2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Sr-Nd ISOTOPE SYSTEMATICS OF CLINOPYROXENNES  IN ULTRAPOTASSIC   VOLCANIC ROCKS FROM EAST AFRICAN RIFT: ROLE OF MANTLE HETEROGENEITY

 N.S.Muravyeva*, B.V. Belyatsky**, V.G. Senin*

*Институт геохимии и аналитической химии им.В.И.Вернадского РАН, Москва, Россия

**  ФГУП ВНИИОкеангеология, Санкт-Петербург, Россия

natash@geokhi.ru

Ultrapotassic magmatism, the deepest type magmatism, which has several characteristics of mantle source enrichment of rare incompatible elements. A classical area distribution ultrapotassic rocks is the western branch of the East African rift. The presence of a relatively small area of ​​rocks, differing in modal and chemical composition reflects the heterogeneity of the upper mantle at the kilometer scale. The most common model is the source of potassium magmatism - lherzolite (harzburgite?) mantle with numerous pyroxenite veins and layers. New evidence of source heterogeneity of potassic magmas from Western Rift were obtained while studying isotopic composition of strontium and neodymium in mineral - phenocrysts of volcanic rocks.
         Despite many years of geological work in this region for kamafugite group of rocks known only a few published Sr-Nd and Pb isotope analysis (Davies, Lloyd, 1989; Rosenthal et. Al., 2009), but data on the isotopic composition of rock-forming minerals is unknown to us. Some researching were carried out for East Africa for nephelinite lavas from Napak volcano, eastern Uganda (Simonetti A. and Bell K., 1993) and  Mount Elgon volcano, eastern Uganda-western Kenya (Simonetti A. and Bell K., 1995). In this work, we study the isotopic composition of strontium and neodymium of forming minerals (clinopyroxene and mica) and estimate the composition of the kamafuge magmas source.

          The object of our study are ultrapotassic volcanic rocks from the provinces of Toro-Ankole and Virunga in the northern part of the western branch of the East African rift. Most of the samples were collected within a volcanic field  Bunyaruguru. For comparison, we studied samples kamafugite (ugandite) and its possible derivatives – melaleucitite and leucitite from volcano Visoke (Virunga Province).

         Isotope systematics of strontium and neodymium studied kamafugites Toro-Ankole (⁸⁷Sr/⁸⁶Sr: 0.704629-0.705356; ¹⁴³Nd/¹⁴⁴Nd: 0.512488 -0.512550) (Muravyeva et. al., 2009; Muravyeva, Belyatsky, 2009; Muravieva et al, 2009) suggests that their mantle source similar in composition to astenospheric source of oceanic islands basalts EM1 (Hofmann, 2003; Stracke et. al., 2005). At the same time, the isotopic composition of lead in the studied rocks ²⁰⁶Pb/²⁰⁴Pb: 18.998 - 19.566; ²⁰⁷Pb/²⁰⁴Pb: 15.686 - 15.737; ²⁰⁸Pb/²⁰⁴Pb: 39.303 - 40.264 (Muravieva et al, 2009)  reveals the similarity of the source with the characteristics of volcanic oceanic islands EM2 and regional anomalies DUPAL. Such a variety of kamafugite isotopic characteristics explains by influence of long mantle metasomatism processes on the source substance.

          In the present work was determined the isotopic composition of strontium and neodymium phenocrysts of clinopyroxene and mica from rocks of different composition: four kamafugite samples fromToro Ankole and two samples (ugandite and leytsitite) from Visoke volcano Virunga Province. Measurements were made on TRITON mass spectrometer in static mode recording of the selected monomineralic fractions (purity better than 99%). Comparing the results to the isotopic composition of host rocks has shown that if the relative isotopic composition of neodymium phenocrysts in general equilibrium with the surrounding rocks, whereas in the ⁸⁷Sr/⁸⁶Sr ratio of phenocrysts is isotopically non-equilibrium. Nature of equilibrium for the studied clinopyroxenes relative to host rock is very different: there is a depleted compositions (the majority) and enriched by strontium (Fig.1).

         Type of isotopic disequilibrium of strontium is in good agreement with the chemical composition and structural features of the studied clinopyroxenes. Composition of clinopyroxene was identified by us on microprobe SX 100 firms САМЕСА. Within each sample there was an interval in the major elements content, sometimes significant (eg, 0,45-0,88 for Mg ^#), as from grain to grain, and between central and marginal parts of the phenocrysts. Different grains of clinopyroxenes exhibit zoning (normal and reverse), which is the main chemical and petrographic indication of disequilibrium. Two trends changing compositions of kamafugite clinopyroxenes, most clearly shown in the graphs Na₂O-Mg ^# and TiO₂-Mg ^#, metasomatic and "magmatic" corresponded to the high and low pressure, ie associated with decreasing in pressure and temperature. Comparing the data of isotopic and chemical composition shows that the trend of high-pressure matches isotopically enriched clinopyroxene, whereas nizkobarny - depleted. Clinopyroxene phenocrysts from mafurite in which reverse zoning prevails, are enriched in radiogenic strontium relative to the whole rock.

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         Attention is drawn to a close resemblance to the strontium isotopic characteristics of clinopyroxenes from different rocks. In contrast to the hydrous minerals of clinopyroxene phenocrysts in effusives are the most resistant to any process of secondary alteration (including metasomatic), which explains the preservation of primary isotopic labels pyroxenes. Thus clinopyroxenes preserve isotopic labels of melts from which they were formed. The difference in the degree of isotopic equilibrium the clinopyroxenes with enclosing rock in the neodymium and strontium can be explained by comparing the rates of diffusion of these elements and the residence time of magmatic melts in the source (Jackson et. Al., 2009). The rate of establishment of diffusion equilibrium for Nd is much lower than that for Sr or Pb. Thus, without the introduction of new batches of isotopically distinct melts in the original magmatic body, the ⁸⁷Sr/⁸⁶Sr isotope ratio in clinopyroxenes will experience diffusion reequilibration with the host melt in the time scales typical for the "life" of magma. Therefore, the observed Sr isotopic disequilibrium clinopyroxenes indicates a rapid change in the composition of magmatic melt, and that the time between mixing of isotopically various magmas and the eruption may be very short. Later melts, which are mixed with clinopyroxene-bearing magma appears to have been much magnesium, which can be judged on the composition of the marginal parts of the phenocrysts. Mg^# of marginal parts for the majority of clinopyroxene with reverse zonation is independent of the composition of the central zone and is 0.80 - 0.88 for obr.11503 and 0,75-0,80 for obr.11530 that, according to the distribution coefficient K_d^Fe-Mg _Cpx-Liq corresponds to the equilibrium melts with Mg ^# ≈ 0,7 ≈ 0,68 and respectively.

         Comparison of the isotopic characteristics of clinopyroxene and mica within the same species (Uganda, Volcano Temple) shows that the isotopic composition of neodymium phenocrysts of pyroxene and mica in equilibrium with the surrounding rocks, and on the composition of strontium mica dramatically different from clinopyroxene. In this case, the isotopic composition of strontium mica phenocrysts reflects the composition of the melt (whole rock), whereas the isotopic composition of pyroxene is obviously close to the characteristics of mantle source. Given the "heap" location points clinopyroxenes in the diagram, we can assume the existence of a single source for all investigated samples with the isotopic composition of strontium close to the value of ⁸⁷Sr/⁸⁶Sr - 0.7046. Such a source is probably the number of older magma chambers, located approximately at the same depth on the boundary of the asthenosphere and lithosphere. These melts were formed and "freezed" there at an early magmatic (or metasomatic?) event and may have had carbonatite composition.

         Isotopic compositions of the samples studied plotted in ⁸⁷Rb/⁸⁷Sr - ⁸⁷Sr/⁸⁶Sr diagram, can be divided into three areas, differing in the degree of radiogenic strontium enrichment ⁸⁷Sr: two - for kamafugite Toro-Ankole region of volcanic rocks and volcano Visoke (Virunga). It is interesting to note that in a small field, which form a point consisting of all the studied clinopyroxenes fall and most depleted in strontium, the least radiogenic calcium-rich mafurite (obr.11503) and katungite. This apparently indicates that the equilibrium of the primary melts of these rocks with pyroxenite source. It can be assumed that rocks rich in clinopyroxene with the most radiogenic Sr were produced from metasomatized mantle source, which indirectly confirms the results of modeling of major elements (Muravyeva et. Al., 2009). The high content of strontium in the rock (2888 ppm) and the presence of phenocrysts of carbonates indicates that the agent of metasomatizm was carbonatite melt-fluid (Kogarko et. Al., 2001).
          Based on these results likely scenario for the origin of the studied rocks are 1) the formation of primary melts during the melting of a heterogeneous pyroxenite-peridotite mantle source and 2) mixing of magmas of the two lithospheric and asthenosphere horizons during rapid ascending to the surface.

References:

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