ALBERT

Description: To evaluate compaction and interstitial melt expulsion during cumulate formation, a 20 m cumulate section including the UG2 and UG3 chromitites from a 264 m drill core through the Upper Critical Zone of the Bushveld Complex (South Africa) has been studied. The cumulates in the studied section are as follows: 3 m plagioclase pyroxenite to pyroxenite, pegmatoid footwall pyroxenite at the lower contact to UG2, 0·7 m UG2 chromitite, 6·8 m pyroxenite, 0·24 m UG3 chromitite, 2·0 m plagioclase-rich pyroxenite changing locally to norite, the two 5 cm leader stringers UG3a and UG3b, and 7 m total of olivine pyroxenites grading into plagioclase pyroxenites. All pyroxenites are dominated by orthopyroxene (opx) and the cumulate sequence is topped by mottled anorthosite grading into norite. Stratigraphic concentrations of major and trace elements of 52 bulk-rock samples were determined. Bulk-rock Mg-numbers are 0·79–0·81 throughout the silicate cumulate units, and 0·40–0·46 in the chromitite layers. The stratigraphic distribution of six incompatible trace elements (K, Rb, Ba, Cs, Zr and Th) has been used to determine the amount of trapped liquid ( F TL ) or paleo-porosity in the cumulate rocks. Final porosities (volume fractions), based on averages from the six trace elements, are 0·06–0·33 in the pyroxenites. In chromitite layers, trapped melt fractions of 0·12–0·36 are calculated from incompatible trace element concentrations, but bulk SiO 2 concentrations and X-ray tomography yield 0·04–0·17 higher porosities. Hence, the bulk silicate fraction in the chromitites may not necessarily correspond to the trapped liquid fraction, as poikilitic opx was crystallizing while the silicate melt still equilibrated. Using a previously derived experiment-based model for compaction time scales, gravitationally driven chemical compaction in the UG2–UG3–pyroxenite section is calculated to occur within 1–10 years. This time frame corresponds to the times necessary to cool a 20 m layer by 10–50°C, the temperature interval argued to encompass the liquidus and almost complete solidification. Compaction within a decade can in fact easily develop the paleo-porosities indirectly observed today and is probably stopped by crystallization of the interstitial liquid. Contrary to previous assertions, melt expulsion from the cumulate pile does not hinder compaction; calculated permeabilities would allow for the migration of an order of magnitude higher amount of melt than has to be expelled from the 20 m pile of cumulate. The pegmatoid zones in the chromitite footwalls enriched in incompatible trace elements are consistent with a collection of interstitial melts expelling from the underlying compacting pyroxenites. Their entrapment below the chromitite layers suggests that these act as permeability barriers. This is in part due to their finer grain size compared with the pyroxenites, but is mainly due to the crystallization of large poikilitic opx during compaction.

Description: Although alkali-alkali earth carbonates have not been reported from mantle-derived xenoliths, these carbonates may have a substantial role in mantle metasomatic processes through lowering melting temperatures. On the Na 2 Mg(CO 3 ) 2 –K 2 Mg(CO 3 ) 2 join only the Na-end-member eitelite ( R space group), was reported in nature. The K-end-member ( R m ) readily hydrates even at low temperatures, therefore, only baylissite, K 2 Mg(CO 3 ) 2 ·4H 2 O, has been observed. Because of the role of (K,Na)Mg-double carbonates in mantle metasomatism, we performed high P-T experiments on K 2 Mg(CO 3 ) 2 , (K 1.1 Na 0.9 ) 2 Mg(CO 3 ) 2 , and Na 2 Mg(CO 3 ) 2 . Structure refinements were done upon compression of single crystals from 0 to 9 GPa at ambient temperature employing synchrotron radiation. Fitting the compression data to the second-order Birch-Murnaghan EoS resulted in V 0 = 396.2(4), 381.2(5), and 347.1(3) Å 3 and K 0 = 57.0(10), 54.9(13), and 68.6(13) GPa for K 2 Mg(CO 3 ) 2 , (K 1.1 Na 0.9 ) 2 Mg(CO 3 ) 2 , and Na 2 Mg(CO 3 ) 2 , respectively. These compressibilities are lower than those of magnesite and dolomite. The KMg-double carbonate transforms into a monoclinic polymorph at 8.05 GPa; the high- P phase is 1% denser than the low- P polymorph. The NaMg-double carbonate has a phase transition at ~14 GPa, but poor recrystallization has prevented structure refinement. The parameters for a V-T EoS were collected at 25–600 °C and ambient pressure and are α 0 = 14.31(5) x 10 –5 K –1 and 16.73(11) x 10 –5 K –1 for K 2 Mg(CO 3 ) 2 and Na 2 Mg(CO 3 ) 2 , respectively. Moreover, fitting revealed an anisotropy of thermal expansion along the a - and c -axis: α 0 ( a ) = 2.84(6) x 10 –5 and 4.78(5) x 10 –5 K –1 and α 0 ( c ) = 10.47(11) x 10 –5 and 8.72(5) x 10 –5 K –1 for K 2 Mg(CO 3 ) 2 and Na 2 Mg(CO 3 ) 2 , respectively.

Description: The more than 500 fossil Ca-carbonatite occurrences on Earth are at odds with the only active East African Rift carbonatite volcano, Oldoinyo Lengai (Tanzania), which produces Na-carbonatite magmas. The volcano’s recent major explosive eruptions yielded a mix of nephelinitic and carbonatite melts, supporting the hypothesis that carbonatites and spatially associated peralkaline silicate lavas are related through liquid immiscibility. Nevertheless, previous eruption temperatures of Na-carbonatites were 490–595 °C, which is 250–450 °C lower than for any suitable conjugate silicate liquid. This study demonstrates experimentally that moderately alkaline Ca-carbonatite melts evolve to Na-carbonatites through crystal fractionation. The thermal barrier of the synthetic Na-Ca-carbonate system, held to preclude an evolution from Ca-carbonatites to Na-carbonatites, vanishes in the natural system, where continuous fractionation of calcite + apatite leads to Na-carbonatites, as observed at Oldoinyo Lengai. Furthermore, saturating the Na-carbonatite with minerals present in possible conjugate nephelinites yields a parent carbonatite with total alkali contents of 8–9 wt%, i.e., concentrations that are realistic for immiscible separation from nephelinitic liquids at 1000–1050 °C. Modeling the liquid line of descent along the calcite surface requires a total fractionation of ~48% calcite, ~12% apatite, and ~2 wt% clinopyroxene. SiO 2 solubility only increases from 0.2 to 2.9 wt% at 750–1200 °C, leaving little leeway for crystallization of silicates. The experimental results suggest a moderately alkaline parent to the Oldoinyo Lengai carbonatites and therefore a common origin for carbonatites related to alkaline magmatism.

Description: Quantifying the time scales of magmatic differentiation is critical for understanding the rate at which silicic plutonic and volcanic rocks form. Directly dating this process is difficult because locations with both clear evidence for fractional crystallization and the accessory phases necessary for radiometric dating are rare. Early zircon saturation, however, appears to be characteristic of many high-K, arc-related melts due to their generally elevated initial Zr concentrations. Thus, high-K plutonic series are ideal candidates to study the time scales of magmatic differentiation using zircon U-Pb geochronology. This study focuses on the Dariv Igneous Complex in western Mongolia where early saturation of zircon in a suite of cogenetic, upper crustal (〈0.5 GPa) igneous rocks ranging from ultramafic cumulates to evolved granitoids allows us to date magmatic differentiation. Crystallization ages from six samples across the sequence indicate that magmatic fractionation from a basalt to high-silica (〉65 wt% SiO 2 ) melt occurred in ≤590 ± 350 k.y. This estimate is greater than modeled time scales of conductive cooling of a single intrusion and physical segregation of minerals from a melt, suggesting that continued influx of heat through magmatic activity in the complex may have prolonged cooling and thus time scales associated with the production of silica-enriched melts.

Description: Our understanding of the mode of transfer and evolution of arc magmas in the lower arc crust is limited by the accessibility of arc roots, which are mainly documented by remote geophysical methods. At the same time, the fractionation processes of primitive parental melts defining a liquid line of descent from basalt to dacite are well defined by experimental petrology. However, the structural evidence for transfer of magmas evolving during their ascent remains basically uncharacterized. The Sapat Complex represents a lower crust segment of the exhumed Kohistan paleo-island arc and exposes kilometer-sized pyroxenite bodies that grew at the expense of host metagabbroic sill sequences. The largest of these pyroxenite bodies are mainly composed of wehrlite to olivine-clinopyroxenite, whereas the smaller bodies show a sequence of cumulative rocks, from ol-clinopyroxenite through various gabbros to tonalite. Inside the bodies, vertical magmatic and reactional structures indicate magma ascent accompanied by cumulate formation. Altogether, cumulates document the evolution of an initially primitive basaltic melt (at ~7 kbar) that contained ≥5 wt % H 2 O. After cotectic olivine and clinopyroxene fractionation, the appearance of hornblende at the expense of clinopyroxene marks a stepping stone in the melt evolution. From this point, the appearance of orthopyroxene and hornblende at the expense of olivine drives the magma towards an andesitic composition, from which the crystallization of An-rich plagioclase and hornblende drives the melt to evolve further. During peritectic hornblende crystallization fluid-precipitated assemblages occur showing that the melts have reached water-saturation while they were crystallizing and percolating, thus degassing H 2 O-rich fluids. Structural observations, mineral and bulk-rock compositions, and calculated seismic P-wave velocities identify the ultramafic pipe-shaped bodies as magmatic conduits in which melt ascended from the mantle through the lower crust to feed upper crustal magma chambers and volcanic systems.

Description: Compound-specific radiocarbon analysis (CSRA) of benzene polycarboxylic acids (BPCAs) yields molecular-level, source-specific information necessary to constrain isotopic signatures of pyrogenic carbon. However, the purification of individual BPCAs requires a multistep procedure that typically results in only microgram quantities of the target analyte(s). Such small samples are highly susceptible to contamination by extraneous carbon, which needs to be minimized and carefully accounted for in order to yield accurate results. Here, we undertook comprehensive characterization and quantification of contamination associated with molecular radiocarbon (14C) BPCA analyses through systematic processing of multiple authentic standards with both fossil and modern 14C signatures at various concentrations. Using this approach, we precisely apportion the contribution of extraneous carbon with respect to the four implemented subprocedures. Assuming a constant source and quantity of extraneous carbon we correct and statistically evaluate uncertainties in resulting 14C data. Subsequently, we examine the results of triplicate analyses of reference materials representing four different environmental matrices (sediment, soil, aerosol, riverine natural organic matter) and apportion their BPCA sources in terms of carbon residues derived from biomass or fossil fuel combustion. This comprehensive approach to CSRA facilitates retrieval of robust 14C data, with application in environmental studies of the continuum of pyrogenic carbon.