Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6155.pdf

MINERAL DIAGENESIS OF CLAYS AND OPALINE SILICA ACROSS MARS: EVIDENCE FOR CRUSTAL FLUIDS AND EXTENDED AQUEOUS ACTIVITY. V. Z. Sun, Jet Propulsion Laboratory, Califor- nia Institute of Technology, Pasadena, CA (Vivian.Sun@jpl.nasa.gov).

Introduction: Fe/Mg clays and hydrated/opaline Methods: Results from approximately 900 CRISM silica represent the vast majority of hydrated mineral cubes and their overlapping HiRISE and CTX images detections on Mars1-4, thus a substantial portion of are represented in this study. Fe/Mg smectites have Mars’ aqueous history is encoded in these two mineral narrow absorptions at 2.3 µm, and are distinguished groups. Both clays and opals are unique in that they from chlorites, which have a more complex absorption can undergo mineral diagenesis, where varying expo- centered at 2.32-2.35 µm. Opaline silica has a diagnos- sure to water will produce different mineral forms that tic 2.2 µm absorption, but it is its 1.4 µm band position can be detected via orbital near-infrared spectroscopy. that distinguishes opal-A (absorptions <1.41 µm) from Fe/Mg smectites will alter to chlorite with exposure to opal-CT and quartz (>1.41 µm)9. water and heat5,6, and amorphous opal-A will convert Burial Diagenesis of Fe/Mg Clays in the Crust: to more crystalline opal-CT and quartz in the presence Our first investigation focuses on Fe/Mg clays detected of water alone7. This attribute of both clays and opals within uplifted materials in central peak craters4. We allows us to infer not only the past presence of water, find that chlorite occurrences increase with crater uplift but also the persistence of aqueous conditions. depth while smecitite occurrences decrease (Fig. 2A), Until recently, it was generally thought that aque- suggesting smectite-to-chlorite diagenesis in the crust. ous conditions were limited and largely ceased after the The transition from a smectite-dominated to a chlorite- widespread formation of clays on Mars1,8. However, dominated crust occurs at 4 km depth, and chlorite is recent orbital studies have identified localities with found to be uplifted from depths as great as 17 km4. more geochemically mature phases9,10, and landed mis- Importantly, these results suggest that fluids were sions (Opportunity, Curiosity) have also observed evi- present within the crust, possibly at significant depths dence for postdepositional fluids in the form of veins, up to 17 km, in order to enable diagenesis. The depths concretions, and other diagenetic features11,12. at which chloritization occurs (Fig. 2A) also carries Here we present evidence for widespread diagene- implications for determining the geothermal gradient sis on Mars based on detections of geochemically ma- within the crust, as the conversion of smectite to chlo- ture phases like chlorite and opal-CT, which transform rite process requires heat in addition to water5,6. from their immature counterparts, smectite and opal-A, Diagenesis of Opaline Silica: Results from our in the presence of (e.g., diagenetic) fluids (Fig. 1). We second investigation show that opal-A and opal-CT are discuss implications for crustal fluids, the timing of both present on Mars (Fig. 2B) but appear globally widespread mineral diagenesis on Mars, and relations correlated with geologic setting. Opal-A tends to occur to observations by landed missions. in bedrock units, while opal-CT is associated with mo-

Fig. 1. Global distribution of geochemically mature minerals – chlorite and opal-CT (bold circles) – suggesting extended aqueous alteration, and their geochemically immature coun- terparts (smectite and opal-A; pastel triangles). Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6155.pdf

bile, unconsolidated deposits9. This suggests either that the opal-CT-bearing deposits represent older, eroded material, or that opal maturation was facilitated by H2O trapped within particulates in the deposits. In the latter scenario, trapped fluids may intermittently interact with the silica, e.g., through freeze-thaw cycles, resulting in slow opal maturation through Ostwald ripening. Inter- estingly, there is an overall lack of opal-CT associated with bedrock, even in uplifted outcrops, which con- trasts with the observations of chlorite in uplifted units. Timeline of Extended Aqueous Activity: Alt- hough geochemically mature chlorite and opal-CT are both observed across Mars, they are associated with different geologic settings and therefore likely formed in different ways. If this hypothesis is true, what does this imply for the availability and persistence of water throughout Mars’ history? These observations may reflect fluids and aqueous alteration during different time periods. Clays are typi- cally understood to have formed during the Noachian, and opaline silica during the Hesperian2,3. Indeed, some of the largest craters uplifting chlorite in our study are as old as 3.8-3.9 Ga, suggesting that this chloritization process in the crust must have occurred prior to this excavation age4. These observations indi- cate that there was substantial burial diagenesis of clays occurring early in Mars’ history, but water availability Fig. 2. A) Distribution of uplifted clays, showing a trend of in the crust eventually decreased as we observe incom- increasing chlorite (red) with uplift depth. Larger craters (not plete chloritization at shallower depths (Fig. 2A). An shown) uplift chlorites from up to 17 km depth. B) Martian opal detections occur as either opal-A associated with bed- overall decrease in water availability may also explain rock, or opal-CT associated with unconsolidated deposits. why opal-bearing outcrops have largely not been con- verted to opal-CT or quartz, as most opaline silica like- 1) When did these global, crustal diagenetic pro- ly formed later during the drier Hesperian3. Instead, cesses occur, especially relative to the global environ- conversion to opal-CT may have only occurred through mental changes from the Noachian to the Hesperian1? slow opal maturation facilitated by pockets of fluids 2) When did the local-scale diagenetic processes at trapped within particulates (see above section). Meridiani and Gale occur, relative to the broader-scale This trend of decreasing water availability may also diagenesis processes inferred from the orbital data? explain why widespread chloritization is not observed 3) How can observations of mineral diagenesis in in the Noachian/Hesperian-aged Gale crater despite the crust be used to infer geothermal gradients during clay-bearing deposits being buried under several kilo- the Noachian and Hesperian? meters of sediment, although this appears contrary to These answers will inform our understanding of Curiosity’s observations of abundant diagenetic fea- Mars’ geochemical and aqueous history, as well as its 12 tures at scales below the resolution of orbital data . surface and subsurface habitability through time. Summary and Outstanding Questions: We have References: [1] Bibring et al. (2006), Science 312, 400- presented evidence for diagenetic processes acting on 404. [2] Carter et al. (2013), JGR 118, E004145. [3] Eh- both Fe/Mg clays and opals on Mars, suggesting that lmann & Edwards (2014), Ann. Rev. Earth Planet. Sci. 42, aqueous conditions, though still limited compared to on 291-315. [4] Sun & Milliken (2015), JGR 120, E004918. [5] Earth, were more extensive than previously thought8. Inoue et al. (1992), App. Clay Sci. 7, 131-145. [6] Środoń This orbital perspective is corroborated by observa- (1999), Ann. Rev. Earth Planet. Sci. 27, 19–53. [7] Williams tions by landed missions, as Opportunity and Curiosity & Crerar (1985), Jrnl. of Sed. Pet. 55, 312-321. [8] Tosca & have both found extensive evidence for diagenetic pro- Knoll (2009), EPSL 286, 379-386. [9] Sun & Milliken cesses and postdepositional fluids11,12, albeit at a dif- (2018), GRL 45, GL078494. [10] Smith et al. (2013), Icarus ferent scale of detection than orbital observations. 223, 633-648. [11] McLennan et al. (2005), EPSL 240, 95- However, several questions still remain: 121. [12] Grotzinger et al. (2015), Science 350, aac7575.