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: Phase assemblages, melting relations and melt compositions of a dry carbonated pelite (DG2) and a carbonated pelite with 1·1 wt % H 2 O (AM) have been experimentally investigated at 5·5–23·5 GPa and 1070–1550°C. The subsolidus mineralogies to 16 GPa contain garnet, clinopyroxene, coesite or stishovite, kyanite or corundum, phengite or potassium feldspar (≤8 GPa with and without H 2 O, respectively), and then K-hollandite, a Ti phase and ferroan dolomite/Mg-calcite or aragonite + ferroan magnesite at higher pressures. The breakdown of clinopyroxene at 〉16 GPa causes Na-rich Ca-carbonate containing up to 11 wt % Na 2 O to replace aragonite and leads to the formation of an Na-rich CO 2 fluid. Further pressure increase leads to typical Transition Zone minerals such as the CAS phase and one or two perovskites, which completely substitute garnet at the highest investigated pressure (23·5 GPa). Melting at 5·5–23·5 GPa yields alkali-rich magnesio-dolomitic (DG2) to ferro-dolomitic (AM) carbonate melts at temperatures 200–350°C below the mantle geotherm, lower than for any other studied natural composition. Melting reactions are controlled by carbonates and alkali-hosting phases: to 16 GPa clinopyroxene remains residual, Na is compatible and the magnesio- to ferro-dolomitic carbonate melts have extremely high K 2 O/Na 2 O ratios. K 2 O/Na 2 O weight ratios decrease from 26–41 at 8 GPa to 1·2 at 16 GPa when K-hollandite expands its stability field with increasing pressure. At 〉16 GPa, Na is repartitioned between several phases, and again becomes incompatible as at 〈3 GPa, leading to Na-rich carbonate melts with K 2 O/Na 2 O ratios 1. This leaves the pressure interval of c . 4–15 GPa for ultrapotassic metasomatism. Comparison of the solidus with typical subducting slab-surface temperatures yields two distinct depths of probable carbonated pelite melting: at 6–9 GPa where the solidus has a negative Clapeyron slope between the intersection of the silicate and carbonate melting reactions at ~5 GPa, and the phengite or potassium feldspar stability limit at ~9 GPa. The second opportunity is related to possible slab deflection along the 660 km discontinuity, leading to thermal relaxation and partial melting of the fertile carbonated pelites, thus recycling sedimentary CO 2 , alkalis and other lithophile and strongly incompatible elements back into the mantle.

Description: Carbonatite and silicate rocks occurring within a single magmatic complex may originate through liquid immiscibility. We thus experimentally determined carbonatite/silicate melt partition coefficients ( D carbonate melt/silicate melt , hereafter D ) for 45 elements to understand their systematics as a function of melt composition and to provide a tool for identifying the possible conjugate nature of silicate and carbonatite magmas. Static and, when necessary, centrifuging piston cylinder experiments were performed at 1–3 GPa, 1150–1260°C such that two well-separated melts resulted. Bulk compositions had Na K, Na ~ K, and Na K; for the latter we also varied bulk H 2 O (0–4 wt %) and SiO 2 contents. Oxygen fugacities were between iron–wüstite and slightly below hematite–magnetite and were not found to exert significant control on partitioning. Under dry conditions alkali and alkaline earth elements partition into the carbonatite melt, as did Mo and P ( D Mo 〉8, D P = 1·6–3·3). High field strength elements (HFSE) prefer the silicate melt, most strongly Hf ( D Hf = 0·04). The REE have partition coefficients around unity with D La/Lu = 1·6–2·3. Transition metals have D 〈 1 except for Cu and V ( D Cu ~ 1·3, D V = 0·95–2). The small variability of the partition coefficients in all dry experiments can be explained by a comparable width of the miscibility gap, which appears to be flat-topped in our dry bulk compositions. For all carbonatite and silicate melts, Nb/Ta and Zr/Hf fractionate by factors of 1·3–3·0, in most cases much more strongly than in silicate–oxide systems. With the exception of the alkalis, partition coefficients for the H 2 O-bearing systems are similar to those for the anhydrous ones, but are shifted in favour of the carbonatite melt by up to an order of magnitude. An increase of bulk silica and thus SiO 2 in the silicate melt (from 35 to 69 wt %) has a similar effect. Two types of trace element partitioning with changing melt composition can be observed. The magnitude of the partition coefficients increases for the alkalis and alkaline earths with the width of the miscibility gap, whereas partition coefficients for the REE shift by almost two orders of magnitude from partitioning into the silicate melt ( D La = 0·47) to strongly partitioning into the carbonatite melt ( D La = 38), whereas D La / D Lu varies by only a factor of three. The partitioning behavior can be rationalized as a function of ionic potential ( Z / r ). Alkali and alkaline earth elements follow a trend, the slope of which depends on the K/Na ratio and H 2 O content. Contrasting the sodic and potassic systems, alkalis have a positive correlation in D vs Z / r space in the potassic case and Cs to K partition into the silicate melt in the presence of H 2 O. For the divalent third row transition metals on the one hand and for the tri- and tetravalent REE and HFSE on the other, two trends of negative correlation of D vs Z / r can be defined. Nevertheless, the highest ionic strength network-modifying cations (V, Nb, Ta, Ti and Mo) do not follow any trend; understanding their behavior would require knowledge of their bonding environment in the carbonatite melt. Strong partitioning of REE into the carbonatite melt ( D REE = 5·8–38·0) occurs only in H 2 O-rich compositions for which carbonatites unmix from evolved alkaline melts with the conjugate silicate melt being siliceous. We thus speculate that upon hydrous carbonatite crystallization, the consequent saturation in fluids may lead to hydrothermal systems concentrating REE in secondary deposits.

Description: The origin of pyroxenites and their relation to melt migration in the mantle have been investigated in two pyroxenite-rich zones in the Beni Bousera massif. Based on combined field, microtextural, mineralogical and geochemical observations, the pyroxenites were separated into four types. Type-I Cr-diopside websterites contain bright green diopside and have primitive bulk Ni, Cr and Mg-number. Their trace element systematics are characterized by slight light rare earth element (LREE) enrichment compared with the middle (MREE) and heavy (H)REE, and negative high field strength element (HFSE) anomalies in bulk-rock and mineral compositions suggesting that they result from melting of metasomatized mantle. Trace element concentrations of melts calculated to be in equilibrium with Type-I cpx have a subduction-like signature and show a close similarity to certain lavas erupted in the Alboran Basin. Calculated mineral equilibration temperatures of ~1200 to 1350°C are close to the basalt liquidus and higher than for other pyroxenite types in Beni Bousera, which generally yield 〈1100°C. Type-II spinel websterites are also primitive, but contain augitic clinopyroxene; their whole-rock compositions are characterized by high Ti, Ni, and Mg-number, intermediate Cr and trace element patterns with LREE depletion over the MREE and HREE. Type-III garnet pyroxenites, which include the famous diamond-pseudomorph-bearing garnet pyroxenites, are more evolved than Types-I and -II and have low and variable Mg-number correlating with an Fe-enrichment trend. High bulk-rock and garnet HREE to LREE ratios result from high-pressure fractionation of garnet and augitic cpx at calculated pressures of 〉45 to 20–30 kbar. Type-III pyroxenites display strong variations of LREE and HFSE depletion and strong bulk Nb/Ta fractionation. Calculated melts in equilibrium with augitic cpx are variably enriched in incompatible trace elements similar to intraplate basalts. Type-IV pyroxenites are composed of green diopside, opx, garnet and plagioclase and/or spinel. Whole-rocks have high Na 2 O, CaO and Al 2 O 3 concentrations and high Mg-number, are HREE depleted, and have positive Eu and Sr anomalies. Garnets are characterized by low HREE/MREE and positive Eu anomalies. The absence of bulk-rock HREE enrichment indicates a metamorphic origin for this garnet, which is corroborated by the presence of Al-rich metamorphic spinels. Relict magmatic plagioclase indicates a shallower (〈10 kbar) crustal origin for these pyroxenites. Their metamorphic assemblage yields temperatures and pressures of 800–980°C and 14 kbar, indicating a pressure increase during the metamorphic overprint. The whole-rock geochemistry of Type-IV pyroxenites is comparable with that of rocks from the lower crustal section of the Kohistan (northern Pakistan) paleo-arc, indicating a possible origin of these rocks as cumulates in the deeper arc crust and subsequent delamination into the underlying mantle.

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.