Supersolidus Conditions Research Articles

Coupled U–Pb and 40Ar/39Ar chronology of late‐stage intrusions at Elba Island (Italy) supports late Miocene long‐lived magma reservoirs in the Tyrrhenian upper crust

AbstractThe late Miocene Monte Capanne and Porto Azzurro plutons are investigated by means of coupled U–Pb zircon and 40Ar/39Ar white mica dating to test the occurrence of long‐lived magmatic systems in the upper crust. Zircon crystallized for >1 Ma in both plutonic systems, with supersolidus conditions overlapping for ~220 ka indicating previously unrecognized co‐existence of the two reservoirs. The development of the Porto Azzurro high T‐aureole is post‐dated by continuous igneous zircon crystallization until ~6.0 Ma. By linking crystallization to post‐emplacement cooling of late‐stage pulses in both western and eastern Elba we constrain long‐lived sizeable reservoirs (possibly the same reservoir) in the Tyrrhenian upper crust between ~8 and 6 Ma.

Consistency of the activity–composition models of Holland, Green, and Powell (2018) with experiments on natural and synthetic compositions: A comparative study

AbstractThe recent publication of new activity–composition models by Holland, Green and Powell (2018; Journal of Petrology 59: 881–900), with a melt model calibrated for source compositions ranging from peridotitic to granitic, opens the door to the modelling of multiple petrogenetic processes at supersolidus conditions in which the composition of the melt phase changes considerably, without having to change the melt model. This melt model is also the first one using the internally consistent thermodynamic databases published by T. Holland and R. Powell that contains TiO2 and Fe2O3, further expanding the application of this model to more realistic geological scenarios. The accompanying mineral models are also the first in containing some minor elements, like TiO2 in garnet and K2O in clinopyroxene. Consequently, it is relevant to test the applicability of these new models to a large P–T–X range of conditions before they can be used in full. Thermodynamic calculations made with the software Perple_X using these models were compared to experimental results, namely the modal proportions and the composition of the melt and several mineral solution phases. The experiments chosen for the comparative study covered a wide range of source compositions (from mafic to felsic), pressure (from 0.3 to 2.1 GPa), temperature (from 700 to 1,150°C) and total and added water content (structural water: 0.15–1.48 wt%; added water: 0–8 wt%; total water: 0.15%–8.15%). The results indicate that the extended melt model reproduces well the composition of the experimental melts, with an inverse correlation between component amount and fit: the best match is found for SiO2 (−0.8% on average) and the worst match is found for those elements with the lowest amounts, TiO2 and MgO (+241% and +235% on average, respectively; values indicate calculated minus experimental, times 100 and divided by experimental). The TiO2 content in the melt model increases dramatically with increasing pressure, from +90% for P < 1.5 GPa to +593% for P > 1.5 GPa. No comparison was made on the Fe2O3 content, as the published iron contents of the experimental melts were always reported as FeOt. In some cases, there is a substantial mismatch in the modal proportions between experiments and calculations, with the reactant phases less abundant and product phases more abundant in the calculations, an effect that is attributed to kinetic effects in the experiments and to the selected clinoamphibole model. Finally, the extended melt model was compared to the tonalitic melt model of Green et al. (2016; Journal of Metamorphic Geology, 34: 845–869). Both melt models produce very similar results for SiO2, Al2O3, Na2O and K2O, with slightly better results for the tonalitic melt model in FeOt and MgO and for the extended melt model in CaO. No comparison is made on TiO2 because the tonalitic melt model does not include this component. In summary, the new activity–composition models represent a significant contribution to thermodynamic calculations on the evolution of siliceous magmas where their composition, temperature and pressure changes substantially.

Thin lithosphere beneath the central Appalachian Mountains: Constraints from seismic attenuation beneath the MAGIC array

The passive margin of the eastern coast of the United States is known to be geologically active, with recently rejuvenated topography, intraplate seismicity, and volcanism of Eocene age. This study uses seismic data from the Mid-Atlantic Geophysical Integrative Collaboration (MAGIC) experiment to constrain lateral variations in the attenuation of teleseismic P waves beneath the central Appalachian Mountains to shed light on the structure and dynamics of the upper mantle at this “active” passive margin. We use a Monte Carlo approach to estimate variations in attenuation along with both data and model uncertainties. The quality factor of the upper mantle dramatically decreases over a distance of less than 50 km on the western side of the central Appalachian Mountains, where a low-velocity anomaly has been previously inferred. Extrinsic factors such as scattering or focusing are rejected as explanations for the observations on the basis of finite-difference waveform modeling experiments. The peak in attenuation beneath the crest of the Appalachian Mountains requires that near- to super-solidus conditions occur in the upper mantle and is co-located with volcanism of Eocene age. Our preferred interpretation is that the attenuation reflects the removal of the mantle lithosphere via delamination beneath the mountains, followed by ongoing small-scale convection.

Modelling the partial melting of metasediments in a low-pressure regional contact aureole: the effect of water and whole-rock composition

AbstractLow-pressure regional aureoles with steep metamorphic field gradients are critical to understanding progressive metamorphism in high-temperature metasedimentary rocks. Delicately layered pelitic and psammitic metasedimentary rocks at Mt Stafford, central Australia, record a greenschist- to granulite-facies Palaeoproterozoic regional aureole, associated with S-type granite plutons, reflecting metamorphism in the range 500–800 °C and at ∼3 kbar. The rocks experienced minimal deformation during metamorphism and partial melting. Partial melting textures evolve progressively along the steep metamorphic field gradient from the incipient stages of melting marked by cuspate grains with low dihedral angles, to melt proportions sufficient to form diatexite with schollen. Phase equilibria modelling in the NCKFMASHTO system for pelitic, semi-pelitic and high- and low-ferromagnesian psammitic samples quantitatively illustrates the dependence of partial melting on rock composition and water volume. Pelitic compositions are more fertile than psammitic compositions when the water content in the rocks is low, especially during the early stages of melting. The whole-rock ferromagnesian component additionally influences melt fertility, with ferromagnesian-rich psammite being more fertile than psammite with a lower ferromagnesian component. Subtle variations in free water content can result in obvious changes in melt volume but limited variation in melt composition. Distinct melting histories of pelitic and psammitic rocks inferred from field relationships may be partially attributed to potential differences in water volume retained to super-solidus conditions. Melt composition is more dependent on the rock composition than the variation in water content.

The significance of U–Pb zircon ages in zoned plutons: the case of the Flamenco pluton, Coastal Range batholith, northern Chile

Compositionally zoned plutons, both layered and concentrically arranged, provide granitic exposures where the mechanisms and timing of the magmatic emplacement processes can be studied. The importance of in-situ geochemical differentiation and the magma replenishment rates are revealed by geochemistry and field relations, together with the increasingly accurate U–Pb geochronology, which has promoted the knowledge about the pluton incremental assembly theories.The Flamenco pluton, located in the Coastal Range of northern Chile, is part of the Upper Triassic to Early Cretaceous Andean intrusives formed in the western active margin of South America, and present a normal zoned structure with mafic magmatic facies (mostly gabbros and Qtz-diorites) close to the contacts with the host metasediments, and tonalites, granodiorites and granites in the inner areas. A combined study of the field relations, geochemistry and zircon geochronology of the magmatic facies was applied to determine the emplacement sequence of the Flamenco pluton, revealing three distinguishable domains separated by metasedimentary septa. The SW area is constituted by mostly homogeneous leucocratic granodiorites that yielded an age of 213 Ma as the best estimation for their emplacement age. Distinctive geochemical characteristics, such as the absence of an Eu anomaly, the depletion in HREE, or the highest Sr, Sr/Y and Ce/Yb values among the granodioritic facies of the pluton, involve lower T and/or higher P conditions at the magmatic source according to experimental studies. These conditions were established during an early stage of the Andean magmatic arc building that is firstly defined here as Upper Triassic. The NW and E domains of the pluton were sequentially emplaced between 194 Ma and 186 Ma and both the field relations and the detailed geochronological results suggest that the mafic facies intruded latter in the emplacement sequence. To the NW, Qtz-dioritic and gabbroic externally emplaced pulses gave a younger crystallization age of 186.3 ± 1.8 Ma, and promoted the granoblastic textures and metamorphic zircon overgrowths that characterize the granodiorites located in the contact with the intermediate and felsic inner magmas, which yielded a best estimation of their emplacement age of 192 ± 1.5 Ma. On the other hand, in the eastern domain, magma-magma relations are observed between gabbros and previously intruded tonalites and granodiorites. Both the mafic and intermediate facies show two main subgroups of ages that yielded 194.7 ± 1.5 Ma to 188.3 ± 2.1 Ma and 193.1 ± 2.2 Ma to 185.5 ± 1.4 Ma respectively. These differences are related to the variations in the magmatic addition rates, which may extend the super-solidus conditions in the eastern domain of the magmatic reservoir as is confirmed by the wider age ranges yielded by these magmatic facies. Zircon overgrowths in the host rocks yield similar ages (around 220 Ma and 205 Ma) than the oldest results obtained in the intrusive facies, indicating that metamorphism correlates with the initial stages of plutonic emplacement.Geochronological results differ between 9 Myr and 41 Myr in the eight studied samples for non-inherited ages and gave very close mean ages (within analytical uncertainty) for all the intrusive units. However, we examine other characteristics such as zircon morphology, internal structure, geochemistry and statistical data to assess if the scattering of the geochronological data may be related to the different processes involved in the construction of the Flamenco pluton. We concluded that this detailed study of U–Pb zircon ages, including individual and significative groups of analyses, is useful to determine accurately the emplacement sequence and the genetic relation between the intrusive units, together with the evidences depicted by the geochemistry and field relations.

Trans-Amazonian U-Th-Pb monazite ages and P-T-d exhumation paths of garnet-bearing leucogranite and migmatitic country rock of the southeastern Tandilia belt, Rio de la Plata craton in Argentina

A garnet-bearing leucogranite and two country rocks from the Transamazonian Tandilia belt of the Rio de la Plata craton were studied in detail. The leucogranite contains garnet with homogeneous composition of pyr6(gros+andr)2spes5alm87. In a garnet-biotite migmatite, the core and rim compositions of garnet are pyr1.7(gros+andr)5spes5.6alm87.7 and pyr1.2(gros+andr)5.5spes6.7alm86.6, respectively. These compositions in a sillimanite-garnet-muscovite migmatite are pyr4(gros+andr)2.7spes2.7alm90.6 and pyr2.7(gros+andr)4spes3.2alm90.1, respectively. We used this information to decipher the P-T evolution of the rocks applying P-T and T-H2O pseudosections with the PERPLE_X computer software package taking into consideration deformational microstructures. The leucogranite records an isothermal decompression from 5.3 to 3.8kbar at 665°C. The garnet-biotite migmatite was exhumed from 5.5kbar at 630°C to 4.3kbar at 615°C and the sillimanite-garnet-muscovite migmatite from supersolidus conditions of 670°C and 3.6kbar to 625°C at 2.4kbar. Late andalusite formed in this rock.Seventy four analyses of 28 monazite grains of the country rocks yielded three groups of U-Th-Pb ages which were related to a collisional event (I: ca. 2.13–2.14Ga.), a postcollisional thermal overprint (II: ca. 2.01Ga) and slow cooling of the orogen (III: 1.80–1.90Ga). Inherited ages of 2.28 and 2.25Ga could refer to an early accretionary stage of the orogen. An age of 2.41Ga indicates the presence of recycled Siderian continental crust. Synkinematic crystallization of melts and the subsolidus development of an S2-foliation, demonstrated by deformational microstructures, occurred during the exhumation of the studied area from depths of ~18km to ~8km in the time interval 2.01–1.90Ga.

Geochemistry of continental subduction-zone fluids

The composition of continental subduction-zone fluids varies dramatically from dilute aqueous solutions at subsolidus conditions to hydrous silicate melts at supersolidus conditions, with variable concentrations of fluid-mobile incompatible trace elements. At ultrahigh-pressure (UHP) metamorphic conditions, supercritical fluids may occur with variable compositions. The water component of these fluids primarily derives from structural hydroxyl and molecular water in hydrous and nominally anhydrous minerals at UHP conditions. While the breakdown of hydrous minerals is the predominant water source for fluid activity in the subduction factory, water released from nominally anhydrous minerals provides an additional water source. These different sources of water may accumulate to induce partial melting of UHP metamorphic rocks on and above their wet solidii. Silica is the dominant solute in the deep fluids, followed by aluminum and alkalis. Trace element abundances are low in metamorphic fluids at subsolidus conditions, but become significantly elevated in anatectic melts at supersolidus conditions. The compositions of dissolved and residual minerals are a function of pressure-temperature and whole-rock composition, which exert a strong control on the trace element signature of liberated fluids. The trace element patterns of migmatic leucosomes in UHP rocks and multiphase solid inclusions in UHP minerals exhibit strong enrichment of large ion lithophile elements (LILE) and moderate enrichment of light rare earth elements (LREE) but depletion of high field strength elements (HFSE) and heavy rare earth elements (HREE), demonstrating their crystallization from anatectic melts of crustal protoliths. Interaction of the anatectic melts with the mantle wedge peridotite leads to modal metasomatism with the generation of new mineral phases as well as cryptic metasomatism that is only manifested by the enrichment of fluid-mobile incompatible trace elements in orogenic peridotites. Partial melting of the metasomatic mantle domains gives rise to a variety of mafic igneous rocks in collisional orogens and their adjacent active continental margins. The study of such metasomatic processes and products is of great importance to understanding of the mass transfer at the slab-mantle interface in subduction channels. Therefore, the property and behavior of subduction-zone fluids are a key for understanding of the crust-mantle interaction at convergent plate margins.

Phase equilibria constraints on melting of stromatic migmatites from Ronda (S. Spain): insights on the formation of peritectic garnet

AbstractStromatic metatexites occurring structurally below the contact with the Ronda peridotite (Ojén nappe, Betic Cordillera, S Spain) are characterized by the mineral assemblage Qtz+Pl+Kfs+Bt+Sil+Grt+Ap+Gr+Ilm. Garnet occurs in low modal amount (2–5 vol.%). Very rare muscovite is present as armoured inclusions, indicating prograde exhaustion. Microstructural evidence of melting in the migmatites includes pseudomorphs after melt films and nanogranite and glassy inclusions hosted in garnet cores. The latter microstructure demonstrates that garnet crystallized in the presence of melt. Re‐melted nanogranites and preserved glassy inclusions show leucogranitic compositions. Phase equilibria modelling of the stromatic migmatite in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2–O2–C (MnNCaKFMASHOC) system with graphite‐saturated fluid shows P–T conditions of equilibration of 4.5–5 kbar, 660–700 °C. These results are consistent with the complete experimental re‐melting of nanogranites at 700 °C and indicate that nanogranites represent the anatectic melt generated immediately after entering supersolidus conditions. The P–T estimate for garnet and melt development does not, however, overlap with the low‐temperature tip of the pure melt field in the phase diagram calculated for the composition of preserved glassy inclusions in garnet in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) system. A comparison of measured melt compositions formed immediately beyond the solidus with results of phase equilibria modelling points to the systematic underestimation of FeO, MgO and CaO in the calculated melt. These discrepancies are present also when calculated melts are compared with low‐T natural and experimental melts from the literature. Under such conditions, the available melt model does not perform well. Given the presence of melt inclusions in garnet cores and the P–T estimates for their formation, we argue that small amounts (<5 vol.%) of peritectic garnet may grow at low temperatures (≤700 °C), as a result of continuous melting reactions consuming biotite.

Effect of KCl on melting in the Mg2SiO4–MgSiO3–H2O system at 5 GPa

To examine the effect of KCl-bearing fluids on the melting behavior of the Earth’s mantle, we conducted experiments in the Mg2SiO4–MgSiO3–H2O and Mg2SiO4–MgSiO3–KCl–H2O systems at 5 GPa. In the Mg2SiO4–MgSiO3–H2O system, the temperature of the fluid-saturated solidus is bracketed between 1,200–1,250°C, and both forsterite and enstatite coexist with the liquid under supersolidus conditions. In the Mg2SiO4–MgSiO3–KCl–H2O systems with molar Cl/(Cl + H2O) ratios of 0.2, 0.4, and 0.6, the temperatures of the fluid-saturated solidus are bracketed between 1,400–1,450°C, 1,550–1,600°C, and 1,600–1,650°C, respectively, and only forsterite coexists with liquid under supersolidus conditions. This increase in the temperature of the solidus demonstrates the significant effect of KCl on reducing the activity of H2O in the fluid in the Mg2SiO4–MgSiO3–H2O system. The change in the melting residues indicates that the incongruent melting of enstatite (enstatite = forsterite + silica-rich melt) could extend to pressures above 5 GPa in KCl-bearing systems, in contrast to the behavior in the KCl-free system.

Phase relations in the system KAlSi 3O 8–NaAlSi 3O 8 at high pressure–high temperature conditions and their implication for the petrogenesis of lingunite

Multi-anvil experiments have been made with the composition (K 0.2Na 0.8)AlSi 3O 8 at 14–25 GPa and 1400–2400 °C. These experiments suggest that at subsolidus conditions the stable phase assemblages are hollandite-I + jadeite + stishovite (at pressures up to ∼20 GPa), hollandite-II + jadeite + stishovite (at pressures between ∼ 20 and ∼ 24 GPa), and hollandite-II + calcium ferrite-type NaAlSiO 4 + stishovite (at pressures > ∼ 24 GPa), where hollandite-I and hollandite-II are distinguished by their chemical compositions and phase diagram topology. The maximum NaAlSi 3O 8 in hollandite occurs at ∼ 22 GPa, and its fraction is positively correlated to temperature, attaining a value of ∼ 51 mol% at 2200 °C. At supersolidus conditions the solidus and liquidus phases around 14 GPa are hollandite-I and jadeite, respectively; at ∼ 22 GPa, however, the solidus phase is jadeite while the liquidus phase is either stishovite or hollandite-II with a very low fraction of NaAlSi 3O 8. These experimental results therefore suggest that lingunite with the hollandite structure and ∼ 90 mol% NaAlSi 3O 8, found in some meteorites, is a disequilibrium product. It is similarly proposed that the phase with the structure of jadeite but the composition of plagioclase, found along with lingunite, is also a disequilibrium phase.

Understanding cratonic flood basalts

The origin of continental flood basalts has been debated for decades. These eruptions often produce millions of cubic kilometers of basalt on timescales of only a million years. Although flood basalts are found in a variety of settings, no locale is more puzzling than cratonic areas such as southern Africa or the Siberian craton, where strong, thick lithosphere is breached by these large basaltic outpourings. Conventionally, flood basalts have been interpreted as melting events produced by one of two processes: 1) elevated temperatures associated with mantle plumes and/or 2) adiabatic-decompression melting associated with lithospheric thinning. In southern Africa, however, there are severe problems with both of these mechanisms. First, the rifting circumstances of several well-known basaltic outpourings clearly reflect lithospheric control rather than the influence of a deep-seated plume. Specifically, rift timing and magmatism are correlated with stress perturbations to the lithosphere associated with the formation of collisional rifts. Second, the substantial lithospheric thinning required for adiabatic decompression melting is inconsistent with xenolith evidence for the continued survival of thick lithosphere beneath flood basalt domains. As an alternative to these models, we propose a new two-stage model that interprets cratonic flood basalts not as melting events, but as short-duration drainage events that tap previously created sublithospheric reservoirs of molten basalt formed over a longer time scale. Reservoir creation/existence (Stage I) requires long-term (e.g. ≫ 1 Ma) supersolidus conditions in the sublithospheric mantle that could be maintained by an elevated equilibrium geotherm (appropriate for the Archean), a slow thermal perturbation (e.g. thermal blanketing or large-scale mantle upwelling), or a subduction-related increase in volatile content. The drainage event (Stage II) occurs in response to an abrupt stress change in the lithosphere, which leads to the initiation and propagation of lithospheric dikes. Such a model accounts for the short eruption time of flood basalts, the evidence for lithospheric control of the eruptions, and the continued survival of cratonic lithosphere following these magmatic events. The most notable consequence of this model is that it implies the existence of large reservoirs of magma, comparable to the eruption volume of flood basalts that have been, and are still likely, present beneath stable continental lithopshere.

STRUCTURAL PETROLOGY OF THE OTHRIS PERIDOTITE (GREECE): MELT IMPREGNATION IN MANTLE LITHOSPHERE

We present the results of a structural and petrological study of mantle rocks of the strongly dismembered Othris Ophiolite (Central Greece). The Othris Ophiolite is a remnant of the Mesozoic Tethyan ocean basin. It overlies a series of thrust sheets, which have the characteristics of an Upper Triassic-Jurassic passive margin sequence overlying Triassic volcanics associated with rifting. Earlier work on the mantle rocks of the Othris Ophiolite (e.g. Menzies, 1973) has revealed a wide range of compositions, from strongly depleted harzburgites to fertile plagioclase-lherzolites. Menzies (1973) interpreted these plagioclase-bearing rocks - which contain abundant melt-relics - as fertile mantle rocks undergoing incipient melting and incomplete meltextraction (‘MORB-source’ rocks). Our mapping of the mantle section of this ophiolite has confirmed its compositional heterogeneity. In addition, we find a large (micro)structural variability. The geometry of the mantle massif is dominated by a km-wide mantle shear zone, juxtaposing two blocks with distinct compositional and microstructural characteristics. The western block comprises coarse grained harzburgites with large (100 m scale) dunite bodies. These harzburgites contain interstitial orthopyroxene, and late stage amphibole. In contrast, the eastern block comprises harzburgitic tectonites, which are generally fine-grained. The top of this block consists of plagioclase- lherzolites and -harzburgites. The plagioclase-lherzolites contain abundant melt relics, such as plagioclase ± clinopyroxene in small lenses and gabbroic dykes. We favour an interpretation in which plagioclase and clinopyroxene in these rocks are the cumulate phases of a basaltic melt which impregnated previously depleted harzburgites, rather than that they represent the products of incipient melting of fertile peridotites. Some of these plagioclaseperidotites are heavily serpentinised, and intruded by (unaltered) doleritic dykes, suggesting that this (seafloor) serpentinisation pre-dated the cessation of magmatic activity, and that the plagioclase-peridotites were emplaced to shallow levels in the ocean. The composition of ophiolitic mantle sections are usually interpreted to reflect degree of melt-extraction. It is generally assumed that mainly harzburgitic mantle sections are formed by a high degree of melt depletion corresponding to fast spreading environments. Ophiolites with lherzolitic mantle sections are thought to correspond to low degrees of melt extraction in slow spreading or rift settings. In Othris, however, we argue that the compositional variability reflects degree of melt-impregnation and refertilisation rather than depletion. Microstructural analysis shows that the melt impregnation recorded in the Othris mantle rocks occurred mainly at sub-solidus temperatures. This indicates that the thermal lithosphere may have reached well into the mantle during magmatic activity, suggesting the presence of a thin crust and/or a thick lithosphere at the Othris paleo-ridge. Freezing of melt in the mantle part of the oceanic lithosphere will lead to incomplete separation of residual mantle and magmatic crust, and a thick Moho Transition Zone (MTZ). Such a situation can be expected at a ridge with a relatively cold structure, i.e. at a slow spreading ridge, in particular near a transform fault (Fig. 1). In contrast, at fast spreading ridges - such as the Oman paleo-ridge - super-solidus conditions exist up to the Moho level and possibly into the lower crust during magmatism. This leads to a narrow MTZ, as observed in the Oman Ophiolite (Boudier and Nicolas, 1995), a well-developed cumulate sequence and a thick crust.

The Transition from Carbonate to Silicate Melts in the CaO--MgO--SiO2--CO2 System

The compositions of melts in equilibrium with a lherzolite mineral INTRODUCTION assemblage were determined in the analogue system CaO– The common association of carbonatites with silicaMgO–SiO2–CO2 at 3 GPa. Carbonate liquids coexist with olivine undersaturated alkaline igneous rocks has led to many and two pyroxenes between the solidus for carbonated lherzolite at speculative petrogenetic schemes for their generation. 1250°C and 1450°C. The Ca/(Ca + Mg) ratio of these melts These include limestone assimilation (Shand, 1945) to is 0·64 and the main eVects of rising temperature are increasing produce silica-undersaturated rocks, crystal fractionation SiO2 content (from 1700°C) is approached. The progression from carbonate to from isotopic studies (e.g. Powell et al., 1966; Bell & silicate liquids is, therefore, abrupt and the field of transitional Blenkinsop, 1987a, 1987b) that the primitive magma is compositions (10–30% SiO2) is restricted to very narrow temof mantle origin. An understanding of the genesis of perature intervals at pressures greater than the solidus ledge. All associated carbonate and silicate rocks therefore depends liquids appear to be miscible. In the context of upwelling magma, on determining the compositions of melts that are proour results provide possible insight into the origins of complexes that duced from peridotitic compositions under upper-mantle are considered to contain primary carbonatites. The solidus ‘ledge’ conditions. This study is aimed at the elucidation of between 2·5 and 3 GPa acts as a filter for both carbonatites and melting behaviour in mantle analogue systems. transitional melt compositions. Carbonatites, which have a wide Qualitative interpretation of the behaviour of carstability field at 3 GPa, may rise through the mantle if they are bonated peridotite at solidus and supersolidus conditions isolated from lherzolite by wallrock reaction and production of can be derived from the model system CaO– wehrlite. Transitional carbonate–silicate melts must also, however, MgO–SiO2–CO2 (CMS–CO2) (Fig. 1). Decarbonation react with the mantle at low pressures. This fact, combined with reactions (Wyllie & Huang, 1975a, 1975b, 1976; Eggler, the small range of physical conditions over which they are generated 1978) define the stability fields of carbonate in lherzolitic and their higher (than carbonatite) viscosity, means that they rarely mantle. The following reaction separates regions in the reach crustal levels. Low-CO2 silicate melts, in contrast, are not mantle where CO2 exists as a free fluid phase and where required to react extensively en route to the surface and are abundant. it resides in mineral phases (dolomite): We suggest that the binary nature of some carbonatite complexes

Practical uses of diamonds

Geothermobarometric calculations for a worldwide database of inclusions in diamond indicate that formation of the dominant harzburgitic diamond association occurred predominantly (90%) under subsolidus conditions. Diamonds in eclogitic and lherzolitic lithologies grew in the presence of a melt, unless their formation is related to strongly reducing CHO fluids that would increase the solidus temperature or occurred at pressure–temperature conditions below about 5 GPa and 1050 °C. Three quarters of peridotitic garnet inclusions in diamond classify as “depleted” due to their low Y and Zr contents but, based on LREEN–HREEN ratios invariably near or greater than one, they nevertheless reflect re-enrichment through either highly fractionated fluids or small amounts of melt. The trace element signatures of harzburgitic and lherzolitic garnet inclusions are broadly consistent with formation under subsolidus and supersolidus conditions, respectively. Diamond formation may be followed by cooling in the range of ~ 60–180 °C as a consequence of slow thermal relaxation or, in the case of the Kimberley area in South Africa, possibly uplift due to extension in the lithospheric mantle. In other cases, diamond formation and final residence took place at comparable temperatures or even associated with small temperature increases over time.Diamond formation in peridotitic substrates can only occur at conditions at least as reducing as the EMOD buffer. Evaluation of the redox state of 225 garnet peridotite xenoliths from cratons worldwide indicates that the vast majority of samples deriving from within the diamond stability field represent fO2 conditions below EMOD. Modeling reveals that less than 50 ppm fluid are required to completely reset the redox state of depleted cratonic peridotite to that of the fluid. Consequently, the overall reduced state of diamond stable peridotites implies that the last fluids to interact with the deep cratonic lithosphere were generally reducing in character. A further consequence of the extremely limited redox buffering capacity of cratonic peridotites is that redox reactions with infiltrating fluid/melt likely cannot produce large diamonds or high diamond grades. Evaluating the shift in maximum carbon content in CHO fluids during either isobaric cooling or ascent along a cratonic geotherm, however, reveals that isochemical precipitation of carbon from CHO fluids provides an efficient mode of diamond crystallization. Since subsolidus fluids are permissible in harzburgites only, and supersolidus melts in lherzolite we suggest that CHO fluid metasomatism may explain the long observed close association between diamonds and harzburgitic garnets. In the absence of thermodynamic data we cannot evaluate if supersolidus carbonate-bearing melts, stable at fO2 conditions below EMOD, would experience a similar decrease in maximum carbon solubility during cooling or ascent along a geotherm. The absence of a clear association between diamond and lherzolitic garnets, however, suggests that this is not the case. A very strong association between diamond and eclogite likely relates to the fact that the transition from carbonate to diamond stable conditions occurs at redox conditions that are at least about 1 log unit more oxidizing than EMOD. At this time we cannot quantitatively evaluate the redox buffering capacity of cratonic eclogites but given their much higher Fe contents it has to be significantly higher than for peridotites. Alternatively, diamond in eclogite may precipitate directly from cooling carbonate-bearing melts that may be too oxidizing to crystallize diamond in olivine-bearing lithologies.