Stable isotopes are very versatile and effective in tracing geological and geochemical processes. This dissertation uses stable Fe isotopes to trace Fe oxidation and transport on planetary surfaces and uses stable Rb isotopes to trace the depletion of moderately volatile elements in planetary bodies. The iron UV photo-oxidation process is studied by performing lab UV photo-oxidation experiments and analyzing Fe isotopes in the lab-derived samples and natural banded iron formation samples. It will be demonstrated that this process is a highly possible Fe oxidation mechanism to precipitate banded iron formations and martian hematite spherules (also known as martian “blueberries”). Hematite spherule samples collected in Hawaii are an ideal terrestrial analogue of martian hematite spherules. Here they are studied for their Fe isotopes to investigate their detailed formation history, providing insights into the formation of these enigmatic martian hematite spherules (Chapter 2). This dissertation also presents a new Rb purification and isotope measurement method that has been developed over the past three years. Rubidium is a new isotopic tracer, and studies on Rb isotopes are very limited due to the difficulty of these measurements. The outlined procedure is capable of achieving high-precision Rb isotopic analyses of even Rb-depleted samples. By performing high-precision Rb isotopic analyses of terrestrial, lunar, martian, and chondrite samples, it will be shown that the depletion of moderately volatile elements in the Moon is related to the status of the protolunar disk after the Moon-forming giant impact. The heavy Rb isotopic composition of the bulk Moon relative to the Earth argues against models of partial condensation as the cause for lunar volatile element depletion. In contrast, a protolunar disk with a vapor layer, which transports volatile elements towards the Earth and is replenished by the underlying magma layer, can quantitatively explain the moderately volatile element depletion and isotope fractionation of the Moon (Chapter 3).