Shock conditions recorded in NWA 8159 martian augite basalt with implications for the impact cratering history on Mars

NWA 8159 is an augite-rich martian basalt, formed by cooling of a relatively evolved, Ca-rich, Ti-poor and LREE-depleted lava, under relatively oxidizing conditions, during the early Amazonian. In addition to its distinct igneous petrogenesis and high fO_2, NWA 8159 is also set apart from most martian shergottites with respect to the low degree of shock metamorphism required to preserve crystalline igneous plagioclase (An_(50–65)). In this study, mineral transformations within and adjacent to shock veins in NWA 8159 were investigated using scanning electron microscopy, Raman spectroscopy and transmission electron microscopy to better constrain the unusal shock history of this meteorite. The transformation of olivine to ahrensite (Fe-ringwoodite) along shock vein margins, and tissintite and coesiteformed from igneous mineral (labradorite and silica) grains entrained as clasts within shock veins has been documented in this study. We report on a previously unidentified mineral assemblage of Ca-Na-majoritic garnet, sodic-clinopyroxene and stishovite crystallized from shock melt. This mineral assemblage indicates a crystallization pressure of approximately 16 GPa, which is within the range of previous shock pressure estimates for this meteorite (15–23 GPa). The presence of a majoritic garnet-bearing assemblage throughout veins up to 0.6 mm wide indicates that the sample remained at high-pressure throughout the melt vein quench. Based on thermal models, the sample must have remained at high pressure for ∼100 ms. This shock duration is an order of magnitude longer than those experienced by more highly shocked shergottites such as Tissint or Zagami (>30 GPa; 10–20 ms) and would seem to imply a relatively large impact event. Recent numerical models demonstrate that a range of shock pressures and durations are realized by rocks within the ejected spall zone of a hypervelocity impact. The shock conditions experienced by NWA 8159 therefore do not require an impact event distinct from other shergottites. Rather, our findings suggest that this meteorite originated from near the martian surface at the edge of the impact site. The shock history of NWA 8159 provides a picture of Mars consistent with that derived from remote observation; that of a random cratering process that samples a geologically long-lived and complex planet

Sm-Nd and Rb-Sr Isotopic Systematics of a Heavily Shocked Martian Meteorite Tissint and Petrogenesis of Depleted Shergottites

Tissint is a very fresh Martian meteorite that fell near the town of Tissint in Morocco on July 18, 2011. It contains abundant olivine megacrysts (~23%) in a fine-grained matrix of pyroxene (~55%), maskelynitized plagioclase (~15%), opaques (~4%) and melt pockets (~3%) and is petrographically similar to lithologies A and C of picritic shergottite EETA 79001 [1,2]. The presence of 2 types of shock-induced glasses and all 7 high-pressure mineral phases that were ever found in melt pockets of Martian meteorites suggests it underwent an intensive shock metamorphism of ~25 GPa and ~2000 C localized in melt pockets [2]. Mineral textures suggest that olivines, pyroxenes and plagioclases probably did not experience such hightemperature. Earlier determinations of its age yielded 596+/-23 Ma [3] and 616+/-67 Ma [4], respectively, for the Sm-Nd system and 583+/-86 Ma for the Lu-Hf system [4], in agreement with the 575+/-18 Ma age of the oldest olivine-phyric depleted shergottite Dho 019 [5]. However, the exposure ages of Tissint (~1 Ma [1, 6, 7]) and Dho 019 (~20 Ma [8]) are very different requiring two separate ejection events. These previously determined Sm-Nd and Lu-Hf ages are older than the Ar-Ar maskelynite plateau age of 524+/-15 Ma [9], reversing the pattern usually observed for Martian meteorites. In order to clarify these age issues and place models for Tissint's petrogenesis on a firm basis, we present new Rb-Sr and Sm- Nd isotopic results for Tissint, and discuss (a) the shock effects on them and the Ar-Ar chronometer, (b) correlation of the determined ages with those of other depleted shergottites, and (c) the petrogenesis of depleted shergottites. Since the meteorite is a recent fall, terrestrial contamination is expected to be minimal, but, the strong shock metamorphism might be expected to compromise the equilibrium of the isotopic systems

Basic principles of forest fuel reduction treatments

Successful fire exclusion in the 20th century has created severe fire problems across the West. Not every forest is at risk of uncharacteristically severe wildfire, but drier forests are in need of active management to mitigate fire hazard. We summarize a set of simple principles important to address in fuel reduction treatments: reduction of surface fuels, increasing the height to live crown, decreasing crown density, and retaining large trees of fire-resistant species. Thinning and prescribed fire can be useful tools to achieve these objectives. Low thinning will be more effective than crown or selection thinning, and management of surface fuels will increase the likelihood that the stand will survive a wildfire. Five empirical examples of such treatment are discussed: Hayfork fires, California, 1987; Tyee fire,Washington, 1994; Megram fire, California, 1999; Hayman fire, Colorado, 2002; and the Cone fire, California, 2002. Applying treatments at an appropriate landscape scale will be critical to the success of fuel reduction treatments in reducing wildfire losses in Western forests.Keywords: fire ecology, fuel treatment, western United States, prescribed fire, thinnin

Basic principles of forest fuel reduction treatments

Successful fire exclusion in the 20th century has created severe fire problems across the West. Not every forest is at risk of uncharacteristically severe wildfire, but drier forests are in need of active management to mitigate fire hazard. We summarize a set of simple principles important to address in fuel reduction treatments: reduction of surface fuels, increasing the height to live crown, decreasing crown density, and retaining large trees of fire-resistant species. Thinning and prescribed fire can be useful tools to achieve these objectives. Low thinning will be more effective than crown or selection thinning, and management of surface fuels will increase the likelihood that the stand will survive a wildfire. Five empirical examples of such treatment are discussed: Hayfork fires, California, 1987; Tyee fire,Washington, 1994; Megram fire, California, 1999; Hayman fire, Colorado, 2002; and the Cone fire, California, 2002. Applying treatments at an appropriate landscape scale will be critical to the success of fuel reduction treatments in reducing wildfire losses in Western forests.Keywords: Prescribed fire, Western United States, Thinning, Fire ecology, Fuel treatmen

Multi-technique equation of state for Fe_(2)SiO_4 melt and the density of Fe-bearing silicate melts from 0 to 161 GPa

We have conducted new equation of state measurements on liquid Fe_(2)SiO_4 in a collaborative, multi-technique study. The liquid density (ρ), the bulk modulus (K), and its pressure derivative (K′) were measured from 1 atm to 161 GPa using 1-atm double-bob Archimedean, multi-anvil sink/float, and shock wave techniques. Shock compression results on initially molten Fe_(2)SiO_4 (1573 K) fitted with previous work and the ultrasonically measured bulk sound speed (C_o) in shock velocity (U_S)-particle velocity (u_p) space yields the Hugoniot: U_S = 1.58(0.03) u_p + 2.438(0.005) km/s. Sink/float results are in agreement with shock wave and ultrasonic data, consistent with an isothermal K_T = 19.4 GPa and K′ = 5.33 at 1500°C. Shock melting of initially solid Fe_(2)SiO_4 (300 K) confirms that the Grüneisen parameter (γ) of this liquid increases upon compression where γ = γ_o(ρ_(o)/ρ)^q yields a q value of –1.45. Constraints on the liquid fayalite EOS permit the calculation of isentropes for silicate liquids of general composition in the multicomponent system CaO-MgO-Al_(2)O_3-SiO_2-FeO at elevated temperatures and pressures. In our model a whole mantle magma ocean would first crystallize in the mid-lower mantle or at the base of the mantle were it composed of either peridotite or simplified “chondrite” liquid, respectively. In regards to the partial melt hypothesis to explain the occurrence and characteristics of ultra-low velocity zones, neither of these candidate liquids would be dense enough to remain at the core mantle boundary on geologic timescales, but our model defines a compositional range of liquids that would be gravitationally stable

Multi‐technique equation of state for Fe 2 SiO 4 melt and the density of Fe‐bearing silicate melts from 0 to 161 GPa

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95366/1/jgrb17308.pd

Tracing the Evolution of Hydrogen in the Martian Crust Through Laboratory Studies of Apatite

Introduction: Northwest Africa (NWA) 7034 and its pairings represent a regolith breccia of basaltic bulk composition, the finegrained matrix of which bears a strong resemblance to the major and trace element composition estimated for the ancient southern highlands crust on Mars. Therefore, NWA 7034 may represent a key sample for constraining the composition of the Martian crust, particularly the ancient highlands. Here we seek to constrain the hydrogen isotopic composition of the Martian crust using apatite [Ca5 (PO4)3(Cl,F,OH)]. Apatites across all lithologic domains in NWA 7034 have been affected by a Pb-loss event at ~1.5 Ga before present and so they are unlikely to have retained magmatic volatile composition and are more likely to have equilibrated with fluids within the Martian crust that may or may not have exchanged with the Martian atmosphere

Shock conditions recorded in NWA 8159 martian augite basalt with implications for the impact cratering history on Mars

NWA 8159 is an augite-rich martian basalt, formed by cooling of a relatively evolved, Ca-rich, Ti-poor and LREE-depleted lava, under relatively oxidizing conditions, during the early Amazonian. In addition to its distinct igneous petrogenesis and high fO_2, NWA 8159 is also set apart from most martian shergottites with respect to the low degree of shock metamorphism required to preserve crystalline igneous plagioclase (An_(50–65)). In this study, mineral transformations within and adjacent to shock veins in NWA 8159 were investigated using scanning electron microscopy, Raman spectroscopy and transmission electron microscopy to better constrain the unusal shock history of this meteorite. The transformation of olivine to ahrensite (Fe-ringwoodite) along shock vein margins, and tissintite and coesiteformed from igneous mineral (labradorite and silica) grains entrained as clasts within shock veins has been documented in this study. We report on a previously unidentified mineral assemblage of Ca-Na-majoritic garnet, sodic-clinopyroxene and stishovite crystallized from shock melt. This mineral assemblage indicates a crystallization pressure of approximately 16 GPa, which is within the range of previous shock pressure estimates for this meteorite (15–23 GPa). The presence of a majoritic garnet-bearing assemblage throughout veins up to 0.6 mm wide indicates that the sample remained at high-pressure throughout the melt vein quench. Based on thermal models, the sample must have remained at high pressure for ∼100 ms. This shock duration is an order of magnitude longer than those experienced by more highly shocked shergottites such as Tissint or Zagami (>30 GPa; 10–20 ms) and would seem to imply a relatively large impact event. Recent numerical models demonstrate that a range of shock pressures and durations are realized by rocks within the ejected spall zone of a hypervelocity impact. The shock conditions experienced by NWA 8159 therefore do not require an impact event distinct from other shergottites. Rather, our findings suggest that this meteorite originated from near the martian surface at the edge of the impact site. The shock history of NWA 8159 provides a picture of Mars consistent with that derived from remote observation; that of a random cratering process that samples a geologically long-lived and complex planet

The Mineralogical Record of Oxygen Fugacity Variation and Alteration in Northwest Africa 8159: Evidence for Interaction Between a Mantle Derived Martian Basalt and a Crustal Component(s)

A prominent geochemical feature of basaltic magmatism on Mars is the large range in initial Sr isotopic ratios (approx. 0.702 - 0.724) and initial epsilon-Nd values (approx. -10 to greater than +50). Within this range, the shergottites fall into three discreet subgroups. These subgroups have distinct bulk rock REE patterns, mineral chemistries (i.e. phosphate REE patterns, Ni, Co, V in olivine), oxygen fugacity of crystallization, and stable isotopes, such as O. In contrast, nakhlites and chassignites have depleted epsilon-Nd values (greater than or equal to +15), have REE patterns that are light REE enriched, and appear to have crystallized near the FMQ buffer. The characteristics of these various martian basalts have been linked to different reservoirs in the martian crust and mantle, and their interactions during the petrogenesis of these magmas. These observations pose interesting interpretive challenges to our understanding of the conditions of the martian mantle (e.g. oxygen fugacity) and the interaction of mantle derived magmas with the martian crust and surface. Martian meteorite NWA 8159 is a unique fine-grained augite basalt derived from a highly depleted mantle source as reflected in its initial epsilon-Nd value, contains a pronounced light REE depleted pattern, and crystallized presumably under very oxidizing conditions. Although considerably older than both shergottites and nahklites, it has been petrogenetically linked to both styles of martian magmatism. These unique characteristics of NWA 8159 may provide an additional perspective for deciphering the petrogenesis of martian basalts and the nature of the crust of Mars