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Rosevkiet t1_ix6lxxr wrote

Reply to comment by PBJ_ad_astra in Why does Moana Kea have so many cinder cones? by OlderThanMyParents

They are observed all along the Hawaiian chain, with Kea on the ne side of the Hawaiian arch and loa on the sw. I’m not sure what the origin of the geochemical differences are between the two, or the reason there are two tracks. The geochemical differences are in radiogenic isotope ratios derived from decay series like Sm-Nd, U-Pb, and Rb-Sr. In mantle derived rocks there are variations attributed to different mantle components, usually considered to represent recycled crust that was carried to the deep mantle, entrained in a plume, and preferentially melted. The kea and loa trends have different proportions of different mantle components.

My reading on the subject is out of date, s may be up to lunch on this, but I think it may mean that the spatial variability of the Hawaiian plume is constant through time, which is pretty neat to think about.

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CrustalTrudger t1_ix7vpui wrote

The two geochemical tracks in Hawaii (and as observed in many other plume related hotspot tracks) are thought to be related to some sort of heterogeneity in the mantle plume itself. As described in the recent review on mantle plumes by Koppers et al., 2021, there are three basic models to explain this:

  1. An unzoned, but heterogeneous plume where the double tracks reflect different components with different melting temperatures.

  2. A concentrically zoned plume with the hottest and densest portion of the plume material in the center.

  3. A bilaterally zoned plume where one half of the plume is more a direct sampling of the source LLSVP and, since most plumes originate from the edge of LLSVPs at the core-mantle boundary, the other half incorporates more "ambient" mantle.

At present, the bilateral model is more favored as a general explanation, but it doesn't explain all of the observations of double tracks at all hotspots, so there may not be a single mechanism. Specific to Hawaii though, the bilateral plume model is the favored one (e.g., Williamson et al., 2019).

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