Deformation and thermal histories of ordinary chondrites: Evidence for post-deformation annealing and syn-metamorphic shock

Abstract-Mason Gully, the second meteorite recovered using the Desert Fireball Network (DFN), is characterized using petrography, mineralogy, oxygen isotopes, bulk chemistry, and physical properties. Geochemical data are consistent with its classification as an H5 ordinary chondrite. Several properties distinguish it from most other H chondrites. Its 10.7% porosity is predominantly macroscopic, present as intergranular void spaces rather than microscopic cracks. Modal mineralogy (determined via PS-XRD, element mapping via energy dispersive spectroscopy [EDS], and X-ray tomography [for sulfide, metal, and porosity volume fractions]) consistently gives an unusually low olivine/orthopyroxene ratio (0.67À0.76 for Mason Gully versus~1.3 for typical H5 ordinary chondrites). Widespread "silicate darkening" is observed. In addition, it contains a bright green crystalline object at the surface of the recovered stone (diameter % 1.5 mm), which has a tridymite core with minor a-quartz and a rim of both low-and high-Ca pyroxene. The mineralogy allows the calculation of the temperatures and ƒ(O 2 ) characterizing thermal metamorphism on the parent body using both the two-pyroxene and the olivine-chromite geo-oxybarometers. These indicate that MG experienced a peak metamorphic temperature of~900°C and had a similar ƒ(O 2 ) to Kernouv e (H6) that was buffered by the reaction between olivine, metal, and pyroxene. There is no evidence for shock, consistent with the observed porosity structure. Thus, while Mason Gully has some unique properties, its geochemistry indicates a similar thermal evolution to other H chondrites. The presence of tridymite, while rare, is seen in other OCs and likely exogenous; however, the green object itself may result from metamorphism.

Section: Absence Of Evidence For Shock Heatingmentioning

confidence: 72%

“…These are also absent in MG; however, it has been argued that these features can be removed by subsequent annealing of the shocked material (Rubin ), particularly if these impacts occurred while the parent body was still hot (≥800–1000 °C) (Ruzicka et al. ).…”

Section: Discussionmentioning

confidence: 99%

Characterization of Mason Gully (H5): The second recovered fall from the Desert Fireball Network

Dyl

Benedix

Bland

et al. 2016

“…The mechanisms are, in most cases, recorded in petrofabric (e.g., distribution of minerals), textural relationships of minerals (e.g., grain size distribution), and nanostructure (e.g., intracrystalline deformations). In the specific case of meteorites, the static metamorphism during accretion and impact-related processes operate under extremely different conditions; therefore, the mechanisms are usually well recorded in the highly contrasting textural relationships (e.g., Tomkins 2009;Tait et al 2014;Guignard and Toplis 2015;Krzesi nska 2017), fabric intensities (e.g., Gattacceca et al 2005;Friedrich 2008;Krzesi nska et al 2015;Forman et al 2017;Krzesinska and Almeida 2019), or micro-and nanostructural details of minerals (e.g., Hanna et al 2015;Ruzicka et al 2015;Ruzicka and Hugo 2018).…”

Section: Introductionmentioning

confidence: 99%

Petrogenesis of ungrouped enstatite meteorite Zakłodzie: Fabric, texture, and nanostructure analysis for identification of mechanisms responsible for chondrite–achondrite transition

Krzesińska

Wirth

Kusiak

2019

Zakłodzie is an enstatite meteorite of unknown petrogenesis. Chemically, it resembles enstatite chondrites, but displays an achondrite‐like texture. Here we report on fabric and texture analyses of Zakłodzie utilizing X‐ray computed tomography and scanning electron microscopy and combine it with a nanostructural study of striated pyroxene by transmission electron microscopy. With this approach we identify mechanisms that led to formation of the texture and address the petrogenesis of the rock. Zakłodzie experienced a shock event in its early evolution while located at some depth inside a warm parent body. Shock‐related strain inverted pyroxene to the observed mixture of intercalated orthorhombic and monoclinic polymorphs. The heat that dissipated after the peak shock was added to primary, radiogenic‐derived heat and led to a prolonged thermal event. This caused local, equilibrium‐based partial melting of plagioclase and metal‐sulfide. Partial melting was followed by two‐stage cooling. The first phase of annealing (above 500 °C) allowed for crystallization of plagioclase and for textural equilibration of metal and sulfides with silicates. Below 500 °C, cooling was faster and more heterogeneous at cm scale, allowing retention of keilite and quenching of K‐rich feldspathic glass in some parts. Our study indicates that Zakłodzie is neither an impact melt rock nor a primitive achondrite, as suggested in former studies. An impact melt origin is excluded because enstatite in Zakłodzie was never completely melted and partial melting occurred during equilibrium‐based postshock conditions. Texturally, the rock represents a transition of chondrite and achondrite and was formed when early impact heat was added to internal radiogenic heat.

“…; Ruzicka et al. ; Tomioka and Miyahara ; Ruzicka and Hugo ), and computational simulations (e.g., Baer and Trott ) have addressed the effects of shock exposure. The most common macroscopic and microscopic manifestations of shock wave exposure in meteorites are deformational, transformational, or a combination of both (Sharp and DeCarli ).…”

Section: Introductionmentioning

confidence: 99%

“…The observable effects of such shock exposure are numerous and varied, depending not only on collisional parameters but also on the nature and composition of the target rock, the in situ surroundings and depth of the ejected meteorite, and the distinguishing behavior of constituent phases in reaction to shock wave subjection. Many studies, based on experimental results (St€ offler et al [1991] and references therein), natural observations (Chen et al 1996;Sharp 1997;Langenhorst and Greshake 1999;Langenhorst and Poirier 2000;Xie et al 2006;Zhang et al 2006;Bl€ aß et al 2010;Acosta-Maeda et al 2013;Ruzicka et al 2015;Tomioka and Miyahara 2017;Ruzicka and Hugo 2018), and computational simulations (e.g., Baer and Trott 2002) have addressed the effects of shock exposure. The most common macroscopic and microscopic manifestations of shock wave exposure in meteorites are deformational, transformational, or a combination of both .…”

Section: Introductionmentioning

confidence: 99%

Absence of olivine orientation fabric in highly shocked Martian dunite

Tkalcec

Brenker

2018

Shock is often given as the cause for many observations in meteorites due to the assumed previous exposure of most meteorites to at least one impact event that ultimately led to their ejection from their parent body. Here we present electron backscatter diffraction (EBSD) results on a substantially shocked dunitic achondrite, chassignite Northwest Africa (NWA) 8694, and question the general culpability of shock exposure for the formation of preferred orientation fabrics of meteoritic olivine crystals. Despite the ubiquitous presence of substantial shock indicators, the EBSD results for NWA 8694 reveal an absence of preferred orientation of olivine crystals, displaying instead an overall random fabric. We propose that the passage of shock waves through olivine crystals within a solid, crystalline, dunitic rock does not produce an overall preferred orientation, nor is it likely to actively form a whole‐rock random fabric but instead has likely no bearing on the formation of olivine orientation fabrics.