Well,

The uniform and low 3He/4He ratios of HIMU basalts as evidence...

Regards

Ciao Gabriele,

what you ask would need a complete graduate course of geochemistry applied to igneous petrology to be explained. Simple replies could be too obvious and not scientifically sounded. First of all it should be defined what the HIMU end-member does indicate and how many HIMU end-members do exist. Then you should move to helium isotopes, trying to understand what is their field of application in igneous petrology, for example what are the differences between MORB and OIB. In several cases you will find conclusions that are at odd with the classical interpretations by plumers.

There is a fundamental paradox in the application of He isotopes in HIMU-OIB.

1) HIMU-OIB are considered to derive from regions identified in the D" (Core-Mantle Boundary Layer), mostly because of the high time necessary to 238U to decay in 206Pb (a couple of Gyr). This is based on rheological and geophysical arguments rather than on geochemical and petrological considerations. Indeed, classical views of Earth's mantle assume that this large volume is fully convecting with the exception of the lowermost sectors, close to the core. There are no strong evidences for this simple way of thinking.

2) HIMU-OIB are considered to reflect primitive mantle compositions, but their Sr-Nd isotopic ratios are nearly indistinguishable (or hardly distinguishable) from those of N-MORB. This is a first paradox.

3) The low 3He/4He isotopic ratios of classical HIMU basalts (i.e., St. Helena) are completely different from the Baffin Bay pricrites, characterized by anomalously high 3He/4He, considered to reflect the "real" primitive (undegassed) mantle plume signal. The authors who propose this seem to forget that Baffin Bay picrites have 87Sr/86Sr < BSE and 143Nd/144Nd > ChUR, indicating derivation from depleted mantle sources. The distribution of incompatible elements in Baffin Bay picrites confirm the derivation from such a depleted mantle source.

4) To deal with carbonatitic magmas add further confusion to the story, because we should explain what a carbonatite is. I can assure you that all the areas I have worked on have revealed that the rocks originally classified as carbonatites were instead the result of interaction of silicatic magma with limestones/dolostones.

Some additional comment on the isotopic history can be found in an article I wrote with the late Don Anderson five years ago.

Cheers and pass my greetings to Silvio and Antonio.

Dear professor Michele Lustrino , first of all i want to thank you for answering my question and given the detailed answer you gave me back i think it was a good question.

Thank also for share with me your work, i already know it and it's very interesting.

I hope to contribute to solve some doubts on carbonatite genesis from geochemical and petrological point of view, especially oceanic carbonatites that are so rare, but i'm still on my first PhD year.

I will pass your greetings to professor Rotolo (my PhD cordinator) and Antonio (my PhD tutor); i'm collaborating also with Vincenzo Stagno.

Thanks again