). Additional pressure amplification occurred due to the complete closure of the DCs and the shock compression of troilite grains or dunite fragments located within them. The added thermal input likely tipped the Ta over its melting point.

In conclusion, the configuration of DCs enabled the creation of localized melting, whereas the dunite primarily experienced fracturing without melting elsewhere in the sample disk. These findings are significant for shock-recovery experiments aiming to produce melt, melt breccias, or shock veins. Strategically doping DCs with materials of interest enables targeted melting effects. Moreover, the design of metallic sample holders with embedded DCs may allow for controlled and simultaneous shock-loading of multiple materials in a single experimental run.

 

References: 

[1] Stöffler et al. (2018) Meteorit. Planet. Sci. 53, 5–49. [2] Langenhorst and Deutsch (1994) Earth Planet. Sci. Lett. 125, 407–420. [3] Langenhorst and Hornemann (2005) EMU Notes Mineral. 7, 357–387. [4] Kohout T. et al. (2014) Icarus, 228, 78–85. [5] Heikura P. et al. (2010) Geological Survey of Finland, report M06/4723/2009/68, 50 pp. [6] Moreau et al. (2021) Meteorit. Planet. Sci., 57(3), 588–602. [7] Wünnemann K. et al. (2006) Icarus, 180, 514-527. [8] Pati J. K. and Reimold W. U. (2007) J. Earth Syst. Sci., 116(2), 81–98.