Origin of high-Mg# orthopyroxene-rich cratonic mantle: Insights from the Mogok peridotites (Myanmar)

Cratonic peridotites are typically depleted but have overall higher modal orthopyroxene than young oceanic and continental peridotites. The origin of this enrichment remains debatable. Here we focus on a spinel harzburgite block from the Mogok metamorphic belt, Myanmar, presenting major and trace element data for 27 harzburgite samples. Twelve samples are clinopyroxene-free but have high modal orthopyroxene (mostly 25.3–30.4%); The remaining fifteen are clinopyroxene-bearing (<4%), with only 10.8–22.7% orthopyroxene. The clinopyroxene-free samples display higher Mg# (91.8–92.5) than those with clinopyroxene (91.1–92.1). All samples yield a positive correlation between modal orthopyroxene and bulk Mg#, overlapping with the trend defined by refractory cratonic peridotite xenoliths. This correlation is unlikely explained by post-melting metasomatism, mechanical sorting, or serpentinization. Instead, it is consistent with non-pyrolitic, silica-rich mantle melting. Thermodynamic modeling shows that high-pressure melting (∼15–35 kbar) of the silica-rich mantle proceeds through an orthopyroxene-forming peritectic reaction, leaving residues with higher Mg# compared to those produced at lower pressures. Our harzburgite samples are compatible with this model, with high-Mg# orthopyroxene-rich samples formed at higher pressures (∼20–40 kbar) than the orthopyroxene-poor ones (∼10–20 kbar). We suggest that high-Mg# orthopyroxene-rich cratonic peridotites are likely an important component of the primordial cratonic mantle. Their formation might occur through anhydrous extensive melting of the silica-rich mantle at relatively high pressures, corresponding to the elevated potential temperatures characteristic of the Archean mantle. Progressive mantle cooling from the Archean to the present can account for the rarity of young analogues of high-Mg# orthopyroxene-rich cratonic mantle.