Question

(7). Mauna Loa volcano in Hawaii starts to erupt while you are on your Regional Geology field trip (GEOL 740), and you collect some of the first basalt that emerges from the volcano. You calculate the bulk distribution coefficient (D) for Ni (nickel) in this basaltic magma and De 10, based on a fractionating assemblage of plagioclase, olivine, and clinopyroxene. You return to the volcano several years later as a prestigious post-doctoral research, by which time, the magma in the magma chamber has differentiated through continued fractional crystallization of these same minerals that you observed originally, supplying ever more silicic lavas to erupt at the surface. Eventually, the volcano stops erupting, but by this time the lava has flowed through a housing development, destroying the vacation home you purchased at the end of your first trip! (13 pts total) (a) What is a distribution coefficient (ie. Ko as opposed to a bulk D) and how do you calculate it (i.e. write the equation)?) (1) (b) if the Ko for Ni in olivine is 15 (also referred to as Dy = 15), is Ni considered a compatible or incompatible element in this system? Would you expect the residual magma to contain more or less Ni after fractional crystallization of olivine? (2) (c) The first magma to erupt in this event has 30 ppm Rb. If the bulk Da 0.01 for the fractionating assemblage listed above, would you expect the house-destroying lava flow (i.e. the last lava to erupt from the differentiated magma chamber) to have a higher or lower concentration of Rb than the first lava to erupt? Explain your logic (5) (d) For your post-doctoral research you have been recruited to join a high-profile research team that's trying to determine whether the mantle source of the Hawaiian volcanoes is from convecting upper mantle (e.g. MORB-source) or from recycled oceanic crust and sediments, returned to the surface via plume magmatism (e.g. EMI or EMII). What geochemical tools do you think would be most useful to you in distinguishing these mantle components and why? (5)

Answer 1

a)

The distribution coefficient is also known as the partition coefficient.

Kd (a partition coefficient) is a ratio of solid phase to solute concentrations

KD = CS/CL

This constant is oftentimes referred to when dealing with trace elements.

b) According to my knowledge, it would be compatible with solid. The reason is through definition, incompatible trace elements have less than one equilibrium constant value since trace elements have higher melt concentrations than solids. This implies compatible elements are worth D greater than 1. Incompatible elements are therefore concentrated in the melt, while compatible elements prefer to focus in the solid. Compatible elements with D greater than 1 are highly fractional and have very low liquid phase concentrations.

Yes, Nickel is compatible with the element of olivine (Mg, Fe) and it can replace the element easily. So. residual magma contains more Ni after fractionation crystallization of olivine.

c) Since Rb has larger in size, so technically it is compactible towards the melt. So, it would not be disturbed by the system at all as D values also seem to be very small. There might be other factors depend which change the compactibilty.