Download PHOSPHORUS GEOCHEMISTRY OF MARS: EVIDENCE FOR AN

Seventh International Conference on Mars
3228.pdf
PHOSPHORUS GEOCHEMISTRY OF MARS: EVIDENCE FOR AN EARLY ACIDIC HYDROSPHERE.
J. P. Greenwood 1, R. E. Blake2, V. Barron 3 and J. Torrent3 1Dept. of Earth & Environmental Sciences, Wesleyan
University, Middletown, CT 06459 ([email protected] ), 2Dept. of Geology & Geophysics, Yale University, New Haven, CT 06520, 3Departamento de Ciencias y Recursos Agrícolas y Forestales, Universidad de Córdoba, Apdo. 3048, 14080 Córdoba, Spain
Introduction: The last ten years of research on
Mars and Martian meteorites has led to a greater understanding and appreciation for the important role
that phosphorus and phosphate minerals play in our
overall knowledge of the Red Planet. Prior to 1997,
there were no measurements of phosphorus on the
Martian surface, and the majority of SNC studies did
not delve very deeply into how the phosphate minerals formed. This has changed radically in the past
decade with in situ measurements first from Mars
Pathfinder, and then from the MER rovers which
have found up to 5 wt. % P 2O5 in Martian materials
such as Paso Robles [1]. More importantly, the role
that phosphorus plays in Martian geochemistry is
readily apparent in that phosphorus appears to be
correlated with sulfur in soils from Meridiani Planum
and Gusev Crater and with chlorine of rocks and soils
in the Gusev plains (Figures 1 and 2; [2]). Here we
review basic processes that may affect phosphorus on
Mars and show that the correlation of phosphorus and
sulfur in Martian soils is due mainly to weathering of
phosphate minerals from igneous rocks under acidic
conditions. This points to an acidic hydrosphere on
early Mars as being the only reasonable method of
obtaining enhanced phosphorus in the global soils.
Origin of high S, Cl, and P in Martian soils:
The high sulfur contents of Martian soils have been
previously ascribed to volcanic degassing [3], hydrothermal alteration by volcanic acid-sulfate fluids
[4], and aqueous oxidation of sulfides [5]. High chlorine has been suggested to arise from either volcanic
degassing [3] or alteration by neutral-chloride fluids
[4]. It has recently been shown that massindependent sulfur isotope signatures in SNC’s are
indicative of atmospherically-processed sulfur, such
as volcanogenic SO2 [2], which delineates an unambiguous contribution of atmospheric sulfur to the
Martian surface. In light of evidence from SNC’s, an
origin of Martian surface sulfur via volcanic degassing of SO 2 seems likely. This rules out models
that explain high sulfur abundances in Martian soils
as due primarily to volcanic fluids [4] or weathering
of sulfides [5, 6].
Phosphorus is not predicted to form volumetrically significant gaseous species during magmatic
degassing under reasonable estimates of fO2 for SNC
or terrestrial magmas [7]. An origin of high phosphorus in Martian soils as a result of volcanic de-
gassing is untenable. Phosphorus additions to the
Martian salt component must ultimately be as a result
of weathering of igneous rock phosphate minerals,
such as merrillite and apatite, the common phosphate
minerals in Martian meteorites [e.g. 8]. Dreibus et al.
[9] have shown that SNC phosphates are easily dissolved in acidic fluids, and even circum-neutral fluids
can extract phosphates readily from SNC meteorites
[10]. Thus, igneous phosphate minerals in Martian
rocks should be easily weathered out under acidic
conditions.
Rieder et al. [11] have favored weathering of
rocky material by acidic fluids to form acidic brines,
including the dissolution of phosphate minerals, at
Meridiani. This is a reasonable explanation for the
Meridiani outcrops. This explanation does not explain high and constant P/S of Martian soils.
Phosphorus aqueous geochemistry:
Solubility and Dissolution rates. Phosphate minerals are generally highly insoluble in neutral to basic
pH fluids. Calcium-phosphate minerals display loglinear increases in solubility with decreasing pH at
pH < 7 [2]. Apatite dissolution rates follow a similar
relationship with pH; dissolution rates increase logarithmically with pH < 6, but change little in the circum-neutral to basic range [2]. Experimental apatite
dissolution rates at low pH can be orders of magnitude faster than dissolution rates of silicates under
similar conditions, and are very similar to dissolution
rates of magnesite (MgCO3). This suggests that the
low amounts of carbonate on the Martian surface, if
due to acidic weathering, may correlate with low
abundances of Ca-phosphate minerals.
Sorption. Sulfur is found to correlate strongly
with the nano-phase oxide component (Np-ox) determined by Mössbauer [12, 13]. Chlorine and phosphorus are not significantly correlated with Np-ox
[2]. Chlorine and phosphorus should display very
different sorption behavior onto iron oxides as chlorine should not adsorb strongly to iron oxides, due to
its forming outer-sphere complexes on iron ‘oxide’
surfaces, rather than the much more strongly bonded
inner-sphere complexes formed by phosphate and
possibly sulfate. Phosphate and sulfate should be
effectively adsorbed by iron oxide phases in aqueous
systems, thus the similar behavior of phosphorus and
chlorine suggests sorption may not be the dominant
Seventh International Conference on Mars
mechanism for correlations of P, S, and Cl in Martian
soils.
P in Martian soils: The high phosphorus contents of Martian soils, and its correlation with sulfur
and chlorine, rather than the rock-forming elements,
is strong evidence that phosphorus behaved as a
soluble element in Martian fluids [2, 11]. If P/S ratios of Martian soils are indicative of these ratios in
Martian fluids, these aqueous fluids must have been
acidic. We have yet to find an acid saline fluid on
Earth that has P/S similar to what has been found in
Martian rocks and soils. Analyses of acid saline fluids are generally depleted heavily in phosphorus (K.
C. Benison and J. C. Varekamp, pers. comm.. and
their unpubl. data). The soils at Gusev and Meridiani
both display this behavior of high P/S, thus, a global
phenomenom of weathering of Ca-phosphate minerals from igneous rocks must have occurred, or be
occurring. Two possibilities present themselves: (1)
An acidic hydrosphere was active on ancient Mars,
which led to large-scale weathering processes, and
the addition of phosphate to Martian fluids occurred
readily over much of the planet. (2) The formation of
Martian bright soils (which are enriched in the nanophase oxide-, sulfur-, chlorine-, phosphorus-rich
component) has been a slow, but highly acidic process, which has led to the preferential leaching of
phosphates (relative to other minerals) and their enrichment in the bright soil component. Clearly, both
are reasonable hypotheses. We explore these possibilities below.
Acid Thin-Film or Acid-Fog weathering vs.
Early Acidic Hydrosphere: The MER team has
published several papers indicating that both acid
thin-film weathering [14] and acid-fog weathering
have affected rocks and/or soils at the two landing
sites [15]. Hurowitz et al. [15] demonstrate that float
rocks in the Columbia Hills, Gusev Crater have
leached alteration zones on the order of several millimeters which show evidence of Ca-phosphate depletion in these zones. Yen et al. [14] postulate that
diurnal or seasonal temperature variations in the current climactic regime drive hydrolysis of ions in salts
3228.pdf
which lead to mobilization of the highly soluble element Br in acidic films and are ultimately responsible
for Br heterogeneities in Martian soils. Clearly, both
of these processes must be important in the formation
of Martian fines, but how important? An answer to
this question can be found in the Meridiani sulfaterich outcrop rocks.
The Meridiani outcrop rocks are rich in phosphorus, unlike sulfate-rich rocks on Earth. While Martian soils may have been affected by acid-fog or acidfilm weathering, the Meridiani rocks were bathed in
acidic fluids at one time. They were not formed by
acid-fog or acid-film weathering, yet they also display high phosphorus contents. Clearly, a weathering
mechanism involving acidic fluids was active on
early Mars which allowed these phosphorus-rich,
sulfate-rich rocks to form. One could argue that the
igneous protolith that was weathered to form the Meridiani outcrop rocks could have been leached of its
phosphate minerals, and that this was a local process
(as originally argued by Rieder et al. [11]). Yet the
discovery of phosphorus-rich, sulfur-rich rocks and
soils at Gusev Crater [1] point to a more global, and
large-scale, acidic weathering environment [2].
References: [1] Gellert R. et al. (2006) JGR, 111,
E02S05, doi:10.1029/2005JE002555. [2] Greenwood
J. P. and Blake R. E. (2006) Geology, 34, 953-956.
[3] Settle M. (1979) JGR, 84, 8343-8353. [4] Newsom H. E. et al. (1999) JGR, 104, 8717-8728. [5]
Burns R. G. and Fisher D. S. (1990) JGR, 95, 1416914173. [6] Zolotov M. Y. and Shock E. L. (2005)
GRL, 32, L21203, doi10.1029/2005GL024253. [7]
Holland, H. D. (1984) The chemical evolution of the
atmosphere and oceans. Princeton, 582p. [8]
Greenwood J. P. et al. (2003) GCA, 67, 2289-2298
[9] Dreibus G. et al. (1996) LPSC XXVII, 323-324.
[10] Mautner M. N. and Sinaj S. (2002) GCA, 66,
3161-3174. [11] Rieder R. et al. (2004) 306, 17461749. [12] Klingelhöfer G. et al. (2004) Science, 306,
1740-1745. [13] Morris R. V. et al. (2004) Science,
305, 833-836. [14] Yen A. et al. (2005) Nature, 436,
49-54. [15] Hurowitz J. A. et al. (2006) JGR, 111,
E02S19, doi:10.1029/2005JE002515.
Seventh International Conference on Mars
3228.pdf
10
S (wt%)
8
S MER-B soil
S MER-B rock
S MER-A soil (GP,WS)
S MER-A rock (GP,WS)
6
4
2
0
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
P (wt%)
Figure 1. S (wt.%) vs. P (wt.%) in Martian soils (closed symbols) and rocks (open symbols) from Meridiani Planum
(red) and Gusev Crater (blue). Linear regressions are shown for undisturbed Gusev Plains (GP) and West Spur
(WS) soils (solid blue line; y=-0.81+9.27x, R=0.69) and rocks (dashed blue line; y=-0.84+8.21x, R=0.84) and MERB soils (solid red line; y=-1.31+9.62x, R=0.67). Undisturbed GP and WS rocks are included because the high sulfur
is likely due to airfall dust on the rock surfaces.
1
Cl (wt%)
0.8
0.6
0.4
0.2
0
0.2
Cl MER-B soil
Cl MER-B rock
Cl MER-A soil (GP,WS)
Cl MER-A rock (GP)
0.25
0.3
0.35
0.4
0.45
0.5
0.55
P (wt.%)
Figure 2. Cl (wt. %) vs. P (wt.%) in Martian soils (closed symbols) and rocks (open symbols) from Meridiani
Planum (red) and Gusev Crater (blue). Linear regressions are shown for undisturbed Gusev Plains and West Spur
soils (solid blue; y=-0.03+2.21x, R=0.58) and Gusev Plains rocks (dashed blue; y=-0.01+2.01x, R=0.70) and Meridiani soils (solid red; y=0.07+1.04x, R=0.41). Chlorine and phosphorus are not significantly correlated in Meridiani soils.