Paleohydrology on Mars constrained by mass balance and mineralogy of pre-Amazonian sodium ...

22 Pages·2017·1.63 MB·English

PUBLICATIONS
Journal of Geophysical Research: Planets
RESEARCH ARTICLE Paleohydrology on Mars constrained by mass
10.1002/2017JE005319 balance and mineralogy of pre-Amazonian
sodium chloride lakes
KeyPoints:
(cid:129) Halitelakedepositssuggestvery
limitedwater-rockinteractiononearly M.MelwaniDaswani1 andE.S.Kite1
Mars,differenttotheextensive
alterationevidentatGaleCrater 1DepartmentoftheGeophysicalSciences,UniversityofChicago,Chicago,Illinois,USA
(cid:129) Lakeswere>100mdeepandlasted
>(101–103)years
(cid:129) Punctuatedhighratesofvolcanism Abstract Chloride-bearing deposits on Mars record high-elevation lakes during the waning stages of
raisingtemperaturesabovefreezing
couldhavesuppliedthechlorinefor Mars’wetera(mid-NoachiantolateHesperian).Thewatersourcepathways,seasonality,salinity,depth,
thesaltdeposits lifetime,andpaleoclimaticdriversofthesewidespreadlakesareallunknown.Herewecombine
reaction-transportmodeling,orbitalspectroscopy,andnewvolumeestimatesfromhigh-resolutiondigital
SupportingInformation: terrainmodels,inordertoconstrainthehydrologicboundaryconditionsforformingthechlorides.
(cid:129) SupportingInformationS1 ConsideringaT=0°Csystem,wefindthat(1)individuallakeswere>100mdeepandlasteddecadesor
(cid:129) TableS2
longer;(2)ifvolcanicdegassingwasthesourceofchlorine,thenthewater-to-rockratioorthetotalwater
volumewereprobablylow,consistentwithbriefexcursionsabovethemeltingpointand/oraridclimate;
Correspondenceto:
M.MelwaniDaswani, (3)ifthechlorinesourcewasigneouschlorapatite,thenCl-leachingeventswouldrequirea(cumulative)time
[email protected] of>10yearsatthemeltingpoint;and(4)Clmasses,dividedbycatchmentarea,givecolumndensities
0.1–50kgCl/m2,andthesecolumndensitiesbrackettheexpectedchlorapatite-Clcontentforaseasonally
Citation: warmactivelayer.Deepgroundwaterwasnotrequired.Takentogether,ourresultsareconsistentwithMars
MelwaniDaswani,M.,andE.S.Kite havingausuallycold,horizontallysegregatedhydrospherebythetimechloridesformed.
(2017),PaleohydrologyonMars
constrainedbymassbalanceand
mineralogyofpre-Amazoniansodium
chloridelakes,J.Geophys.Res.Planets, 1. Introduction
122,doi:10.1002/2017JE005319.
Ancient mineral deposits on planetary surfaces can serve as records of past climatic and environmental
Received3APR2017 conditions.Discretechloridemineralbearingsedimentaryunitsoccuronmid-NoachiantoearlyHesperian
Accepted6AUG2017 (3.9–3.5Gyrold)crustonthesouthernhighlandsofMars(Figure1)andhavebeenvariouslyinterpretedas
Acceptedarticleonline11AUG2017
aresultofevaporationofpondsandlakesfedbysurfacerunoff[Osterlooetal.,2008;Hyneketal.,2015]or
groundwaterupwelling[Osterlooetal.,2008;Rueschetal.,2012;El-Maarryetal.,2013,2014],orpossiblya
combinationofboth[Osterlooetal.,2010;Glotchetal.,2016].Hereweuseanovel,physicallyandchemically
self-consistentmethodtoconstrainthepaleohydrologyofMarswhenthechloride-bearingdepositsformed
by combining (1) reaction-transport geochemical modeling of aqueous alteration on the surface of Mars,
(2)massbalanceandgeologicalconstraintsontheoriginofthechlorineinthechloride-bearingdeposits,and
(3)geomorphologicanalysesofthedepositsandthebasinsinwhichtheywereemplaced.Thiscombination
of geochemical modeling and basin analysis is enabled by recently published constraints from remote
sensingandlaboratoryworkonthechlorides[Glotchetal.,2016].
As detected by thermal emission spectroscopy from orbit (Thermal Emission Imaging System (THEMIS))
[Christensenetal.,2004],thechloride-bearingdeposits(hereafterCBDs)tendtooccurinlocallowsorbasins,
appeartobethick(ontheorderofmeters[Osterlooetal.,2010;Hyneketal.,2015]),andoftenexhibitvertical
polygonalfracturesthathavebeeninterpretedasdesiccationcracks[Osterlooetal.,2008,2010;El-Maarry
etal.,2013,2014].Occasionally,theCBDsoccurininvertedchannelsand/orinfillingsmallcraters[Osterloo
etal.,2010].Assuch,thedepositsappeartobepaleoplayasorpaleolakesandinconsistentwithsaltefflores-
cenceformingthinsurficialcrusts[Osterlooetal.,2008].
Whilethechloride-bearingsinuousridgesaresuggestiveofsurfacerunoffasthemechanismfortransporting
thebrineswhichledtotheCBDs,someregionsthatlackfluvialvalleynetworks(e.g.,SNoachisTerra)have
abundantCBDs[Osterlooetal.,2010].(Thisdoesnotexcludeacorrelationwithrunoff,becausesurfacerunoff
doesnotnecessarilyresultinchannelformation,andsmall>3.5Gyrchannelsmightnolongerbevisible.)
Asanalternativetorunoffasawatersource,hydrothermalbrinescouldhaveupwelledattopographiclows
andsubsequentlyevaporated,ashasbeeninterpretedfortheoriginofthehalogenenrichment(~2wt%Cl
©2017.AmericanGeophysicalUnion.
AllRightsReserved. comparedto~0.5wt%Clinlessalteredbasalts)analyzedbytheMarsExplorationRover(MER)SpiritatHome
MELWANIDASWANIANDKITE PALEOHYDROLOGYOFCHLORIDELAKESONMARS 1
Journal of Geophysical Research: Planets 10.1002/2017JE005319
Figure1.Mapshowingthelocationofthechloride-bearingdeposits(blackpolygons)observedonthesurfaceofMars
[Osterlooetal.,2010].Thelocationsofthefoursitesstudiedhereareshowninred.Inthebackground,adustcover
indexmapoverlaystheshadedreliefmap,whichusesMOLAtopography[Smithetal.,2001].Thedustcoverindexisa
measureoftheamountofsilicatedustobscuringthesurfacetoorbitalspectroscopy,makinguseoftheemissivitymea-
suredat1350–1400cm(cid:1)1bytheMarsGlobalSurveyorThermalEmissionSpectrometer.Highernumbersmeanlessdust
cover[RuffandChristensen,2002].
Plate,Gusevcrater[Schmidtetal.,2008].However,attheCBDsites,noevidencehasbeenreportedforan
associated hydrothermal mineralogical assemblage [Osterloo et al., 2010], although this may have been
obfuscated by, e.g., wind erosion. Additionally, a widespread process of deep groundwater upwelling is
inconsistent with the observation that some CBDs are found in local topographic lows at high elevation,
whiletopographiclowsatlowerelevationsoftenlackCBDs[Osterlooetal.,2010].
While the chloride-bearing depositional facies are scattered across Mars, their distinctive geomorphology
andlocal topographic setting suggestsa single formationmechanism.However,thesedataoffer nocon-
straints on the duration, intensity, and number of wet events that led to CBD formation [Osterloo et al.,
2010]. A key provenance constraint is the nondetection of other evaporite minerals (sulfates, carbonates,
silica, and other halides) in close proximity, overlain by, or in “association” with the CBDs [Osterloo et al.,
2010]. In fact, new emissivity scatter modeling and laboratory experimental validation of THEMIS spectra
showthattheCBDsarecomposedof10–25vol%halite[Glotchetal.,2016],with<5wt%gypsumorcalcite
[YeandGlotch,2016].Assumingnooutflow,thissignifiesthatthemolarCl/Sratioofthefluidthatfilledthe
lakeswas≲11(foramolarvolumeof74.7cm3forgypsumand27.0cm3forhalite).Evaporationofsaline
bodies of water on Earth typically forms sequences of evaporitic minerals defined by the fluid chemistry
andthesolubilitiesofsaltspresent[EugsterandHardie,1978];broadlysimilarsequencesarepredictedfrom
theevaporationoffluidsonMars[ToscaandMcLennan,2006].Hence,thefluidsthatformedtheCBDsmust
havecontainedlittleScomparedtoCl.
AcartoonschematicofhowtheCBDscouldhavebeenformedisshowninFigure2.Runoffandgroundwater
dischargeintoatopographiclowwouldhavepondedafluid.Evaporativeconcentrationwouldhavethen
precipitatedchlorideminerals.CBDcatchmentareas,andlowerlimitsonCBDvolumes,canbeaccurately
mapped today by combining digital terrain models (DTMs) and orbiter thermal emission spectroscopy
[e.g.,Hyneketal.,2015].Thisisbecauselandscapemodificationbywinderosionhasbeengenerallysubdued
sincetheformationoftheCBDs;the
overall topography of the Southern
Highlands has changed little since
the CBDs formed [Nimmo and
Tanaka, 2005]. This enables a mass
balance analysis that is not possible
for more ancient aqueous deposits
whose watersheds are less well pre-
Figure2.(a–c)Cartoondepictingthegeologicalcontextofchloride-bearing served[Murchieetal.,2009].
lakesonMars.Fluidsflowassurfacerunoffandgroundwaterfromleftto
Brinefractionationisnotinvestigated
right.AtFigure2athefluidsreactwiththebasinrocksandleachanionsand
inthisstudy,butitleadstopaleohy-
cationsfromtherocks,includingClandSdepositedonthesurfacebyvol-
canicgases.AtFigure2b,thesalt-bearingfluiddischargesatatopographic drology conclusions similar to those
low,andatFigure2c,saltsprecipitateafterthepondingfluidhasevaporated. of our T = 0°C investigation. NaCl
MELWANIDASWANIANDKITE PALEOHYDROLOGYOFCHLORIDELAKESONMARS 2
Journal of Geophysical Research: Planets 10.1002/2017JE005319
brineswitheutecticpointsbelowthefreezingpointofwatercouldhavebeenresponsibleforthechloride
enrichmentintheobserveddeposits[BurtandKnauth,2003;Clarketal.,2005].Inthiscase,theabsenceof
sulfatesaltsattheCBDs[Osterlooetal.,2010;Glotchetal.,2016]couldbetheresultofbrinefractionation
since chloride brines have lower freezing points than sulfate brines [e.g., Reeburgh and Springer-Young,
1983;Marion,2001;TonerandSletten,2013].
Regardlessofthephysicalprocessthatledtotheformationofthefacies,thegeochemicalsourceofchlorine
andcations(namelysodium,basedonlaboratoryspectraandspectralmodeling[Glotchetal.,2016])hasnot
beenconstrained.Possibleoriginsforthechlorinecouldbeweatheringandleachingofthedrainagebasin
rock(section2.1)[e.g.,Eugster,1980;Warren,2010],volcanicoutgassing(section2.2)[ToscaandMcLennan,
2006;Osterloo et al., 2008; Glotch et al., 2016; Zolotov and Mironenko, 2016], reworking (i.e., dissolution
and transport) of previously formed evaporites [e.g., Salvany et al., 1994; Warren, 2010] (section 2.3),
and cometary/extraterrestrial delivery by a halite-rich body (section 2.4) (e.g., H-chondrites like Zag and
Monahans[Rubinetal.,2002]).Althoughsomesodiummayhavebeenderivedfromvolcanicdegassingor
exsolutionfrommagma[e.g.,Webster,2004],mostNawasprobablyderivedfromtheinteractionofafluid
withtheMartianbasalticcrust[ToscaandMcLennan,2006;ZolotovandMironenko,2016].OnEarth,thevast
majorityofClintheoceansisderivedfromvolcanicdegassing(includingsubmarinevents)asopposedto
continental weathering [Rubey, 1951; Spencer and Hardie, 1990], and maxima in Cl in ~500 km resolution
MarsOdysseyGammaRaySpectrometermapsarespatiallyassociatedwithmodernMarsvolcanoes[Keller
etal.,2007].Incontrast,Clinterrestrialendorheiclakesislargelyleachedandmobilizedfromsurrounding
basinrocks[EugsterandHardie,1978;Eugster,1980].Drydepositionfromvolcanicgasesisunlikelytodirectly
source chloride deposits on Mars. For example, the regions where pyroclastic deposits were predicted to
form[Kerberetal.,2012,2013]arenotbroadlycoincidentwiththelocationofthechloridedepositsobserved
byTHEMIS.
Insection2,wediscussthepossiblegeologicalandgeochemicaloriginsofthechlorineintheCBDsandrelate
thesetopaleohydrologyandpaleoclimatology.Insection3,wedescribetheworkflowweusedtoexplore
possibleoriginsofchlorine,usingcompositionalanalysesoftheMartiansurface,fourdifferentCBD-hosting
basins(oneareevaluationofpreviousworkbyHyneketal.[2015]),andtwohydrologicalscenariosthatpoten-
tiallyfedthelakeswheretheCBDsarefound:surfacerunoffandgroundwaterdischarge.Insection4,wepre-
sent our results, and finally, in light of the results, we discuss the implications on Mars’ paleoclimate and
paleohydrologyinsection5.
2. Physicochemical Controls onPlausible Chlorine Sources
HydrologicscenariosfortheCBDformingevent(s)canbeparametrizedusingthreevariables:(1)water-to-
rockmassratio(hereafterW/R;aunitlessmeasurementofthemassofwaterreactingwithrock,dividedby
thetotalmassofrockthathasreactedwithwater;seeReed[1997,1998]foradetaileddiscussion),(2)the
duration of the individual warming event (or events) that allowed liquid water to exist and form the Cl-
bearingpools(τ,inMarsyears,where1Marsyear≈5.94×107s),and(3)thetotalwatervolume(TWV,in
kgH O)flowingthroughthebasinanddischargingintothetopographiclowthatwillformalake.
2
W/R is linked to the mobility of elements and the “open” versus “closed” behavior in aqueous alteration
systemswithrespecttochemicalexchangewithdifferentreservoirs[e.g.,Ehlmannetal.,2011].W/Rcontrols
the reaction path progress between the crust and the fluid. High W/R conditions occur at the interface
betweenthefluidandtherock(e.g.,fracturesandtheatmosphere-rockinterface),whereadilutefluidcomes
incontactwiththerocksurface,andlittlerockisdissolved.LowW/Roccurswherefluidchemistryisdomi-
natedbysolutesleachedfromtherocks,e.g.,attheendofalonggroundwatertravelpathwheremuchof
thewaterpermeatingthroughtherockhasbeenconsumedtoformsecondaryminerals(e.g.,phyllosilicates
inaweatheringprofile).W/Ralsocontrolstheformationandfractionationofbrines:chloridesaltsareusually
moresolublethansulfatesaltsandrequirelesswaterperunitmasstodissolve,so,e.g.,lowW/Rcouldforma
chloride-onlybrinefromamixedchlorideandsulfate-bearingreactant,whereashighW/Rwouldputboth
saltsinsolution.Similarly,theevaporationofwaterfromamixedchloride-sulfatebrinewillcausesulfatesalts
toprecipitatebeforechloridesalts[e.g.,Eugster,1980;ToscaandMcLennan,2006].
Oursecondhydrologicvariableisτ.ClimatemodelssuggestthatMarsinthelateNoachiantoearlyHesperian
had an equilibrium surface temperature below the freezing point of water on average [e.g., Wordsworth,
MELWANIDASWANIANDKITE PALEOHYDROLOGYOFCHLORIDELAKESONMARS 3
Journal of Geophysical Research: Planets 10.1002/2017JE005319
Table 1. Composition of the Reactant Rock Used in the Geochemical 2016]. Assuming that this is correct,
Models, Based on Mazatzal Basalt Analyzed by Spirit at Gusev Crater,
the availability of liquid water for
UsingAPXSandMössbauer[McSweenetal.,2004,2006]a
water-rock interactions is controlled
Component RockComposition(wt%) Mineral(CIPW) wt% by the duration (τ) of the individual
SiO2 46.22 Plagioclase(An43.7) 39.98 event or events that allowed
Al2O3 10.88 Orthoclase 0.65 temperatures above the freezing
Fe2O3 2.14 Diopside 16.49 point of water (see sections 2.1 and
FeO 17.08 Hypersthene 6.03
2.2). We consider that “short”
MnO 0.44 Olivine(Fo52.7) 31.63
CaO 8.35 Magnetite 3.1 durations are always <0.5 Mars
Na2O 2.66 Apatite 1.48 years, such that the water required
K2O 0.11 Pyrite 0.64 to form the chloride deposits could
P2O5 0.64 Total 100 have been sourced from seasonal
FeS 0.84
melting of snow and/or ice. We
Cl 0.15
Total 100 assume the availability of snow or
aTiO2 and Cr2O3 exist in low concentration in Mazatzal (combined ground ice for episodic melting,
~1wt%)andarenotincludedbecauseoflackingTiandCrmineralsin consistent with the geomorphologic
theSOLTHERMthermodynamicdatabaseandtheirlowsolubility. [e.g., Fassett and Head, 2008, 2011]
and isotopic [e.g., Mahaffy et al.,
2015;Villanuevaetal.,2015]evidencethatearlyMarshadabundantwater.
Totalwatervolume(TWV)influencesthegeomorphologyandthedegreeofchemicalinteractionbetween
the rock and the fluid. High TWV typically allows larger amounts of rock to be dissolved, leading to the
formation of an assemblage of secondary minerals formed from the rock-derived solutes. A low TWV
wouldprecludedeeplakesbutmightpermitshallowpools.LowTWVallowsonlythemostsolubleminer-
als in the precursor rock to be dissolved, altering both fluid chemistry and the resultant
mineralogical assemblage.
2.1. BasaltWeatheringasaSourceofChlorine
Martian basalts are enriched in Cl by a factor of ~2.5 compared to terrestrial basalts and mantle rocks
[FilibertoandTreiman,2009].Clisincompatible,soitisenrichedinlate-stagevolatile-richminerals(apatite
andamphibole)formedincrystallizingmagmas,ordegassed[e.g.,Aiuppa,2009].Basaltscontainingthese
mineralswillreleaseCluponweatheringbyalterationfluids.Chlorapatite(Ca (PO ) Cl)couldthereforeact
5 43
asa source of Cl[Guidry andMackenzie,2003;Adcock etal., 2013]forthebrines formingtheCBDs. As an
illustration, we consider a single wet event that lasts just long enough to dissolve apatite at the surface.
The dissolution rate of chlorapatite depends on pH, mass, and surface area of the apatite grains. For soil
thatisclosetothesurface,inequilibriumwitha60mbarpCO atmosphere(pH≈4.5),andassumingthat
2
theapatitegraindiameterintheMartianregolithissimilartotheapatitesizefoundinMartianmeteorites
(10(cid:1)5–10(cid:1)4 m in the basaltic breccia Northwest Africa 7034 [Wittmann et al., 2015]), we calculate that
chlorapatite in the regolith dissolves completely in 0.04–0.4 Mars years, using a dissolution rate of
4.2 × 10(cid:1)9 mol chlorapatite m(cid:1)2 s(cid:1)1 derived from Adcock et al. [2013]. (See Appendix A for further
details.) A single wet event of this duration suggests a weathering depth controlled by vertical diffusion
of a top-down warming pulse through the surface soil/regolith:
pffiffiffiffiffi
L≈2:32 κτ (1)
where L is the depth reached by a warming pulse originating at the atmosphere-regolith interface (m), κ
isthethermaldiffusivityoftheregolith(assumedheretobetypicalforsilicates,i.e.,7×10(cid:1)7m2s(cid:1)1),and
τ isthetime(inseconds)ofasingleunfreezingevent[e.g.,TurcotteandSchubert,2002,section4.15].(We
ignore the latent heat associated with melting a body of ice.) Thus, we calculate 3–10 m as the implied
depthofunfreezingintheregolithinasingleeventlasting0.04–0.4Marsyears.Inthisscenario,apatiteat
the top of the weathering profile will be completely dissolved and apatite at the bottom will have just
started dissolving. Assuming that the composition of basalts encountered by the MER rover Spirit at
Gusevcrater[McSweenetal.,2004,2006]isrepresentativeofthecompositionoftheMartiansurfaceelse-
where on Mars, on average, Martian surface rocks contain ~0.15 wt % Cl (Table 1) or 2.5–4.35 kg Cl m(cid:1)3
(usingdensitiesof1650kgm(cid:1)3forMars200μmgrainsandand2900kgm(cid:1)3forMarsbasaltfromMellon
et al. [2008]). Therefore, a priori, it appears that apatite dissolution could potentially supply significant
MELWANIDASWANIANDKITE PALEOHYDROLOGYOFCHLORIDELAKESONMARS 4
Journal of Geophysical Research: Planets 10.1002/2017JE005319
Figure3.Possiblesourcesofchlorineinthechloride-bearingdepositsshowingpermittedandexcludedcombinationsof
water-to-rockratio(W/R;unitless),durationofthewarmingevent(s)abovethefreezingpointofwater(τ;Marsyears),
andtotalwatervolume(TWV;kgH2O).Thegreenareasarepermittedscenarios,thepinkareasareexcluded,andthe
orangeareaisdisfavoredbutnotexcluded(seetext).
chlorinetoformtheCBDsviameltingofice/snowwithinaseason.TheexactClrequirementswilldepend
on the amount of Cl in the CBDs and on the ratio of catchment area to CBD volume.
Figure3ashowsamatrixofoutcomeswhenconsideringprimaryigneousapatiteweatheringasthesourceof
ClfortheCBDs.Whileapatitedissolutionisrapid,theregolithismainlybasaltic,andinfiltratingfluidswould
bebufferedtohigherpH,whichwoulddecreasethedissolutionrateofapatite[e.g.,Adcocketal.,2013].Short
durations of warming (τ) may not thaw a sufficient depth in the active layer to mobilize sufficient anions
andcations.
For medium durations of warming, fluids will be able to dissolve sufficient basalt to meet thecation and
anionmassrequirementsoftheCBDs,butathighW/RandlowTWV,themassofthepondingfluidswould
beinsufficientandtoodilutetoformtheCBDs.Thisscenariocouldcorrespondtothemeltingofalimited
amountofnear-surfaceice.LowW/RwouldpreventhighconcentrationsofS(relativetoCl)fromreaching
thelake,whereasatintermediateW/R,Sinsolutionderivedfromdissolvingprimarysulfidesinthebasalt
wouldformsecondarymineralsalongthereactionpathandpreventS-enrichedbrinesfromreachingthe
pondingsite.AthighW/R,however,Swouldnotprecipitatealongthereactionpathandwouldpondwith
Cl (along with carbonate if the W/R is high enough) [e.g., Eugster and Hardie, 1978] at the lake site.
Therefore,thisparametercombinationisunlikely,becauseofthenondetectionofsulfatesandcarbonates
associatedwiththeCBDs.
MELWANIDASWANIANDKITE PALEOHYDROLOGYOFCHLORIDELAKESONMARS 5
Journal of Geophysical Research: Planets 10.1002/2017JE005319
2.2. EvaluatingaVolcanicSource
forChlorine
VolcanicgasesfromtheNoachianto
the mid-Hesperian most likely con-
tained HCl [e.g., Wänke et al., 1994;
Smith et al., 2014]. Cl sourced from
volcanic gases could build up in the
shallowregolithvia(1)drydeposition
of Cl-bearing molecules or mineral
phases(subsequentlyremobilizedby
a fluid) and (2) wet deposition, in
whichchlorine-bearinggasesaredis-
solvedinatmosphericwater,andthen
reactwiththeshallowregolith.
The mantle source regions of the
shergottite-nakhlite-chassignite
(SNC)Martianmeteoriteswouldhave
contained 25 ± 8 ppm Cl and
56±71ppmH O;i.e.,theywereon
2
Figure4.CumulativechlorinedegassedbycrustformationonMarsoverthe
average drier but halogen enriched
rangeofagesofCBD-hostingterrain(3.96to3.0Gyr[Osterlooetal.,2010]).
I/E=intrusivetoextrusiveratio,DCl=gas-meltmolarpartitioncoefficient compared to Earth [Filiberto and
forchlorine.Seesection2.2andAppendixBfordetails. Treiman, 2009; Filiberto et al., 2016].
(These results supersede previous
estimates,whichsuggestedthat8ppmHCland0.5wt%H OwerepresentinMartianpreeruptivebasaltic
2
magmas[Craddock andGreeley, 2009].)Weuse theestimated volatile contentof theparentalmelt of the
Shergottymeteoritetocalculatetheamountofchlorinedegassedtothesurfaceovertime.Shergottyisa
basaltderivedfrom10to15%partialmeltingofalightrareearthelement-enrichedmantleregion[Stolper
andMcSween,1979]whichcontained12–23ppmCland36–73ppmH O[McCubbinetal.,2016].Thepartial
2
meltwouldhavecontained363–484ppmH Oand116–155ppmCl[McCubbinetal.,2016],whilethebulk
2
meteoritecontains~108ppmCland~280ppmH O[Lodders,1998].
2
Thesolubilityofvolatilesinmagmacontrolstheirreleaseintothegasphase.Forbasaltmeltscontaining≤1wt
%H O,aswasprobablythecaseforShergotty,thesolubilityofchlorideishighlydependentontheconcentra-
2
tionofcationsinthemeltabletocomplexwithCl(e.g.,Al,Na,Ca,andMg)[Websteretal.,1999].Assumingneg-
ligible loss of cations during and after fractional crystallization of Shergotty, and whole rock composition
(cid:1)
reportedbyLodders[1998],wecalculateaCl solubilityof1.0wt%at2kbarand0.75wt%at1bar.Thevalue
weuseforchlorineconcentrationinthe~3.0–3.9Gyrmantleisalowerlimitbecausethe1.35–0.17Gyr[Nyquist
etal.,2001]SNCssampleayoungmantle,anddegassinghasremovedClfromthemantlereservoirovertime.
Volcanicdegassingisproportionaltomagmaflux.Despitehighearlymagmaproductionrates,thevolumeof
intrusiveproductioninthecrustcouldbeafactorof3–750timestheextrusivebasalts[GreeleyandSchneid,
1991;Lillisetal.,2009;Filibertoetal.,2014;BlackandManga,2016].GaseousHClisthemaincarrierofClin
volcanicgases [e.g., Aiuppa et al., 2009]. HClabundance is controlled by a poorly understood mechanism
involvingvapor-meltpartitioningdependentonmeltandfluidcompositions,temperature,pressure,redox
state of the melt, crystallization, partial melting, and/or open versus closed modes of degassing [Aiuppa,
2009;Aiuppaetal.,2009;Edmondsetal.,2009;MétrichandWallace,2009].Absentamechanisticunderstand-
ing,ourbestguideisgivenbysemiempiricalmodelsusingmeltinclusionanalysesoferuptedterrestrialsam-
ples,solubilityexperiments,andS/Clrelationshipsobservedinventingvolcanicgases[Websteretal.,1999;
Aiuppa,2009;Edmondsetal.,2009;PyleandMather,2009].Weadoptedseparatemolarpartitioncoefficients
frommagmaintothegasphaseforCl(D )forintrusive(D =0–0.25)andextrusive(D =0.9)bodies,con-
Cl Cl Cl
sistentwithpublishedsemiempiricalmodels(AppendixB).
Figure4 showsthecalculatedmass of accumulated CldegassedonMarsovertherangeof ages of CBD-
hosting terrain (3.96 to 3.0 Gyr [Osterloo et al., 2010]). We use the solubility of Cl in a Shergotty-like melt
(see above), intrusive to extrusive ratios of 3 (maximal outgassed Cl) and 10 (minimal outgassed;
MELWANIDASWANIANDKITE PALEOHYDROLOGYOFCHLORIDELAKESONMARS 6
Journal of Geophysical Research: Planets 10.1002/2017JE005319
consistentwithintrusiveMartiangabbroNWA6963[Filibertoetal.,2014]),D appropriateforintrusivesand
Cl
extrusives,andacrustalproductionmodelspecifictoMars’compositionofKieferetal.[2015]andKiefer[2016,
personalcommunication].
Intheminimaldegassingscenario(I/E=10,andnoCldegassedfromintrusivebodies),volcanicdegassing
between3.96and3.0Gyrcouldproducesufficientchlorinein<1Myr(Figure4)toaccountforthemassof
theobservedglobalinventoryofCBDs,assumingthatthevolcanicClisalldepositedinthecatchmentsof
theobservedCBDs,whichcontain~1.85×1013kgCliftheCBDsare4mthick[Hyneketal.,2015],covera
surface area of ~1.41 × 104 km2 [Osterloo et al., 2010], and contain 10–25 vol % NaCl (density = 2165 kg
m(cid:1)3)[Glotchetal.,2016].At3Gyrthecumulativeminimal-scenariodegassedClovertheplanetwouldhave
been~300kg/m2(Figure4).
Figure3bshowsamatrixofoutcomeswhenconsideringvolcanicdegassingasthesourceofchlorine,based
onthehydrologicalparametersdescribedatthebeginningofsection2.Forshortdurationsofthewarming
event(s),volcanicvolatilesdepositedintheshallowregolithcouldbemobilizedeasilybyfluids,butcationsin
lowconcentrationinvolcanicgases(Na,K,Mg,Ca,andFe)requiredtoultimatelyformchloridesaltswould
needtobesourcedfromsurfacerocks/soil.Forashortwarmingevent,lowTWVcoupledwithlowW/Rcould
resultfromnearsurfacemeltingofsmallamountsoficereactingwithnear-surfacevolcanicClandlocalrock
(e.g.,seasonalmelting).Inthiscase,transportofthefluidanditssolutesintoapondwouldnotoccur,since
water-rockreactionwouldconsumethesmallamountofwaterlocally,andanyresidualwaterwouldfreeze
orevaporateattheendofthewarmingevent.
ThemeltingofalargevolumeofsubsurfaceporeicecouldresultinhighTWVandlowW/R.ThehighTWV
wouldpreventthefluidfrombeingcompletelyconsumedbyreactions,butthelowW/Rfluidswillcontain
largecation,Si,andSconcentrationsfromthedissolutionattherock-poreinterface.Ifmeltinganddraining
occursrepeatedly,thenthecation,Si,andSconcentrationmayexceedthatdemandedbytheavailablevol-
canicCltoformtheCBDs,andsulfatesorclaymineralsmaycooccurwithchlorides.Boththeseoutcomesare
intensionwithobservations.
Forwarmperiodsontheorderofayear,atlowW/R,oneorafewwettingeventswithlowTWVcouldcause
therequired fluidmassand compositiontopond attopographic lows. (Sulfatescould alsoprecipitatefar
fromthelakes.)Ontheotherhand,apersistentlywarm/wetclimate(highTWV,ormanywettingeventsat
lowTWV)couldpotentiallymobilizethesoilintheregolithbeforeClfromvolcanicdegassingcanaccumulate
inthesoil,sincevolcanicCldegassingisrelativelyslow(Figure4).
AthighW/R,regardlessoftheTWV,sulfatesandchlorideswouldcooccuratthesiteofevaporiteprecipita-
tion.ThisisbecausehighW/Rreactionofsurfacesoils/rockswouldnotonlydissolveandtransportthechlor-
ineandsulfur-bearingphasesdepositedfromvolcanicgasesbutalsoweatherprimaryigneoussulfidesinthe
near-subsurfaceandtransportSspeciesdownstream.Sincethechloridedepositsarenot“associated”with
sulfates[e.g.,Osterlooetal.,2010],thisscenariomustberuledout.
ThedissolutionrateofbasalticchlorapatiteandtheamountofchlorapatiteintheMartiancrustcontrolthe
upperlimitforthedurationofthewarmingevent(s)inavolcanicCl-sourcescenario,sinceextendedperiods
ofliquidwateravailabilitycouldcauseClderivedfromchlorapatitedissolutiontodilutethesignalofvolca-
nicallyderivedClintheCBDs.Chlorapatitedissolvesin0.04–0.4Marsyears,correspondingtoaweathering
depthof3.0–10m(seesection2.1).Thismeansthatinalow-endCldegassingscenario(I/E=10,withonly
extrusivebodiesdegassing),crustalproductiononMarsinthetimescalerelevanttotheCBDsproducedon
average3.2×10(cid:1)7±2.1×10(cid:1)7kgClm(cid:1)2yr(cid:1)1(Figure4),whichwouldoutweightheClpresentinbasaltic
apatitein7.9–13.8Myr(4.3–7.5×106Marsyears)ofvolcanicdegassing.Thistimeismuchshorterthanthe
rangeofformationagesinferredfortheCBDs[Osterlooetal.,2010].Furthermore,apatitedissolutionwould
havebeenhinderedasthepHofthefluidinitiallyinequilibriumwiththeatmospherewasbufferedbybasalt.
Therefore,thedissolutionofprimarymineralsrequiresamorerealisticapproach,whichwecarryoutwith
geochemicalmodels(section3).
2.3. Reworking(DissolutionandTransport)ofPreviouslyFormedEvaporites
Thelow-elevation,low-latitudesulfatedepositsofMarshavebeensuggestedtobereworkedNoachianeva-
porites [Milliken et al., 2009; Zolotov and Mironenko, 2016]. However, reworking of preexisting massive
MELWANIDASWANIANDKITE PALEOHYDROLOGYOFCHLORIDELAKESONMARS 7
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evaporitesisunlikelytobethesourceoftheClfortheobservedCBDs.Forexample,thechloridesareoften
foundinperchedbasinsonhighelevations,farfromanyplausibleancientmarinebasin.
2.4. ChlorineFromtheSky:MeteoriticDeliveryofChlorine?
Readily soluble Cl-bearing phases might be supplied to the shallow regolith by meteorites. The observed
globalinventoryofNaClfromtheCBDsis1.4–14km3iftheCBDsare1–4mthick[Hyneketal.,2015]and
10–25vol%NaCl[Glotchetal.,2016].RareHchondritesZagandMonahanscontainpercentlevelsofextra-
terrestrialhalite[Rubinetal.,2002]—thecumulativevolumeofZag-likeimpactorsdeliveringNaClwouldhave
tobeatleast28–563km3(for5vol%–1vol%NaClintheimpactors).Thisaveragesoutto0.03–0.11kgCl/m2if
itwasdeliveredsolelyacrossNoachianandHesperianhighlandterrains(6.79×107km2[Tanakaetal.,2014]).
Webelievethatmeteoriticdelivery(or“astrosedimentation”[HesselbrockandMinton,2017])ofCltoformthe
CBDs is unlikely. One reason is the much greater efficiency of volcanism (section 2.2). Another is that the
CBDspostdatebasin-formingimpactsonMars[Toonetal.,2010;Robbinsetal.,2013].Ongoinginsituisotopic
analysesofMarsCl[Farleyetal.,2016]mightallowthemeteoritic-sourcehypothesistobetested.
3. Methods
3.1. Geochemical1-DFlow-ThroughandFlushModeling
Tobetterunderstandtheoriginofthechloride-bearingdeposits,weusedreaction-transportmodeling[e.g.,
vanBerkandFu,2011;Bridgesetal.,2015].Specifically,weusedprogramCHIM-XPT[Reed,1998]tocompute
simulationsofgeochemicalweatheringoftheMazatzalbasaltanalyzedbySpiritatGusevcraterwithAlpha-
ParticleX-raySpectrometer(APXS)andMössbauer(Table1)[McSweenetal.,2004,2006].Mazatzalisrepre-
sentativeofthebasalticcrustalcompositionofMars[McSweenetal.,2009].Themodelinputcomposition
we used was obtained from a brushed and abraded surface rock in which the interference of Mars dust
andsoilwasminimized,wascorrectedforthesulfurandchlorineallocthonoustotherock,andisconsistent
withmeasuredbasalticMartianmeteoritecompositions[McSweenetal.,2004,2006].Additionally,ourmodel
compositioninputisinsensitivetolocalelementalredistribution(basaltsattheGusevplainshaveexperi-
encedlimited,isochemicalweathering[Haskinetal.,2005;Hurowitzetal.,2006])sinceourmodelinputuses
thewholerockbulkcomposition,ratherthanthecalculatedinferredmineralogy(Table1).Adetailedexpla-
nationofthemodelingtechniquesisgivenbyReed[1998].Weusetheupdatedthermodynamicdatabase
SolthermBRGM,whichincludesdataforlow-temperaturegeochemistryfromtheBRGMThermoddemdata-
base[Blancetal.,2012]amongothersources.
Inthe1-Dflow-throughmodel,waterinequilibriumwitha60mbarpCO +0.15mbarpO atmosphere(~10×
2 2
and17×themodernatmosphericCO andO partialpressure,respectively[Mahaffyetal.,2013])permeates
2 2
theatmosphere/rockinterfaceandtraversesthebasalticcrust.Thefluidequilibrateswiththeparcelofrockit
isincontactwithbutisoutofequilibriumwiththeprecedingparcelsofrock(fromwhichithasbeenfractio-
nated).Asthefluidmovestoequilibratewithsuccessiveparcelsofrock(Figure5a),thefluidisconsumedin
water-rockreactionsalongthefluidpathandthesystemevolvesfromhighW/RtolowW/R.TheactualW/Rof
theinflowingfluidattheCBDlakesisunknown,sowevarytheW/Rinthemodelandcomparetheresulting
modeloutputtoobservedmineralogy.
FlushmodelswereusedtoconstrainthetotalamountofwaterrequiredtoproduceafluidwithahighCltoS
ratio,inaccordancewithobservationsthatthechloridedepositsarenotassociatedwithsulfatesorother
evaporites [e.g., Osterloo et al., 2010; Glotch et al., 2016]. In flush models, a parcel of rock in contact with
theatmosphereisflushedinconsecutive“wettingevents”byafreshfluidequilibratedwiththeatmosphere
(Figure5b).Theamountoffluidallowedtoenterandleavetherockiscontrolledinthefirst“wettingevent”
bythedesiredW/Randinsubsequenteventsbytheresultingporosity.Wechosetoallow10%porevolume
spaceoftheequilibratedfluidtoexittheparcelofrockaftereachwettingeventandtoinfusetherockwith
10%porevolumespaceoffreshfluid.InitialW/Rratioschosenwere1,5,10,50,100,500,and1000.Porosity
evolvedinourflushmodelsasmineralswithdifferentdensitiesdissolvedandprecipitated,andsoluteswere
removedfromthesystemwithfluidextractedateverystep.
3.2. DepositionalBasinAnalyses
FourCBDsitespreviouslycataloguedbyOsterlooetal.[2010]wereselectedforthisstudy:(1)thedepositstu-
diedby Hynek et al. [2015] west of Miyamoto Crater, (2) Terra Sirenum, (3)west of Knobel Crater, and (4)
MELWANIDASWANIANDKITE PALEOHYDROLOGYOFCHLORIDELAKESONMARS 8
Journal of Geophysical Research: Planets 10.1002/2017JE005319
Figure5.Schematicofthetwotypesofgeochemicalreaction-transportmodelsusedinourstudy.(a)1-Dflow-through
modelusedtostudythebasaltweatheringhypothesisfortheoriginoftheCBDs.Freshwaterequilibratedwiththe
atmospherereactsandequilibrateswithaparcelofbasalt.Porosityoftherocksintheflowpathchangesassecondary
mineralsprecipitateandrockisdissolved.Waterisconsumedinwater-rockreactionsalongthepath,decreasingW/Rasthe
fluidmovesthroughtothenextparceloffreshunreactedbasalt.(b)Flushmodelusedtostudythevolcanicorigin
hypothesisforthechlorineintheCBDs.Freshwaterequilibratedwiththeatmospherereactswithsurfacebasalt+ClandS
fromvolcanicdrydeposition.Aproportion(afixedpercentageoftherock’sporosityvolume,herechosenas10vol%)of
fluidequilibratedwiththerockexitsthesystem.Thesamerockparcelisreactedwithfreshwaterinsubsequentsteps.
Flushingformsalterationmineralsanddissolvestherock,whilealteringtheporosityofthesystem(allowingmoreorless
watertoenterinsubsequentsteps)andthecompositionoftheexitingfluid.
southeastofBunnikCrater(Figure1andTableS1inthesupportinginformation).Siteswereselectedonthe
basisofHighResolutionImagingScienceExperiment(HiRISE)stereopaircoverage.Ateachsite,CBDswere
mapped using high-resolution orthorectified HiRISE image data (25 cm/pixel [McEwen et al., 2007]) and
digital terrain models (DTMs; 1 m/pixel) produced by David P. Mayer using Ames Stereo Pipeline (ASP)
[Moratto et al., 2010] and the University of Chicago ASP scripts [Mayer and Kite, 2016]. Additionally,
orthorectified CTX images (~6 m/pixel [Malin et al., 2007]) and CTX DTMs (24 m/pixel) were used to
manuallydelineatewatershedsandtheextentofthepaleolakesencompassingtheCBDs.TheMarsOrbiter
Laser Altimeter (MOLA) [Smith et al., 2001] gridded elevation product (~463 m/pixel) was used to
determinemaximumdepthsofthepaleolakeswhereHiRISEandCTXDTMcoveragewasnotavailable.We
computed the thicknesses of the CBDs using a three-step procedure. First, we identified points in HiRISE
orthoimages on top of the deposits and underneath the deposits, using craters excavating through the
CBDs (as in Hynek et al. [2015]), erosional features, and the edges of the CBDs (Figure 6). Second, we
subtracted the elevations of the bottom of the deposits from the top using the HiRISE DTMs to calculate
localdepositthicknesses.Finally,weinterpolatedacrossirregularlyspacedthicknessmeasurementsusing
aninversedistanceweightingfunctioninArcGISinordertoestimatethicknessacrosseachCBD.
MELWANIDASWANIANDKITE PALEOHYDROLOGYOFCHLORIDELAKESONMARS 9
Journal of Geophysical Research: Planets 10.1002/2017JE005319
Where CBDs within a basin lay out-
side our HiRISE DTM coverage (e.g.,
threeHiRISEDTMsatTerraSirenum),
the mean of the interpolated thick-
nesses on each of the HiRISE DTMs
was used to assign the thickness of
the CBDs outside of the HiRISE
Figure6.Cartoondepictingchloride-bearingdepositmeasurements.Using DTMs. Average extents and thick-
HiRISEDTMs,thechloride-bearingdeposit(wavylines)thicknesswas
nesses of the CBDs are reported in
calculatedusingerosionalwindows(greendottedline,e.g.,craters)into
Table2.Minimumandmaximumlake
thesubstratebelowthechloride-bearingdepositandattheedgesofthe
deposit(greenstars).Themeasuredthicknesspointswereinterpolated volumes were calculated using the
(seesection3.2)toobtainanoverallthickness.Theminimumlakeextent Cut/FilltoolonArcGIS,wherethelake
(redlongdashedline)andvolumewerecalculatedbasedonthemaximum bottomtopographyderivedfromthe
heightofthechloride-bearingdeposit,andthemaximumlakeextent
MOLADTM(oraCTXDTMinthecase
(blueshortdashedline)andvolumewerecalculatedbasedonthepour-
oftheCBDwestofMiyamotoCrater)
pointelevationofthelake.
was subtracted from a constant ele-
vation raster that defined the lake depth at (1) its minimum extent, from the maximum elevation of the
CBD, and (2) at its maximum extent, from the maximum elevation of the lake overflow/tipping point
(Figure6).
CalibrateddaytimethermalinfraredemissionimagesfromTHEMIS(~100m/pixel[Christensenetal.[2004])
wereusedtoidentifytheextentoftheCBDs.WhereTHEMISmultispectraldatawereavailable,decorrelation
stretch(DCS)imageswereproducedwithspectralbands8,7,and5(“8/7/5”);9/6/4;and6/4/2mappedtored,
green,andbluechannelsrespectively,asdescribedbyOsterlooetal.[2010]toallowimprovedidentification
andmappingofCBDs.
4. Results
4.1. GeochemicalModels
4.1.1. 1-DFlow-ThroughModelforBasaltWeathering
Asthefluidreactedwithlargermassesoffreshbasalt,thefluidcompositionevolvedfromhighW/Rtolow
W/R, and secondary minerals were precipitated along the reaction-transport path (Figure 7 and Table S2
for details on specific mineral species). Cl concentration increased in the fluid with decreasing W/R as it
wasdissolvedfromthereactantbasaltandnotincorporatedintosecondarymineralprecipitates.Cl/Sratios
abovetheminimumCl/ScalculatedtobeintheCBDs(~11)occurredonlyatW/R≤2.1.
ThemostnoticeablechangetothefluidcompositionoccurredbetweenW/R6000to500,wheremostofthe
Cinsolutionwasconsumed(Figure7a)toformcarbonates(Figure7c)(sideriteformedatW/R=6000–2500,
ankeriteatW/R=5000–700,andcalciteatW/R=700–350),thusbindingcations(FeandCa;Figure7b)lea-
chedfromthereactantrockbythefluidatearlierstages(Figure7a).
AtmoderatetolowW/R,themainmineralsinksforcations(Si,Al,Mn,Fe,Mg,Ca,K,andNa)andwaterwere
phyllosilicates(kaolinite,montmorillonite,minnesotaite,chlorite,sepiolite,saponite,andgreenalite)andzeo-
lites(clinoptilolite,phillipsite,andchabazite)(Figures7band7c).AtlowerW/R(W/R<280)thecementcal-
ciumsilicatehydrateconsumedCa,silica,andwater,whereasatverylowW/R(W/R<10),hematitewasthe
mainstableFe-bearingphase.Phosphatewasprecipitatedoutofthesolutionashydrousphosphateminerals
stable at low temperature (vivianite, MgHPO , MnHPO , and Ca H(PO ) ·3H O (octocalcium phosphate), a
4 4 4 43 2
metastable precursor to authigenic apatite [Gunnars et al., 2004; Oxmann and Schwendenmann, 2015]).
Sulfur was mainly taken up by pyrite, which lead to a fluid composition with increasing Cl/S at W/R ≤ 10
(Figure7a).
4.1.2. FlushModelforVolcanicDepositReworking
W/RandTWVcontrolthemolarCl/SconcentrationratioofthefluidproducedbyflushingarockofMazatzal’s
composition(Figure8).ForW/RandTWVcombinationsthatstabilizesulfatesandsulfides,sulfurisfixedin
mineralphases,whiletheClwasflushedout.Cl/SwasalwaysbelowtherelevantratiofortheCBDs(~11)
atinitialW/R>400andwithflushingby>400kgofH O.Cl/S≥11occurredintheinitialwettingevents,
2
when less than ~15 kg TWV had flushed through the basalt. Above ~15 kg TWV flushed, sulfur released
MELWANIDASWANIANDKITE PALEOHYDROLOGYOFCHLORIDELAKESONMARS 10