Abstracts Travel
Program Organizing committee

Geochemical and genetic types of abyssal alkaline ultrabasic magmas (illustrated by imberlites )

Lapin A.V.

The Institute of Mineralogy, Geochemistry and Crystal Chemistry of Rare Elements, ul. Veresaeva,15, Moscow, 121357 Russia; e-mail:lapin@imgre.ru


A number of kimberlite occurrences, that have been revealed during last decades in Russia, differ from classical representatives of this magmatic formation by some significant properties i.e. the absence of ilmenite, as well as by relatively small quantity of minerals- satellites of diamond, and that of pyrope, first of all. These occurrences include kimberlites of the Arkhangelsk diamond-bearing province and the Nakyn field in the Middle Markha region in Yakutia. It can be concluded from the cited petrochemical data, that the most strong evidence of abnormal character of these manifestations is the high degree of contrast of their geochemical parameters relative to rock parameters, that are traditionally considered as the formation petrotype of diamond-bearing kimberlites. Contrast differences are established, first of all, for contents of rare and radioactive HFS elements (Ce, Nb, Zr, U, Th) and for the ratios Ce/Y, Nb/Zr. They are manifested by individual features of correlation relations of elements (Ce-Y; Ce-P; Nb-Ti and oth.) in two kimberlite groups (Lapin, Tolstov. 2008).

High degree of geochemical contrast in two kimberlite groups, which apparently has deep geonetic reasons, allows to distinguish a special geochemical type of these rocks, that is characterized by negative anomalies of HFS rare and radioactive elements (kimberlites of D-type) and by opposition to the rocks of the formation petrotype of kimberlites (kimberlites of N-type), that are characterized by moderately high HFS elements contents.

The main factor, determining distribution of noncoherent HFS elements in the mantle kimberlite source, is abyssal metasomatism, caused by juvenile fluids flows, which is always accompanied by significant introduction of alkalies, titanium, highly charged rare and radioactive elements. Thus, there is a good reason to consider the lithosphere substrate, metasomatized under the influence of juvenile abyssal fluids, as a mantle source of kimberlites, belonging to the geochemical type N. It is also clear, that generation of D-type kimberlites takes place under different conditions. Geochemical properties of this type of kimberlites and their confinement to the collision margins of ancient cratons and terrains allows to think, that these rocks with negative HFS elements and titanium anomaly are generated from the lithosphere substrate, that was metasomatized under the influence of volatiles, introduced from the subducted oceanic crust.

Thus, to the present, two possible methods of metasomatic preparation of the mantle substrate are outlined. One of them is associated with primary differentiation of the Earths matter the process of the planet degassion; it occurs in the flows of juvenile fluid emanations, containing carbonic acid, phosphor pentoxide, alkalies and rare metals. The other includes secondary redistribution of the differentiated planet matter during subduction of the oceanic crust, accompanied by recycling of its matter and of its volatile components, first of all.

 It is expected that similar geochemical  and genetic dualism is peculiar not only for Kimberlites but also for other types of abyssal  alkaline ultrabasic magmas, including those accompanied by carbonatites.

References  Lapin A.V., Tolstov A.V. Geochemical  types of   Kimberlites  and tleir mantle sources. In:  Deep..seated magmatism, its sources and plumes. Ed, by Vladykin. Vladivostok Irkutsk. 2008. . 147-168.