USING MAHLI IMAGES FOR DETECTION OF GYPSUM IN THE …USING MAHLI IMAGES FOR DETECTION OF GYPSUM IN...
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USING MAHLI IMAGES FOR DETECTION OF GYPSUM IN THE MURRAY FORMATION AT GALE CRATER, MARS. Zalmai Yawar 1 , Juergen Schieber 1 , David Bish 1 , and Michelle Minitti 2 , 1 Dept. Geol. Sci, Indi-ana Univ., 1001 E 10 th Str., Bloomington, IN 47405; 2 Framework, Silver Spring, MD; [email protected]. Introduction: Calcium sulfates are a ubiquitous con- stituent of most outcrops of sedimentary strata in Gale crater. They occur most conspicuously as vein fills [1], and occur in three varieties, gypsum, anhydrite, and bassanite [2, 3]. Whereas the CheMin X-ray diffrac- tion (XRD) instrument on the MSL rover is best suited to differentiate between these various Ca-sulfate phas- es, it requires a drilled or scooped sample and thus there are limitations on its application. In addition, a malfunction of the drill prevented collection of XRD data from a substantial stratgraphic interval as the rover ascended Mt. Sharp. Whereas Ca-sulfates likely are part of the matrix of many rocks within the Mt. Sharp stratigraphy, macro- scopic occurences exist almost exclusively in veins that occur throughout the sediment fill of the crater [2]. In places the Ca-sulfate fill of these veins shows unique parallel striations (Fig. 1). We interpret these striations as due to the cleavage planes of gypsum, and propose that they can be used as a visual indicator for the pres- ence of gypsum. Figure 1: Ca-sulfate filled vein in the Blunts Point Member of the Murray Formation (Sol 2083). The white vein fill shows two sets of well developed paral- lel “striations” (yellow arrows) that do not parallel the vein walls and thus are unlikely to reflect incremental mineral growth on the wall rock. The two distinct sets of “striations” are interpreted as reflections of the cleavage planes of two large gypsum crystals with dif- ferent orientations. Interpretation as Gypsum: Although known to occur in macroscopic crystals and to display cleavage, most natural occurences of both anhydrite and bas- sanite are finely crystalline and reveal cleavage or tex- ture only under the microscope. They both form pri- marily via dehydration of gypsum, a process that re- sults in large volume changes and typically produces fine-grained deposits that exhibit little outward macro- scopic manifestation of cleavage. Gypsum, in contrast, commonly displays excellent (010) cleavage and often forms macroscopic crystals. Gypsum dehydration can also result in topotactic replacement, creating a fibrous morphology parallel to the gypsum (010) cleavage on a scale of 10’s of µm [4]. As Roberton and Bish (2013) showed through ex- perimental studies [5], gypsum is likely to be the the dominant Ca-sulfate mineral over much of Mars’s sur- face. Although bassanite may be thermodynamically favored in near-equatorial regions at higher tempera- tures, dehydration kinetics are sufficiently sluggish that gypsum is likely to predominate except in localized areas affected by volcanism or impacts. Gypsum col- lected from the surface does dehydrate under the very low relative humidity and comparatively warm condi- tions inside the rover [2], a direct confirmation of its stability under current surface conditions. Thus, stability considerations in combination with (1) the apparent coarse crystallinity suggested by “striation” sets (Fig. 1) and (2) the perfect parallelism of “striations” that is consistent with cleavage, provide strong indication that “striated” Ca-sulfate is gypsum. Confirmation of this inference come from Mahli imag- es of drill sites where gypsum was identified by Che- Min (Fig. 2). Figure 2: Oudam drill hole with two adjacent Ca- sulfate veins. Yellow arrows point to sets of variably oriented striations. XRD analysis indicates 3.3% gyp- sum in the rock that was drilled adjacent to the veins. Occurrence and Stratigraphic Range: Systemat- ic examination of MAHLI images starting at Pahrump (Sol 760) and extending up to the VRR (Sol 2247) has produced a list of MAHLI images that are candidates for additional gypsum occurrences. A selection of these is marked on the stratigraphic section in Fig. 3. 2007.pdf 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)
Transcript of USING MAHLI IMAGES FOR DETECTION OF GYPSUM IN THE …USING MAHLI IMAGES FOR DETECTION OF GYPSUM IN...
USING MAHLI IMAGES FOR DETECTION OF GYPSUM IN THE MURRAY FORMATION AT GALE CRATER, MARS. Zalmai Yawar1, Juergen Schieber1, David Bish1, and Michelle Minitti2, 1Dept. Geol. Sci, Indi-ana Univ., 1001 E 10th Str., Bloomington, IN 47405; 2Framework, Silver Spring, MD; [email protected].
Introduction: Calcium sulfates are a ubiquitous con-stituent of most outcrops of sedimentary strata in Gale crater. They occur most conspicuously as vein fills [1], and occur in three varieties, gypsum, anhydrite, and bassanite [2, 3]. Whereas the CheMin X-ray diffrac-tion (XRD) instrument on the MSL rover is best suited to differentiate between these various Ca-sulfate phas-es, it requires a drilled or scooped sample and thus there are limitations on its application. In addition, a malfunction of the drill prevented collection of XRD data from a substantial stratgraphic interval as the rover ascended Mt. Sharp.
Whereas Ca-sulfates likely are part of the matrix of many rocks within the Mt. Sharp stratigraphy, macro-scopic occurences exist almost exclusively in veins that occur throughout the sediment fill of the crater [2]. In places the Ca-sulfate fill of these veins shows unique parallel striations (Fig. 1). We interpret these striations as due to the cleavage planes of gypsum, and propose that they can be used as a visual indicator for the pres-ence of gypsum.
Figure 1: Ca-sulfate filled vein in the Blunts Point Member of the Murray Formation (Sol 2083). The white vein fill shows two sets of well developed paral-lel “striations” (yellow arrows) that do not parallel the vein walls and thus are unlikely to reflect incremental mineral growth on the wall rock. The two distinct sets of “striations” are interpreted as reflections of the cleavage planes of two large gypsum crystals with dif-ferent orientations.
Interpretation as Gypsum: Although known to occur in macroscopic crystals and to display cleavage, most natural occurences of both anhydrite and bas-sanite are finely crystalline and reveal cleavage or tex-ture only under the microscope. They both form pri-marily via dehydration of gypsum, a process that re-sults in large volume changes and typically produces fine-grained deposits that exhibit little outward macro-scopic manifestation of cleavage. Gypsum, in contrast, commonly displays excellent (010) cleavage and often
forms macroscopic crystals. Gypsum dehydration can also result in topotactic replacement, creating a fibrous morphology parallel to the gypsum (010) cleavage on a scale of 10’s of µm [4].
As Roberton and Bish (2013) showed through ex-perimental studies [5], gypsum is likely to be the the dominant Ca-sulfate mineral over much of Mars’s sur-face. Although bassanite may be thermodynamically favored in near-equatorial regions at higher tempera-tures, dehydration kinetics are sufficiently sluggish that gypsum is likely to predominate except in localized areas affected by volcanism or impacts. Gypsum col-lected from the surface does dehydrate under the very low relative humidity and comparatively warm condi-tions inside the rover [2], a direct confirmation of its stability under current surface conditions.
Thus, stability considerations in combination with (1) the apparent coarse crystallinity suggested by“striation” sets (Fig. 1) and (2) the perfect parallelismof “striations” that is consistent with cleavage, providestrong indication that “striated” Ca-sulfate is gypsum.Confirmation of this inference come from Mahli imag-es of drill sites where gypsum was identified by Che-Min (Fig. 2).
Figure 2: Oudam drill hole with two adjacent Ca-sulfate veins. Yellow arrows point to sets of variably oriented striations. XRD analysis indicates 3.3% gyp-sum in the rock that was drilled adjacent to the veins.
Occurrence and Stratigraphic Range: Systemat-ic examination of MAHLI images starting at Pahrump (Sol 760) and extending up to the VRR (Sol 2247) has produced a list of MAHLI images that are candidates for additional gypsum occurrences. A selection of these is marked on the stratigraphic section in Fig. 3.
2007.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)
Figure 3: Summary stratigraphic section with gypsum detected in drill samples (blue circles) and in outcrop MAHLI images (via “striations”; blue squares). Section adapted from version distributed by C. Fedo.
An example from near the top of the section is shown in Figure 4.
Figure 4: Multiple sets of variable oriented “striations” in an image from Sol 2044, near the top of the strati-graphic section shown in Fig. 3.
Conclusions: The “striations observed in multiple MAHLI images of Ca-sulfate veins most likely reflect cleavage planes in gypsum that were enhanced by eoli-an abrasion and possibly incipient volume loss due to dehydration. On earth, similar features have been ob-served on calcite due to differential weathering of twinned crystals. The distribution of this feature through the stratigraphy suggests that gypsum is a common and even dominant Ca-sulfate mineral present in the veins from about -4450 m, somewhat above the Pahrump outcrop near the base of the Murray Formation.
It appears that gypsum occurs in multiple locations above that elevation, either as the sole vein fill or in-termingled with anhydrite. The absence of gypsum in drill samples and outcrop samples below the -4450 m elevation may indicate that this elevation marks the burial diagenetic gypsum-anhydrite conversion depth [1], a situation observed in many sedimentary succes-sions on earth.
Recognizing the significance of “striations” as a visual cue to to the presence of gypsum allows us to document the presence of gypsum over the stratigraph-ic interval traversed by the rover while the drill was inoperable (Sol 1495 to Sol 2057). It also allows eval-uation of the gypsum distribution between widely spaced drill samples.
References: [1] Schieber et al. (2017) Sedimentol-ogy, v. 64, p. 311-358. [2] Vaniman, D.T., et al. (2018) American Mineralogist, v. 103, p. 1011-1020. [3] Rapin, W., et al. (2016) EPSL, v. 452, p. 197-205.[4] Mees, F. and Tursina, T.V. (2010) Salt Minerals inSaline Soils and Salt Crusts, Elsevier. [5] Robertson,K., and Bish, D. (2013) Icarus, v. 223, p. 407-417.
2007.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)
