Jürgen Konzett

THE TIMING OF MARID METASOMATISM IN THE KAAPVAAL MANTLE : AN ION PROBE STUDY OF ZIRCONS FROM MARID XENOLITHS

Abstract The precise timing of metasomatic events in the Kaapvaal craton lithospheric mantle is poorly constrained and therefore it has proven difficult to relate mantle metasomatism in the source to igneous activity at the surface. A suite of mica–amphibole–rutile–ilmenite–diopside-bearing xenoliths (MARID) represents the product of such metasomatic fluids or melts. In this paper we report results of a SHRIMP and cathodoluminescence study on zircons from four MARID samples from the Kimberley cluster of pipes. Textures combined with experimental evidence suggest that zircons are late crystallizing phases in MARIDs that form close to the solidus where the major precipitating phases are phlogopite and K-richterite. Cathodoluminescence patterns reveal a complex growth history of the zircons involving strong deformation, repeated events of crystallization, resorption and/or modification of the zircon composition, possibly over several millions of years. Many of the U–Pb ages are significantly older than the intrusion age of the group I kimberlites of the Kimberley cluster. Zircon ages in a MARID xenolith from the Wesselton kimberlite vary in the range 113±3 Ma to 142±3 Ma, compared with a pipe age of 84±3 Ma. In two samples from the De Beers, Bultfontein or Dutoitspan pipes, ages range from 85±5 to 119±2 Ma in one and 80±4 to 90±2 Ma in the other. The ages obtained for the latter are indistinguishable from the pipe age of kimberlites belonging to the younger group I kimberlite event (ca. 80–95 Ma). Zircons in this sample, as well as the younger ages in the other, may reflect real ages but most likely have lost radiogenic Pb as a result of deformation and fluid or melt interaction. The presence of pre-group I ages in two of the samples demonstrates unambiguously that the metasomatism that yielded the older crystallization ages cannot be related to the group I kimberlite event. Instead, minimum ages for the MARID zircons coincide with the period of group II kimberlite magmatism in the central to southeastern Kaapvaal craton. This coincidence in ages would be consistent with the contention based on experimental evidence, that MARID-type rocks can be derived from group II precursor magmas by olivine fractionation and exsolution of a carbonatitic component. An alternative possibility, not excluded by the zircon minimum ages, is a genetic relation between MARID metasomatism and Karoo magmatism.

Incommensurate structure of Ca2Al2O5 at high temperatures – structure investigation and Raman spectroscopy

A high-temperature X-ray diffraction study revealed that brownmillerite-type Ca(2)Al(2)O(5) transforms to an incommensurately modulated structure at elevated temperatures. Single crystals of Ca(2)Al(2)O(5) were synthesized in an end-loaded piston cylinder press at 2.5 GPa and 1273 K. The diffraction pattern observed at 1090 (10) K by in situ single-crystal diffraction experiments can be indexed by an I-centred orthorhombic cell and a modulation wavevector of q = 0.595 (1)c(*). A (3 + 1)-dimensional model in superspace group Imma(00gamma)s00 was used to refine the modulated structure. The structure is assembled from two building units: (i) layers of corner-sharing [AlO(6)] octahedra, stacked along b alternate with (ii) layers of zweier single chains of [AlO(4)] tetrahedra running along a. The modulated structure arises from an aperiodic sequence of two different configurations of the chains within the tetrahedral layers. The modulated high-temperature phase of Ca(2)Al(2)O(5) is isotypic to the modulated high-temperature modification of Ca(2)Fe(2)O(5). A large hysteresis was found in the phase-transition temperature. On heating, the transition occurs at ca 1075 (10) K; on cooling, satellite reflections can be observed down to 975 (10) K. The characterization of Ca(2)Al(2)O(5) is completed by Raman spectroscopy, including a partial interpretation of the spectra.

Lime Mortar with Natural Hydraulic Components: Characterisation of Reaction Rims with FTIR Imaging in ATR-Mode

Reaction rims of natural hydraulic relicts in historic mortars were investigated using a novel technology; a FTIR-spectrometer equipped with a focal plane array detector enabling in ATR-mode IR imaging with a spatial resolution of 1.0 μm. IR spectra show two regions with main absorption bands at 1,280–1,580 cm−1 and 900–1,120 cm−1. Bands at 1,450 and 1,396 cm−1 correspond to the asymmetric stretching of CO 3 2− , indicating two different forms of CaCO3; the 900–1,120 cm−1 group of bands is assigned to Si-O stretching vibrations indicating C–S–H phases. The ratio of the integral absorbance of these two main regions of absorption bands shows an inhomogeneous spatial distribution in the reaction rim. From this variation we conclude that the reaction rims consist of areas containing both calcite and aragonite in addition to C–S–H phases and areas containing aragonite and C–S–H phases, the latter with a lower Ca/Si ratio and a higher degree of polymerization. SiO2 gel is present in both areas.

Structural studies on a stuffed framework high pressure polymorph of CaAl2O4

The crystal structure of a high pressure polymorph of CaAl2O4 has been investigated using laboratory X-ray powder diffraction data. The compound was prepared from a ceramic precursor in a piston cylinder apparatus at 800 °C and 20 kbar. The quenchable so-called phase-II of monocalcium aluminate adopts the monoclinic space group P21/c (a = 7.97187(7) Å, b = 8.62844(7) Å, c = 10.26276(10) Å, β = 94.801(4)°, V = 703.44(1) Å3, Z = 4, Dcalc = 2.99 g/cm3). The main structural features of the high-P form are layers of AlO4-tetrahedra perpendicular to the a-axis. Stacking of the layers parallel to [100] results in a three-dimensional framework containing channels, where the calcium cations are accommodated for charge compensation. Individual sheets can be described as being built by condensation of ditrigonally-shaped six-membered rings (S6R). The sequence of up (U) and down (D) pointing apices within a single ring is UUDUDD. The calcium cations in the tunnels are coordinated by six to seven oxygen ligands. Topologically the framework belongs to the group of three-dimensional 4-connected nets. The coordination sequences (4-11-24-41-63-91-123-160-202-249) and the vertex symbols (61.61.62.41.61.61) are identical for the four crystallographically independent Al-atoms. The framework density has a value of 17.06 T-atoms/1000 Å3.

From phosphates to silicates and back: An experimental study on the transport and storage of phosphorus in eclogites during uplift and exhumation

Abstract High P-T experiments have shown that in major rock types such as MORBs, peridotites, and pelitic sediments, increasing P (and T) leads to a gradual transfer of P from phosphates mostly represented by apatite to silicates with garnet as most important silicate P carrier. This is due to the formation of a Na3Al2(PO4)3 phase component in garnet via [8]Na[4]P[8]M-î[4]Si_1 that is strongly P-, and to a lesser extent T-dependent and creates garnets with significant P and Na at P as low as 2–3 GPa. Based on this experimental evidence, one would expect to routinely find P-Na-rich garnets in UHP-rocks with a wide range in composition. With very few exceptions, however, this is not the case. This discrepancy indicates that both P and Na are effectively released from garnet and re-distributed within the garnet matrix during uplift and exhumation. To explore the mechanisms of this P-Na release, P-Na-rich garnet pre-synthesized at 7 GPa and 1200 °C, containing 0.7 wt% P2O5 and 0.3 wt% Na2O, was exposed to P-T conditions of 2 GPa and 800–1000 °C in a simplified, H2O-bearing, model eclogitic bulk composition. The experiments show that at subsolidus temperatures of 850–975 °C, and in the presence of a hydrous fluid, apatite quickly forms from garnet breakdown involving consumption of coexisting quartz and clinopyroxene. The apatites usually appear as rounded to lath-shaped isolated grains scattered in the garnet + clinopyroxene ± orthopyroxene + quartz + rutile matrix. More rarely, single apatite inclusions, or clusters of inclusions, may form in clinopyoxene or garnet. The observed apatite grain size is in the range ≤ ~1 × 1 to 24 × 6 μm with the largest grains occasionally containing clinopyroxene inclusions. Combined garnet breakdown and neo-formation, using pre-existing garnet as a nucleation site, may form zoned garnets with Na-P-depleted and Ti-enriched rims that represent an approach to a garnet composition typical for mid- to shallow crustal P-T conditions. Partial melting experiments indicate that eclogites containing P-rich garnet may produce P-rich and apatite-undersaturated melts for moderately SiO2-rich melt compositions. These melts can crystallize abundant apatite during solidification and, thus, would be effective agents for P-extraction during partial melting. Due to their very low apatite saturation concentration, the P-transport and storage capacity of granitic melts would be much more limited. An unexpected finding of this study are the substantial P and Mg contents in kyanite with 0.17–0.20 wt% P2O5 and 0.20–0.56 wt% MgO, respectively. The combined P-Mg incorporation into kyanite is consistent with the coupled substitution [4]Si4+ + [6]Al3+ = [4]P5+ + [6]Mg2+ and with a strong preference of P for orthosilicate structures. The results of this study suggest that some to even all of the apatite now present in eclogites that underwent deep subduction formed by chemical adjustment of the eclogite garnet to decreasing Na3Al2(PO4)3 solubility during uplift and exhumation. This results in the appearance of apatite as a new phase in a hitherto apatite-free assemblage. Rapid transport to the surface and/or a lack of suitable reactants can suppress this re-equilibration and explain the occasionally high P- and Na-contents of eclogitic garnet-inclusions in diamond or garnets from diamondiferous eclogites sampled by kimberlites. Similarly, concurrent decreasing Ti-solubility in garnet may lead to rutile-saturation in eclogites that did not contain this phase under peak-P conditions as evidenced by the joint occurrence of (oriented) rutile and apatite inclusions in the eclogitic garnets. For the application of geothermobarometry, this delayed Ti (+P) saturation is important to be kept in mind.

Re-equilibration of a high-pressure metamorphic fluid: evidence from tourmaline-, apatite- and quartz-hosted fluid inclusions in an Eoalpine eclogite from the Eastern Alps

Fluid inclusions in tourmaline, apatite and quartz from an eclogite of the Polinik Complex, as part of the Koralpe-Wolz high-pressure (H P ) nappe system (Eastern Alps), have been investigated. All three minerals are interpreted as part of the eclogite- to post-eclogite-facies mineral assemblage which formed during Cretaceous Eoalpine metamorphism at ~ 20 kbar and ~ 650°C. Tourmaline, apatite and quartz contain two types of fluid inclusions: (1) a fluid in the system of H 2 O–NaCl–CaCl 2 –CO 2 –CH 4 –N 2 system, and (2) a fluid in the system of H 2 O–NaCl–CaCl 2 ± MgCl 2 system. Type (1) dominates in tourmaline, and type (2) in apatite and quartz. Fluid inclusions of type (1) occur individually and as clusters. In addition to isolated inclusions, type (2)-inclusions also arranged along intragranular/transgranular fluid inclusion planes – but not at grain contacts. The clear distinction in their textural occurrence enables the reconstruction of the H P fluid evolution from a supposed precursor fluid to stages of retrogression accompanied with re-equilibration and preferential loss of the aqueous fluid phase along a proposed exhumation path. Calculated fluid densities of type (1) indicate conditions that reach 14 kbar in tourmaline and 12 kbar in quartz when linked with peak temperatures of ~ 650°C. Conditions are interpreted as minimum conditions during formation of the host minerals. Fluid-inclusion densities from the studied aqueous system type (2) are coherent in the studied host minerals and reach pressures of about 8–10 kbar. A possible reason for the differences in estimated pressure between type (1) and type (2) is, beside textural arguments, the preferred loss of water during recrystallization of the studied minerals at post-peak stages. This hypothesis is supported by the same fluid chemistry of the aqueous phase (H 2 O–NaCl–CaCl 2 ± MgCl 2 ) in type (1)- and type (2)-inclusions. Additionally, different wetting behaviour of fluids containing dominantly polar [type (1)] and non-polar [type (2)] species promoted recrystallization, especially in quartz, and led to widespread decrepitation of large fluid inclusions in quartz during decompression. Hence, the fluid evolution documented in the Polinik eclogites can be reconstructed through effects of physical and chemical changes of the host minerals after peak metamorphism.

High-pressure synthetic (Na0.97Mg0.03)(Mg0.43Fe0.173+Si0.40)Si2O6, with six-coordinated silicon, isostructural with P2/n omphacite

The title compound, (sodium magnesium) [magnesium iron(III) silicon] disilicate, (Na0.97Mg0.03)(Mg0.43Fe0.17 3+Si0.40)Si2O6, is isotypic with ordered P2/n omphacite. Its structure is characterized by single chains of corner-sharing SiO4 tetrahedra, extending along the c axis, which are crosslinked by bands of edge-sharing octahedra (site symmetry 2), statistically occupied by (Mg2+ + Fe3+ + Si4+). Between the bands built up of the octahedra are two non-equivalent highly distorted six-coordinated sites (site symmetry 2), statistically occupied by (Na + Mg). In contrast to omphacites, the great differences in size and charge between Mg2+ and Si4+ result in complete, rather than partial, ordering of Mg and Si into two distinct octahedral sites, whereas Fe3+ is disordered between the two sites. The octahedron filled by (Mg + Fe) is larger and markedly more distorted than that occupied by (Si + Fe). The average (Mg + Fe)—O and (VISi + Fe)—O bond lengths are 2.075 and 1.850 Å, respectively.

Genesis of chromium-rich kyanite in eclogite-facies Cr-spinel-bearing gabbroic cumulates, Pohorje Massif, Eastern Alps

Abstract Natural kyanites with Cr2O3 contents >1 wt% are very rare and known only from high-P environments, for example, eclogite-facies gabbroic cumulates containing Cr-spinel from the Pohorje Massif, Eastern Alps, Slovenia. In these rocks, turquoise-colored Cr-rich kyanites are present in two different textural types. A first type has formed as blocky crystals of several hundred micrometers in size around clusters of small drop-like Cr-spinels. This kyanite shows a highly irregular Cr distribution and may contain up to 15.6 wt% Cr2O3, which is one of the highest Cr2O3 contents reported so far. A second type is present as part of reaction coronas around large red-brownish Cr-spinel and forms deep-blue needle-like crystals that rarely exceed 100 μm in size. This kyanite contains up to 8.2 wt% Cr2O3 and is associated with Cr-rich corundum (≤9.1 wt% Cr2O3) and Cr-Al-rich pargasite (≤3.9 wt% Cr2O3). The formation of kyanite around Cr-spinel droplets is interpreted to be the result of increasing P-T conditions during prograde metamorphism where Cr-spinel and plagioclase or quartz react to Crkyanite, ±garnet, ±omphacite. In contrast, the formation of kyanite associated with Cr-rich corundum and Cr-rich pargasite within coronas around Cr-spinel occurred in an early stage of the retrogressive evolution of the gabbroic cumulates at eclogite-facies conditions of ~2.5 GPa and 750–800 °C triggered by the influx of H2O-rich fluids. The driving force for developing these coronas was an increase in the chemical potential of silica caused by the infiltrating hydrous fluid phase. P-T estimates using matrix mineral assemblage place the peak metamorphic conditions close to the quartz/coesite transition with temperatures in the range of 750–810 °C and pressures of ~2.9 GPa.

Na8.25Y1.25Si6O18 and its family of zwölfer ring silicates

Abstract The crystal structure of a sodium yttrium silicate with nominal composition Na9YSi6O18 was determined from laboratory X-ray powder diffraction. The actual composition of the crystals was found to be Y-enriched, and a corrected nominal composition of Na8.25Y1.25Si6O18 was established. The compound is cubic with space group Pa−3 and unit cell parameter of a = 15.120 Å. The structure consists of twelve-membered rings of corner-connected SiO4-tetrahedra, which are arranged around the 3-fold axes of the structure. It is isostructural with K4SrGe3O9 and Na4CaSi3O9. A profound disorder results from the absence of any stochiometric composition. The existence of a simultaneous occurrence of two different conformations of the rings relates to the various ordered and disordered cation positions in the structure. Micro-Raman spectra obtained from the polycrystalline material confirm the observed disorder. A model explaining the conformational variations appearing in this structure type was derived from a systematic comparison of isostructural compounds.