EFFECTS OF PHOSPHORUS ON PARTIAL MELTING OF A MODEL MARTIAN MANTLE: IMPLICATIONS FOR GENERATION OF MARTIAN IGNEOUS CRUST

Phosphorus is known to influence mantle-derived silicate partial melt compositions along with the liquidus field of various minerals, promoting minerals like pyroxene over that of olivine (1-2). Phosphorus concentration is estimated to be 10 times higher in the martian mantle than that of Earth’s (3), which can have significant influence on the petrogenesis of basaltic, igneous crust on Mars.

To constrain the effects of phosphorus on martian mantle melting phase relations and melt compositions, we performed two sets of partial melting experiments using a piston-cylinder at 2 GPa on a martian primitive mantle composition (4), one containing 0 wt.% P₂O₅ (LF0) and another containing 0.5 wt.% P₂O₅ (LF0.5). All experiments produced an assemblage of ol + opx ± cpx ± sp ± melt depending on the melting degree. Comparing our experimental samples with those obtained under the same conditions using the same starting composition with a bulk P₂O₅ content of 0.2 wt.% (LF0.2; 4), an enhancement of residual orthopyroxene over olivine is evidenced in LF0.2 and LF0.5, with a depletion of residual olivine up to twice as much in LF0.5 compared to LF0. This leads to compositional variations in the melt including a depletion of SiO₂ content up to 10 wt.% in LF0.2 compared to LF0 at 10 wt.% melting. Our experiments demonstrate that the mantle P₂O₅ is sufficient to dramatically modify the petrogenesis of rocks produced by partial melting. We combined the difference in major element melt compositions (e.g., SiO₂, FeO*, CaO/Al₂O₃) caused by variation in starting P contents with the previously constrained effects of pressure (0.5-5 GPa) on martian mantle-derived melt compositions. Fractionation-corrected shergottites and Gusev basalts thought to be representative of a primary melt mainly correspond to melt compositions expected when the starting P₂O₅ is 0.2-0.5 wt.%, suggesting that the martian mantle composition with P₂O₅ of 0.16-0.17 wt.% (3-4) might not be high enough to produce surface compositions of Mars.

(1) Kushiro.(1996) Earth Process:Reading the Isotopic Code, 109-122. (2) Toplis et al.(1994) GCA,58,2,797-810. (3) Dreibus and Wänke.(1985) Meteoritics, 20,267-381. (4) Lodders & Fegley.(1997) Icarus, 126(2), 373–394. (5) Ding et al.(2020) JGR:Planets, 125(4).