@misc{10481/69960,
year = {2021},
month = {4},
url = {http://hdl.handle.net/10481/69960},
abstract = {The dichotomy between partial melting and metasomatism is a paradigm of mantle geochemistry since the
pioneering work of Frey and Prinz (1978) on the occurrence of LREE-enriched harzburgites. However, the
thermo-chemical implications of such two-stage scenarios are often poorly considered, and the latter fail to explain
why trace-element enrichment and major-element depletion are often proportional.We here re-envisage
the petrogenesis of the famous San Carlos peridotites based on new petrographic observations and detailed
modal, major- and trace-element compositions. The lherzolites (and pyroxenites) are characterized by homogeneously
fertile mineral chemistry and LREE-depleted patterns consistent with low degrees of partial melting of
the lherzolitic protolith. Bulk compositions and mineral zoning suggest that opx-rich pyroxenites formed by
pressure-solution creep during melt-present deformation, locally accompanied by magmatic segregations of
cpx. The harzburgites are characterized by stronger mineral zoning with low-Mg# and Na-, Al- and Cr-rich cpx
rims, and can be discriminated in a low-Jd and high-Jd cpx groups. The high-Jd group is interpreted as the result
of local elemental redistribution in the presence of a low-degree hydrous melt, in good agreement with their
wide range of LREE enrichment. In contrast, the MREE-to-HREE fractionation and increasing Cr# in spinel of
the low-Jd group indicate that they experienced higher degrees of melting. Open-system melting simulations
of trace-element fractionation during hydrous flux melting suggests that the high-Jd harzburgites are the result
of low fluid influx producing poorly extracted melt, while higher influx led to higher melting degrees and efficient
melt extraction in the low-Jd harzburgites; the lherzolites mostly remained below or near solidus during
that process. The lithological and chemical heterogeneity of San Carlos mantle is thus compatible with a
single-stage evolution, which is also supported by the striking consistency between Fe-Mg exchange and REE
thermometric estimates (1057 and 1074 °C on average, respectively), indicating that harzburgites and lherzolites
probably followed a similar P-T path and relatively little sub-solidus re-equilibration. These interpretations suggest
that the development ofmelt extraction pathways promoted by reactive channeling instability is able to produce
complex lithological heterogeneities during hydrous flux melting. This process provides a self-consistent
explanation for the systematic enrichment of harzburgites observed in many mantle terranes and xenoliths
worldwide. We argue that San Carlos is one of such examples where a ca 1.5-Ga continental lithosphere experienced
localized flux melting and deformation during the tectonic reactivation of a Proterozoic subduction zone,
providing new constraints on the mantle sources of volcanic activity in the Jemez Lineament.},
organization = {Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT)
Japan Society for the Promotion of Science},
publisher = {Elsevier},
keywords = {Flux melting},
keywords = {Open-system melting model},
keywords = {Reactive channeling instability},
keywords = {Pressure-solution creep},
keywords = {Tectonic reactivation},
keywords = {Jemez Lineament},
title = {Systematic LREE enrichment of mantle harzburgites: The petrogenesis of San Carlos xenoliths revisited},
doi = {10.1016/j.lithos.2021.106195},
author = {Tilhac, Romain},
}