In some impact melt (IM) glasses in the shergottites such as EET79001, Shergotty and Tissint, recently showed that secondary mineral assemblages having large sulfur excesses cannot be produced in-situ by impact shock melting of the host rock constituents. Instead, these putative secondary minerals inferred to be present in IM glasses were produced somewhere else in the shergottite source region and were subsequently mobilized into the host rock voids (by lava erosion or aolian activity) prior to impact ejection. In this abstract, we examine the aqueous conditions (pH and water/rock ratios) under which the acid sulfate solutions could have interacted with the primary minerals in the basaltic rocks and precipitated the secondary minerals such as Fe-sulfates in some cases and Ca- and Al-sulfates in other cases under favorable conditions at the shergottite provenance on Mars

Nyquist, L. E.

Rao, M. N.

Ross, D. K.

Wentworth, S. J.

NASA Technical Reports Server

ACID- SULFATE WEATHERING  ENVIRONMENT AT SHERGOTTITE PROVENANCE  ON MARS.  M. 
N.  Rao
1
, L. E.  Nyquist
2 
, D. K. Ross
3,4
 and S. J. Wentworth
5
, 
1
SCI, Johnson Space Center, Houston. TX. 77058. (e-
mail: sitarao@sbcglobal.net), 
2
XI/NASA Johnson Space Center, Houston, TX. 77058, 
3
Jacob JETS, NASA Johnson 
Space Center, Houston, TX. 77058  and 
4
UTEP- CASSMAR., 
5
HEPCO, Jacobs Engineering, Johnson Space Center, 
Houston. TX. 77058. 
 
Introduction:  In some impact melt (IM) glasses in 
the shergottites such as EET79001, Shergotty and 
Tissint, recently [1] showed that secondary mineral 
assemblages having large sulfur excesses cannot be 
produced in-situ by impact shock melting of the host 
rock constituents.  Instead, these putative secondary 
minerals inferred to be present in IM glasses were pro-
duced somewhere else in the shergottite source region 
and were subsequently mobilized into the host rock 
voids (by lava erosion or aolian activity) prior to im-
pact ejection.  In this abstract, we examine the aqueous 
conditions (pH and water/rock ratios) under which the 
acid sulfate solutions could have interacted with the 
primary minerals in the basaltic rocks and precipitated 
the secondary minerals such as Fe-sulfates in some 
cases and Ca- and Al-sulfates in other cases under fa-
vorable conditions at the shergottite provenance on 
Mars. 
The Model: The shergottite source region  on Mars 
apparently consists of two different types of rock com-
plexes, (a) One hosting olivine phyric types from which 
shergottites such as Tissint, DAG 476, EET79001, Lith 
A and others were launched. These rocks have olivine, 
pyroxene and feldspar as major primary minerals.  (b) 
A second region consisting of pyroxene phyric types 
from which meteorites such as Shergotty, Zagami,  
EET79001, Lith B and others were derived.  These 
rocks consist of pyroxene and feldspar (no olivine) as 
major primary minerals. The rocks were located near 
the Martian surface [1] and were pervasively covered 
with Martian soil/dust that had high sulfate content 
(~8% SO3) and low chloride content (~0.7% Cl) [2-5].  
The large overabundance of sulfur observed in 
Martian soils is generally attributed to chemical altera-
tion caused by the interaction of sulfate-rich acidic 
fluids with rock and dust near the Martian surface [2-
5].  The acid sulfate fluids originated from SO2 emitted 
by volcanic activity into the Martian atmosphere 
where, after oxidation, it combined with water vapor 
producing H2SO4 aerosols.  In turn, these aerosols rain 
down to the Martian surface and mix with water avail-
able there.  If there is little water in some regions (low 
water to rock ratios), the pH of the solutions remain 
highly acidic (pH= 0 to 1).  On the other hand, if more 
water is available  at some other places (higher water to 
rock ratios), the pH of the solutions becomes moderate-
ly acidic (pH= 3 to 5).    
In the shergottite source region, the acid sulfate so-
lutions percolating through the rocks initiate mineral 
dissolution where easily soluble mineral phases dis-
solve first and go into solution preferentially leaving 
behind  relatively refractory minerals chemically unaf-
fected [4,5,12].  In the case of olivine phyric rocks, the 
easily soluble mineral olivine first goes into solution 
releasing the major cations Fe and Mg into solution.  
On the other hand, when such fluids flow through the 
pyroxene phyric rock complexes, feldspar goes into 
solution preferentially (no olivine in this case) by re-
leasing major cations Ca and Al into solution [4-6].   
The occurrence of jarosite has not been reported in 
the shergottite impact melt glasses studied so far 
[1,7.8,9] , thus making the highly acidic low pH, (i.e.,~ 
0 to 1) and low water to rock ratio sulfate solutions less 
likely to have percolated through the overlying Martian 
soil  at the shergottite provenance.  Alternatively, the 
pH of the solutions that interacted with the rocks in this 
region could have been moderately high, i.e., ~3 to 5 
(with high water to rock ratios) as another possibility.   
Secondary Minerals – Olivine phyric rocks: 
From moderately acidic (pH = ~3 to 5) sulfate solu-
tions that interacted with  the shergottite rocks, second-
ary minerals such as Fe
3+
 hydroxy sulfates (olivine 
dissolution) and  Al – hydroxy sulfates together with 
gypsum (feldspar dissolution) likely precipitated as 
poorly crystalline aggregates because of limited inter-
action time between fluid and rock.  This deposition is 
dependent on the saturation state of the particular ion 
concentration under consideration with respect to the 
specific mineral phase. The highly insoluble poorly- 
crystalline minerals deposited near the shergottite 
source region  presumably serve as mineral  reservoirs 
from which the altered materials were subsequently 
mobilized and  incorporated into the host rock voids.  
Dissolution of olivine  by the moderately acidic flu-
ids releases Fe and Mg along with other soluble chlo-
rides of Na and K into solution.  When Fe concentra-
tion levels exceed the solubility product constant, the 
highly insoluble Fe- hydroxy sulfates precipitate.  [8] 
reported the occurrence of copious amounts of  Ferric 
sulfates mixed with phosphates in a vug in QUE 
94201. This result is consistent with the Fe sulfate dep-
osition discussed above. Also, the highly soluble min-
eral phases such as Mg-sulfate and chlorides of Na and 
K brought into solution remained without precipitating. 
1950.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)
https://ntrs.nasa.gov/search.jsp?R=20190001834 2019-08-30T22:24:21+00:00Z
Thus the soluble mineral phases were decoupled from 
the insoluble ones and were ion-transported away from 
the reaction sites to other locations as these fluids 
flowed down the topographic lows in this region. This 
inference is consistent with the result that we found no 
MgSO4 added into the #507 glass precursors as shown 
in Fig.4A given in [1]. Also, the chloride contents in 
EET79001, Shergotty and Zagami impact melts deter-
mined by EPMA are found to be close to the detection 
limits [10].   
The Solutions:  In Fig.3  given in [1], the FeO-SO3 
plot shows positive correlation whereas Al2O3 -SO3 
shows a corresponding negative correlation. On close 
examination of each data point in the plot, one notices 
that when the Fe
3+
 content is high the Al
3+
 content is 
correspondingly low, and vice-versa in #507, suggest-
ing an inverse elemental relation between Fe
3+
 and Al
3+
 
in this sample.  It is likely that this feature indicates 
substitutive solid solution behavior in this system and 
was possibly established during the hydroxy sulfate 
precipitation.  In the case of alunite–jarosite sub-group 
minerals belonging to the recently altered volcanic 
deposits of Cerro Negro, significant solid solution mix-
ing between Fe
3+
 and Al
3+
 occurred during precipita-
tion from acid sulfate solutions at pH = ~0 to 1 [12].  A 
similar solid solution mixing between Fe and Al could 
have also occurred when the glass precursor minerals 
of EET79001,507 precipitated from acid sulfate solu-
tions at pH= ~3 to 5 at the shergittite source region.   
Elemental Ratios: The Fe/S (atomic) ratios de-
termined in the Fe- S bearing mineral phases in the IM 
glass precursors likely provide clues regarding their 
characterization although these inferences are not 
uniquely definitive because of unknown structural pa-
rameters associated with the constituent mineral grains 
because they were subjected to  impact shock defor-
mation [11].  To learn about the composition of Ferric 
sulfates, we determined the Fe/S ratios in blebs in 
EET79001, 507 and Tissint IM glasses. They range 
from 1.04 to 1.12 indicating that they are not related to 
either jarosite (Fe/S = 1.5) or schwertmanite (Fe/S = 
8).  The Fe-S blebs also are not related to pyrrhotite  
(Fe/S=0.92).  In this context, note that [11] studied 
several secondary ferric sulfate minerals in Paso Ro-
bles class soils at Gusev and suggested  a chemical 
formula of the type (Fe
3+
)2 (SO4)3 . nH2O for them.  It 
is likely that , at the shergottite source region also, sim-
ilar kind of ferric sulfates could have precipitated from 
the acid sulfate solutions.  Thus, the Fe sulfates in #507 
glass precursors discussed could be Ferric hydroxy 
sulfates (Fe/S=1) admixed with small amounts of goe-
thite or hematite.  
Moreover, the occurrence of silica and Fe-sulfates 
(proxy for SO3) together in #507 glass precursors as 
indicated by the anti-correlation shown in Fig. 2 of [1] 
suggests that acid sulfate solutions with moderate pH  
interacted with the shergottite rocks because SiO2 is 
highly soluble if the pH of the solutions is 0 to 1 and it 
won’t precipitate from these fluids [13].    
Pyroxene-phyric rocks: When acid-sulfate solu-
tions attack pyroxene phyric rocks, feldspar preferen-
tially dissolves releasing the major cations Ca and Al 
into solution [4,5,12,13].  Here, the precipitation of 
gypsum from these solutions is pH independent where-
as Al-hydroxy sulfate precipitation is pH dependent [6, 
13]..     
When the ion activity product (IAP) exceeds the 
solubility product constant (SP) for the given phase of 
interest (e.g., CaSO4), Ca precipitates as gypsum or 
anhydrite.  The gypsum / anhydrite deposition is ac-
companied by the precipitation of Al as Al – hydroxy 
sulfate from the acid sulfate solutions at pH = ~ 3 -5 
[5].  Both these insoluble precipitates were left behind 
on the pyroxene phyric rocks as the acid sulfate solu-
tions moved away from the reaction sites transporting 
the soluble ions.  The poorly crystalline minerals gyp-
sum and Al – hydroxy sulfates precipitated here were 
later mobilized by aeolian activity into the host rock 
voids (e.g. EET79001).  This inference is consistent 
with the finding that the impact- melt glasses #506, #27 
and #54 in EET79001, Lith A show  positive correla-
tion between CaO and SO3 (indicating Ca sulfates) as 
well as Al2O3 and SO3 correlation (indicative of Al -
hydroxy sulfates) as shown in Fig.5 given in [1].  
Conclusions: The results presented here and in [1] 
suggest that acid sulfate solutions of moderate pH (with 
high water/rock ratios) percolated through rocks and 
soil in the shergottite source region more recently than 
~180 Myr ago, the time of shergottite crystallization. 
 
References: [1] Rao M.N. et al. (2018) Meteoritics 
& Planet. Sci., 53, No.12. [2] Clark B.C. et al. (2005) 
EPSL, 240, 73 – 94. [3] Newsom H. E. et al. (1999) J. 
G. R., 104, 8717-8728. [4] Tosca N. J. et al. (2004) 
J.G.R. 109, doi: 10.1029/2003JE002218. [5] Hurowitz 
J.A.etal.(2005)J.G.R.,110,doi:10.1029/2004JE 002391. 
[6] Golden D.C.etal.(2005)J.G.R.110,doi: 10.1029 / 
2005JE002451. [7] Gooding J. L. et al. (1988) G. C. 
A. 52, 909-915.  [8] Wentworth S. J. et al. (2005) Ica-
rus, 174, 383-395. [9] Schrader C. M. etal. (2016) 
MAPS,51,663-680.[10]RaoM.N.etal.(2008)J.G.R. 113, 
doi:10.1029/2007JE002958.[11]YenA.S.etal.(2008)J.
G.R.,113,doi:10.1029/2007JE002798.[12]McCollomT. 
M. et al.(2013)J.G.R.,118,doi:10.1002/jgre.20044. [13] 
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Acid-Sulfate Weathering Environment at Shergottite Provenance on Mars

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