Angelika Kalt

Nd, Sr, and Pb isotopic evidence for diverse lithospheric mantle sources of East African Rift carbonatites

Abstract Carbonatites may provide valuable information on mantle source compositions as their isotopic ratios are insensitive to crustal contamination. In order to place constraints on mantle sources, nineteen samples from three Miocene to Quaternary carbonatite areas in the East African Rift were analysed for their Sr, Nd, and Pb isotopic compositions. The samples from Kerimasi (northern Tanzania), Homa Mountain, and Wasaki Peninsula (both Lake Victoria, Kenya) as a whole show considerable variations in their isotope ratios (0.70327–0.70502 for 87Sr/86Sr, 0.51249–0.51283 for 143Nd 144Nd, 18.72–20.41 for 206Pb/204Pb, 15.52–15.78 for 207Pb/204Pb, and 39.22–40.47 for 208Pb/204Pb) that lie between the inferred compositions for HIMU (high 238U/204Pb mantle) and EM I (enriched mantle I) components in most isotope plots. The internal isotopic variations of the three carbonatite areas define distinct arrays and diverse trends in isotope diagrams. Although the isotope data define linear arrays in Sr Nd and Pb Pb diagrams, which suggest binary mixing between HIMU and EM I mantle components, neither the isotopic compositions of the carbonatites as a whole nor the compositional ranges for individual carbonatite occurrences can be explained by such a process. This clearly emerges from the absence of linear data trends in Sr Pb and Nd Pb isotope plots and from the lack of consistent endmember compositions. These features are also displayed by previously published isotope data for East African carbonatites. It is therefore suggested that carbonatite complexes within the East African Rift have isotopically distinct and small mantle sources that are probably not adequately described in terms of the mantle components defined for oceanic basalts. Most likely, these sources are located in a heterogeneous lithospheric mantle and were produced by enrichment and depletion processes at different times and degrees.

Petrology and geochronology of eclogites from the Variscan Schwarzwald (F.R.G.)

The Moldanubian basement of the Schwarzwald contains basic to ultrabasic rocks of both crustal and mantle origin which display high-pressure mineral assemblages or relics of such. In order to constrain the P-T-t evolution of the crustal high-pressure rocks, petrological and geochronological studies have been carried out on three eclogite samples. Geothermobarometric estimations indicate minimum metamorphic pressures of 1.6 GPa and equilibration temperatures of 670 750°C. Reaction textures document various metamorphic stages during exhumation of the high-pressure rocks. The age of high-pressure metamorphism is constrained by Sm-Nd isochrons of 332±13 Ma, 334±11 Ma, and 337±6 Ma defined by garnet, whole rock and clinopyroxene. For one sample, large garnets show prominent growth zoning in terms of major elements, Sm, Nd, and inclusions, dividing the grains into two growth stages. Sm-Nd isotope analyses on these garnets indicate that the time span between the two growth stages is too small to be resolved, reflecting a rather rapid metamorphic evolution. This result is further constrained by a Rb-Sr isochron age of 325±6 Ma on retrograde biotite and whole rock on the same sample. For one of the studied eclogites, formation of the magmatic precursor rocks is possibly approximated by the Ordovician U-Pb upper intercept age of a discordia from zircons.

Complete solid solution between magnesian schorl and lithian excess-boron olenite in a pegmatite from the Koralpe (eastern Alps, Austria)

At the margins of a plagioclase-quartz pegmatite near Stoffhutte (Koralpe, Austroalpine realm, Austrian Alps), a new solid solution series within the tourmaline group (XY 3 Z 6 [T 6 O 18 ](BO 3 ) 3 V 3 W) has been found, namely between magnesian schorl and lithian excess-boron olenite. At the pegmatite contact with mylonitic garnet-biotite schists, a tourmalinite band developed. Massive tourmaline cores are Mg-rich schorl with normal boron contents of 3.0 cations per formula unit that most likely formed by a reaction of melt with garnet. Within the pegmatite, only olenitic crystals were found. Within a transition zone, the Mg-rich schorl crystals become zoned and develop olenitic rims of increasing size, most likely formed from a fluid via the reaction plagioclase \({+}\ H_{2}O\ {+}\ B_{2}O_{3}\ =\ olenite\ {+}\ quartz\) . The major compositional changes are decreasing Fe, Mg and Si contents and X Mg values as well as increasing B, Al and Li contents. Over a distance of less than five centimetres, tourmaline composition determined by secondary ion microprobe (B, Li, Be, H) and electron microprobe (remaining elements, total Fe as Fe 2+ ) varies continuously from \((Na_{0.90}Ca_{0.11})(Fe_{1.61}Mg_{1.10}Mn_{0.01}Ti_{0.11}Zn_{0.01}Li_{0.01}Al_{0.10}{\square}_{0.05})Al_{6.00}[Si_{6.01}O_{18}]\ (B_{2.99}O_{9})(OH_{3.02}F_{0.23}O_{0.75})\ to\ (Na_{0.42}Ca_{0.31}{\square}_{0.27})(Fe_{0.14}Mg_{0.03}Mn_{0.01}Ti_{0.06}Li_{0.33}Be_{0.01}Al_{2.49})Al_{6}[Si_{4.91}Al_{0.21}B_{0.88}O_{18}](B_{3.00}O_{9})OH_{3.38}F_{0.07})\) . In contrast to the proton-deficient ideal olenite, the olenitic tourmaline from Stoffhutte contains 3.0–3.4 hydrogens per formula. The main substitutions are most likely \(^{[Y]}Al^{3{+}}\ {+}\ ^{[T]}B^{3{+}}\ =\ ^{[T]}Si^{4{+}}\ {+}\ ^{[Y]}(Mg,\ Fe)^{2{+}}\) , which is a modified Tschermaks9 substitution, \(^{[Y]}Al^{3{+}}\ {+}\ ^{[Y]}Li^{{+}}\ =\ 2\ ^{[Y]}(Mg,\ Fe)^{2{+}}\) , \(^{[X]}Ca^{2{+}}\ {+}\ ^{[X]}{\square}\ =\ 2\ ^{[X]}Na^{{+}},\ ^{[Y]}Al^{3{+}}\ {+}\ ^{[X]}{\square}\ =\ ^{[X]}Na^{{+}}\ {+}\ ^{[Y]}(Mg,\ Fe)^{2{+}}\ and\ ^{[Y]}Fe^{2{+}}\ =\ ^{[Y]}Mg^{2{+}}\) . Coexisting muscovite and feldspar have considerable B (0.42–0.55 wt% and 71–134 ppm, respectively), Be (49–140 ppm and 79–169 ppm, respectively) and Li contents (6–9 ppm and 9–126 ppm, respectively). The apparent partition coefficient for total B between tourmaline and muscovite (D B = c tur / c ms ) varies between 6.5 and 16.8, but the values for tetrahedrally coordinated B (D B(T) ) range from 0 to 13.0.

METAMORPHIC EVOLUTION OF ULTRAHIGH-PRESSURE GARNET PERIDOTITES FROM THE VARISCAN VOSGES MTS. (FRANCE)

Abstract In the Central Vosges Mts. (France) of the Variscan belt, MgCr garnet peridotite bodies occur within the uppermost tectonometamorphic unit (Leptynitic granulites) as lenses in low-pressure/high-temperature metamorphic rocks. Neglecting late-stage serpentinization, the metamorphic evolution of these rocks was characterized by four stages. During stage I, the rocks were equilibrated at high pressures and temperatures (> 4.9 GPa/ > 950°C, in most cases > 1100°C), either within or near to the stability field of diamond. Stage II is documented by the formation of coronas around relict garnet with the assemblage orthopyroxene ± clinopyroxene ± amphibole + spinel ± plagioclase. During stage III, the remaining garnet was transformed to very fine-grained kelyphite consisting of orthopyroxene + amphibole + spinel ± plagioclase. Small relict garnet grains are preserved in one peridotite only. Stage IV corresponds to the late formation of tremolitic hornblende and chlorite which partially replaced the pseudomorphs after garnet or occur along cracks in the matrix of some rocks. Compositional zoning patterns of pyroxene porphyroclasts suggest that initial decompression was either accompanied by a moderate increase in temperature or nearly isothermal. Garnet breakdown textures and compositions of minerals grown during stages II and III also suggest rapid decompression at still elevated temperatures (1000-720°C).

Cordierite channel volatiles as evidence for dehydration melting an example from high-temperature metapelites of the Bayerische Wald (Variscan belt, Germany)

The H 2 O and CO 2 channel contents of cordierite in migmatites from the Bayerische Wald (Variscan Belt, Germany) were measured by infrared spectroscopy, directly from thin polished rock sections. Cordierite is characterized by low H 2 O and CO 2 contents of 0.38 to 0.74 and 0.04 to 0.10 wt%, respectively. Errors are mainly introduced by the varying orientations of the cordierite sections and are estimated to be 10–12% relative for the average total H 2 O content of a sample. The orientation effects are even more pronounced for CO 2 . Whereas the total cordierite H 2 O contents vary between the samples within the given range, the H 2 O contents of cordierite grains within a single sample show no variations, neither between cordierite formed by different reactions, nor between cores and rims. Only in one sample could an increase be observed in total H 2 O contents from cores to rims. The CO 2 contents vary between the samples and in some cases also within the samples. The low volatile contents are not consistent with equilibration of cordierite with H 2 O-CO 2 fluids. Most likely, they are due to equilibration with peraluminous water-undersaturated melts. The melt H 2 O contents calculated with the cordierite H 2 O contents of the Bayerische Wald, using recently published partition coefficients, are in the range of melt H 2 O contents from dehydration melting experiments. The variation of cordierite H 2 O contents between the studied samples can be explained by locally differing melting reactions and melt percentages. The lower range of the H 2 O concentrations of cordierite could also be due to dehydration, assuming the most unfavourable circumstances. However, the presence of Na, CO 2 and N 2 acting as channel blockers, the 1:1 ratio of H 2 O I: H 2 O II and rapid cooling of the rocks down to 500°C argue against significant dehydration. Therefore, in-situ FTIR spectroscopy on cordierite can be a useful tool to determine fluid/melting conditions in migmatites where clear evidence of partial melting is lacking, and calculated temperatures are between the H 2 O-saturated and the dehydration-melting solidus.

Metamorphic evolution of garnet-spinel peridotites from the Variscan Schwarzwald (Germany)

Garnet-spinel peridotites form small, isolated, variably retrogressed bodies within the low-pressure high-temperature gneisses and migmatites of the Variscan basement of the Schwarzwald, southwest Germany. Detailed mineralogical and textural studies as well as geothermobarometric calculations on samples from three occurrences are presented. Two of the garnet-spinel peridotites have equilibrated at 680–770°C, 1.4–1.8 GPa within the garnet-spinel peridotite stability field, one of the samples having experienced an earlier stage within the spinel peridotite stability field (790°C, <1.8 GPa). The third sample, with only garnet and spinel preserved, probably equilibrated within the garnet peridotite stability field at higher pressures. These findings are in line with the distinction of two groups of ultramafic garnet-bearing high-pressure rocks with different equilibration conditions within the Schwarzwald (670–740°C, 1.4–1.8 GPa and 740–850°C, 3.2–4.3 GPa) which has previously been established (Kalt et al. 1995). The equilibration conditions of 670–770°C and 1.4–1.8 GPa for garnet-spinel peridotites from the Central Schwarzwald Gneiss Complex (CSGC) are similar to those for eclogites of the Schwarzwald and also correspond quite well to those for garnet-spinel peridotites from the Moldanubian zone of the Vosges mountains and of ecologites from the Moldanubian s.str. of the Bohemian Massif.

Li, Be, and B abundances in minerals of peridotite xenoliths from Marsabit (Kenya): Disequilibrium processes and implications for subduction zone signatures

The light elements Li, Be, and B have been analyzed in situ in minerals from three groups of peridotite xenoliths hosted in Quaternary basanites from the Marsabit volcanic field (northern Kenya). Group I and II are fertile lherzolites that experienced deformation, decompression, and cooling in the context of Mesozoic rifting (Group I), followed by heating, static recrystallization, and associated cryptic metasomatism (Group II) as a result of Tertiary-Quaternary rifting and magmatism. Group III xenoliths are spinel harzburgites and dunites that experienced strong cryptic and modal metasomatism. The Li-Be-B systematics in minerals of Group I and II are similar to unmetasomatized subcontinental lithospheric mantle. In contrast, Group III samples are characterized by significant enrichment in all light elements and disequilibrium partitioning between different phases. Light element concentrations levels are similar to that expected for mantle rocks metasomatized by melts and fluids released from subducting slabs, while light element/rare earth element ratios (especially Li/Yb) approach those of typical Island Arc basalts. However, detailed investigation of textures and chemical zoning shows that at least Li concentrations in primary minerals were modified (i.e., decoupled from Yb) during late-stage melting and/or fluid percolation related to Tertiary-Quaternary alkaline magmatism in Marsabit (formation of melt pockets consisting of silicate glass, clinopyroxene, olivine, and chromite), ultimately followed by xenolith entrapment and transport to the surface. Mass balance calculations show that the melt pockets formed at the expense of earlier metasomatic phases. During this process the melt pockets mostly preserved the B, Be, and rare earth element budget of the precursor phase assemblage, whereas Li was added. Elevated B/Be and low Ce/B of metasomatic phases prior to late melting could result from metasomatism by a slab fluid. However, similar characteristics are expected for evolved Si- and CO2-rich fluids derived from basanite melt-peridotite interaction, not related to any subduction zone process. The results of this study imply that the inference of a “slab signature” exclusively based on trace element data of metasomatized peridotite is ambiguous.