Inclusion Suite Research Articles

Ferropericlase inclusions in ultradeep diamonds from Sao Luiz (Brazil): high Li abundances and diverse Li-isotope and trace element compositions suggest an origin from a subduction mélange

The most remarkable feature of the inclusion suite in ultradeep alluvial and kimberlitic diamonds from Sao Luiz (Juina area in Brazil) is the enormous range in Mg# [100xMg/(Mg + Fe)] of the ferropericlases (fper). The Mg-richer ferropericlases are from the boundary to the lower mantle or from the lower mantle itself when they coexist with ringwoodite or Mg- perovskite (bridgmanite). This, however, is not an explanation for the more Fe-rich members and a lowermost mantle or a “D” layer origin has been proposed for them. Such a suggested ultra-deep origin separates the Fe-rich fper-bearing diamonds from the rest of the Sao Luiz ultradeep diamond inclusion suite, which also contains Ca-rich phases. These are now thought to have an origin in the uppermost lower mantle and in the transition zone and to belong either to a peridotitic or mafic (subducted oceanic crust) protolith lithology. We analysed a new set of more Fe-rich ferropericlase inclusions from 10 Sao Luiz ultradeep alluvial diamonds for their Li isotope composition by solution MC-ICP-MS (multi collector inductively coupled plasma mass spectrometry), their major and minor elements by EPMA (electron probe micro-analyser) and their Li-contents by SIMS (secondary ion mass spectrometry), with the aim to understand the origin of the ferropericlase protoliths. Our new data confirm the wide range of ferropericlase Mg# that were reported before and augment the known lack of correlation between major and minor elements. Four pooled ferropericlase inclusions from four diamonds provided sufficient material to determine for the first time their Li isotope composition, which ranges from δ7Li + 9.6 ‰ to −3.9 ‰. This wide Li isotopic range encompasses that of serpentinized ocean floor peridotites including rodingites and ophicarbonates, fresh and altered MORB (mid ocean ridge basalt), seafloor sediments and of eclogites. This large range in Li isotopic composition, up to 5 times higher than ‘primitive upper mantle’ Li-abundances, and an extremely large and incoherent range in Mg# and Cr, Ni, Mn, Na contents in the ferropericlase inclusions suggests that their protoliths were members of the above lithologies. This melange of altered rocks originally contained a variety of carbonates (calcite, magnesite, dolomite, siderite) and brucite as the secondary products in veins and as patches and Ca-rich members like rodingites and ophicarbonates. Dehydration and redox reactions during or after deep subduction into the transition zone and the upper parts of the lower mantle led to the formation of diamond and ferropericlase inclusions with variable compositions and a predominance of the Ca-rich, high-pressure silicate inclusions. We suggest that the latter originated from peridotites, mafic rocks and sedimentary rocks as redox products between calcite and SiO2.

PGE–(REE–Ti)-Rich Micrometer-Sized Inclusions, Mineral Associations, Compositional Variations, and a Potential Lode Source of Platinum-Group Minerals in the Sisim Placer Zone, Eastern Sayans, Russia

We report the results of a mineralogical investigation of placer samples from the upper reaches of the Sisim watershed, near Krasnoyarsk, in Eastern Sayans, Russia. The placer grains are predominantly Os–Ir–(Ru) alloys (80%) that host various inclusions (i.e., platinum-group elements (PGE)-rich monosulfide, PGE-rich pentlandite, Ni–Fe–(As)-rich laurite, etc.) and subordinate amounts of Pt–Fe alloys. Analytical data (wavelength- and energy-dispersive X-ray spectroscopy) are presented for all the alloy minerals and the suite of micrometer-sized inclusions that they contain, as well as associated grains of chromian spinel. The assemblage was likely derived from chromitite units of the Lysanskiy mafic–ultramafic complex, noted for its Ti–(V) mineralization. In the Os–Ir–(Ru) alloys, the ratio Ru/Ir is ≤1, Ir largely substitutes for Os, and compositional variations indicate the scheme [Ir + Ru] → 2Os. In contrast, in the laurite–erlichmanite series, Ir and Os are strongly and positively correlated, whereas Ir and Ru are negatively correlated; Ru and Os are inversely correlated. These compositions point to the scheme [Os2+ + 2Ir3+ + □] → 4Ru2+ or alternatively, to Os2+ + Ir2+ → 2Ru2+. We deduce a potential sequence of crystallization in the parental rock and address the effects of decreasing temperature and increasing fugacity of sulfur and arsenic on the assemblage. Inclusions of Ti-rich minerals in the alloy grains are consistent with the Lysanskiy setting; the complete spectrum of chromite–magnesiochromite compositions indicates that an important part of that complex was eroded. A localized fluid-dominated micro-environment produced the unique association of laurite with monazite-(Ce), again considered a reflection of the special attributes of the Lysanskiy complex.

Interaction of peridotite with Ca-rich carbonatite melt at 3.1 and 6.5 GPa: Implication for merwinite formation in upper mantle, and for the metasomatic origin of sublithospheric diamonds with Ca-rich suite of inclusions

We performed an experimental study, designed to reproduce the formation of an unusual merwinite + olivine-bearing mantle assemblage recently described as a part of a Ca-rich suite of inclusions in sublithospheric diamonds, through the interaction of peridotite with an alkali-rich Ca-carbonatite melt, derived from deeply subducted oceanic crust. In the first set of experiments, we studied the reaction between powdered Mg-silicates, olivine and orthopyroxene, and a model Ca-carbonate melt (molar Na:K:Ca = 1:1:2), in a homogeneous mixture, at 3.1 and 6.5 GPa. In these equilibration experiments, we observed the formation of a merwinite + olivine-bearing assemblage at 3.1 GPa and 1200 °C and at 6.5 GPa and 1300–1400 °C. The melts coexisting with this assemblage have a low Si and high Ca content (Ca# = molar 100 × Ca/(Ca + Mg) > 0.57). In the second set of experiments, we investigated reaction rims produced by interaction of the same Ca-carbonate melt (molar Na:K:Ca = 1:1:2) with Mg-silicate, olivine and orthopyroxene, single crystals at 3.1 GPa and 1300 °C and at 6.5 GPa and 1400 °C. The interaction of the Ca-carbonate melt with olivine leads to merwinite formation through the expected reaction: 2Mg2SiO4 (olivine) + 6CaCO3 (liquid) = Ca3MgSi2O8 (merwinite) + 3CaMg(CO3)2 (liquid). Thus, our experiments confirm the idea that merwinite in the upper mantle may originate via interaction of peridotite with Ca-rich carbonatite melt, and that diamonds hosting merwinite may have a metasomatic origin. It is remarkable that the interaction of the Ca-carbonate melt with orthopyroxene crystals does not produce merwinite both at 3.1 and 6.5 GPa. This indicates that olivine grain boundaries are preferable for merwinite formation in the upper mantle.

Mineral inclusion assemblage and detrital zircon provenance

Mineral inclusions are common in magmatic zircon and a potentially rich source of petrologic information. Controls on the relative proportions of inclusion phases, specifically early-crystallizing minerals such as apatite and late-crystallizing phases such as quartz, K-feldspar, and muscovite, have not been systematically studied. For instance, apatite dominates many magmatic zircon inclusion suites, and selective replacement of apatite over other phases has been proposed as a mechanism for generating apatite-poor inclusion assemblages in detrital zircons. However, the extent to which apatite inclusion abundance is influenced by source rock composition has not been established. The preservation of characteristic minerals in granite series, such as differences in magnetite and ilmenite abundances due to varying redox, have also not been systematically explored as inclusion phases in zircon. We surveyed zircon inclusion assemblages in Phanerozoic granitoids of a range of compositions and found a broadly inverse relationship between the presence of apatite in the inclusion suite and whole-rock SiO2 content. Selective loss of apatite is evident from deficits in apatite content among inclusions in contact with cracks in both detrital zircons and some granitoid zircons with independent evidence for fluid ingress (i.e., secondary phases filling open cracks). In cases where microstructural observations can identify primary inclusion assemblages (versus those impacted by fluid ingress), the relative occurrence of apatite can be used to broadly predict source rock SiO2 content. There is little relationship between whole-rock chemistry and the abundance of late-crystallizing phases or the relative proportions of quartz, plagioclase, and alkali feldspar. The presence of ilmenite not in contact with cracks in the host zircon likely points to origins in ilmenite-series (i.e., reduced) magmas, but the presence of magnetite is less diagnostic. Using the apatite abundance metric, detrital Hadean zircons from Jack Hills (Western Australia), appear to derive from felsic rather than intermediate or mafic granitoids and detrital Eoarchean zircons from Nuvvuagittuq Supracrustal Belt (Quebec) may derive from intermediate to felsic granitoids. No Fe-Ti oxides have been identified in Nuvvuagittuq zircons, but rare ilmenite in Jack Hills zircons points to origins of at least some part of the population in ilmenite-series magmas.

A new indicator mineral methodology based on a generic Bi-Pb-Te-S mineral inclusion signature in detrital gold from porphyry and low/intermediate sulfidation epithermal environments in Yukon Territory, Canada

Porphyry-epithermal and orogenic gold are two of the most important styles of gold-bearing mineralization within orogenic belts. Populations of detrital gold resulting from bulk erosion of such regions may exhibit a compositional continuum wherein Ag, Cu, and Hg in the gold alloy may vary across the full range exhibited by natural gold. This paper describes a new methodology whereby orogenic and porphyry-epithermal gold may be distinguished according to the mineralogy of microscopic inclusions observed within detrital gold particles. A total of 1459 gold grains from hypogene, eluvial, and placer environments around calc-alkaline porphyry deposits in Yukon (Nucleus-Revenue, Casino, Sonora Gulch, and Cyprus-Klaza) have been characterized in terms of their alloy compositions (Au, Ag, Cu, and Hg) and their inclusion mineralogy. Despite differences in the evolution of the different magmatic hydrothermal systems, the gold exhibits a clear Bi-Pb-Te-S mineralogy in the inclusion suite, a signature which is either extremely weak or (most commonly) absent in both Yukon orogenic gold and gold from orogenic settings worldwide. Generic systematic compositional changes in ore mineralogy previously identified across the porphyry-epithermal transition have been identified in the corresponding inclusion suites observed in samples from Yukon. However, the Bi-Te association repeatedly observed in gold from the porphyry mineralization persists into the epithermal environment. Ranges of P-T-X conditions are replicated in the geological environments which define generic styles of mineralization. These parameters influence both gold alloy composition and ore mineralogy, of which inclusion suites are a manifestation. Consequently, we propose that this methodology approach can underpin a widely applicable indicator methodology based on detrital gold.

Mantle-derived trace element variability in olivines and their melt inclusions

Trace element variability in oceanic basalts is commonly used to constrain the physics of mantle melting and the chemistry of Earth's deep interior. However, the geochemical properties of mantle melts are often overprinted by mixing and crystallisation processes during ascent and storage. Studying primitive melt inclusions offers one solution to this problem, but the fidelity of the melt-inclusion archive to bulk magma chemistry has been repeatedly questioned. To provide a novel check of the melt inclusion record, we present new major and trace element analyses from olivine macrocrysts in the products of two geographically proximal, yet compositionally distinct, primitive eruptions from the Reykjanes Peninsula of Iceland. By combining these macrocryst analyses with new and published melt inclusion analyses we demonstrate that olivines have similar patterns of incompatible trace element (ITE) variability to the inclusions they host, capturing chemical systematics on intra- and inter-eruption scales.ITE variability (element concentrations, ratios, variances and variance ratios) in olivines from the ITE-enriched Stapafell eruption is best accounted for by olivine-dominated fractional crystallisation. In contrast, ITE variability in olivines and inclusions from the ITE-depleted Háleyjabunga eruption cannot be explained by crystallisation alone, and must have originated in the mantle. Compatible trace element (CTE) variability is best described by crystallisation processes in both eruptions. Modest correlations between host and inclusion ITE contents in samples from Háleyjabunga suggest that melt inclusions can be faithful archives of melting and magmatic processes. It also indicates that degrees of ITE enrichment can be estimated from olivines directly when melt inclusion and matrix glass records of geochemical variability are poor or absent. Inter-eruption differences in olivine ITE systematics between Stapafell and Háleyjabunga mirror differences in melt inclusion suites, and confirm that the Stapafell eruption was fed by lower degree melts from greater depths within the melting region than the Háleyjabunga eruption.Although olivine macrocrysts from Stapafell are slightly richer in Ni than those from Háleyjabunga, their overall CTE systematics (e.g., Ni/(Mg/Fe), Fe/Mn and Zn/Fe) are inconsistent with being derived from olivine-free pyroxenites. However, the major element systematics of Icelandic basalts require lithological heterogeneity in their mantle source in the form of Fe-rich and hence fusible domains. We thus conclude that enriched heterogeneities in the Icelandic mantle are composed of modally enriched, yet nonetheless olivine-bearing, lithologies and that olivine CTE contents provide an incomplete record of lithological heterogeneity in the mantle. Modally enriched peridotites may therefore play a more important role in oceanic magma genesis than previously inferred.

Calcium and titanium isotope fractionation in refractory inclusions: Tracers of condensation and inheritance in the early solar protoplanetary disk

Measured and modeled Ca and Ti isotopic fractionation effects in a diverse suite of refractory inclusions are used to understand processes of condensation in the solar protoplanetary disk where they and their precursor materials formed. This coordinated approach reveals largely decoupled isotopic signatures and implies that few, if any, of the studied inclusions can be considered primary condensates. All studied inclusions are enriched in light Ca isotopes (∼−0.2 to −2.8‰/amu), but only two show correspondingly light Ti isotopes. Studied inclusions exhibit both heavy and light Ti isotope enrichments (∼0.3 to −0.4‰/amu). These refractory element isotopic signatures, therefore, suggest admixture and reprocessing of earlier formed materials with distinct condensation histories. Along with coordinated measurements of 50Ti isotopic anomalies, which span a range from ∼0 to ∼40 epsilon-unit excesses, the comparison of measured and modeled fractionation of Ca and Ti isotopes provides a powerful approach to understanding primitive nebular processes and environments in the protoplanetary disk. Remarkable evidence for Ca isotopic zoning within a typical Type B1 inclusion exemplifies the potential record of the earliest solar nebula that is likely lost and/or overprinted in the isotopic compositions of more volatile elements (e.g., Mg, Si, and O) by later modification processes.

Petrogenesis of alkaline basalt-hosted sapphire megacrysts. Petrological and geochemical investigations of in situ sapphire occurrences from the Siebengebirge Volcanic Field, Germany

Megacrystic sapphires are frequently associated with alkaline basalts, most notably in Asia and Australia, although basalt is not generally normative in corundum. Most of these sapphire occurrences are located in alluvial or eluvial deposits, making it difficult to study the enigmatic relationship between the sapphires and their host rocks. Here, we present detailed petrological and geochemical investigations of in situ megacrystic sapphires within alkaline basalts from the Cenozoic Siebengebirge Volcanic Field (SVF) in Germany. Markedly, the sapphires show several micrometer thick spinel coronas at the contact with the host basalt, indicating chemical disequilibrium between the sapphire and the basaltic melt, supporting a xenogenetic relationship. However, in situ U–Pb dating of a Columbite Group inclusion within one Siebengebirge sapphire using laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) indicates a close genetic relationship between sapphire crystallization and alkaline mafic volcanism in the SVF. The syngenetic mineral inclusion suite including carbonates, members of the Pyrochlore, Betafite and Columbite Groupe minerals, as well as a high abundance of HFSE and of gaseous low-density CO2 inclusions support a parentage of a highly evolved, MgO and FeO deficient carbonatitic melt. We identified CO2 to be the link between alkaline basaltic volcanism and the xenocrystic sapphires. Only alkaline volcanic suites can build up enough CO2 in this magma chamber upon fractionation so that at high degrees of fractionation a carbonatitic melt exsolves which in turn can crystallize sapphires.

Native gold from the Inagli Pt–Au placer deposit (the Aldan Shield, Russia): geochemical characteristics and implications for possible bedrock sources

The Inagli alluvial Pt–Au placer deposit in the Republic of Sakha (Yakutia), Russia, is linked to the Inagli massif, one of the several Uralian–Alaskan-type alkaline-ultrabasic complexes in the Aldan Shield. Gold from the placer is heterogeneous in composition and is represented by three types. Type 1 gold is the most abundant and is characterized by simple Au–Ag alloys with 4–34 wt% Ag, low Cu (up to 0.08 wt%) and negligible Hg, Pt, and Pd contents, and silver–tellurium sulfosalts (Ag–Cu–Te–S–As compounds) in the inclusion suite. Silicate inclusions are biotite, K-feldspar, Fe–Mg amphibole, chlorite, plagioclase, Fe–Mg pyroxene, zircon, and titanite. Distinctive features of this gold type are most similar to those derived from low-sulfidation systems linked to iron oxide copper–gold or iron skarn types of mineralization. The bedrock source of type 1 gold could be related with monzonite to syenite intrusions surrounding the Inagli massif. Distinctive features of type 2 gold include a wide discontinuous range of Ag content (1–18 wt%), elevated Cu (up to 0.5 wt%), and occasional Pd (up to 0.3 wt%) levels, non-detectable Pt and Hg contents, and rare inclusions of simple sulfides (digenite, pyrrhotite) and Na amphibole. Type 3 gold is distinguished by a narrow range in Ag content (5–8 wt%), elevated Hg (0.5–1 wt%) contents, negligible Cu, Pt and Pd levels, and Au–Pb compounds + K-feldspar inclusions. Microchemical characteristics of type 2 and type 3 gold are interpreted as suggestive of an alkaline–magmatic-related fluid. Based on the grain morphology and microchemical signatures, potential bedrock sources for both gold types could be related to the numerous alkaline veins and potassic alteration zones within the dunite core. A comparison of the Inagli and the Kondyor placer gold allows to generate distinctive generic signatures for gold from Uralian–Alaskan-type alkaline–ultrabasic complexes in the Aldan Shield.

Deep mixing of mantle melts beneath continental flood basalt provinces: Constraints from olivine-hosted melt inclusions in primitive magmas

We present major and trace element compositions of 154 re-homogenised olivine-hosted melt inclusions found in primitive rocks (picrites and ferropicrites) from the Mesozoic Paraná–Etendeka and Karoo Continental Flood Basalt (CFB) provinces. The major element compositions of the melt inclusions, especially their Fe/Mg ratios, are variable and erratic, and attributed to the re-homogenisation process during sample preparation. In contrast, the trace element compositions of both the picrite and ferropicrite olivine-hosted melt inclusions are remarkably uniform and closely reflect those of the host whole-rocks, except in a small subset affected by hydrothermal alteration. The Paraná–Etendeka picrites and ferropicrites are petrogenetically related to the more evolved and voluminous flood basalts, and so we propose that compositional homogeneity at the melt inclusion scale implies that the CFB parental mantle melts were well mixed prior to extensive crystallisation.The incompatible trace element homogeneity of olivine-hosted melt inclusions in Paraná–Etendeka and Karoo primitive magmatic rocks has also been identified in other CFB provinces and contrasts with findings from studies of basalts from mid-ocean ridges (e.g. Iceland and FAMOUS on the Mid Atlantic Ridge), where heterogeneity of incompatible trace elements in olivine-hosted melt inclusions is more pronounced. We suggest that the low variability in incompatible trace element contents of olivine-hosted melt inclusions in near-primitive CFB rocks, and also ocean island basalts associated with moderately thick lithosphere (e.g. Hawaii, Galápagos, Samoa), may reflect mixing along their longer transport pathways during ascent and/or a temperature contrast between the liquidus and the liquid when it arrives in the crust. These thermal paths promote mixing of mantle melts prior to their entrapment by growing olivine crystals in crustal magma chambers. Olivine-hosted melt inclusions of ferropicrites from the Paraná–Etendeka and Karoo CFB have the least variable compositions of all global melt inclusion suites, which may be a function of their unusually deep origin and low viscosity.

2016 Nier Prize for Gregory A. Brennecka

2016 Nier Prize for Gregory A. Brennecka

Characterization of Gold Mineralization in the Northern Cariboo Gold District, British Columbia, Canada, Through Integration of Compositional Studies of Lode and Detrital Gold with Historical Placer Production: A Template for Evaluation of Orogenic Gold Districts

There is a strong association between regions containing orogenic gold mineralization and exploitation of placer gold, although, in many cases, the nature of the source mineralization for these placer deposits remains unclear. This study describes a novel approach to evaluating the economic potential of in situ orogenic gold mineralization through characterization of both lode and placer gold mineralogy, followed by synthesis of this information with records of both mineral occurrences and historical placer mining. The northern Cariboo Gold District in east-central British Columbia, Canada, was chosen as a location for the study because of the gold endowment of the area (1.2 million ounces [Moz] of lode gold and between 0.5 and 3 Moz. of placer gold) and the information available from both placer and lode gold mining. Compositional analysis of 533 gold grains from 21 lode localities and 1,914 gold grains from 30 placer localities from throughout the Cariboo Gold District has identified four main compositional types in terms of their alloy compositions and associated suite of mineral inclusions revealed in polished section. A distinctive low (4–7%)-Ag gold that exhibits a strong Bi association in the mineral inclusion suite is geographically limited to the Wells area, where it is recorded in both lode mineralization and its placer expression. Regionally pervasive mineralization yields gold of binary Au-Ag alloy and a simple inclusion suite of sulfides and sulfarsenides. Gold in most large placers in trunk drainages was derived from multiple (mostly small) occurrences of this type. The nature of compositional variation between gold grains liberated from hypogene ore has informed the history of episodic mineralization and suggests multiple stages of gold emplacement at some localities, while others are dominated by gold deposited in a single stage. The new information from gold grain analysis has been considered in the context of other information. Classification of a placer as either allocthonous or autocthonous both informs interpretation of compositional characteristics of the detrital gold grains and provides information on distance to the hypogene source. Mineral inclusion assemblages observed in sample populations of placer gold grains have been correlated with reports of hypogene vein mineralogy described in mineral occurrence records to clarify the geographical extent of specific mineralization types. The compositional range of alloys of different gold types has been compared to historical records of gold production and gold fineness (Au-Ag ratio) to reconstruct the size and distribution of hypogene sources prior to erosion. Synthesis of gold compositional studies with other publicly accessible data sets provides a new generic approach capable of evaluating the most attractive targets for future exploration and highlighting compositional signatures of placer gold which relate to undiscovered in situ sources.

Mineralogical and geochemical characteristics of emeralds from the Leydsdorp area, South Africa

The world’s oldest emerald deposit occurs in the Gravelotte-Leydsdorp area of the Limpopo Province of South Africa. Emeralds have been mined from the main emerald working, the Cobra pit, since 1927, although emeralds were discovered in the area much earlier. The emerald mineralisation is closely linked with albitite pegmatites, with the emeralds confined almost entirely to metasomatic zones at the margins of strongly deformed pegmatoid bodies in biotite, talc and actinolite schists. While the surrounding Murchison greenstone belt has been extensively studied, very little geochemical information is available for the emeralds in the deposit. This paper documents the geochemical characteristics of 57 low-quality emeralds from the Leydsdorp area, Murchison Greenstone Belt. The emeralds range in size from 3 to 12 mm and were studied using both visual examination and electron microprobe analysis with 960 microprobe analyses on 54 specimens. The emeralds in the present study are subhedral to euhedral with a well-formed prismatic habit, and range in colour from light green to a dark green or bluish green. A number of samples are characterised by colour zoning, typically comprising of a green to dark green transition from core to rim, while small colourless areas within crystals are also seen. Many of the emeralds examined are practically opaque owing to the presence of very abundant mineral (and fluid) inclusions. Numerous small cracks and veins are also present. A wide variety of inclusion species are encountered, comprising dominant biotite, apatite and quartz, as well as a host of other minerals including hematite, pyrite, pyrrhotite, zircon, molybdenite and galena. SiO 2 content varies between 64 and 67 wt%, Al 2 O 3 range from 13 to 17 wt%, MgO concentrations vary between 0.08 and 3.46 wt% and Na 2 O contents range from 0.76 to 2.33 wt%. The Cr content of Leydsdorp emeralds range up to 1.07 wt% Cr 2 O 3 , with an average of 0.23 wt%. Samples are often heterogeneous with a maximum variation of 0.17 to 0.9 wt% Cr 2 O 3 observed in one specimen. Darker green crystals and dark green patches in individual crystals have significantly higher Cr contents compared to light green crystals and crystal sections. An inverse relationship between Al and the amount of Cr, Mg, Na and Cs is seen, while substitution of Al by Fe is also evident. The major and minor element chemistry of the emeralds are broadly similar to other emerald deposits worldwide. The major chromophore appears to be Cr as opposed to V. Na contents are somewhat higher than the world-wide average and this, along with the inclusion suite, could serve to distinguish these emeralds from other emerald deposits.

Linking subsurface to surface degassing at active volcanoes: A thermodynamic model with applications to Erebus volcano

Volcanic plumbing systems are the pathways through which volatiles are exchanged between the deep Earth and the atmosphere. The interplay of a multitude of processes occurring at various depths in the system dictates the composition and quantity of gas eventually erupted through volcanic vents. Here, a model is presented as a framework for interpreting surface volcanic gas measurements in terms of subsurface degassing processes occurring throughout a volcanic plumbing system. The model considers all possible sources of fluid from multiple depths, including degassing of dissolved volatiles during crystallization and/or decompression as recorded in melt inclusions plus any co-existing fluid phase present in a magma reservoir. The former is achieved by differencing melt inclusion volatile contents between groups of melt inclusions saturated at discrete depths. The latter is calculated using a thermodynamic model, which computes the composition of a C–O–H–S fluid in equilibrium with a melt given a minimum of five thermodynamic parameters commonly known for natural systems (T, P, fO2, either fH2 or one parameter for H2O, and either fS2 or one parameter for CO2). The calculated fluids are thermodynamically decompressed and run through a mixing model, which finds all possible mixtures of subsurface fluid that match the chemistry of surface gas within ±2.0 mol%. The method is applied to Mount Erebus (Antarctica), an active, intraplate volcano whose gas emissions, which emanate from an active phonolitic lava lake, have been well quantified by FTIR, UV spectroscopy, and multi-gas sensors over the last several decades. In addition, a well-characterized suite of lavas and melt inclusions, and petrological interpretations thereof, represent a wealth of knowledge about the shallow, intermediate, and deep parts of the Erebus plumbing system. The model has been used to calculate the compositions of seven C–O–H–S fluids that originate from four distinct regions within the Erebus plumbing system and in the lava lake (deep basanite, intermediate, shallow phonolite, and lava lake phonolite equilibrium fluids, plus crystallization-induced degassing of deep, intermediate, and shallow melts). A total of 144 possible mixtures were found. In all cases, ∼60% of the surface gas is sourced from deep degassing. The remaining ∼40% is made up primarily of fluid in equilibrium with the lava lake (∼20%) plus intermediate (∼10%) and phonolite (∼5%) equilibrium fluids and minor to no contribution from all other fluid sources. These results, whereby the surface gas signature is dominated by fluids originating from deep mafic melts, could be representative of any volcanic system comprised of a deep mafic member and shallow evolved fractionates as has been inferred at Yellowstone, Etna, and many others. At Erebus, results of this modeling demonstrate that the degassing of stagnant magma can contribute significant fluid and energy to the system such that the continuous convection and degassing of volatile-rich magma is not necessary to explain the volcano's persistently active nature or the composition of its gas emissions.The C++ model code is open source and is hosted as a github repository at https://github.com/kaylai/Iacovino2015_thermodynamic_model/.

Syn-collapse eclogite metamorphism and exhumation of deep crust in a migmatite dome: The P–T–t record of the youngest Variscan eclogite (Montagne Noire, French Massif Central)

In many orogens, high-pressure (HP) metamorphic rocks such as eclogite occur as lenses in quartzofeldspathic gneiss that equilibrated at much lower pressures. The pressure–temperature–time (P–T–t) history of eclogite relative to host gneiss provides information about mechanisms and timescales of exhumation of orogenic crust. The Montagne Noire of the southern Massif Central, France, is an eclogite–bearing gneiss (migmatite) dome located at the orogen-foreland transition of the Variscan belt. Results of our study show that it contains the youngest eclogite in the orogen, similar in age to migmatite and granite that crystallized under low-pressure conditions. P–T conditions for an exceptionally unaltered eclogite from the central Montagne Noire were estimated using a pseudosection supplemented by garnet–clinopyroxene and Zr-in-rutile thermometry. Results indicate peak P∼1.4 GPa and T∼725°C for Mg-rich garnet rim (50 mol% pyrope) + omphacite (36 mol% jadeite) + rutile + quartz. U–Pb geochronology (LA-ICP-MS) of 16 zoned zircon grains yielded ∼360 Ma (4 cores) and ∼315 Ma (12 rims and cores). Rare earth element abundances determined by LA-ICP-MS for dated zircon are consistent with crystallization of ∼315 Ma zircon under garnet-stable, plagioclase-unstable conditions that we interpret to indicate high pressure; in contrast, the ∼360 Ma zircon core corresponds to crystallization under lower pressure plagioclase-stable conditions. Based on garnet zoning and inclusion suites, rutile textures and Zr zoning, P–T results, and zircon petrochronology, we interpret the ∼315 Ma date as the age of eclogite-facies metamorphism that only slightly preceded dome formation and crystallization at 315–300 Ma. This age relation indicates that eclogite formation at high pressure and migmatite dome emplacement at low pressure were closely spaced in time. We propose that collapse-driven material transfer from the hot orogen to the cool foreland resulted in thickening of the orogen edge, leading to eclogite facies metamorphism of the deep crust. Soon after, the low-viscosity partially molten crust flowed from the plateau toward the foreland, incorporating and exhuming eclogite. The P–T–t history of the Montagne Noire eclogite shows that some dome material ascended from >40 km depth to shallow crustal levels, likely in a single decompression event, and that migmatite domes are therefore very efficient at exhuming the deep crust.

The evolution and storage of primitive melts in the Eastern Volcanic Zone of Iceland: the 10 ka Grímsvötn tephra series (i.e. the Saksunarvatn ash)

Major, trace and volatile elements were measured in a suite of primitive macrocrysts and melt inclusions from the thickest layer of the 10 ka Grímsvötn tephra series (i.e. Saksunarvatn ash) at Lake Hvítárvatn in central Iceland. In the absence of primitive tholeiitic eruptions (MgO > 7 wt%) within the Eastern Volcanic Zone (EVZ) of Iceland, these crystal and inclusion compositions provide an important insight into magmatic processes in this volcanically productive region. Matrix glass compositions show strong similarities with glass compositions from the AD 1783–1784 Laki eruption, confirming the affinity of the tephra series with the Grímsvötn volcanic system. Macrocrysts can be divided into a primitive assemblage of zoned macrocryst cores (An78–An92, Mg#cpx = 82–87, Fo79.5–Fo87) and an evolved assemblage consisting of unzoned macrocrysts and the rims of zoned macrocrysts (An60–An68, Mg#cpx = 71–78, Fo70–Fo76). Although the evolved assemblage is close to being in equilibrium with the matrix glass, trace element disequilibrium between primitive and evolved assemblages indicates that they were derived from different distributions of mantle melt compositions. Juxtaposition of disequilibrium assemblages probably occurred during disaggregation of incompatible trace element-depleted mushes (mean La/Ybmelt = 2.1) into aphyric and incompatible trace element-enriched liquids (La/Ybmelt = 3.6) shortly before the growth of the evolved macrocryst assemblage. Post-entrapment modification of plagioclase-hosted melt inclusions has been minimal and high-Mg# inclusions record differentiation and mixing of compositionally variable mantle melts that are amongst the most primitive liquids known from the EVZ. Coupled high-field strength element (HFSE) depletion and incompatible trace element enrichment in a subset of primitive plagioclase-hosted melt inclusions can be accounted for by inclusion formation following plagioclase dissolution driven by interaction with plagioclase-undersaturated melts. Thermobarometric calculations indicate that final crystal–melt equilibration within the evolved assemblage occurred at ~1140 °C and 0.0–1.5 kbar. Considering the large volume of the erupted tephra and textural evidence for rapid crystallisation of the evolved assemblage, 0.0–1.5 kbar is considered unlikely to represent a pressure of long-term magma accumulation and storage. Multiple thermometers indicate that the primitive assemblage crystallised at high temperatures of 1240–1300 °C. Different barometers, however, return markedly different crystallisation depth estimates. Raw clinopyroxene–melt pressures of 5.5–7.5 kbar conflict with apparent melt inclusion entrapment pressures of 1.4 kbar. After applying a correction derived from published experimental data, clinopyroxene–melt equilibria return mid-crustal pressures of 4 ± 1.5 kbar, which are consistent with pressures estimated from the major element content of primitive melt inclusions. Long-term storage of primitive magmas in the mid-crust implies that low CO2 concentrations measured in primitive plagioclase-hosted inclusions (262–800 ppm) result from post-entrapment CO2 loss during transport through the shallow crust. In order to reconstruct basaltic plumbing system geometries from petrological data with greater confidence, mineral–melt equilibrium models require refinement at pressures of magma storage in Iceland. Further basalt phase equilibria experiments are thus needed within the crucial 1–7 kbar range.

Mineral inclusions in sapphire from the basalt-related deposit in Bo Phloi, Kanchanaburi, western Thailand: indication of their genesis

The Bo Phloi gem field in Kanchanaburi Province, Western Thailand, is closely associated with Cenozoic basalts. Blue and yellow sapphire, black spinel, and minor zircon have been mined for over three decades. The mineral inclusions observed in sapphire samples are alkali feldspar, nepheline, hercynitic spinel, zircon, manganiferous ilmenite, silica-rich enstatite, almandine–pyrope garnet, monazite, calcite, sapphirine, biotite–phlogopite mica, and staurolite. Based on their geochemical affinity, these mineral inclusions can be categorized into two main groups: felsic alkaline and contact-metamorphic, which appear to have originated from different processes. These inclusions provide new evidence for proposing a bimodal genetic model. Felsic alkaline origin is evidenced by the occurrence of a felsic alkaline inclusion suite and the REE geochemistry of sapphire-associated zircon, which indicates that most of the sapphires crystallized from a high-alkali felsic melt (probably, in the lower crust). Contact-metamorphic origin is evidenced by the presence of a contact-metamorphic inclusion suite, suggesting that some of these sapphires might also have originated from metasomatized crustal rocks and a contaminated melt along the contact zone of a basaltic intrusion (probably, in the upper mantle or lower crust).

Eruption style at Kīlauea Volcano in Hawai‘i linked to primary melt composition

Explosive eruptions at basaltic volcanoes have been linked to gas segregation from magmas at shallow depths in the crust. The composition of primary melts formed at greater depths was thought to have little influence on eruptive style. Ocean island basaltic volcanoes are the product of melting of a geochemically heterogeneous mantle plume and are expected to give rise to heterogeneous primary melts. This range in primary melt composition, particularly with respect to the volatile components, will profoundly influence magma buoyancy, storage and eruption style. Here we analyse the geochemistry of a suite of melt inclusions from 25 historical eruptions at the ocean island volcano of Kīlauea, Hawai‘i, over the past 600 years. We find that more explosive styles of eruption at Kīlauea Volcano are associated statistically with more geochemically enriched primary melts that have higher volatile concentrations. These enriched melts ascend faster and retain their primary nature, undergoing little interaction with the magma reservoir at the volcano’s summit. We conclude that the eruption style and magma-supply rate at Kīlauea are fundamentally linked to the geochemistry of the primary melts formed deep below the volcano. Magmas might therefore be predisposed towards explosivity right at the point of formation in their mantle source region. The explosive style of volcanic eruptions has been linked to gas separation from magmas in the shallow crust. Geochemical analysis of magmas erupted over the past 600 years at Kīlauea Volcano, Hawai‘i, now reveal a link between eruption style and the geochemistry of magmas formed at greater depths, implying that some magmas are predisposed towards explosivity.

Diamonds in Earthʼs oldest zircons from Jack Hills conglomerate, Australia, are contamination

The earliest period of Earth history is the Hadean era (4600–4000 Myr) but to date no rocks are known to be preserved from this time. However, abundant detrital zircons from the Archean Jack Hills conglomerate complex of Western Australia have been found with dates in excess of 4000 Myr and there is a significant literature concerning these zircons from which abundant geochemical and mineralogical information has been obtained. The essence of what has been derived from these zircons and their inclusions concerning conditions of formation is that the zircons crystallized from granitic melts at shallow depths. However, in the last decade two papers were published reporting microdiamond inclusions in a suite of zircons ranging in age from ∼4200 Myr to ∼3000 Myr, implying much greater depths and repeated recycling and re-incorporation of diamonds into zircons over at least 1200 Myr. To complicate the issue, subsequent to this discovery, another study surveyed the inclusion suite in a very large sample (>400) of Jack Hills zircons (JHZs) and found no diamonds but abundant inclusions that are consistent with the previous consensus. It is important, therefore, to verify whether or not the reported JHZ diamonds could be laboratory contamination because diamond abrasive paste was used for sample preparation. Here, we report very-high-resolution re-examination of the original diamond-bearing specimens that were kindly provided to us by the authors (see Acknowledgements). We confirm the presence of diamonds in their zircons but the diamonds we find are fragments of polishing compound. These results combined with the lack of diamonds in any other samples of JHZs strongly suggest that there are no indigenous diamonds in the JHZs.

Highly Refractory Lithospheric Mantle beneath the Tanzanian Craton: Evidence from Lashaine Pre-metasomatic Garnet-bearing Peridotites

Here we report the first occurrence of ultra-depleted pyrope garnets with rare earth element (REE) patterns similar to those of hypothetical garnets proposed to have formed in the subcontinental lithospheric mantle prior to those of metasomatic origin. These unique ultra-depleted garnets have not previously been identified in global suites of mantle xenoliths or diamond inclusions. They occur in certain harzburgite members of the xenolith suite from the Lashaine tuff cone, northern Tanzania. The Lashaine ultra-depleted garnets are characterized by low concentrations of CaO (<0·35 wt %), Cr2O3 (2·5 wt %), incompatible trace elements (e.g. Ba, Sr, Ti, Zr and Y), and have chondrite-normalized REE patterns with steep positive middle-to-heavy REE slopes and flat-to-positive, light-to-middle REE slopes [i.e. they are ‘tick’ (√)-shaped]. The ultra-depleted garnets have high Mg# (92·5) and coexist in chemical and textural equilibrium with highly refractory olivine (Fo95·4) and orthopyroxene (Mg# = 96·4), which are all more magnesian than encountered in most global mantle harzburgites and diamond inclusions. The ultra-depleted garnets occur in interconnecting networks around grains of orthopyroxene, which give the rocks a banded appearance. We propose that the ultra-depleted garnets formed by isochemical exsolution from orthopyroxene following a change in geothermal gradient (decrease in temperature and increase in pressure) associated with Archean lithospheric thickening. Metasomatism of refractory mantle is recorded in the Lashaine xenoliths by an almost continuous variation from ultra-depleted to sub-calcic (harzburgitic) to calcic (lherzolitic) garnet compositions. Overall this systematic trend correlates with increasing depth and is accompanied by a change in chondrite-normalized REE patterns, from ‘√’-shaped to sinusoidal to normal (i.e. light REE depleted). We attribute these compositional variations in garnet, together with a general decrease in Mg# in all mineral phases with depth, to a gradual change from low-temperature high-density fluid metasomatism to high-temperature silicate melt-related enrichment. Pressure and temperature estimates suggest that this metasomatism is concentrated at the base of the Tanzanian lithospheric mantle (i.e. between 125 and 160 km depth), which is consistent with the results of previous studies. Harzburgites containing ultra-depleted garnets were entrained from shallower depths (∼125 km) in the lithosphere than most Lashaine lherzolites and we suggest that long-term survival of this refractory mantle reflects its location above the level reached by ascending metasomatic agents. We propose that preservation of widespread highly refractory, low-density and high-viscosity lithospheric mantle at shallow depths beneath the Tanzanian Craton may have ensured its long-term stability and resistance to delamination. The presence of refractory mantle beneath both the core and eastern margin of the Tanzanian Craton may account for the steep gradient in lithospheric thickness relative to other regions of thick stable Archean lithosphere where thinning beneath cratonic margins is more gradual.