Unmixing and Mixing of Carbonatite and Silicate Magma - Evidence from Laacher See Carbonate-Bearing Xenoliths

Hilmar Liebsch Geochemisches Institut, Universität Göttingen, Goldschmidtstr.1, 37077 Göttingen, Germany


Gerhard Wörner Geochemisches Institut, Universität Göttingen, Goldschmidtstr.1, 37077 Göttingen, Germany

Roland Oberhänsli Institut für Geowissenschaften, Universität Potsdam, Germany

The Quarternary Laacher See volcano erupted over 5 km3 of zoned phonolitic tephra: the Lower Laacher See Tephra (LST) corresponds to the top, the Middle LST to the middle, and the Upper LST to the bottom of the magma chamber. The tephra deposit contains rare xenoliths of carbonatite-bearing rocks which are divided into three groups:
(1) Calcite bearing nosean-syenite with < 5 cm-sized sövite droplets are examples coexisting immiscible carbonatite and silicate melts. (2) Heterogeneous sövite-syenites with eutectic intergrowth of calcite, sanidine and nosean are either layered or unlayered and examples for the crystallisation from a mixed silicate-carbonatite melt with up to 60 % carbonate. Finally (3) calcite bearing syenites with interstitial calcite and small rounded inclusions of calcite in minerals occur. The relative abundance of carbonate in the bulk of the xenoliths is about 5 to 10 wt.%. In order to define the relationship between phonolite magma and carbonatite-bearing rocks, we analysed carbon and oxygen isotopes in the carbonate and silicate fractions, major and trace elements of minerals and glasses by electron microprobe, and bulk rocks as well as separated sövitic and syenitic parts of heterogeneous samples by XRF and ICP-MS.

Carbon and oxygen isotopic compostitions of calcite show typical mantle values, but carbonate poor samples have also heavier oxygen- and lighter carbon-isotope values. Whole rock silicate oxygen isotope ratios are similar to the host phonolite. The compositions of silicate minerals in sövite-droplets and related syenite are identical, indicating liquid immiscible carbonatite and silicate melts were in
equilibrium. Compared to the phonolite phenocrysts, the composition of silicate minerals in the carbonatite-bearing rocks is different: plagioclase and sphene are absent while calcite, pyrochlore and orthite are present. Sanidine and nosean of the carbonatite-bearing rocks have higher Na contents than the phonolite phenocrysts; biotite, clinopyroxene, and magnetite have higher Mn contents, apatite has higher REE contents. Thus the compositional range of silicates in the carbonatite-bearing rocks extend beyond those of the most evolved phonolites (Lower LST). The compositions of interstitial silicate glasses cover the compositional range of the phonolite matrix glasses.

The trace elements in the carbonatite-bearing rocks are similar to the phonolite trace element patterns. Both types of rocks show negative spikes of Ba, K, Sr, P, Ti and Sc whereas Zr, Hf, Ta, Th and U are enriched. A negative Eu-anomalie, which only occurs in the most evolved phonolites, is also present in the carbonatite-bearing rocks. Compared to the most evolved Lower LST, the carbonatite-bearing rocks are less MREE depleted, have a smaller negative Eu-anomaly and are less LREE enriched.

Trace element compositions of samples with variable carbonate content allowed us to define the enmember-compositions of the two immiscible syenite and sövite melts. Ca, Mn, Sr, Y, and REE are enriched in the sövite while Si, Al, Na, K, S, Rb, Zr, and Hf are enriched in the syenite. The trace element compositions of the most evolved phonolites are intermediate between the calculated endmembers and show parallel REE-patterns. Mass balance calculations show that a calculated initial phonolite was intermediate between MLST and LLST compositions (but more quite volaite-rich) and produced by unmixing 5 to 10 %wt. of sövite and 90 to 95 %wt. syenite. In this scenario, the carbonate-rich (up to 60 wt.%) sövite-syenites can only be explained if separation concentration and remixing is assumed to enrich the carbonate fraction in the hybrid magma.

We conclude that during the zonation process of the Laacher See magma chamber a portion of the residing magma separated from the main chamber. This separation ocurred after a main phase fractional crystallisation in the phonolite and before the uppermost Lower LST had evolved. The separated magma must have been very volatile-rich and later evolved independently from the main chamber. This magma reached the miscibillity gap between silcate and carbonate melt during its further evolution. About 5 to 10% of carbonatite magma unmixed from syenite melt and coalesced into droplets. This stage is represented by the syenites with sövite droplets (type 1). The carbonatite droplets ascended due to their lower density and became enriched in the upper parts of the system. A change in physical conditions (increasing temperature; decrasing pressure) and the loss of alkalies by a CO2-, F-, S-, Ca-, Mn- and REE-rich fluid caused the remixing of the carbonatite- and silicate melts. Thus the hybrid Sövite-Syenites (type 2) were formed. The calcite-bearing syenites (type 3) represent the plutonic equivalent of initial carbonate-bearing melt, which was depleted in carbonatite-compatible trace elements by unmixing and loss of the carbonate fraction.