Vera Rubin Ridge (VRR) in Gale Crater, Mars, is a ~200 m wide ~6.5 km long northeast- southwest resistant geomorphological feature on the northern slopes of Aeolis Mons (Mt. Sharp). Analysis of Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) orbital data showed that VRR has strong hematite spectral signatures. Hematite was confirmed in-situ at VRR with the Curiosity rover and has been shown to be present throughout the Mur- ray formation. VRR is stratigraphically continu-ous with the underlying Murray formation. Previous thermochemical modelling showed how hematite at VRR could have formed as the result of open-system weathering at high water/rock ratios. Here we use thermochemical modelling to investigate possible reaction pathways for the hematite-clay- bearing assemblage observed at VRR, starting from an identified least-altered (minimum clay content) Murray composition, and a Mars basal brine

Bedford, C.C.

Bridges, J.C.

Fraeman, A.A.

L'Haridon, J.

Mangold, N.

McAdam, A.

Rampe, E.B.

Schwenzer, S.P.

Turner, S.M.R.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThermochemical Modelling of Fluid-Rock Reactions inVera Rubin ridge, Gale Crater, Mars.Conference or Workshop ItemHow to cite:Turner, S.M.R.; Schwenzer, S.P.; Bridges, J.C.; Bedford, C.C.; Rampe, E.B.; Fraeman, A.A.; McAdam, A.;Mangold, N. and L’Haridon, J. (2019). Thermochemical Modelling of Fluid-Rock Reactions in Vera Rubin ridge, GaleCrater, Mars. In: 50th Lunar and Planetary Science Conference, 18-22 Mar 2019, The Woodlands, Houston, Texas,USA.For guidance on citations see FAQs.c© [not recorded]Version: Version of RecordLink(s) to article on publisher’s website:https://www.hou.usra.edu/meetings/lpsc2019/pdf/1897.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukTHERMOCHEMICAL MODELLING OF FLUID-ROCK REACTIONS IN VERA RUBIN RIDGE, GALE CRATER, MARS.  S.M.R. Turner1, S.P. Schwenzer1, J.C. Bridges2, C.C. Bedford1, E.B. Rampe3, A.A. Fraeman4, A. McAdam5, N. Mangold6, J. L’Haridon6. 1The Open University, UK, (stuart.turner@open.ac.uk). 2The University of Leicester, UK. 3NASA Johnson Space Center, USA. 4Jet Propulsion Laboratory, California Institute of Technolo-gy, USA. 5NASA Goddard Space Flight Center, USA. 6Laboratoire de Planétologie et Géophysique de Nantes, Uni-versité de Nantes, France.  Introduction:  Vera Rubin Ridge (VRR) in Gale Crater, Mars, is a ~200 m wide ~6.5 km long northeast-southwest resistant geomorphological feature on the northern slopes of Aeolis Mons (Mt. Sharp). Analysis of Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) orbital data showed that VRR has strong hematite spectral signatures [1]. Hematite was confirmed in-situ at VRR with the Curiosity rover [2,3] and has been shown to be present throughout the Mur-ray formation [4–7]. VRR is stratigraphically continu-ous with the underlying Murray formation [8,9].   Previous thermochemical modelling showed how hematite at VRR could have formed as the result of open-system weathering at high water/rock ratios [10]. Here we use thermochemical modelling to investigate possible reaction pathways for the hematite-clay-bearing assemblage observed at VRR, starting from an identified least-altered (minimum clay content) Murray composition, and a Mars basal brine [11,12]. Modelling method: We used CHIM-XPT software to conduct thermochemical modelling [13]. CHIM-XPT is a program for computing multicomponent het-erogeneous chemical equilibria in aqueous-mineral-gas systems, and has been previously used to model fluid-rock reactions at Gale crater [10–12]. Every calculation step calculates equilibrium between the fluid and the dissolved rock, meaning that each step can be treated and interpreted independently from the direction from which it was reached. In CHIM-XPT, the water/rock ratio (W/R) is the ratio of incoming fluid to reacted rock. The models here focus on 1–100,000 W/R. The Starting Fluid. Gale Portage Water (GPW) [11] was selected as the starting fluid, as this fluid was derived from equilibrium mediation between a brine and rocks of the Gale area. The solution is initially oxidizing (all S species as SO42-), and the redox in the fluid is controlled by the SO42-/HS- pair. The redox of the system throughout each model is dependent on the Fe2+/Fe3+ ratio of the host rock. pH was modelled as a free-parameter throughout. The Starting Rock.  The initial starting composition was selected from Murray drill sample compositions that are mineralogically least altered, based on clay content (assuming the clays are authigenic). The Tele-graph Peak (TP) drill sample was found to be most statistically representative of a bulk lower Murray composition based on a comparison to the average APXS compositions of the Confidence Hills, Mojave 2 and TP CheMin samples [7]. Furthermore, ChemCam bulk contour analysis [14] showed TP to be nearest the overall mean Murray composition. Based on these analyses, TP compositions [7] were selected for the initial starting compositions in our modelling.   Results: Initial modelling focused on establishing a ‘parameter space’ where the bulk, crystalline and amorphous components of TP [7] were reacted with GPW [11]. Modelling progressed to investigate differ-ent dissolution scenarios. Guided by relative mineral solubility [15], we investigated dissolving TP amor-phous and olivine [7]. We then progressed to mixtures of olivine, TP amorphous and TP crystalline [7] to adjust for the lack of Al in the resulting mineral phases compared to VRR drill samples [2,3].  Fig. 1: CHIM-XPT thermochemical model for the reaction of the amorphous component of TP [7] and GPW [11]. The model was run at 50 °C and 0.1 Fe3+/Fetotal.  Initial ‘Parameter Space’ Modelling.  This focused on modelling the individual bulk, crystalline and amor-phous components of TP [7] with GPW [11] over a range of temperatures (20 °C, 50 °C, and 100 °C). The total Fe in the starting composition was expressed in various amounts of Fe2+ and Fe3+ (100% Fe2+, 90% Fe2+ & 10% Fe3+, 50% Fe2+ & 50% Fe3+).  The bulk composition of TP failed to produce any Fe-oxide at 20 °C and 50 °C, but did produce hematite at 100 °C for >100,000 W/R. The crystalline composition of TP only produced Fe-oxide at 0.5 Fe3+/Fetotal, with goethite precipitating at 20 °C (<10 wt.% for ~5,000–100,000 W/R) and 1897.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)hematite precipitating at 50 °C (<10 wt.% ~8,000–100,000 W/R) and 100 °C (<10 wt.% 1,000–100,000 W/R with trace amounts down to ~50 W/R). The amorphous composition of TP produced signif-icant amounts of Fe-oxide. At 20 °C, >10 wt.% goe-thite precipitated between 12,000–100,000 W/R. At 50 °C, >10 wt.% and up to ~30 wt.% hematite precipitates between 5,000–100,000 W/R (Fig. 1). At 100 °C >30 wt.% hematite precipitates between 10,000–100,000 W/R for all modelled Fe2+/Fe3+ ratios.   Fig. 2: CHIM-XPT thermochemical model for a starting composition of 94% olivine and 6% TP amorphous [7] and GPW [11]. The model was run at 50 °C and 0.1 Fe3+/Fetotal.  Fig. 3: CHIM-XPT thermochemical model for a starting composition of TP amorphous (30%), TP crystalline (20%) and olivine (50%) [7], and GPW [11]. The model was run at 50 °C and 0.1 Fe3+/Fetotal.  Olivine and TP Amorphous Starting Composition. Comparisons of the TP [6,7] and VRR [2,3] CheMin mineralogies showed that olivine is present in TP but not in Stoer, Highfield or Rock Hall (Morris et al., this conference [2]). Considering olivine is very reactive, and thus may have been a component that has been dissolved, we decided to mix olivine and the amor-phous component, in the same proportions that they occur in TP [7]. Fig. 2 shows the result of this starting composition reacting with GPW, which shows an in-crease in hematite wt.% between 5,000–100,000 W/R compared to Fig. 1, and a ~2 wt.% hematite precipitat-ing between 100–1,000 W/R that corresponds to an increase in SiO2 wt.%. Olivine, TP Amorphous and TP Crystalline Start-ing Compositions. The next iteration of modelling fo-cused on mixing olivine with both the crystalline and amorphous components of TP [7]. Fig. 3 shows one starting composition mixture reacting with GPW, re-sulting  >10 wt.% hematite precipitating between 100–100,000 W/R peaking at ~44 wt.% at 100,000 W/R.    Discussion: The models presented here indicate a potential formation process for the hematite-clay as-semblage at VRR/Murray of alteration of precursor silicate materials by an oxidizing fluid at temperatures ≥50 °C. In comparison, previous hypotheses for hema-tite formation at VRR include oxidation of an anoxic Fe2+ rich fluid resulting in deposition of hematite [1,16], and alteration of sediments in the shallows of a redox-stratified ancient lake [17].  Our thermochemical modelling assumes that the presently observed mineralogy is what was precipitat-ed. However, it is possible that goethite initially precip-itated and underwent phase transition to hematite over time [18]. Furthermore, the presence of talc in Figs. 1–3 is not observed in drill-hole compositions to date, and presence of jarosite and akaganeite in Stoer and Rock Hall [2] is not currently explained by our models. Further work will be undertaken to refine the predicted mineral assemblages and to include reaction pathways that lead to sulfate and oxide precipitation. Summary: Thermochemical modelling of the reac-tion between TP compositions [7] and GPW [11] has showed that hematite is readily produced at >100 W/R and 50 °C, for all modelled Fe2+/Fe3+ ratios. This sug-gests that the hematite at VRR could have been a con-duit for groundwater flow at temperatures ≥50 °C.  References: [1] Fraeman A.A. et al., (2013) Geology, 41(10), 1103-1106. [2] Morris R.V. et al., (2019) LPSC, 50, (this conf.). [3] Bristow T.F. et al., (2018) AGU Abstract P41A-01. [4] Bristow T.F. et al., (2018) Sci. Adv., 4, eaar3330. [5] Fraeman A.A. et al., (2019) LPSC, 50, (this conf.). [6] Rampe E.B. et al., (2017) EPSL, 471, 172-185. [7] Morrison S.M. et al., (2018) Am. Min., 103, 857-871. [8] Edgar L.A. et al., (2018) LPSC, 49, #1704. [9] Edgar L.A. et al., (2018) AGU Abstract P41A-01. [10] Bridges J.C. et al., (2015) LPSC, 46, #1769. [11] Bridges J.C. et al., (2015) JGR: Planets, 120, 1-19. [12] Schwenzer S.P. et al., (2016) MAPS, 51(11), 2175-2202. [13] Reed M.H. et al., (2010): Users Guide for CHIM-XPT; 71p.; Oregon (University of Oregon). [14] Bedford C.C., (2019) PhD thesis (in prep), The Open University, UK. [15] Gudbrandsson S. et al., (2011) GCA, 75(19), 5496-5509. [16] Fraeman A.A. et al., (2016) JGR: Planets, 121, 1713-1736. [17] Hurowitz J.A. et al., (2017) Science, 356, eaah6849. [18] Cornell R.M. and Schwert-mann U., (2003) The Iron Oxides. Wiley and Sons.  1897.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)

Thermochemical Modelling of Fluid-Rock Reactions in Vera Rubin ridge, Gale Crater, Mars.

Rampe, E. B.

Bridges, J. C.

Fraeman, A. A.

Schwenzer, S. P.

Turner, S. M. R.

Bedford, C. C.

NASA Technical Reports Server

THERMOCHEMICAL MODELLING OF FLUID-ROCK REACTIONS IN VERA RUBIN RIDGE, GALE 
CRATER, MARS.  S.M.R. Turner1, S.P. Schwenzer1, J.C. Bridges2, C.C. Bedford1, E.B. Rampe3, A.A. Fraeman4, 
A. McAdam5, N. Mangold6, J. L’Haridon6. 1The Open University, UK, (stuart.turner@open.ac.uk). 2The University 
of Leicester, UK. 3NASA Johnson Space Center, USA. 4Jet Propulsion Laboratory, California Institute of Technolo-
gy, USA. 5NASA Goddard Space Flight Center, USA. 6Laboratoire de Planétologie et Géophysique de Nantes, Uni-
versité de Nantes, France. 
 
Introduction:  Vera Rubin Ridge (VRR) in Gale 
Crater, Mars, is a ~200 m wide ~6.5 km long northeast-
southwest resistant geomorphological feature on the 
northern slopes of Aeolis Mons (Mt. Sharp). Analysis 
of Compact Reconnaissance Imaging Spectrometer for 
Mars (CRISM) orbital data showed that VRR has 
strong hematite spectral signatures [1]. Hematite was 
confirmed in-situ at VRR with the Curiosity rover [2,3] 
and has been shown to be present throughout the Mur-
ray formation [4–7]. VRR is stratigraphically continu-
ous with the underlying Murray formation [8,9].   
Previous thermochemical modelling showed how 
hematite at VRR could have formed as the result of 
open-system weathering at high water/rock ratios [10]. 
Here we use thermochemical modelling to investigate 
possible reaction pathways for the hematite-clay-
bearing assemblage observed at VRR, starting from an 
identified least-altered (minimum clay content) Murray 
composition, and a Mars basal brine [11,12]. 
Modelling method: We used CHIM-XPT software 
to conduct thermochemical modelling [13]. CHIM-
XPT is a program for computing multicomponent het-
erogeneous chemical equilibria in aqueous-mineral-gas 
systems, and has been previously used to model fluid-
rock reactions at Gale crater [10–12]. Every calculation 
step calculates equilibrium between the fluid and the 
dissolved rock, meaning that each step can be treated 
and interpreted independently from the direction from 
which it was reached. In CHIM-XPT, the water/rock 
ratio (W/R) is the ratio of incoming fluid to reacted 
rock. The models here focus on 1–100,000 W/R. 
The Starting Fluid. Gale Portage Water (GPW) 
[11] was selected as the starting fluid, as this fluid was 
derived from equilibrium mediation between a brine 
and rocks of the Gale area. The solution is initially 
oxidizing (all S species as SO42-), and the redox in the 
fluid is controlled by the SO42-/HS- pair. The redox of 
the system throughout each model is dependent on the 
Fe2+/Fe3+ ratio of the host rock. pH was modelled as a 
free-parameter throughout. 
The Starting Rock.  The initial starting composition 
was selected from Murray drill sample compositions 
that are mineralogically least altered, based on clay 
content (assuming the clays are authigenic). The Tele-
graph Peak (TP) drill sample was found to be most 
statistically representative of a bulk lower Murray 
composition based on a comparison to the average 
APXS compositions of the Confidence Hills, Mojave 2 
and TP CheMin samples [7]. Furthermore, ChemCam 
bulk contour analysis [14] showed TP to be nearest the 
overall mean Murray composition. Based on these 
analyses, TP compositions [7] were selected for the 
initial starting compositions in our modelling.   
Results: Initial modelling focused on establishing a 
‘parameter space’ where the bulk, crystalline and 
amorphous components of TP [7] were reacted with 
GPW [11]. Modelling progressed to investigate differ-
ent dissolution scenarios. Guided by relative mineral 
solubility [15], we investigated dissolving TP amor-
phous and olivine [7]. We then progressed to mixtures 
of olivine, TP amorphous and TP crystalline [7] to 
adjust for the lack of Al in the resulting mineral phases 
compared to VRR drill samples [2,3]. 
 
Fig. 1: CHIM-XPT thermochemical model for the reaction of 
the amorphous component of TP [7] and GPW [11]. The 
model was run at 50 °C and 0.1 Fe3+/Fetotal. 
 
Initial ‘Parameter Space’ Modelling.  This focused 
on modelling the individual bulk, crystalline and amor-
phous components of TP [7] with GPW [11] over a 
range of temperatures (20 °C, 50 °C, and 100 °C). The 
total Fe in the starting composition was expressed in 
various amounts of Fe2+ and Fe3+ (100% Fe2+, 90% 
Fe2+ & 10% Fe3+, 50% Fe2+ & 50% Fe3+).  
The bulk composition of TP failed to produce any 
Fe-oxide at 20 °C and 50 °C, but did produce hematite 
at 100 °C for >100,000 W/R. 
The crystalline composition of TP only produced 
Fe-oxide at 0.5 Fe3+/Fetotal, with goethite precipitating 
at 20 °C (<10 wt.% for ~5,000–100,000 W/R) and 
1897.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)
https://ntrs.nasa.gov/search.jsp?R=20190001847 2019-08-30T22:28:33+00:00Z
hematite precipitating at 50 °C (<10 wt.% ~8,000–
100,000 W/R) and 100 °C (<10 wt.% 1,000–100,000 
W/R with trace amounts down to ~50 W/R). 
The amorphous composition of TP produced signif-
icant amounts of Fe-oxide. At 20 °C, >10 wt.% goe-
thite precipitated between 12,000–100,000 W/R. At 50 
°C, >10 wt.% and up to ~30 wt.% hematite precipitates 
between 5,000–100,000 W/R (Fig. 1). At 100 °C >30 
wt.% hematite precipitates between 10,000–100,000 
W/R for all modelled Fe2+/Fe3+ ratios.  
 
Fig. 2: CHIM-XPT thermochemical model for a starting 
composition of 94% olivine and 6% TP amorphous [7] and 
GPW [11]. The model was run at 50 °C and 0.1 Fe3+/Fetotal. 
 
Fig. 3: CHIM-XPT thermochemical model for a starting 
composition of TP amorphous (30%), TP crystalline (20%) 
and olivine (50%) [7], and GPW [11]. The model was run at 
50 °C and 0.1 Fe3+/Fetotal. 
 
Olivine and TP Amorphous Starting Composition. 
Comparisons of the TP [6,7] and VRR [2,3] CheMin 
mineralogies showed that olivine is present in TP but 
not in Stoer, Highfield or Rock Hall (Morris et al., this 
conference [2]). Considering olivine is very reactive, 
and thus may have been a component that has been 
dissolved, we decided to mix olivine and the amor-
phous component, in the same proportions that they 
occur in TP [7]. Fig. 2 shows the result of this starting 
composition reacting with GPW, which shows an in-
crease in hematite wt.% between 5,000–100,000 W/R 
compared to Fig. 1, and a ~2 wt.% hematite precipitat-
ing between 100–1,000 W/R that corresponds to an 
increase in SiO2 wt.%. 
Olivine, TP Amorphous and TP Crystalline Start-
ing Compositions. The next iteration of modelling fo-
cused on mixing olivine with both the crystalline and 
amorphous components of TP [7]. Fig. 3 shows one 
starting composition mixture reacting with GPW, re-
sulting  >10 wt.% hematite precipitating between 100–
100,000 W/R peaking at ~44 wt.% at 100,000 W/R.    
Discussion: The models presented here indicate a 
potential formation process for the hematite-clay as-
semblage at VRR/Murray of alteration of precursor 
silicate materials by an oxidizing fluid at temperatures 
≥50 °C. In comparison, previous hypotheses for hema-
tite formation at VRR include oxidation of an anoxic 
Fe2+ rich fluid resulting in deposition of hematite 
[1,16], and alteration of sediments in the shallows of a 
redox-stratified ancient lake [17].  
Our thermochemical modelling assumes that the 
presently observed mineralogy is what was precipitat-
ed. However, it is possible that goethite initially precip-
itated and underwent phase transition to hematite over 
time [18]. Furthermore, the presence of talc in Figs. 1–
3 is not observed in drill-hole compositions to date, 
and presence of jarosite and akaganeite in Stoer and 
Rock Hall [2] is not currently explained by our models. 
Further work will be undertaken to refine the predicted 
mineral assemblages and to include reaction pathways 
that lead to sulfate and oxide precipitation. 
Summary: Thermochemical modelling of the reac-
tion between TP compositions [7] and GPW [11] has 
showed that hematite is readily produced at >100 W/R 
and 50 °C, for all modelled Fe2+/Fe3+ ratios. This sug-
gests that the hematite at VRR could have been a con-
duit for groundwater flow at temperatures ≥50 °C.  
References: [1] Fraeman A.A. et al., (2013) Geology, 41(10), 
1103-1106. [2] Morris R.V. et al., (2019) LPSC, 50, (this conf.). [3] 
Bristow T.F. et al., (2018) AGU Abstract P41A-01. [4] Bristow T.F. 
et al., (2018) Sci. Adv., 4, eaar3330. [5] Fraeman A.A. et al., (2019) 
LPSC, 50, (this conf.). [6] Rampe E.B. et al., (2017) EPSL, 471, 
172-185. [7] Morrison S.M. et al., (2018) Am. Min., 103, 857-871. 
[8] Edgar L.A. et al., (2018) LPSC, 49, #1704. [9] Edgar L.A. et al., 
(2018) AGU Abstract P41A-01. [10] Bridges J.C. et al., (2015) 
LPSC, 46, #1769. [11] Bridges J.C. et al., (2015) JGR: Planets, 120, 
1-19. [12] Schwenzer S.P. et al., (2016) MAPS, 51(11), 2175-2202. 
[13] Reed M.H. et al., (2010): Users Guide for CHIM-XPT; 71p.; 
Oregon (University of Oregon). [14] Bedford C.C., (2019) PhD 
thesis (in prep), The Open University, UK. [15] Gudbrandsson S. et 
al., (2011) GCA, 75(19), 5496-5509. [16] Fraeman A.A. et al., 
(2016) JGR: Planets, 121, 1713-1736. [17] Hurowitz J.A. et al., 
(2017) Science, 356, eaah6849. [18] Cornell R.M. and Schwert-
mann U., (2003) The Iron Oxides. Wiley and Sons.  
1897.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)


Thermochemical Modelling of Fluid-Rock Reactions in Vera Rubin Ridge, Galecrater, Mars

Thermochemical Modelling of Fluid-Rock Reactions in Vera Rubin ridge, Gale Crater, Mars.

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