Disentangling the physico-chemical evolution of the melts that give rise to kimberlites during their genesis, ascent through the sub-cratonic lithosphere and emplacement in the crust is challenging. This is because the extensive entrainment and assimilation of, and reaction with, mantle-derived material makes kimberlites mixtures of xenocrystic, magmatic components and alteration minerals, rarely preserving evidence of their original melt composition. Here, a detailed textural and compositional study of coherent and volcaniclastic kimberlite units from the Udachnaya-East pipe (Siberian craton) was performed to reconstruct the pressure-temperature-oxygen fugacity-compositional (P-T-fO2-X) path of kimberlite melts during their ascent through the sub-cratonic lithosphere. Routine and high-precision electron microprobe analyses of olivine, phlogopite and Fe-Ti oxides enabled to discriminate the mantle-derived cargo from the magmatic components, and thus make inferences on the liquid line of descent (LLD) of proto-kimberlite to kimberlite melts. Most of the olivine cores in Udachnaya-East kimberlite are xenocrystic, and record conditions of last-equilibration in the Siberian sub-cratonic lithospheric mantle at T-P ranging from 812-1227°C at 3.1-5.4 GPa, to 871-1170°C at 4.6-7.0 GPa, depending on the chosen model geotherm (35mW/m2vs. 40 mW/m2). Based on their Ni, Cr, Mn, Al and P vs. Mg/Fe systematics, olivine core populations were associated to the sheared, granular garnet-bearing or spinel-bearing cratonic peridotites. The occurrence of olivine Internal Zones (I) having the same composition as Fe-rich cores, as well as rare mantle-derived xenocrystic cores of phlogopite, bear witness of mantle metasomatism preceding kimberlite ascent. The assimilation of mantle material by initially H2O- and P-rich proto-kimberlitic melts is recorded by the cotectic formation of magmatic olivine Internal Zones (II) and phlogopite Internal Zones around resorbed xenocrystic cores. The LLD then evolved by inducing cotectic precipitation of olivine, phlogopite rims and Cr-spinel at P of 1.5-3.0 GPa, T of 1120-1250°C and fO2 from -2.8 to -1.6 ΔFMQ. Ilmenite-magnetite pairs in the groundmass record the later conditions of crystallization, that occurred at P ~1.0 GPa, T from 1133°C down to 1000°C and fO2 of +0.3 to +0.9 ΔFMQ. These results confirm the progressive oxidation of the melt, that finally led to the formation of calcite, apatite, and Mg-rich olivine and phlogopite rinds. The F-enrichment of phlogopite rinds reflects a late decrease of the H2O/CO2 activity of the melt, that evolved towards alkali-carbonated composition. Our results showed that the crystal cargo of kimberlites can be used to track all processes acting in between the melt-rock reactions in the mantle and the emplacement in the crust.
<p>Mafic&#8211;ultramafic lenses embedded in felsic granulites of the Gf&#246;hl Unit, Moldanubian Zone, are considered to be mantle fragments incorporated into mid-crustal levels of the Variscan orogenic crust. We investigated a several 100 m sized mafic lens mainly formed by garnet pyroxenite. The primary mineral assemblage comprises calcium-rich garnet (X<sub>Grs</sub> = 0.4), kyanite, and sodium-rich clinopyroxene (X<sub>Na_M2</sub> = 0.29) (&#177; quartz), which indicates pressures above 1.8 GPa and temperatures around 1000 &#176;C. Towards the margins of the mafic lens, the garnet pyroxenites were increasingly overprinted at lower pressures leading to the destabilization of kyanite, Na-rich clinopyroxene, and garnet. A first decompression phase is represented by garnet-hosted sapphirine&#8211;spinel&#8211;plagioclase symplectites supposedly replacing kyanite and clinopyroxene. A second stage is evident from the partial resorption of garnet by plagioclase and clinopyroxene in the form of a peculiar corrosion tubes penetrating the garnet in a worm-like fashion. Finally, the third stage decompression assemblage is represented by plagioclase&#8211;orthopyroxene&#8211;spinel symplectites partially replacing garnet. In all cases, garnet shows pronounced secondary compositional zoning towards the decompression products. The secondary zoning is qualitatively similar for the sapphirine&#8211;spinel&#8211;plagioclase symplectites and the plagioclase&#8211;clinopyroxene corrosion tubes and is characterized by a strong decrease of the Grs content accompanied by an increase of the Alm and Prp contents towards the decompression products. For the sapphirine&#8211;spinel&#8211;plagioclase symplectite, the garnet composition changes from Alm<sub>14</sub>Prp<sub>42</sub>Grs<sub>44</sub> in the pristine garnet to Alm<sub>22</sub>Prp<sub>63</sub>Grs<sub>15</sub> at the interface to the symplectite. The compositional change towards the corrosion tubes is from Alm<sub>19</sub>Prp<sub>40</sub>Grs<sub>41</sub> to Alm<sub>30</sub>Prp<sub>54</sub>Grs<sub>16</sub>. The secondary zoning towards the plagioclase&#8211;orthopyroxene&#8211;spinel symplectites is characterized by an increase of X<sub>Alm</sub> from 0.19 to 0.27 and a concomitant decrease of X<sub>Prp</sub> from 0.55 to 0.49 at constant X<sub>Grs</sub> of 0.25. In all cases, the compositional changes are gradual suggesting diffusion-mediated re-equilibration of the garnet at decreasing pressures. Time scales for the duration of decompression were estimated by fitting a multicomponent diffusion model to the observed compositional patterns. Depending on the choice of the diffusion coefficients, the time scales vary from several hundreds to hundred thousands of years, whereby the earliest decompression features yield time scales that are five times longer than those obtained from the corrosion tubes and about ten times longer than those obtained from the plagioclase&#8211;orthopyroxene&#8211;spinel symplectites. These timescales reflect the duration from the onset of the different decompression-induced mineral reactions to the time when the rocks cooled below about 700 &#176;C and the composition patterns of the garnet were effectively frozen. The longest timescales obtained from the early decompression reactions are on the order of 100,000 years and the shortest timescales obtained from the late-stage symplectites are on the order of 1,000 years. Considering the regional metamorphic setting of the Moldanubian Zone, such timescales are remarkably short and suggest rapid transport of the mafic&#8211;ultramafic lithologies from mantle depths to the mid-crustal level. Concomitant incorporation into a dominantly felsic environment led to immediate cooling.</p>
<p>Corona microstructures comprised of garnet (grt) and clinopyroxene (cpx) were observed at the contacts between plagioclase (pl) and Fe-rich orthopyroxene (opx) in meta-gabbroic rocks in a several 100 m sized (ultra-)mafic lens embedded in felsic granulite of the Gf&#246;hl unit (Moldanubian zone, Lower Austria).</p><p>The corona microstructures are formed around monomineralic aggregates of opx and they are comprised of two layers, an inner about 100 &#956;m thick<br>layer of polycrystalline cpx and an outer, about 800 &#956;m thick layer of polycrystalline garnet. The corona structures are surrounded by the pl-rich rock matrix. The cpx layer shows a weak but systematic chemical zoning characterized by increasing Mg and decreasing Na and Al contents from the contact with grt towards the contact with opx. The grt layer shows a pronounced and complex chemical zoning. There is a consistent trend of decreasing Mg and increasing Ca contents from the contact with the cpx layer, where the composition is Alm<sub>22 </sub>Prp<sub>67</sub> Grs<sub>11</sub> towards the contact with the rock matrix, where we observe Alm<sub>25</sub> Prp<sub>48</sub> Grs<sub>28</sub>. This pattern is interpreted as a primary growth zoning. Superimposed on the growth zoning there is a secondary zoning, which is evident from a decrease of the Ca content and a concomitant increase of the Mg content from the interior of the individual grains<br>of the grt polycrystal forming the grt layer towards the grt grain boundaries. The secondary zoning is most pronounced in the outermost portions of the garnet layer, where the primary growth zoning shows the highest Ca and the lowest Mg contents. Locally the garnet grains contain abundant primary melt inclusions. In most segments of the corona, secondary opx and pl form layers along the contact between the primary cpx and grt layer, where the opx partially replaces the cpx layer and the pl partially replaces grt. The secondary opx has higher Mg and lower Na, Al, and Ca contents than the opx<br>in the core of the corona structure. The secondary pl has the same composition as the matrix pl. At its outer edge, the garnet layer is locally replaced by spinel bearing cpx-pl symplectites. The primary compositional zoning of the garnet layer could be reproduced in equilibrium assemblage diagrams (pseudosections). Calculated equilibrium phase relations indicate that the grt-cpx corona formed at the contacts between opx and pl at supersolidus HP &#8722; HT conditions of P > 1.8 GPa and T > 900 &#176;C and low H<sub>2</sub>O content. Growth of coronal grt and cpx requires the diffusive transport of Fe and Mg from the opx to the pl and concomitant transport of Ca and Al in the opposite direction. The secondary zoning of garnet, the back reaction forming secondary opx and pl at the contact between the primary grt and cpx layer and the spinel bearing pl-cpx symplectites locally replacing garnet at the outer edge of the grt layer are related to different decompression stages. Preservation of the secondary garnet zoning indicates relatively rapid cooling during late<br>stages of or immediately after decompression.</p>
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