Geochemistry of Noachian Bedrock and Alteration Events, Endeavour Crater, Mars
50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 1100.pdf
GEOCHEMISTRY OF NOACHIAN BEDROCK AND ALTERATION EVENTS, ENDEAVOUR CRATER, MARS. D. W. Mittlefehldt1, R. E. Arvidson2, L. S. Crumpler3, W. H. Farrand4, R. Gellert5, J. A. Grant6, B. L. Jol- liff2, S. vanBommel2, A. S. Yen7, M. C. Bouchard2, and C. Schröder8, 1NASA Johnson Space Center, Houston, TX, USA (david.w.mittlefehldt@nasa.gov), 2Washington University in Saint Louis, Saint Louis, MO, USA 3New Mexi- co Museum of Natural History and Science, Albuquerque, NM, USA, 4Space Science Institute, Boulder, CO, USA, 5University of Guelph, Guelph, Ontario, Canada, 6Smithsonian Institution, Washington, DC, USA, 7Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA, 8University of Stirling, Stirling, UK.
Introduction: Since August 2011, Mars Explora- in varying proportions. Apparent bedding in some out- tion Rover Opportunity has been investigating the ge- crops is expressed as mm- to cm-scale planar lamina- ology of western rim segments of 22 km diameter, tions. Fine-grained clastic rocks can form by impact, Noachian-aged, degraded [1, 2], Endeavour Crater. explosive volcanic, aeolian, or fluvial/lacustrine pro- She is currently located almost halfway down the cesses; the limited exposures do not allow us to distin- back-wasted crater wall in Perseverance Valley, be- guish among these possibilities [5]. tween Capes Tribulation and Byron [3] (Fig. 1). Endeavour rim rocks are basaltic-composition A unique lithology, the Matijevic formation that polymict breccias (Shoemaker formation) composed of crops out in a small region on the inboard side of Cape 1-10 cm-sized, erosion-resistant clasts in a fine-grained York (Fig. 1), has been shown to be pre-Endeavour matrix. Two texturally distinct breccias are present: a bedrock [4, 5]. The Matijevic formation is directly clast-poor unit with typically smaller clasts; a clast- overlain by polymict breccias of the Shoemaker for- rich unit with generally coarser clasts [8]. Clast-poor mation [5, 6]. We have interpreted a lower subunit of breccias form planar, eroded outcrops, while the clast- the Shoemaker formation on Cape Tribulation as distal rich breccias are more resistant, forming ridge crests ejecta from one or more pre-Endeavour craters [7]. on Capes York and Tribulation. The walls of Marathon Sulfate-sandstones of the Burns formation post-date Valley document stratigraphic relationships between Endeavour Crater [5]. The pre-Endeavour bedrock and the two units: clast-rich breccias form the upper unit; associated alteration styles are discussed herein. clast-poor breccias occupy the valley floor [3]. Polymict breccias outside the tectonic rim of Ries Crater (equivalent to Opportunity’s location) show no systematic vertical grain size variation in the deposit [9]. Polymict ejecta within the outer rim of Mistastin Crater are similarly poorly sorted [10]. Stratigraphic relationships between the lower and upper Shoemaker units do not match expectations for single-impact ejec- ta. Mean clast size and abundance decrease with radial distance outward in Ries and Meteor Crater ejecta, and in impact experiments [9, 11]. The stratigraphy in Marathon Valley is consistent with a lower unit of distal ejecta from one or more older craters overlain by proximal ejecta from Endeavour Crater. Investigations by Opportunity [7], and orbital ob- servations of the walls of more youthful Bopolu and Iazu Craters to the south [1, 12], indicate the pre- Burns-formation bedrock in this region is part of the Noachian subdued crater unit that includes compo- nents of lava and pyroclastic flows, impact breccias, impact-melt sheets, and fluvio-lacustrine deposits [13]. Noachian alteration: Several alteration episodes
of Noachian to Early Hesperian age affected the En- Noachian bedrock: The Matijevic formation oc- deavour rim [4-6, 14, 15]. The oldest is Si-Al-rich vein curs as light-toned, planar outcrops with discontinu- formation in the Matijevic formation [15]. Silica- ous, erosionally resistant dark veneers [4, 5]. The Al±Ge-rich veins occur as curvilinear red zones cut- rocks are fine grained, with subrounded particles ≤0.3 ting the lower Shoemaker formation in the Marathon mm in size, and they contain 1-4 mm-sized spherules Valley region [16, 17]. A linear array of Si-Al-rich 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 1100.pdf
pitted rocks occupy a fracture system in Perseverance thermal fluids. Later ferric-smectite-formation is also Valley within an area of lower Shoemaker rocks [3, confined to the regional bedrock.
0 18]. Based on our interpretation of the lower Shoe- Shoemaker fm. a montmorillonite Marathon Valley maker, all occurrences of crosscutting Si-Al-rich veins 60 Perseverance Valley 10 are in pre-Endeavour rocks [7], implying this alteration Matijevic fm. style was restricted to the Noachian bedrock. 50 ) 20 % Espérance le (calculated) Endeavour rim rocks are basaltic in composition o A m l 40 2 ( O O 3 30 ( and overlap the Martian basalt field (Fig. 2). The “pur- 2 m K + o O le 2 % ple rocks” represent a fine-grained, homogeneous li- a N 30 ) + O 40 a thology of uncertain origin found scattered at several C
f 20 eld locations. They might be evolved igneous composi- sp ars 50 : oli vin tions, or an altered composition formed under low wa- e 10 ter/rock, acidic conditions (blue arrows; cf., [19]). 60 basalts
pyroxene The three lithologies that crosscut outcrops show 0 evidence for compositional modification by alteration. 40 50 60 70 80 90 100 MgO+FeOT (mole%) 50 The Lihir/Espérance veins that cut the Matijevic for- b 30
mation show the effects of alteration under higher wa- 40 N 40 a + ) K + ter/rock conditions; two abraded targets plot nearest % 2 le C o 30 a m 50 the field for terrestrial montmorillonites (Fig. 2). We ( ( m /4 o i basalts l S e interpreted the Lihir/Espérance veins to have formed montmorillonite 20 % 60 )
as a mixture of montmorillonite and silica produced by Lihir/Espérance veins Espérance 10 70 (calculated) red zone group moderate-temperature alteration of bedrock by cir- PV pitted rocks purple rocks 0 cumneutral to mildly alkaline fluids [14]. The calculat- 30 40 50 60 70 80 90 100 Mg+Mn+Fe (mole%) ed end-member vein composition [14] plots within the T montmorillonite field. Rocks from red zones show Figure 2. Alteration diagrams for Si-Al-rich crosscut- modest evidence for alteration; they are enriched in Al ting veins in pre-Endeavour bedrock. and Si, and sometimes Ge, compared to host bedrock References: [1] Grant J. A. et al. (2016) Icarus [16]. One of the Perseverance Valley pitted rocks is 280, 22. [2] Hughes M. N. et al. (2018) Geol. Soc. Am. similar to the purple rocks, while others express com- Abst. with Prog. 50, Abstract #320733. [3] Crumpler positional evidence for alteration. L. S. et al. (2019) LPS L, this conference. [4] Arvidson The second oldest alteration event on Endeavour R. E. et al. (2014) Science 343, doi:10.1126/science. rim is low-water/rock alteration along the unconformi- 1248097. [5] Crumpler L. S. et al. (2015) JGR Planets ty between the Matijevic and Shoemaker formations, 120, doi:10.1002/2014JE004699. [6] Squyres S. W. et forming veneers on the former. Ferric smectites are al. (2012) Science 336, 570. [7] Mittlefehldt D. W. et inferred to occur in these veneers [4], which post-date al. (2018) Geol. Soc. Am. Abst. with Prog. 50, Abstract the Si-Al-rich veins [15]. Ferric smectites occur within #318037. [8] Crumpler L. S. et al. (2017) LPS XLVIII, the lower Shoemaker rocks of Marathon Valley [20], Abstract #2276. [9] Hörz F. et al. (1983) Rev. Ge- but their temporal relationship with the red zones has ophys. 21, 1667. [10] Mader M. M. & Osinski G. R. not been established. Thus, ferric smectites are associ- (2018) M&PS 53, 2492. [11] Melosh H. J. (1989) Im- ated with the pre-Endeavour bedrock. In the walls of pact Cratering A Geologic Process, Oxford Univ., 245 Iazu Crater, ~25 km south of Endeavour, ferric smec- pp. [12] Powell K. et al. (2017) JGR Planets 122, tites are observed in Noachian-aged basaltic- 1138. [13] Hynek B. M. & Di Achille G. (2017) USGS composition rocks underlying layered sulfates of the Sci. Investigation Map, #3356. [14] Clark B. C. et al. Burns formation [20], lending support to a Noachian (2016) Am. Min. 101, 1515. [15] Mittlefehldt D. W. et age for ferric smectite alteration. al. (2018) JGR Planets 123, 1255. [16] Mittlefehldt D. Key findings: Clast-poor, fine-grained breccias of W. et al. (2016) LPS XLVII, Abstract #2086. the lower Shoemaker formation are identified as a pre- [17] Bouchard M. C. & Jolliff B. L. (2018) LPS XLIX, Endeavour lithology, expanding the extent of Noachi- Abstract #2950. [18] Crumpler L. S. et al. (2018) Geol. an bedrock, and providing ground-truth for the orbital- Soc. Am. Abst. with Prog. 50, Abstract #318088. ly-defined Noachian subdued crater unit. The oldest [19] Hurowitz J. A. & McLennan S. M. (2007) EPSL identified alteration event is formation of, Si-Al±Ge- 260, 432. [20] Fox V. K. et al. (2016) Geophys. Res. rich veins by circumneutral to mildly alkaline hydro- Lett. 43, 4885.