Two generations of clinopyroxene phenocrysts in Gaussberg volcano lamproiites (E. Antarctica)

 Lamproites are a group of ultra-alkaline rocks enriched with rare lithophilic elements and have high ratios of K₂O/Al₂O₃ and K₂O/Na₂O, the K₂Ocontentcanreach12%. The Quaternary(about56,000years old)lamproites of the Gaussberg subglacialvolcanoareauniquegeologicalobjectlocatedon the continentalmargin[Sushchevskayaetal.,2014]and,thus,provide us an opportunity to study the features of the continentalmantleunder the Antarctic continent. The rocksappear as apillowlavas,tufobreccias and as usual consistofolivine microphenocrysts,clinopyroxene,leuciteandmultipleaccessoryminerals,mainly of primaryorigin as a minororeminerals–ilmenite,titano-magnetite;amphibolesandmicas. The inclusions are up to2mm in sizeand are distributedin the bulk rock,presentedofclinopyroxene,amphibole,phlogopite,apatite,diopside.

The aim of this work was to determine the sequence and conditions of crystallization of ultrakali Gaussberg magmas based on the study of petrographic and mineralogical features of natural samples, as well as our experiments on the crystallization of the initial melt at different temperatures. During the investigation were studied 16 samples of Gaussberg lamproites, selected during the 2nd Soviet Antarctic Expedition of 1956-1958, a detailed description of which was previously presented in our works]. Previously, two groups of clinopyroxene inclusions were identified in Gaussberg lamproites, differ in the form of occurence and in the content of the main petrogenic components: more magnesian grains of diopside composition and more ferrous grains of salic composition (the so-called "green cores").To determine the geochemical features of the selected groups of clinopyroxene grains, we provide a detailed study. In addition to the previously obtained data from the rock layers, a study of the mineral in the cuts was conducted. The contents of the main petrogenic components were determined in the geochemistry of the Russian Academy of Sciences using a TESCAN MIRA 3 scanning electron microscope (ULTIM MAX 100 EMF detector). The content of impurity elements was obtained at GEOHI RAS using inductively coupled plasma mass spectrometry with laser ablation (LA-ICP-MS) on an Element-XR mass spectrometer (Thermo Finnigan) with an UP-213 laser attachment (New Wave Research).