It is interesting that phosphorus was detected in some rocks at the Pathfinder landing site in proportions that were more consistent with sedimentary rocks than igneous rocks:

1999 Fall Meeting
P32A-06
TI: Possible Indication of Sedimentary Products at the Mars Pathfinder Landing Site: Phosphorous in APXS X-Ray Spectra.
AB: A refined method of searching for small peaks in the APXS (Alpha Proton X-ray Spectrometer) x-ray spectra permitted the determination of phosphorous in soils and rocks at the Mars Pathfinder landing site. Surprisingly, only small differences of the P concentration between soils and rocks were found. A mean P content of about 0.3 \% was obtained with the exception of 0.06 \% P measured in Scooby Doo (A8). The soils at Viking and Pathfinder landing sites have high and very similar sulfur and chlorine concentrations when compared to the rocks analyzed with the APXS [1]. The observed good correlation of S versus Mg, Si, Cl, K, and Ti for the Pathfinder samples reflects the fact that sulfur poor rocks are partly covered with sulfur rich soils. The absence of a correlation of P versus sulfur in all Pathfinder samples indicates no enrichment of P in the rocks compared to soil as it was found for the other analyzed incompatible element potassium. Commonly, during magmatic fractionation processes P behaves as an incompatible element like potassium and chlorine. A lack of a correlation of P and K in the Pathfinder rocks would be more in accordance with a sedimentary origin of these rocks rather than an igneous origin. On Earth, a well mixed average sediment is remarkably similar to upper continental crust composition [2]. On Mars, the similar composition of soils at Chryse, Utopia and Ares Vallis is a strong indication of good mixing of surface material. Hence, the average chemical composition of the martian soil can be taken as the average composition of the near-surface crust on Mars [3]. When the high S and Cl content is subtracted, this composition of the martian crust comes close to that of the basaltic martian meteorites: the crustal value of P (0.3 \%) matches that of Shergotty (0.35 \%). Although, this estimated P value is very high when compared to terrestrial concentrations, it is in accordance with the predicted high P concentration in the martian mantle as was derived from the SNC meteorites [4]. Ref.: [1] Rieder, R et al. (1997) Science 178, 1771. [2] Taylor, S R and McLennan, S M (1985) The Continental Crust
http://www.agu.org/cgi-bin/wais?l=P32A-06

Note that in the APXS graph showing high phosphorus in Wishstone rock, the phosphorus is higher than the potassium in Wishstone, but this proportion is reversed in the West Spur rocks:

http://photojournal.jpl.nasa.gov/catalog/PIA07191

Is this analogous to what was found at Pathfinder with phosphorus not following the proportions of potassium, suggesting sedimentary rock?

Bob Clark