PRIMORDIAL FRACTIONATION OF HIGH FIELD STRENGTH ELEMENTS (HFSE) ON STAGNANT-LID PLANET MANTLES

High field strength element ratios (e.g., Zr/Nb, Nb/Th) have been used successfully on Earth for fingerprinting mantle sources for basalts (primitive (PM), enriched (EM), depleted (DM), hydrated (HM) mantles), deciphering the origin and evolution of these mantles, and constraining the timing of the propagation of plate tectonics. In contrast, on stagnant-lid planets such as the Moon and Mars, the high field strength element ratios from some basalts may deviate from primitive mantle and potentially reflect mantle sources that are remnants of the earliest stages of planetary differentiation through magma oceans. Model ages for basalt sources from the Moon and Mars are ancient and indicate silicate differentiation occurred 4380 Ma and 4504 Ma, respectively. This implies the HFSE ratios for these basalts represent ancient processes that are linked to magma ocean differentiation. Most of the lunar basalts have HFSE rations with a PM signature, although many other trace element and isotopes indicate derivation from a depleted source. Only the Apollo 17 high Ti-lunar basalts and Apollo 11 and 17 high-Ti pyroclastic glasses have HFSE ratios that deviate from PM. This suite of basalts has lower Th/Yb and overlapping to slightly higher Nb/Yb. Martian basalts (e.g., shergottites), that define a HFSE array that extends from DM through PM to EM. This is particularly well-illustrated in plots of Nb/Yb versus Th/Yb. We conclude that these primordial mantle signatures for the Moon and Mars reflect the fractionation of HFSE by distinct liquidus phases (e.g., ilmenite, garnet) during planetary magma ocean differentiation, the influence of these phases are dependent upon the depth and thermal history of the magma ocean, and that these signatures are most likely preserved on stagnant-lid planets.