V. G. Malkovets

Diamond, subcalcic garnet, and mantle metasomatism: Kimberlite sampling patterns define the link

A genetic relationship between diamond and subcalcic Cr-pyrope garnet, both being produced by a metasomatic process, can be inferred from the sampling patterns of kimberlites in the Daldyn-Alakit province, Yakutia, Russia. Pressure-temperature estimates for xenoliths and xenocrysts show a strong concentration of highly depleted rocks in a well-defi ned zone 140‐190 km deep; diamond inclusions and diamond-bearing xenoliths show that most diamonds come from harzburgites within this layer. Xenocryst distribution curves indicate that diamondiferous kimberlites have sampled both garnet and chromite from the harzburgitic layer, but low-grade pipes have sampled only chromite. Diamond formation probably is due to the oxidation of methane-rich, silica-bearing fl uids: Fe 2 O 3 (in chromite) + CH 4 → C + H 2 O + FeO (in chromite), accompanied by another reaction: chromite ± olivine ± orthopyroxene + Si, Ca (in fl uid) → low-Ca, high-Cr garnet. The presence or absence of diamond in kimberlites thus refl ects the distribution of metasomatized fl uid conduits in a lithospheric mantle that originally consisted of highly refractory harzburgites containing neither garnet nor diamond.

Inclusions of Cr- and Cr–Nb-Rutile in pyropes from the Internatsionalnaya kimberlite pipe, Yakutia

The results of study of rutile inclusions in pyrope from the Internatsionalnaya kimberlite pipe are presented. Rutile is characterized by unusually high contents of impurities (up to 25 wt %). The presence of Cr2O3 (up to 9.75 wt %) and Nb2O5 (up to 15.57 wt %) are most typical. Rutile inclusions often occur in assemblage with Ti-rich oxides: picroilmenite and crichtonite group minerals. The Cr-pyropes with inclusions of rutile, picroilmenite, and crichtonite group minerals were formed in the lithospheric mantle beneath the Mirnyi field during their joint crystallization from melts enriched in Fe, Ti, and other incompatible elements as a result of metasomatic enrichment of the depleted lithospheric mantle.

Inclusions of crichtonite group minerals in pyropes from the Internatsionalnaya kimberlite pipe, Yakutia

The results of study of crichtonite group minerals in pyropes from the Internatsionalnaya kimberlite pipe are reported. Most of the studied samples are characterized by high concentrations of Sr, Ca, Na, and LREEs in comparison with minerals of the LIMA series from kimberlites of South Africa, whereas the average concentrations of Ba and K are significantly lower. Crichtonite group minerals in pyropes are characterized by predomination of Na over K in most samples and by a high concentration of Al2O3 (up to 4.5 wt %). Findings of inclusions of crichtonite group minerals with high concentrations of incompatible elements provide evidence for the metasomatic origin of host chromium-rich pyropes.

Error sources in single-clinopyroxene thermobarometry and a mantle geotherm for the Novinka kimberlite, Yakutia

Abstract A new suite of 173 clinopyroxene grains from heavy-mineral concentrates of the diamondiferous Novinka kimberlite (Upper Muna field, Yakutia) has been analyzed for major and minor elements with an electron microprobe to perform a thermobarometric study and model the thermal structure of the Archean Upper Muna lithospheric mantle. Scrupulous evaluation of propagation of analytical uncertainties on pressure estimates revealed that (1) the single-clinopyroxene geobarometer can be very sensitive to analytical uncertainties for particular clinopyroxene compositions, and that (2) most clinopyroxenes from Novinka have compositions that are sensitive to analytical uncertainties, notwithstanding their apparent compositional suitability for single-clinopyroxene thermobarometry based on previously proposed application limits. A test on various mantle clinopyroxenes containing different proportions of the sensitive elements Cr, Na, and Al allowed us to identify clinopyroxene compositions that produce unacceptably high propagated errors and to define appropriate analytical conditions (i.e., higher beam currents and longer counting times for specific elements) that allow precise P-T estimates to be obtained for sensitive compositions. Based on the results of our analytical test, and taking into account the intrinsic limitations of the single-clinopyroxene thermobarometer, we have designed a new protocol for optimum thermobarometry, which uses partly revised compositional filters. The new protocol permits precise computation of the conductive paleogeotherm at Novinka with the single-clinopyroxene thermobarometer of Nimis and Taylor (2000). Thermal modeling of the resulting P-T estimates indicates a ~34 mW/m2 surface heat flow, a thermal lithosphere thickness of ~225 km, and an over 100 km thick “diamond window” beneath Novinka in the middle Paleozoic (344–361 Ma). We estimate that appropriate analytical conditions may extend the applicability of single-clinopyroxene thermobarometry to over 90% of clinopyroxene-bearing garnet peridotites and pyroxenites and to ~70% of chromian-diopside inclusions in diamonds. In all cases, application to clinopyroxenes with Cr/(Cr+Al)mol < 0.1 is not recommended. We confirm the tendency of the single-clinopyroxene barometer to progressively underestimate pressure at P > 4.5 GPa.

Petrogenesis of Ilmenite-Bearing Symplectite Xenoliths from Vitim Alkaline Basalts and Yakutian Kimberlites, Russia

Ilmenite-bearing symplectite, typically ilmenite-clinopyroxene, occurs in rare mantle xenoliths from kimberlite and other alkaline rocks. Two major hypotheses have been proposed for their origin: (1) exolution from high-pressure pyroxene or garnet; and (2) cotectic or eutectic co-precipitation from evolved “proto-kimberlite” melt, producing megacrysts. Here we present new findings about these ilmenite-clinopyroxene symplectites in the Miocene picrobasalts and Pleistocene basanites of the Vitim volcanic field, central Siberia. For comparison, we have described unusual ilmenite-clinopyroxene symplectites from Yakutian kimberlites: the Mary pipe (Kuoisk field) and the Mir pipe (Malo-Botuobinsk field). Symplectite clinopyroxene from Vitim alkaline basalts corresponds to the low-Mg end of the clinopyroxene megacryst trend. They have an Mg# of 70-72 and contain 1.5-2.0 wt% TiO2. Ilmenites from Vitim symplectites are Cr-poor and contain 5.3-6.3 wt% MgO, whereas ilmenites from kimberlite symplectites contain 0.1-1.4 wt% Cr2O3 and 8-13 wt.% MgO. However, trace-element patterns of symplectitic clinopyroxene in both kimberlites and alkaline basalts are similar. Clinopyroxenes from Vitim symplectites have more evolved trace-element patterns (Lan = 3.1-3.8, normalized to primitive mantle) relative to the majority of megacrysts (Lan = 1.0-3.8). Modeling of fractional crystallization of clinopyroxene from alkaline basaltic melt indicates that clinopyroxenes from symplectites correspond to the residual melt fraction of 20-40%, which is high for eutectic crystallization. Symplectites from the Mary pipe can be divided into Cr-rich (Cr2O3 cpx = 0.20-0.39 wt.% and Cr2O3 ilm = 0.7-1.3 wt%) and Cr poor (Cr2O3 cpx = 0.04-0.16 wt.% and Cr2O3 ilm = 0.1-0.4 wt.%) subgroups. However trace-element patterns in clinopyroxenes from both groups of symplectites are similar. This may indicate a symplectite origin from different pulses of magma. Cr-enrichment in the Cr-rich group can be related to contamination by adjacent peridotites. Compositional and textural variations in symplectites from both the alkaline basalts and kimberlites reveal similarities in genesis and a clear connection with megacryst assemblages. We suggest that ilmenite-clinopyroxene symplectites characterize the initial stage of simultaneous precipitation of ilmenite and clinopyroxene. They crystallize after the majority of the clinopyroxene megacrysts and before normal Ilm-Cpx intergrowths and ilmenite megacrysts. This indicates a possible cotectic rather than eutectic precipitation of ilmenite and clinopyroxene in symplectites. Pressure and temperature estimations indicate that Ilm-Cpx symplectites from the Mir and Mary kimberlite pipes formed at pressures of 40-50 kbar, whereas those from Vitim basalts formed at lower pressures of 12-17 kbar. Coexisting Fe-Ti oxides in Vitim ilmenites from symplectites and megacrysts suggest equilibration temperatures of 690-1025°C at fO2 = 0.22-1.88 log units below the QFM buffer. This redox state is similar to that of spinel peridotites in the same localities.