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The plot above is about the concentration of siderophile elements in the terrestrial mantle. From the logarithmic plot, we can deduce that the concentration of highly siderophile elements is quite uniform. Why ?

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    $\begingroup$ You are misreading the graph. It does not say that the concentration of highly siderophile elements is uniform. Platinum is much more abundant than is rhenium in the bulk silicate Earth and in the solar system (CI chondrites in particular). What the graph does say is, for example, that the depletion of platinum in the bulk silicate Earth relative to platinum's abundance in CI chondrites is about the same as is the depletion of rhenium in the bulk silicate Earth relative to rhenium's abundance in CI chondrites. $\endgroup$ – David Hammen Jun 2 at 13:59

First, note that this plot is normalized to a reference, here the composition of CI chondrites (your version lacks the Y axis label, it's figure 2.1 from Walker 2016). So if the concentration of highly siderophile elements (HSE) seems uniform, it's partly because it is a relative concentration, not an absolute one. If you look at the raw data (Table 1.1), you'll see that the absolute concentrations vary by more than one order of magnitude depending on the element (from 0.35 ppb for Re to 7.6 for Pt). Also, as you noted the scale is logarithmic, which tends to flatten the differences.

Now, why do HSE have the same relative abundance in the mantle compared to chondrites? Because elements from the same family show the same behavior—actually it's why they form a family in the first place. HSE all are, well, highly siderophile, so they all went into the core during the core segregation, and now they all are highly depleted in the mantle compared to chondrites.

Note that despite this depletion, HSE are actually still too abundant in the mantle: there should be even less of them. Geochemists have long debated this issue. This nice paper (Walker 2009) reviews the different hypotheses that might explain this HSE "enrichment": incomplete core segregation, lower metal/silicate partitioning, late accretion of chondritic material after core segregation...

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    $\begingroup$ ahh, the late veneer. In the past couple of years there were some new ideas regarding this. Some people started suggesting based on some experiments, that the currently measured HSE are actually too low, not too high, because they aren't as siderophile as we thought initially. Then they invoke a "suldide matte" that absorb the excess. Fun times. $\endgroup$ – Gimelist Jun 3 at 0:22

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