Martian meteorites and Martian magnetic anomalies: A new perspective from NWA 7034

Northwest Africa 7533, a polymict Martian breccia, consists of fine‐grained clast‐laden melt particles and microcrystalline matrix. While both melt and matrix contain medium‐grained noritic‐monzonitic material and crystal clasts, the matrix also contains lithic clasts with zoned pigeonite and augite plus two feldspars, microbasaltic clasts, vitrophyric and microcrystalline spherules, and shards. The clast‐laden melt rocks contain clump‐like aggregates of orthopyroxene surrounded by aureoles of plagioclase. Some shards of vesicular melt rocks resemble the pyroxene‐plagioclase clump‐aureole structures. Submicron size matrix grains show some triple junctions, but most are irregular with high intergranular porosity. The noritic‐monzonitic rocks contain exsolved pyroxenes and perthitic intergrowths, and cooled more slowly than rocks with zoned‐pyroxene or fine grain size. Noritic material contains orthopyroxene or inverted pigeonite, augite, calcic to intermediate plagioclase, and chromite to Cr‐bearing magnetite; monzonitic clasts contain augite, sodic plagioclase, K feldspar, Ti‐bearing magnetite, ilmenite, chlorapatite, and zircon. These feldspathic rocks show similarities to some rocks at Gale Crater like Black Trout, Mara, and Jake M. The most magnesian orthopyroxene clasts are close to ALH 84001 orthopyroxene in composition. All these materials are enriched in siderophile elements, indicating impact melting and incorporation of a projectile component, except for Ni‐poor pyroxene clasts which are from pristine rocks. Clast‐laden melt rocks, spherules, shards, and siderophile element contents indicate formation of NWA 7533 as a regolith breccia. The zircons, mainly derived from monzonitic (melt) rocks, crystallized at 4.43 ± 0.03 Ga (Humayun et al. ) and a 147Sm‐143Nd isochron for NWA 7034 yielding 4.42 ± 0.07 Ga (Nyquist et al. ) defines the crystallization age of all its igneous portions. The zircon from the monzonitic rocks has a higher Δ17O than other Martian meteorites explained in part by assimilation of regolith materials enriched during surface alteration (Nemchin et al. ). This record of protolith interaction with atmosphere‐hydrosphere during regolith formation before melting demonstrates a thin atmosphere, a wet early surface environment on Mars, and an evolved crust likely to have contaminated younger extrusive rocks. The latest events recorded when the breccia was on Mars are resetting of apatite, much feldspar and some zircons at 1.35–1.4 Ga (Bellucci et al. ), and formation of Ni‐bearing pyrite veins during or shortly after this disturbance (Lorand et al. ).

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“…; Gattacceca et al. ). Magnetic anomalies are restricted spatially but the 3 μm absorption indicating dust rich in Fe 3+ is widespread on Mars.…”

Section: Discussionmentioning

confidence: 98%

“…Abundant fine‐grained magnetite and maghemite explain the breccia's magnetic properties especially the high saturation remanence (Gattacceca et al. ).…”

Section: Discussionmentioning

confidence: 99%

Regolith breccia Northwest Africa 7533: Mineralogy and petrology with implications for early Mars

Hewins

Zanda

Humayun

et al. 2016

Meteorit & Planetary Scien

138

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“…; Gattacceca et al. ). The acquisition of a chemical remanent magnetization (CRM) or a thermo‐chemical remanent magnetization (TCRM) within a breccia (or within in situ crust) would have depended upon the relative timing of oxide growth and dynamo cessation.…”

Section: Discussionmentioning

confidence: 98%

Influence of redox conditions on the intensity of Mars crustal magnetic anomalies

Brachfeld

Shah

First

et al. 2015

Meteorit & Planetary Scien

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We evaluate the relationship between the intensity of remanent magnetization and fO 2 in natural and synthetic Mars meteorites. The olivine-phyric shergottite meteorite Yamato 980459 (Y-980459) and a sulfur-free synthetic analog (Y-98*) of identical major element composition were analyzed to explore the rock magnetic and remanence properties of a basalt crystallized from a primitive melt, and to explore the role of magmatic and alteration environment fO 2 on Mars crustal anomalies. The reducing conditions under which Y-980459 is estimated to have formed (QFM-2.5;Shearer et al. 2006) were replicated during the synthesis of Y-98*. Y-980459 contains pyrrhotite and chromite. Chromite is the only magnetic phase in Y-98*. The remanence-carrying capacity of Y-980459 is comparable to other shergottites that formed in the fO 2 range of QFM-3 to QFM-1. The remanencecarrying capacity of these low fO 2 basalts is 1-2 orders of magnitude too weak to account for the intense crustal anomalies observed in Mars's southern cratered highlands. Moderately oxidizing conditions of >QFM-1, which are more commonly observed in nakhlites and Noachian breccias, are key to generating either a primary igneous assemblage or secondary alteration assemblage capable of acquiring an intense remanent magnetization, regardless of the basalt character or thermal history. This suggests that if igneous rocks are responsible for the intensely magnetized crust, these oxidizing conditions must have existed in the magmatic plumbing systems of early Mars or must have existed in the crust during secondary processes that led to acquisition of a chemical remanent magnetization.

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“…Gattacceca et al. () investigated the magnetic properties of NWA 7034, which revealed that the meteorite contains 15 wt% magnetite and maghemite, which makes it the most oxidized Martian meteorite so far studied. Furthermore, these authors speculate that low‐temperature hydrothermal alteration of NWA 7034 on Mars, possibly impact‐induced, produced some of the magnetite and altered magnetite to maghemite.…”

Section: Introductionmentioning

confidence: 99%

Petrography and composition of Martian regolith breccia meteorite Northwest Africa 7475

Wittmann

Korotev

Jolliff

et al. 2015

Meteorit & Planetary Scien

110

196

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The Northwest Africa (NWA) 7475 meteorite is one of the several stones of paired regolith breccias from Mars based on petrography, oxygen isotope, mineral compositions, and bulk rock compositions. Its inventory of lithic clasts is dominated by vitrophyre impact melts that were emplaced while they were still molten. Other clast types include crystallized impact melt rocks, evolved plutonic rocks, possible basalts, contact metamorphosed rocks, and siltstones. Impact spherules and vitrophyre shards record airborne transport, and accreted dust rims were sintered on most clasts, presumably during residence in an ejecta plume. The clast assemblage records at least three impact events, one that formed an impact melt sheet on Mars ≤4.4 Ga ago, a second that assembled NWA 7475 from impactites associated with the impact melt sheet at 1.7–1.4 Ga, and a third that launched NWA 7475 from Mars ~5 Ma ago. Mildly shocked pyroxene and plagioclase constrain shock metamorphic conditions during launch to >5 and <15 GPa. The mild postshock‐heating that resulted from these shock pressures would have been insufficient to sterilize this water‐bearing lithology during launch. Magnetite, maghemite, and pyrite are likely products of secondary alteration on Mars. Textural relationships suggest that calcium‐carbonate and goethite are probably of terrestrial origin, yet trace element chemistry indicates relatively low terrestrial alteration. Comparison of Mars Odyssey gamma‐ray spectrometer data with the Fe and Th abundances of NWA 7475 points to a provenance in the ancient southern highlands of Mars. Gratteri crater, with an age of ~5 Ma and an apparent diameter of 6.9 km, marks one possible launch site of NWA 7475.

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Martian meteorites and Martian magnetic anomalies: A new perspective from NWA 7034

Cited by 54 publications

References 32 publications

Regolith breccia Northwest Africa 7533: Mineralogy and petrology with implications for early Mars

Regolith breccia Northwest Africa 7533: Mineralogy and petrology with implications for early Mars

Influence of redox conditions on the intensity of Mars crustal magnetic anomalies

Petrography and composition of Martian regolith breccia meteorite Northwest Africa 7475

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