Sediment-rich mélange diapirs have been suggested to transport key chemical slab signatures and volatiles to arc magma sources. Here, we assess the phase equilibria, buoyancy and implications for chemical geodynamics of a previously unexplored hydrous shaly-rich mélange (5-10 wt.% H2O) with minor ultramafic component (9:1 ratio) from deep forearc to subarc depths (2-3 GPa and 700-1150°C). Their solidi lie between <645 to ~700°C and upon partial melting, produce dacitic to rhyolitic melts (water-free basis) in coexistence with abundant biotite, pargasitic-amphibole, and quartz (low dense minerals), garnet, and enstatite (rutile/Ti-magnetite ± apatite) that favors the onset of diapirs in thinner mélange channels (<100m) with lower mélange and mantle viscosities in all slab geotherms. At >850°C the low dense mineral abundance decreases, and the mélange loses buoyancy, requiring thicker mélange channels (>100-800m) with higher mélange and mantle viscosities. Thinner and thicker mélanges form smaller (<1 km radius) and larger diapirs (>1 km radius), respectively. While smaller are more susceptible to mineralogical equilibration, lose buoyancy and stall, larger diapirs may sustain buoyancy and relaminate under the arc crust. High LILE/HFSE signatures in most arc lavas may be explained by aqueous fluids and low degree mélange partial melting (<850°C) within the channel or a diapir close to the channel where rutile/apatite/Ti-Fe-oxides minerals are stable, and the presence of garnet impart high LILE/HREE. Although high degree partial melts due to diapirism or slab tears explain dacitic to rhyolitic arc lavas, they would not fractionate HFSEs from LILEs to explain the high HFSE/LILE arc lava signature.

A document by Arkadeep Roy. Click on the document to view its contents.

Abstract The key to evaluating the formation history and evolution of the Moon lies in understanding the current state of its interior. We used a multidisciplinary approach to explore the current day lunar structure and composition with the aim of identifying signatures of formation and early evolution. We constructed a large number of 1D lunar interior models to explore a wide range of potential structures and identified those models that match the present day mass, moment of inertia, and bulk silicate composition of the Moon. In an advance on previous studies, we explicitly calculate the physical and elastic properties of the varying mineral assemblages in the lunar interior using multicomponent equations of state. We considered models with either a compositionally homogeneous mantle or a stratified mantle that preserved remnants of magma ocean crystallization, and tested thermal profiles that span the range of proposed selenotherms. For the models that reproduced the observed mass and moment of inertia, we found a narrow range of possible metallic (iron) core radii (269-387 km) consistent with previous determinations. We explored the possibility of an ilmenite bearing layer both below the crust and at the core-mantle boundary as a potential tracer of magma ocean solidification and overturn. We observed a trade-off between the mass of the upper and lower ilmenite-bearing layers and structures that have undergone mantle overturn are both consistent with present observations. Plain Language Summary In order to understand how the Moon formed, along with the following history including the processes that change and shape it, the current state of the lunar interior offers a lot of valuable information or clues. We used several different computer simulation tools from different disciplines to calculate the Moon’s interior structure. We then compared our calculations with observations of the Moon’s mass and moment of inertia (a measure of how its weight is distributed through the interior) and the average composition and chemistry of the Moon. We considered a Moon that is well mixed and one that has preserved layers from its early history and tried different temperature structures. We find that the Moon has to have a small dense iron core and that it may have a hot soft layer just above the core that can dampen moonquakes.