ISTEP (Institut des Sciences de la Terre de Paris)

 K-richterite/phlogopite-bearing peridotite xenoliths and MARID inclusions have been found in Late Cretaceous (67±0.2 million years) monchiquites and an olivine nephelinite from North Eastern Morocco. It is the first evidence of MARID rocks and K-richterite/phlogopite-bearing peridotites outside the kimberlitic context. In the hydrous xenoliths, textural features suggest that K-richterite, phlogopite and Al-poor diopside are replacement minerals. K-richterites contain 2–5 wt% FeO, 0.1–1.5 wt% TiO2 and <0.5 wt% Cr2O3. Micas contain 5.4–7.4 wt% FeO and 0.3–2.2 wt% TiO2, with Cr and Ni contents <0.2 wt%. Diopsides are Al-poor (<0.2 wt% Al2O3) and contain 0.1–0.2 wt% TiO2, 0.9–1.1 wt% Na2O and 1.3–1.7 wt% Cr2O3. Compared to known K-richterites and micas from metasomatised peridotite nodules (PKP types), the Moroccan minerals are more Fe rich, K-richterites have higher Ti and micas less Cr and Ni. They are thus closer to MARID than to PKP minerals. K-richterites and mica from the MARID inclusions show typical characteristics, e.g. high FeO (4.3–4.7 wt% in richterite and 7.2 wt% in mica), low NiO and/or Cr2O3 and the incomplete filling of the tetrahedral site by Si+Al. Ion probe D/H analyses of amphiboles and micas from both xenolith types give high δD values ranging from –8 to –73, with large variations within single grains (up to 50‰). Both the D-enrichment and the δD variations are inherited from the mantle. The similar chemical composition and δD values of K-richterite/phlogopite from the hydrous peridotites and MARID minerals suggest a genetic link between the two types of xenoliths. The conditions required for producing MARIDs and K-richterite/phlogopite-bearing peridotites may thus exist in contexts other than stable cratonic settings. MARID rocks and the associated metasomites may result from a hydrous fluid interaction with a peridotite, the metasomatic agent being characterised by a high K and low Al signature and a high δD value. A D-rich source is involved in the metasomatic event producing the hydrous minerals, and the scatter observed in the δD values suggests a mixing between this source and another one with typical upper mantle D/H composition. As indicated by the low δD (–74) values of micas from the host lava, metasomatism predated and is unrelated to the alkaline volcanism.

Sector-zoned micas are described in three occurrences of igneous rocks. Basal sectors (001) and lateral sectors (010) are well defined. Analyses of probably isochronous growth points indicate that there are consistent chemical composition differences between the sectors, the (010) sector being richer in Si and poorer in Ti, Fe, Al and Ba relative to the (001) sector. Within each sector type Fe/ (Fe + Mg) increases from core to rim. These differences vary in amplitude from one occurrence to the other, but are systematic for micas with extremely different tetrahedral cations. Possible factors influencing this type of crystallization are briefly reviewed: growth rate, geometry of protosites and bulk composition of the liquid.

... Michel Fialin 1 ,2 ,* , Antoine Bézos 2 , Christiane Wagner 3 , Véronique Magnien 4 and Eric ... de Globe, 4 place Jussieu, 75252 Paris cedex 5, France 3 Laboratoire de Pétrologie, Modélisation ... types of processes can lead to multiple-vacancy states: Auger and Coster-Kronig (CK ...

Mantle-derived xenoliths commonly show reaction textures, mostly reaction rims on orthopyroxene, glass-bearing sieve-textured clinopyroxene and spinel, and glassy pockets around spinel and amphibole. The origin of these textures remains controversial, either in relation to various processes that occur in the upper mantle, melting [1, 2]) and metasomatism [3, 4], or during the xenoliths transport to the surface [5, 6]. As the textural modifications of whatever origin may be very similar, the crucial point for assessing mantle processes - particularly mantle metasomatism- is thus to identify the mantle-originated textures and to discard any possible imprint of xenolith-host magma interactions. This contribution is part of a study of reaction textures in a suite of anhydrous and hydrous (with amphibole and minor mica) spinel lherzolites from the French Massif Central, which have been modally and/or cryptically metasomatized in the mantle and show evidence of progressive metasomatism im...