Numerical simulation of temperature effects at fissures due to shock loading

Experimental shock synthesis of diamonds in a graphite gneissAbstract-The occurrence of diamonds in terrestrial impact craters and meteorites is related to dynamic shock loading during hypervelocity impacts. To understand the mechanism of impact diamond formation in natural rocks, shock-recovery experiments with graphite gneiss were carried out at shock pressures between 35 and 79 GPa. This is the first report on the successful shock synthesis of microdiamonds in a natural rock. Micrometer-size diamonds and a wide range of intermediate, presently unclassified, amorphous, and disordered carbon phases were observed within vesiculated biotite melts in the vicinity of relic graphite grains using microRaman spectrometry. We explain these findings by jetting mechanisms of carbon and graphite clusters, originating at the edges of graphite grains, into the very hot and volatile rich biotitic melt veins during shock loading. This environment enabled the thermally activated crystallization of diamonds during shock compression in a period of less than 0.5 µsec. Regraphitization of diamonds during pressure release was widespread and caused the formation of the amorphous to disordered carbon phases recorded frequently with microRaman spectroscopy. The surviving diamonds must have cooled down to 2000 K during the compression phase at local thermal sinks and cooler interfaces to avoid regraphitization.

Section: Resultsmentioning

confidence: 99%

Section: Proposed Scenario Of Diamond Formationmentioning

confidence: 99%

Experimental shock synthesis of diamonds in a graphite gneiss

Kenkmann

Hornemann

Stöffler

2005

“…Therefore, these authors suggested lower shock pressures for their zircon-reidite sample than indicated by petrography. However, heterogeneities in shock are commonly induced by positive pressure and temperature excursions from the equilibrium shock conditions (Stˆffler and Langenhorst 1994;Heider and Kenkmann 2003). For example, micrometer-size hotspots that can occur during the shock compression of porous solids (DeCarli et al 2002) must be considered for zircon, which is prone to developing porosities from auto-irradiation.…”

Section: Planar Microstructures and Reiditementioning

confidence: 99%

Shock‐metamorphosed zircon in terrestrial impact craters

Wittmann

Kenkmann

Schmitt

et al. 2006

Self Cite

155

201

Shock-metamorphosed zircon in terrestrial impact cratersAbstract-To ascertain the progressive stages of shock metamorphism of zircon, samples from three well-studied impact craters were analyzed by optical microscopy, scanning electron microscopy (SEM), and Raman spectroscopy in thin section and grain separates. These samples are comprised of well-preserved, rapidly quenched impactites from the Ries crater, Germany, strongly annealed impactites from the Popigai crater, Siberia, and altered, variably quenched impactites from the Chicxulub crater, Mexico. The natural samples were compared with samples of experimentally shock-metamorphosed zircon. Below 20 GPa, zircon exhibits no distinct shock features. Above 20 GPa, optically resolvable planar microstructures occur together with the high-pressure polymorph reidite, which was only retained in the Ries samples. Decomposition of zircon to ZrO 2 only occurs in shock stage IV melt fragments that were rapidly quenched. This is not only a result of post-shock temperatures in excess of ∼1700 °C but could also be shock pressure-induced, which is indicated by possible relics of a high-pressure polymorph of ZrO 2 . However, ZrO 2 was found to revert to zircon with a granular texture during devitrification of impact melts. Other granular textures represent recrystallized amorphous ZrSiO 4 and reidite that reverted to zircon. This requires annealing temperatures >1100 °C. A systematic study of zircons from a continuous impactite sequence of the Chicxulub impact structure yields implications for the post-shock temperature history of suevite-like rocks until cooling below ∼600 °C.

“…Melting occurs due to pressure peaks caused by differing physical properties at phase boundaries (Fritz et al 2005a;Stewart et al 2007) and is especially pronounced, when open pore space is present (Stöffler et al 1991;Heider and Kenkmann 2003;Stewart et al 2007). Melt formation is important here, because melting changes the diffusion parameters of the phases completely.…”

Section: The Role Of Shock-induced Melting For the He Retentivitymentioning

confidence: 99%

Helium loss from Martian meteorites mainly induced by shock metamorphism: Evidence from new data and a literature compilation

Schwenzer

Fritz

Stöffler

et al. 2008

available online at http://meteoritics.org Helium loss from Martian meteorites mainly induced by shock metamorphism:Evidence from new data and a literature compilation Abstract-Noble gas data from Martian meteorites have provided key constraints about their origin and evolution, and their parent body. These meteorites have witnessed varying shock metamorphic overprinting (at least 5 to 14 GPa for the nakhlites and up to 45-55 GPa (e.g., the lherzolitic shergottite Allan Hills [ALH] A77005), solar heating, cosmic-ray exposure, and weathering both on Mars and Earth. Influences on the helium budgets of Martian meteorites were evaluated by using a new data set and literature data. Concentrations of 3 He, 4 He, U, and Th are measured and shock pressures for same sample aliquots of 13 Martian meteorites were determined to asses a possible relationship between shock pressure and helium concentration. Partitioning of 4 He into cosmogenic and radiogenic components was performed using the lowest 4 He/ 3 He ratio we measured on mineral separates ( 4 He/ 3 He = 4.1, pyroxene of ALHA77005). Our study revealed significant losses of radiogenic 4 He. Systematics of cosmogenic 3 He and neon led to the conclusion that solar radiation heating during transfer from Mars to Earth and terrestrial weathering can be ruled out as major causes of the observed losses of radiogenic helium in bulk meteorites. For bulk rock we observed a correlation of shock pressure and radiogenic 4 He loss, ranging between ~20% for Chassigny and other moderately shocked Martian meteorites up to total loss for meteorites shocked above 40 GPa. A steep increase of loss occurs around 30 GPa, the pressure at which plagioclase transforms to maskelynite. This correlation suggests significant 4 He loss induced by shock metamorphism. Noble gas loss in rocks is seen as diffusion due to (1) the temperature increase during shock loading (shock temperature) and (2) the remaining waste heat after adiabatic unloading (post shock temperature). Modeling of 4 He diffusion in the main U,Th carrier phase apatite showed that post-shock temperatures of ~300 °C are necessary to explain observed losses. This temperature corresponds to the post-shock temperature calculated for bulk rocks shocked at about 40 GPa. From our investigation, data survey, and modeling, we conclude that the shock event during launch of the meteorites is the principal cause for 4 He loss.