Figure 8.08. Mauna Loa Data Variation Diagrams. Mauna Loa data from Mike Rhodes (2015) show a linear trend with a negative slope on an MgO-SiO2 variation diagram. Click on the "DiFoAn Data Added" button to see how this trend compares with the DiFoAn laboratory-constrained data for removal of olivine as observed in Figure 8.05. The linear trend for the Mauna Loa data does not pass through forsterite, but the trend is consistent with removal of olivine with a composition near Fo(80), which you can see by clicking on the "Interpretive Lines Added" button.
Clicking on other radio buttons will show the Mauna Loa Data on other variation diagrams without or with the DiFoAn known trends of olivine fractionation. These diagrams support the interpretation that fractional crystallization of olivine with a composition near Fo(80) produced the observer linear trends in Mauna Loa data.
Because olivine is a solid solution mineral, olivines that grow at different temperatures might have different compositions, affecting the simple linear trends of our ideal model. The effects of olivine compositional variations can be removed by switching to mole units and by choosing axes so that ALL Fe/Mg olivine compositions plot at the same point. One such diagram is shown by the "MgO+FeO)-SiO2-Others" radio button. On that diagram, the Mauna Loa data and the DiFoAn data are along lines that radiate from the ALL Olivine Compositions point.
