Ultramafic Material Research Articles

Immobilisation of chromium in magnesium carbonate minerals

AbstractHexavalent chromium (Cr6+) is a toxic carcinogenic pollutant that might be released by the mining and processing of ultramafic rocks and nickel laterites and which requires permanent removal from the contaminated biosphere. Ultramafic material can also serve as a feedstock for the sequestration of CO2 resulting from the growth of new minerals, raising the intriguing proposition of integrated sequestration of both pollutants, CO2 and chromium, into magnesium carbonates. Such a synergistic process downstream of ore recovery and mineral processing could be an elegant proposition for more sustainable utilisation and management of the Earth's resources. We have therefore carried out an experimental and microanalytical study to investigate potentially suitable carbonate minerals. Uptake of chromium in carbonate phases was determined, followed by identification of the crystalline phases and characterisation of the local structural environment around the incorporated chromium centres. The results suggest that neither nesquehonite nor hydromagnesite have the structural capacity to incorporate Cr6+ or Cr3+ significantly at room temperature. We therefore propose that further research into this technology should focus on laboratory assessments of other phases, such as layered double hyroxides, that have a natural structural capacity to uptake both chromium and CO2.

Historical diamond mine waste reveals carbon sequestration resource in kimberlite residue

Mined sub-aerially stored kimberlite provided a natural laboratory in which to examine the potential for carbon sequestration in ultramafic materials. A 15 cm hand sample of ∼50-year-old ‘cemented’ coarse residue deposit (CRD) collected from a cemented surface layer in the Cullinan Diamond Mine tailings in Gauteng, South Africa, demonstrated the encouraging effects of weathering on mineral carbonation of kimberlite. The examination of petrographic sections using light microscopy, X-ray fluorescence microscopy (XFM) and backscatter electron – energy dispersive spectroscopy demonstrated that weathering produced extensive, secondary Ca/Mg carbonates that acted as an inter-granular cement, increasing the competency of the CRD, i.e., producing a hand sample. Nearly every grain in the sample, including primary, un-weathered angular carbonate clasts were coated in secondary, μm- to mm-scale carbonate layers, which are interpreted as secondary materials. DNA analysis of an internal, aseptic sample of secondary carbonate revealed that the weathered kimberlite hosts a diverse microbiome consistent with soils, metal cycling and hydrocarbon degradation that was found within the secondary carbonate, interpreted as a biomaterial. The formation of secondary carbonate demonstrates that ‘waste kimberlite’ from diamond mining can serve as a resource for carbon sequestration.

Fluid processes in the early Earth and the growth of continents

Water is an essential ingredient in transforming primitive mantle-derived (mafic) rocks into buoyant (felsic) continental crust, thereby driving the irreversible differentiation of Earth's lithosphere. The occurrence in Archaean cratons of sodic granites of the tonalite–trondhjemite–granodiorite (TTG) series, high-MgO variolitic basalts, high-Mg diorites (sanukitoids) and diamonds with harzburgitic inclusion assemblages, all require the presence of hydrous fluids in Earth's deep crust and upper (lithospheric) mantle since at least the Paleoarchaean (3.6–3.2 billion years ago). However, despite its importance, where and how water was stored in Archaean crust, and how some water was transported into the upper mantle, are poorly understood. Here, we investigate Archaean crustal fluid budgets through calculated phase equilibria for three protolith compositions — a low-MgO mafic (basaltic) composition, a high-MgO (picritic) composition and an ultrahigh-MgO ultramafic (komatiitic) composition — that are representative of mafic to ultramafic magmatic rocks in Archaean greenstone belts. We show that the mode and stability of hydrous minerals, in particular chlorite, is positively correlated with protolith MgO content, such that high-MgO basalts can store up to twice the amount of crystal-bound H2O than low-MgO basalts. Importantly, ultrahigh-MgO rocks such as komatiite can store four times as much H2O, most of which is retained until temperatures exceeding 700°C. Warmer geotherms in the early Archaean favoured dehydration of hydrated high-MgO and ultramafic rocks in the deep crust, leading to hydration and/or fluid-fluxed melting of overlying basaltic rocks to produce ‘high-pressure’ TTG magmas. Burial of Archaean mafic–ultramafic crust along cooler geotherms resulted in dehydration of ultramafic material within the lithospheric mantle, providing the source of enriched Archaean basalt that was parental to large volumes of ancient TTG-dominated continental crust.

U-Pb geochronology and isotopic geochemistry of adakites and related magmas in the Ediacaran arc section of the SW Iberian Massif: The role of subduction erosion cycles in peri-Gondwanan arcs

In the peri-Gondwanan Ediacaran arc section outcropping in the SW of the Iberian Massif (Mérida region), changes in the subduction angle and subduction rate, variable participation of highly modified mantle sources together with the role of old crustal material and juvenile mafic rocks driven by the incoming slab, occur with a recognizable cyclicality and can be traced according to the geochronology, geochemistry and isotopic sources of the magmatic bodies. Our results support the initial existence of a mantle wedge highly modified due to percolation of significant volumes of variably old crustal materials, whose isotopic sources are almost identical to those found in the oldest known metasedimentary sequence described in SW Iberian Massif, the Serie Negra Group. Old crustal sources, analogous to those of this series were introduced to high depths by common subduction an also probably by significant subduction erosion. During the oldest period recorded in the study arc section, between c. 602 and c. 550 Ma, subduction episodes that involve high rates of crustal material along with an eventual incorporation of mafic and ultramafic materials from the incoming oceanic plate, favoured by low subduction angles, lead to the generation of more silicic magmas with adakitic geochemical affinity and isotopic signature (Nd-Sr) with crustal tendency. However, an increase in the subduction angle and change to a roll-back stage dated at c. 540 Ma, caused significant decrease in the access of cortical material to the subduction channel, favouring generation of typical calc-alkaline magmas derived from the modified mantle wedge, which is also consistent with more juvenile Nd-Sr ratios. The correlation found between the magmatic events and the involved tectonic processes suggests that subduction erosion mechanisms have been very likely underestimated as active dynamic processes along the peri-Gondwana margin during at least Neoproterozoic to Early Cambrian times.

METEORITOS MARCIANOS REALMENTE VIERAM DE MARTE?

Os meteoritos marcianos, em sua maioria, são considerados rochas ígneas oriundas de materiais basálticos e ultramáficos de Marte. Atualmente, os meteoritos oriundos de Marte são divididos em 4 grupos (Shergottito, Nakhlito, Chassignito e Ortopiroxênito) de acordo com sua estrutura e composição química. Isto é importante, pois o agrupamento desses materiais facilita a buscar o possível ambiente do qual eles se originaram, ou em outras palavras, seria como saber a localização do “afloramento” e seu “contexto regional” para a geologia. Dessa forma, o objetivo principal aqui é desenvolver uma revisão ampla e sintetizada sobre os meteoritos SNC. Uma das justificativas é que em 2021, mais precisamente em fevereiro, chegará em Marte a perseverance, que irá explorar um paleolago profundo de 250 m cerca de 3,9 bilhões a 3,5 bilhões de anos atrás. Sendo assim, este pode ser um momento oportuno para reunir conteúdo em português e avaliar criticamente o que sabemos e o que precisamos ainda aprender sobre Marte. Outra justificativa é que embora exista uma quantidade substancial de artigos em inglês sobre meteoritos SNC muitos deles estejam desatualizados e a literatura em português sobre SNC é escassa, praticamente inexistente. Assim, espera-se que este artigo possa ser utilizado como referência para alunos de graduação e Pós-graduação, proporcionando informações relevantes para o desenvolvimento e aprendizagem da meteorítica, mais especificamente dos SNC, tal como fazer a difusão de seus conceitos para futuros trabalhos de iniciação científica, monografias, dissertações ou até mesmo teses.

Aqueous mineral carbonation of ultramafic material: a pre-requisite to integrate into mineral extraction and tailings management operation.

Ex situ aqueous mineral carbonation of ultramafic mining waste is an evolving technology for the CO2 sequestration from small- to medium-scale emitters. The mineral ores or mine wastes of associated ultramafic mineralogy are a suitable feedstock for mineral carbonation. The aqueous mineral carbonation at ambient temperature is motivating and attractive from an energy-saving perspective. This study has investigated the CO2 sequestration potential of a locally available ultramafic material generated from a nickel ore mine with a futuristic scope of integrating the method into an ongoing mineral extraction and/or tailing management operation. The mineral characterization and experimental results indicate that the tested material has CO2 sequestration potential and underwent carbonation at ambient temperature. The carbonate conversion efficiencies obtained for Ca and Mg from the dissolved ionic forms at optimum conditions are 60% and 25%, respectively. The material was able to sequestrate about 0.12 gCO2 per g solid at this efficiency. Aragonite and hydromagnesite are the major products that evolved out from the aqueous carbonation. Based on the mineral carbonation results, the novel concept of integrating the evolved method to existing mineral extraction and/or tailings management operation is discussed.

Inverse modeling constrained by potential field data, petrophysics, and improved geologic mapping: A case study from prospective northwest Tasmania

The Heazlewood-Luina-Waratah area is a prospective region for minerals in northwest Tasmania, Australia, associated with historically important ore deposits related to the emplacement of granite intrusions and/or ultramafic complexes. The geology of the area is poorly understood due to the difficult terrain and dense vegetation. We have constructed an initial high-resolution 3D geologic model of this area using constraints from geologic maps and geologic and geophysical cross sections. This initial model is improved upon by integrating results from 3D geometry and physical property inversion of potential field (gravity and magnetic) data, petrophysical measurements, and updated field mapping. Geometry inversion reveals that the Devonian granites in the south are thicker than previously thought, possibly connecting to deep sources of mineralization. In addition, we identified gravity anomalies to the northeast that could be caused by near-surface granite cupolas. A newly discovered ultramafic complex linking the Heazlewood and Mount Stewart Ultramafic Complexes in the southwest also has been modeled. This implies a greater volume of ultramafic material in the Cambrian successions and points to a larger obducted component than previously thought. The newly inferred granite cupolas and ultramafic complexes are targets for future mineral exploration. Petrophysical property inversion reveals a high degree of variation in these properties within the ultramafic complexes indicating a variable degree of serpentinization. Sensitivity tests suggest maximum depths of 2–3 km for the contact aureole that surrounds major granitic intrusions in the southeast, whereas the Heazlewood River complex is likely to have a deeper source up to 4 km. We have demonstrated the value of adding geologic and petrophysical constraints to 3D modeling for the purpose of guiding mineral exploration. This is particularly important for the refinement of geologic structures in tectonically complex areas that have lithology units with contrasting magnetic and density characteristics.

Petrogenesis of Late Carboniferous intrusions in the Linglong area of Eastern Tianshan, NW China, and tectonic implications: Geochronological, geochemical, and zircon Hf–O isotopic constraints

The Linglong porphyry Cu deposit is located in the Dananhu–Tousuquan Arc Belt, adjacent to the large Tuwu–Yandong Cu deposits in Eastern Tianshan, northwest China. In this study, zircon U–Pb, whole rock geochemistry, and zircon Hf–O isotopic analyses were carried out on the Linglong intrusions (i.e., quartz albite porphyry and diorite porphyry). New SIMS zircon U–Pb dating and previous data indicate that the quartz albite porphyry occurred in the Late Carboniferous (318.6 ± 3.0 Ma), younger than emplacement of the diorite porphyry (338–340 Ma) and tonalite porphyry (332–335 Ma) in this area, as well large-scale copper mineralization (331–335 Ma) in Eastern Tianshan. The geochemistry of the quartz albite porphyry intrusions is consistent with calc-alkaline normal arc magmatism, characterized by high SiO2 (74.82–77.89 wt%), obvious Eu anomalies (0.42–0.52), high Y (21–25.9 ppm) and Yb (2.98–3.67 ppm) concentrations, moderate Mg# values (35–40), but relatively low Sr/Y (6.86–9.52)and low 10,000 Ga/Al values. In situ Hf–O isotopic analyses on zircons show that the quartz albite porphyry rocks have variable εHf(t) (+9.0 to + 13.9) and low δ18O values (4.72 to 5.84‰), suggesting a juvenile lower crust derived composition. However, the diorite porphyry intrusions show features of low SiO2 (57.51–61.12 wt%), minor Eu anomalies (0.82–1.15), strong depletion in high field strength elements (e.g., Nb, Ta, P and Ti), but enrichment in large ion lithophile elements (e.g., Rb, Ba, K and Pb). They also exhibit normal island arc magma geochemical signatures, with high Y (20.9–40.2 ppm), and Yb (2.75–4.64 ppm) concentrations, high Mg# (40–51), and relatively low (La/Yb)N (2.07–4.10) and Sr/Y (5.75–12.13) values, suggesting the melt was derived from the subduction-modified mantle mafic to ultramafic materials. Combined with regional tectonic evolution and our new data, we suggest that the Late Carboniferous magma was generated during the period of the northward subduction of the Paleo-Tianshan ocean plate beneath the Dananhu–Tousuquan island arc.

PGE mineralization in andesite explosive breccias associated with the poperechny iron-manganese deposit (Lesser Khingan, Far East Russia): Whole-rock geochemical, 190Pt-4He isotopic, and mineralogical evidence

We report in this paper an unusual occurrence of platinum-group minerals in evolved explosive breccia associated with the Poperechny iron-manganese deposit (Lesser Khingan Range, Far East Russia). PGMs in andesite breccia are represented by Fe-Pt solid solutions (85%) and PGM (mostly Os-Ir-Ru) solid solutions, sulfides and sulfarsenides (15%). Textural and compositional variations in PGM assemblages suggest that Pt-Fe and Os-Ir-Ru solid solutions, as well as erlichmanite-laurite series sulfides were formed during high-temperature fractionation of mantle-derived mafic parental melt (similar to Alaskan-type complexes) and were entrained in the evolved andesitic melt during its emplacement in the crust. Pd-Pt plumbostannide and copper-gold solid solutions reflect late magmatic re-crystallization and metasomatism. Early Cretaceous (~125 Ma) age of ferroplatinum in the explosive breccia suggests that PGM-bearing ultramafic material could have been sampled during regional slab-window tectonics related to the Late Mesozoic subduction of Izanagi plate along southern margin of the North Asian continent.

Experimental Modeling of the Formation of the Metallic Core of the Moon by the Method of High-Temperature Centrifuges

At early stages of the formation of planetary bodies, silicate, metallic, and sulfide phases, which have been formed in the course of the partial or complete fusion of the initial planetary substance, were subject to a gravitational differentiation. The latter is related to the separation of the crystalline and molten phases in density and composition. Althose in recent years a number of papers deal with the experimental and theoretical determination of the processes of the percalation of metallic melts through the crystalline silicate matrix and the physical properties of the metallic and sulfide melts. For the Moon, the problem of the existence of the metallic or sulfide core still remains unsolved. There is stil the nessity of further investigation in this direction. The possible origin of the Moon’s metallic core at the precipitation of iron-sulfide phases during the partial melting of ultramafic material under various redox conditions was experimentally modeled by partially melting the model system olivine (85 wt %)+ ferrobasalt (10 wt %)+ metallic phase Fe95S5 (5 wt %) in a high-temperature centrifuge at 1430-1450°C.

The compositions of the lunar crust and upper mantle: Spectral analysis of the inner rings of lunar impact basins

The innermost ring in impact basins exposes material originating from various depths, and can be used to study the composition of the lunar crust with depth. In this study, we conduct quantitative mineralogical analyses of the innermost ring in 13 lunar impact basins using reflectance data from the Kaguya Multiband Imager and radiative transfer modeling. We use results from recent hydrocode modeling to calculate the depth of origin of the material exposed by the innermost rings. We find that the most abundant rock type on the innermost ring of most basins is anorthosite. The mafic assemblages are dominated by olivine in some cases, but most often by pyroxene. The impact modeling suggests that the innermost ring material was excavated from a wide range of depths. Here we focus on two mean depths: a crustal component and a mantle component. The crustal component largely dominates the innermost ring material, and the mantle component is present on the innermost ring of 9 of the basins we studied. On these 9 rings, the abundance of low-calcium pyroxene decreases with the proportion of crustal component, suggesting a dominantly mantle origin. However, as we do not detect exposures of ultramafic material, such mantle material is possibly present at the sub-pixel scale (<62 m). This quantitative study reassesses the composition of the lunar crust and upper mantle, which is of great importance for understanding the formation of the Moon.

Elemental Concentration in Serpentinitic Soils over Ultramafic Bedrock in Sierra Bermeja (Southern Spain)

Although the presence of potentially toxic metals in soils is normally associated with human soil pollution, these elements also appear naturally in environments in which the lithological base contains ultramafic rocks such as peridotites. Serpentinitic soils tend to develop on substrates of this kind, often containing metals with few or no known biological functions, which in some cases are toxic for most plants. This study assessed the level of potentially toxic metals and other elements in an endorheic basin discovered in Sierra Bermeja (Southern Spain), one of the largest peridotite outcrops on Earth. In this location—of particular interest given that basins of this kind are very rare on peridotites—six geomorphoedaphic sub-units on three different substrates were identified. The distribution of microelements in these sub-units was analyzed, and stratified random sampling was performed to identify the major ions with essential functions for living organisms and the potentially toxic metals. The lowest values for macronutrients appeared in the soils formed on ultramafic materials. When analyzing the load of potentially toxic metals, no significant differences were detected between the soils formed on serpentinite and peridotite substrates, although different values were obtained in the soils formed over acidic rocks.

Characteristics of fragipan B horizons developed on different parent material in North-Western Italy

ABSTRACTA Fragipan (Bx) is a soil horizon hard when dry and brittle when moist, that undergoes slaking upon water immersion, forming a barrier to roots and limiting land use. Brittleness and slaking depend on soil porosity and particle arrangement, but still no agreement exists on the inorganic components responsible for such arrangement. We hypothesized that the same kind of particle arrangement may originate from different soil components, ultimately depending on the lithology of parent material, and evaluated the soil and clay characteristics that best differentiate Bx from B horizons. Thirty-six samples were taken from Typic Fragiudalfs developed on the two sides of an alluvial fan characterized by different amounts of ultramafic materials. Discriminant Analysis evidenced that pedogenic Fe oxides were fundamental in discriminating Bx from B horizons on pure ultramafic parent material, while clay mineralogy was more important in soils with less ultramafic materials. In the first case, the association between clay minerals and self-assembling oxides systems may lead to brittleness, while in the second case the higher abundance of kaolinite may contribute to weak associations between pH-dependent sites and negatively charged surfaces. The only common feature to fragipans on both river sides was a higher degree of weathering.

Using mineral geochemistry to decipher slab, mantle, and crustal input in the generation of high-Mg andesites and basaltic andesites from the northern Cascade Arc

To better understand the role of slab melt in the petrogenesis of North Cascades magmas, this study focuses on petrogenesis of high-Mg lavas from the two northernmost active volcanoes in Washington. High-Mg andesites (HMA) and basaltic andesites (HMBA) in the Cascade Arc have high Mg# [molar Mg/(Mg+Fe 2+ )] relative to their SiO 2 contents, elevated Nd/Yb, and are Ni- and Cr-enriched. The rock units examined here include the Tarn Plateau HMBA (51.8–54.0 wt% SiO 2 , Mg# 68–70) and Glacier Creek HMA (58.3–58.7 wt% SiO 2 , Mg# 63–64) from the Mount Baker Volcanic Field, and the Lightning Creek HMBA (54.8–54.6 SiO 2 , Mg# 69–73) from Glacier Peak. This study combines major and trace element compositions of minerals and whole rocks to test several petrogenetic hypotheses and to determine which, if any, are applicable to North Cascades HMA and HMBA. In the Tarn Plateau HMBA, rare earth element (REE) equilibrium liquids calculated from clinopyroxene compositions have high Nd/Yb that positively correlates with Mg#. This correlation suggests an origin similar to that proposed for Aleutian adakites, where intermediate, high Nd/Yb slab-derived melts interact with the overlying mantle to become Mg-rich, and subsequently mix with low Nd/Yb, mantle-derived mafic magmas with lower Mg#. In the Glacier Creek HMA, elevated whole-rock MgO and SiO 2 contents resulted from accumulation of xenocrystic olivine and differentiation processes, respectively, but the cause of high Nd/Yb is less clear. However, high whole-rock Sr/P (fluid mobile/fluid immobile) values indicate a mantle source that was fluxed by an enriched, hydrous slab component, likely producing the observed high Nd/Yb REE signature. The Lightning Creek HMBA is a hybridized rock unit with at least three identifiable magmatic components, but only one of which has HMA characteristics. Cr and Mg contents in Cr-spinel and olivine pairs in this HMA component suggest that its source is a strongly depleted mantle, and high whole-rock Sr/P values indicate mantle melting that was induced through hydration, likely adding the component responsible for the observed high Nd/Yb REE pattern. The elevated SiO 2 contents (54.6 wt%) of the HMA component resulted from differentiation or high degrees of partial melting of ultramafic material through the addition of H 2 O. Therefore the Lightning Creek HMBA is interpreted to have originated from a refractory mantle source that underwent melting through interaction with an enriched slab component. Our results indicate that in addition to slab-derived fluids, slab-derived melts also have an important role in the production of HMA and HMBA in the north Cascade Arc.

Crustal Structure Along Sunda-Banda Arc Transition Zone from Teleseismic Receiver Functions

We analyzed receiver function of teleseismic events recorded at twelve Indonesian-GEOFON (IA-GE) broadband stations using nonlinear Neighbourhood Algorithm (NA) inversion and H-k stacking methods to estimate crustal thickness, Vp/Vs ratios and S-wave velocity structure along Sunda-Banda arc transition zone. We observed crustal thickness of 34–37 km in Timor Island, which is consistent with the previous works. The thick crust (> 30 km) is also found beneath Sumba and Flores Islands, which might be related to the arc-continent collision causing the thickened crust. In Timor and Sumba Islands, we observed high Vp/Vs ratio (> 1.84) with low velocity zone that might be associated with the presence of mafic and ultramafic materials and fluid filled fracture zone. The high Vp/Vs ratio observed at Sumbawa and Flores volcanic Islands might be an indication of partial melt related to the upwelling of hot asthenosphere material through the subducted slab.

Effect of redox conditions on iron metal phase segregation during experimental high-temperature centrifuge modeling of the origin of the Moon’s core

The possible origin of the Moon’s metallic core at the precipitation of iron–sulfide phases during the partial melting of ultramafic material under various redox conditions was experimentally modeled by partially melting the model system olivine (85 wt %) + ferrobasalt (10 wt %) + metallic phase Fe95S5 (wt %) in a high-temperature centrifuge at 1430–1450°C. The oxygen fugacity fO2 was determined from the composition of the quenched experimental silicate melts (glasses). A decrease in fO2 is proved to be favorable for the segregation of iron–sulfide melt from the silicate matrix. The metallic phase is most effectively segregated in the form of melt droplets, and these droplets are accumulated in the lower portions of the samples under strongly reduced conditions, at fO2 ∼ 4.5–5.5 orders of magnitude lower than the iron–wustite buffer.

Magnetic modelling and interpretation of the Hay-Booligal Zone and its basement

SUMMARY Little is known about the basement of the Hay–Booligal Zone (located in NSW). Magnetic modelling of long wavelength anomalies within the Hay–Booligal Zone indicates that the Hay–Booligal basement consists of serpentinised ultramafic material at depths of 6 to 12 km. This supports interpretation that the Hay–Booligal basement could be similar to the Selwyn Block in Victoria and rather than a crystalline microcontinent. Also, interpretation of short wavelength semi-parallel TMI anomalies near the previous boundary of the Hay–Booligal Zone has lead to the adjusted the boundary of the Hay– Booligal and Bendigo zones.

The Rhodope Zone as a primary sediment source of the southern Thrace basin (NE Greece and NW Turkey): evidence from detrital heavy minerals and implications for central-eastern Mediterranean palaeogeography

Detrital heavy mineral analysis coupled with a regional geological review provide key elements to re-evaluate the distribution of the Rhodope metamorphic zone (SE Europe) in the region and its role in determining the evolution of the Thrace basin. We focus on the Eocene–Oligocene sedimentary successions exposed in the southern Thrace basin margin to determine the dispersal pathways of eroded crustal elements, of both oceanic and continental origins, as well as their different contributions through time. Lithological aspects and tectonic data coupled with geochemistry and geochronology of metamorphic terranes exposed in the area point to a common origin of tectonic units exposed in NW Turkey (Biga Peninsula) with those of NE Greece and SE Bulgaria (Rhodope region). The entire region displays (1) common extensional signatures, consisting of comparable granitoid intrusion ages, and a NE-SW sense of shear (2) matching zircon age populations between the metapelitic and metamafic rocks of the Circum-Rhodope Belt (NE Greece) and those of the Camlica–Kemer complex and Cetmi melange exposed in NW Turkey. Detrital heavy mineral abundances from Eocene–Oligocene sandstones of the southern Thrace basin demonstrate the influence of two main sediment sources mostly of ultramafic/ophiolitic and low- to medium-grade metamorphic lithologies, plus a third, volcanic source limited to the late Eocene–Oligocene. Detrital Cr-spinel chemistry is used to understand the origin of the ultramafic material and to discriminate the numerous ultramafic sources exposed in the region. Compositional and stratigraphic data indicate a major influence of the metapelitic source in the eastern part (Gallipoli Peninsula) during the initial stages of sedimentation with increasing contributions from metamafic sources through time. On the other hand, the western and more external part of the southern Thrace margin (Gokceada, Samothraki and Limnos) displays compositional signatures according to a mixed provenance from the metapelitic and metamafic sources of the Circum-Rhodope Belt (Camlica–Kemer complex and Cetmi melange). Tectonic restoration and compositional signatures provide constraints on the Palaeogene palaeogeography of this sector of the central-eastern Mediterranean region.

Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

Abstract. Allochthonous accreted terranes are exotic geologic units that originated from anomalous crustal regions on a subducting oceanic plate and were transferred to the overriding plate by accretionary processes during subduction. The geographical regions that eventually become accreted allochthonous terranes include island arcs, oceanic plateaus, submarine ridges, seamounts, continental fragments, and microcontinents. These future allochthonous terranes (FATs) contribute to continental crustal growth, subduction dynamics, and crustal recycling in the mantle. We present a review of modern FATs and their accreted counterparts based on available geological, seismic, and gravity studies and discuss their crustal structure, geological origin, and bulk crustal density. Island arcs have an average crustal thickness of 26 km, average bulk crustal density of 2.79 g cm−3, and three distinct crustal units overlying a crust–mantle transition zone. Oceanic plateaus and submarine ridges have an average crustal thickness of 21 km and average bulk crustal density of 2.84 g cm−3. Continental fragments presently on the ocean floor have an average crustal thickness of 25 km and bulk crustal density of 2.81 g cm−3. Accreted allochthonous terranes can be compared to these crustal compilations to better understand which units of crust are accreted or subducted. In general, most accreted terranes are thin crustal units sheared off of FATs and added onto the accretionary prism, with thicknesses on the order of hundreds of meters to a few kilometers. However, many island arcs, oceanic plateaus, and submarine ridges were sheared off in the subduction interface and underplated onto the overlying continent. Other times we find evidence of terrane–continent collision leaving behind accreted terranes 25–40 km thick. We posit that rheologically weak crustal layers or shear zones that were formed when the FATs were produced can be activated as detachments during subduction, allowing parts of the FAT crust to accrete and others to subduct. In many modern FATs on the ocean floor, a sub-crustal layer of high seismic velocities, interpreted as ultramafic material, could serve as a detachment or delaminate during subduction.

An appraisal of an iterative construction of the endmembers controlling the composition of deep-sea manganese nodules from the Central Indian Ocean Basin

This paper describes an estimation of endmember compositions followed by the assessment of those results by log-ratio variance analysis. As an appraisal, it deals only with the first objective of an endmember analysis namely, to identify endmembers if they exist by estimating their compositions. Following the creation of the endmember estimates, the computation of an array of log-ratio variances was a key innovation in this type of study. Log-ratio variances revealed intrinsic linear associations between the dominant elements on each of the estimated endmember compositions, largely confirming the endmember analysis. The dataset under study contained the concentrations of 16 elements in 93 samples of deep-sea manganese nodules from the Central Indian Ocean Basin. Many previous analyses of these nodules were undertaken to assess the economic potential of the deposits. This study by contrast, quantified the inter-element associations that account for the nodule compositions. Four endmembers were identified. The elements loaded on each were: (1) Mn, Zn, Ni, Cu, Mn-rich, (2) Fe, Ti, P, Co, Fe-rich, (3) Si, Al, Na, K, clay minerals, (3) Mg, ultramafic material, possibly including Mn, Cr, V, Ca, Na. These latter elements were also detected by their log-ratio variances to be associated with Mg on the 4th endmember.