Dunitic Zone Research Articles

A long section of serpentinized depleted mantle peridotite.

The upper mantle is critical for our understanding of terrestrial magmatism, crust formation, and element cycling between Earth's solid interior, hydrosphere, atmosphere, and biosphere. Mantle composition and evolution have been primarily inferred by surface sampling and indirect methods. We recovered a long (1268-meter) section of serpentinized abyssal mantle peridotite interleaved with thin gabbroic intrusions. We find depleted compositions with notable variations in mantle mineralogy controlled by melt flow. Dunite zones have predominantly intermediate dips, in contrast to the originally steep mantle fabrics, indicative of oblique melt transport. Extensive hydrothermal fluid-rock interaction is recorded across the full depth of the core and is overprinted by oxidation in the upper 200 meters. Alteration patterns are consistent with vent fluid composition in the nearby Lost City hydrothermal field.

Double-Front Crystallization in the Chapesvara Ultramafic Subvolcanic Complex, Serpentinite Belt, Kola Peninsula, Russia

Dunite–harzburgite–olivine-bearing orthopyroxenite successions in the subvolcanic Chapesvara-I and Chapesvara-II intrusions in the Serpentinite Belt, western Kola Peninsula, are notably magnesian. The mean Mg# value (whole-rock) is 86.6, and the olivine is Fo84−89. The upper contact facies (UCF) displays a lower Mg# (81.6). It consists of grains of Fo92 and abundant chromian spinel, implying rapid crystallization of an almost unfractionated melt. On average, the whole-rock Al2O3/TiO2 value is 22.45, close to 22.9 (UCF) and to the primitive mantle, ~22. The rise of primitive ultramafic magma presumably occurred in a special tectonic setting at the boundary of the Paleoproterozoic Lapland Granulite Terrane and the Belomorian Composite Terrane of Archean age. The Chapesvara suite resembles examples of the Al-undepleted komatiites in the Barberton Belt, South Africa, with magmas of up to 30–35% MgO. The UCF rock yields an anomalously low molar MgO/SiO2 value, close to that of dunitic rocks located at the center of the Chapesvara-II body. This rock is the most primitive, as indicated by the maximum Fo content of olivine, the lowest value of (Gd/Yb)N, 0.52, and the lowest abundances of middle to heavy rare-earth elements (REE) in the chondrite-normalized spectrum. The crystallization of the Chapesvara-II sill-like intrusion likely proceeded in two stages, which are evident from the olivine compositions varying from the maximum Fo92 (UCF) to Fo≤89.5 (the central dunite zone). At Stage 1, the UCF rock (Fo92) crystallized first, close to the upper contact. The area of crystallization then shifted to a central portion of the Chapesvara-II body, in which the dunitic zone (Fo89.5) formed in situ (Stage 2). The compositional variations in chromian spinel are consistent with this suggestion. Two crystallization trends were recognized. The type-1 trend displays a relative maximum or minimum close to the center, and then diverges into two linear subtrends directed upward and downward. This pattern is manifested in the variations of Mg# in olivine and chromian spinel, the whole-rock contents of Al and Ca, and in levels of incompatible elements: Ti, V, Zr, Y, and Hf. The type-2 trend decreases or increases uniformly from top to bottom. Variations in amount of Ni in olivine, the Fe3+# index in chromian spinel, and in values of Mg# (rocks), follow a type-2 trend. Variations in total amounts of REE, Nb, and Th, which gradually increase downward, are also related to a type-2 trend. Thus, a contrasting development and possible interference of the two types of evolutionary trends were observed in the crystallization history of the Chapesvara-II sill-like body. A double-front crystallization, hitherto unreported, involved two fronts moving upward and downward, respectively. The upward subtrend appeared to be of subordinate importance, whereas the extent of fractional crystallization of the downward front was much greater. Crystallization proceeded from the top to the bottom, presumably because of the preferential loss of heat at the roof. Variations in the Fe3+# index indicate that the level of fO2 also increased downward with progressive crystallization. Convection cells were presumably the key mechanism of accumulation of the crystallizing olivine grains to form the central dunite zone close to the center of the sill-like intrusion. The observed characteristics of the Chapesvara complex indicate the existence of a primitive-mantle source and imply a highly magnesian composition of intruding magma not only for Chapesvara, but also for the Pados-Tundra layered complex and associated suites of the Serpentinite Belt in the Kola Peninsula.

Chromite, platinum group elements and nickel mineralisation in relation to the tectonic evolution of the Dun Mountain Ophiolite Belt, east Nelson, New Zealand

ABSTRACTIn the Dun Mountain Ophiolite Belt of east Nelson, New Zealand, chromite segregations (chromitites) occur in a lower zone of serpentinised harzburgite and dunite from Wooded Peak (Dun Mountain) to Little Ben, and in dunite-rich zones in the Red Hills massif and Baldy Ridge outlier. Electron probe microanalyses of chromite grains from the chromitites and also the Red Hills peridotite reveal a range in chromite compositions that are consistent with depletion of peridotite and reaction with island arc tholeiite and boninitic melts in a forearc setting of a supra-subduction zone. Podiform chromitite samples in the harzburgite zone show elevated Ir and Ru values, similar to the levels of enrichment of these platinum group elements (PGE) found in podiform chromite deposits in ophiolites worldwide. The PGE values in dunite-hosted chromitite from Baldy Ridge (Matakitaki) have elevated Pt, Pd and Rh, as is found in cumulate dunite zones in some other ophiolites. Disseminated Ni mineralisation as awaruite ± magnetite ± ferritchromit ± heazlewoodite ± pentlandite ± native Cu occurs in serpentinised peridotites, and was likely produced as a by-product of hydration as a result of detachment faulting allowing penetration of seawater into underlying mantle peridotite.

Melilite-derived mineral inclusions in chromite from the Gaositai complex: Implications for an extensional tectonic setting in Early Permian at the north North China Craton

The Early Permian mafic-ultramafic concentrically zoned Gaositai intrusion at Chengde, on the northern margin of the North China Craton (NCC), is a cumulative complex emplaced along a giant fracture that penetrates deeply into the continental lithosphere. Melt inclusions are present in chromite crystals from the inner dunite and chromitite zones of the Gaositai complex. The melt inclusions have experienced post-trap crystallization and resulted in multiple mineral phases, including melilite, garnet, phlogopite, magnesite and apatite, which can indicate the liquidus minerals of the primitive magma. The characteristics of the melilite+melanite+clinopyxene assemblage indicate that the primary parental magma was highly undersaturated and derived from an alkali-rich mantle source. The crystallization of phlogopite, magnesite and apatite suggests a primary magma rich in K, H2O and CO2. When compared with experimental data, the primary magma of the Gaositai intrusion is concordant with a kamafugite magma originating from partial melting of enriched mantle with H2O and CO2 at pressures greater than 2.7 GPa. This magmatic process would have been related to extensional thinning of the continental lithosphere. The Gaositai primary magmas have high Nb/La ratios, which are similar to those of ocean island basalts, but different from arc-related magmas. This suggests that the northern margin of the NCC was not an active continental margin of the Paleo-Asian Ocean subduction zone during the Early Permian: an extensional tectonic setting during the emplacement of the Gaositai intrusion is more likely.

Biogeochemical study of chromite bearing zones in Forumad area, Sabzevar ophiolite, Northeastern Iran

Chromite ores in Forumad area from Sabzevar ophiolite belt, northeastern Iran are surrounded by magmatic dunite that is rich in nickel as compared with other ultramafics. Chromium is intensively bonded within chromite and is not an essential element for plant's life cycle, whereas Ni is more bioavailable and is an essential constituent of plant enzymes. Determined bio-concentration factor (BCF) for Ni in Salsola orientalis S.G. Gmel., the predominant plant species on the soils developed on dunite zones from 20 locations (average 0.81) indicated that Ni can be successfully used as a pathfinder for tracing the chromite ore bodies hosting dunites in the studied area. In soils developed on serpentinized ultramafics, maximum total concentrations of 719, 1150, 181, 617, 4481, 2314, and 20,790μgg−l were determined for Ni, Cr, Co, Mn, Fe, Mg, and Ca, respectively by using Atomic Absorption Spectroscopy (AAS). Dry climate, low rainfall and high temperature of the area lead to grow a specific flora including annual and perennial herbaceous plants. The chemical data showed excellent correlation coefficient between Cr and Ni (0.91) in soils developed on dunite zones. Investigation on 107 species from 30 families, growing on the studied soils indicated that S. orientalis could consider as promising plant for prospecting of chromite ores in Sabzevar ophiolite belt in reconnaissance survey.

Petrology and Geochemistry of the Crust–Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific

The Massif du Sud ophiolite, New Caledonia, SW Pacific, is one of the largest exposed ultramafic bodies on Earth. The ophiolite consists of a mantle section of ultra-depleted tectonite harzburgite, overlain by a large dunite zone, which is separated by a transition zone from the gabbros at the top of the massif. Profiles through the stratigraphy of the mantle section show complex geochemical and petrological variations. The harzburgites are characterized by a high degree of partial melting and document a complex evolution from mantle exhumation towards a supra-subduction zone environment. Olivine and spinel compositions suggest that the harzburgites are residual after boninite-like melt extraction at 5–10 kbar. The lower dunite zone is analogous to the replacive dunite channels in the harzburgite, in that it is formed by pyroxene consumption. In contrast, the upper dunite zone has chemical and textural characteristics of cumulus olivine crystallized from primitive mafic melts. The upper dunite zone grades into a transition zone with pyroxenite cumulates that are intruded by gabbro sills. The crystallization sequence and mineral compositions indicate that pyroxenites and gabbros formed from hydrous, oxidized primitive basaltic magmas at ∼1250°C and 2–4 kbar. Their geochemistry indicates that these parental melts are transitional between boninites and primitive arc magmas and carry a fore-arc basalt signature. The Massif du Sud therefore represents a crust–mantle section in a nascent arc. The cryptic transition between residual mantle rocks and crustal cumulates highlights the difficulty of estimating the thickness and average composition of an arc crust by seismic methods.

Review of platinum-group element distribution and mineralogy in chromitite ores from southern Iran

The platinum-group element (PGE) potential of four chromite mining districts in southern and south-eastern Iran was investigated using geochemical and mineralogical methods. A total of 20 mines and prospects were sampled in the Neyriz ophiolite, the Abdasht and Sikhuran complexes in the Esfandagheh district, and the Faryab district. Chromitite and dunite were analysed for major, trace element and PGE geochemistry, and the platinum-group element mineralogy (PGM). In all mining districts, chromites analysed from mantle tectonite, probable transition zone and cumulate zone chromitites are refractory, low in TiO2 (<0.3wt.%) as well as Cr- and Mg-rich, with Cr# [100∗Cr/(Cr+Al)] ranging from 70 to 81 and Mg# [100∗Mg/(Mg+Fe2+)] ranging from 40 to 83. Associated olivine is highly magnesian (Mg# 89–98) and Ni-rich (0.1–1.0wt.% NiO).PGE concentrations range from <100 to >5000ppb, with median values around 200ppb in all complexes investigated. The 187Os/188Os isotope ratios of chromitite indicate chondritic to slightly suprachondritic initial Os. Mantle-normalised Pt/Ir ratios are generally low (<0.2), but are up to 0.5 in small chromite pods in the harzburgitic mantle section of the Neyriz ophiolite. PGM, mainly laurite–erlichmanite, mostly occur as small (<1 to 30μm) inclusions in chromite. They are associated with hydrous silicates (calcic amphibole), base metal sulphides (Ni- and Cu-sulphides) and other PGM, such as cuproiridsite, Ni–Ir sulphides, and Os–Ir alloys. PGMs interstitial to chromite are represented by irarsite, hollingworthite, Ru–Os–Ir alloys, tolovkite, RhNiAs, Pt–Fe alloy, probable Ru-rich oxides, and others.In the Sikhuran complex, pods of sulphide-bearing chromitite (0.2–1.2% sulphides) occur in a dunite zone interpreted as an ultramafic cumulate zone. Chromites in chromitite are characterised by high Cr# (75–81) and intermediate Mg# numbers (45–65); in associated dunite, chromites (Cr#, 74–80) are more Fe-rich (Mg#, 29–50). Chromitite and dunite carry up to 5.2ppm total PGE and are characterised by high normalised Pt/Ir (up to 5), Pt/Pd<1, and radiogenic Os (moderately suprachondritic initial Os). The interstitial sulphides are dominated by pentlandite that carries on average 0.45% Co, 25ppm Os, 11ppm Ir and 8ppm Rh. Palladium and Pt concentrations in pentlandite are below the detection limit of the Laser Ablation–ICP-MS technique. Pentlandite is associated with rare grains of Pd- and Pt-minerals such as stibiopalladinite and Pt–oxide, and with Cu-rich phases.Large degrees of partial melting in a supra-subduction zone geotectonic setting, probably in a two-stage melting process, produced boninitic melts that crystallised refractory, Cr–Mg rich and Ti-poor chromite. In some cases, sulphide saturation was achieved during and after chromite formation. Saturation during chromite crystallisation is evidenced by Ni–Cu-rich sulphide droplets preserved in some chromitites. Sulphur saturation after chromite crystallisation governed the formation of Ni-rich sulphide liquids that exsolved monosulphide solid solution, which later crystallised to form PGE-bearing pentlandite. Platinum and Pd did not partition into pentlandite but crystallised as Pt- and Pd-rich PGM associated with the pentlandite.

Reactive Melt Flow as the Origin of Residual Mantle Lithologies and Basalt Chemistries in Mid-Ocean Ridges: Implications from the Red Hills Peridotite, New Zealand

A reactive flow geochemical model based on pMELTS thermodynamic calculations explains the observed modal, major, and trace element variations in the Red Hills peridotite, New Zealand. The model also reproduces the major and trace element chemical variations in mid-ocean ridge basalts (MORB) observed in present-day spreading ridges. The Red Hills peridotite is thought to originate from palaeo-MOR magmatic processes in the mantle–Moho transition zone. The peridotite body consists of a harzburgite matrix and dunite channels. The harzburgite forms the Lower Unit and is intruded by replacive dunite channels in the Upper Unit. This lithology gradually turns into a massive dunite zone in which disseminated to lenticular clinopyroxene aggregates are present. The rare earth element (REE) abundances in the peridotite samples vary greatly depending on their lithologies. In the Lower Unit, REE are extremely depleted, whereas in the Upper Unit they are relatively enriched, in contradiction to the depleted lithologies. Our model consists of two stages. The first stage assumes melting of depleted MORB source mantle in the garnet stability field, and the second assumes reactions between residual solids and the melts from the first stage in the spinel stability field in an open system. The model explains the formation of depleted harzburgite and the formation of dunite channels in the harzburgite matrix well. The major and trace element compositions of the melts calculated by the model vary from ultra-depleted MOR melts in harzburgite to normal MORB in dunite, suggesting that these lithologies are residues of a palaeo-MOR. The model also explains the origins of the local and global geochemical trends observed in MORB and the geochemical variation in abyssal peridotite samples. Our model confirms the important role of reactive flow in the mantle–Moho transition zone beneath MORs.

The Dovyren intrusive complex: Problems of petrology and Ni sulfide mineralization

This paper presents a review of petrological-geochemical studies at the Yoko-Dovyren Massif with an emphasis on relations between parameters of the parental magma, a model for the genesis of the lower con- tact zone, and the nature of Ni sulfide ore mineralization, including the evaluation of the possible ore potential. Arguments are presented in support of the conclusion that the Dovyren magma brought much intratelluric oli- vine of the composition Fo 85-87 into the chamber, and the composition of the initial melt corresponded to gab- bronorite or moderately magnesian basite with no more than 10 wt % MgO. The probable temperature of the parental magma was approximately 1200-1250°e , and the sulfur solubility did not exceed 0.10-0.12 wt % ( P = 1 kbar, WM buffer). The comparison of this estimate with the average S contents in the bottom plagioperi- dotites ( 0.12 ± 0.06 wt %) indicates that the initial magma was saturated with a sulfide phase. For the first time the problem of the composition of contaminated dunites was formulated (these rocks occur in the Layered Series and contain more magnesian olivine Fo 87-92 ). The reason for the increase in the mg# of olivine is thought to be the partial melting and compaction of the original cumulates due to the infiltration of intercumulus melt enriched in volatile components. The volatiles were presumably provided by the thermal decomposition of car- bonate xenoliths, a process that resulted in an increase in the CO 2 pressure and the transfer of calcite-magnesite components of carbonates into the melt. This follows from (1) the occurrence of magnesian skarn developing after carbonates, (2) high CaO contents in olivine form the contaminated dunite, (3) the appearance of olivine- bearing pyroxenites and wehrlites in the upper part of the dunite zone, (4) correlation between the olivine and chromite composition in the contaminated and uncontaminated dunites, (5) broad variations in the oxygen iso- topic composition of olivine and plagioclase from rocks of the Layered Series, (6) experimental data on the dis- solution of carbonates in alkali basalt melts, and (7) analogies with isotopic-geochemical characteristics of rocks from the Jinchuan ultramafic complex. Petrological implications of the interpretation of the Dovyren chamber are discussed with reference to closed and flow-through (during an initial stage) magmatic systems. A petrological-geological model is proposed for the genesis of the Synnyr-Dovyren volcanic-plutonic complex and related Ni sulfide ore mineralization. The potential resources of Cu-Ni sulfide ores in the plagioperidotites are evaluated with regard to the still-unexposed part of the massif.

Forearc extension and sea-floor spreading in the Thetford Mines Ophiolite Complex

Abstract The Ordovician Thetford Mines Ophiolite Complex (TMOC) is an oceanic terrane accreted to the Laurentian margin during the Taconic Orogeny and is affected by syn-obduction (syn-emplacement) deformation and two post-obduction events (Silurian backthrusting and normal faulting, and Acadian folding and reverse faulting). The southern part of the TMOC was tilted to the vertical during post-obduction deformation and preserves a nearly complete cross-section through the crust. From base to top we distinguish cumulate Dunitic, Pyroxenitic and Gabbroic Zones, a hypabyssal unit (either sheeted dykes or a subvolcanic breccia facies), and an ophiolitic extrusive-sedimentary sequence, upon which were deposited sedimentary rocks constituting the base of a piggy-back basin. Our mapping has revealed the presence of subvertically dipping, north-south- to 20°-striking faults, spaced c. 1 km apart on average. The faults are manifested as sheared or mylonitic dunites and synmagmatic breccias, and correspond to breaks in lithology. The fault breccias are cut by undeformed websteritic to peridotitic intrusions, demonstrating the pre- to synmagmatic nature of the faulting. Assuming that rhythmic cumulate bedding was originally palaeo-horizontal, kinematic analysis indicates that these are normal faults separating a series of tilted blocks. In the upper part of the crust, the north-south-striking fault blocks contain north-south-striking dykes that locally constitute a sheeted complex. The faults correspond to marked lateral changes in the thickness and facies assemblages seen in supracrustal rocks, are locally marked by prominent subvolcanic breccias, and have upward decreasing throws suggesting that they are growth faults. The base of the volcano-sedimentary sequence is a major erosional surface in places, which can penetrate down to the Dunitic Zone. The evidence for coeval extension and magmatism, and the discovery of a locally well-developed sheeted dyke complex, suggest that the TMOC formed by sea-floor spreading. The dominance of a boninitic signature in cumulate and volcanic rocks suggests that spreading occurred in a subduction zone environment, possibly in a forearc setting.

CHEMISTRY OF MANTLE PERIDOTITES FROM TROODOS OPHIOLITE

A detailed geochemical mapping of the mantle peridotite section in the Troodos ophiolite massif has revealed a kmscale compositional heterogeneity. Based on the Cr# (Cr/(Cr+Al)) of spinel and modal abundance of clinopyroxene (cpx) we subdivided the mantle section into two units: 1) spinel lherzolite with a Cr# of 0.22-0.28, and 5-7 modal % cpx and 2) harzburgite with a Cr# 0.51-0.70, and 0.5-1 modal % cpx. The boundary between spinel lherzolite and harzburgite units coincides with that separating Wilson’s (1959) limit between zone abundant chrysolite veinlets and rare chrysolite veinlets. Small-scale compositional heterogeneity is present within both lherzolite and harzburgite units. The spinel lherzolite contains numerous dunite bodies. Zones of cpx-bearing harzburgites are located between the spinel lherzolites and the dunite bodies and have higher Cr# of spinel (0.32-0.45) and lower modal cpx 1.5-4.5 % than surrounding spinel lherzolite. These harzburgites will be refereed later on as a harzburgites-II to distinguish them from the harzburgites of the main unit (harzburgites-I). The dunites display the highest Cr# of spinel (between 0.70-0.84). Contacts between spinel lherzolite and harzburgite-II are gradational. Contacts between harzburgite-II and dunite are sharp. While cpx is present in the contact zone between harzburgite- II and dunite, dunites themselves are cpx free. Clinopyroxenite and enstatitite veins are associated with dunite bodies. The harzburgite-I unit underlies the ultramafic metacumulates representing the “petrologic” Moho (Allen, 1975; etc). Dunite, plagioclase lherzolite bodies and gabbroic impregnation zones are abundant within the main harzburgite. Spinel lherzolites display a narrow compositional range over the entire unit, well within compositional variations of abyssal peridotites in respect to both trace element concentrations of cpx and mineral chemistry. The incompatible element patterns of cpx are very close to those of abyssal peridotites with only difference in a slight upward inflection of La and Ce of former (Fig. 1A). Cpx from harzburgite- II surrounding the dunite bodies within the lherzolite unit have lower HREE concentrations and extremely low (close to SIMS detection limit) MREE concentrations (Fig. 1B). They show very significant upward inflection of La and Ce relative to Nd. Cpx from the enstatitite vein shows the same trace element pattern. Cpx from the clinopyroxenite veins differs from those of harzburgite-II by having higher MREE concentration (Fig. 1B). All these rocks are close in their cpx trace element composition and mineral chemistry to forearc peridotites and most depleted Oman harzburgite (Fig. 1B). Cpx from the harzburgite-I unit are extremely depleted in HREE (Fig. 1A). Their MREE concentrations are below detection limit of SIMS. The close association of cpx with opx and grains morphology suggests that cpx formed by exsolution from opx during recrystallization accompanying solid-state deformation (Sobolev and Batanova, 1995). The modeling suggest that spinel lherzolite represents the residue of 12% critical melting of a depleted (MORB-like) mantle source with a low residual mantle porosity (0.1%) The upward inflection of La and Ce could be explained by subsequent melt percolation event. The structure of cpx-bearing harzburgite – dunite zones, their position within the spinel lherzolite and their association with magmatic pyroxenite veins allows us to suggest that these zones could be produced by reaction of spinel lherzolite with percolating melts. Dunite bodies and surrounding cpx-bearing harzburgite-II represent pathways of the melts in the mantle. Some of these melts were close to ultra-depleted boninites from Troodos upper pillow lavas. However the majority of analyzed clinopyroxenes suggest a percolation of melts significantly more depleted in MREE than any known so far. Such melts could form by second stage melting of refractory lherzolite but require addition of LREE and Zr enriched component likely of subduction related origin. A more plausible mechanism for the production of the harzburgite-I can be regarded as an open-system reaction between infiltrating melt and the initial spinel lherzolite.

Dunite of the Ioko-Dovyrensk massif

The dunite zone of the Ioko-Dovyrensk massif is 15 km long and 870 m thick. The dunites are fresh and possess physicochemical properties that make them a suitable raw material for the production of magnesiasilicate refractories. The weathering crust can be a source of molding sand. The North-Baikal Geological Prospecting Expedition (GPE) and the Locomotive and Car Repair Plant (Ulan-Ude) have tested four technological samples of olivine sands, three of which tests gave positive results. The creation of a base for the production of magnesia-silicate refractories in Siberia will make it possible to reduce the import of raw materials and even to export domestic products. The possibility of practical use of the dunites, the discovery of a low-sulfide metallic platinum mineralization, and the considerable growth of the prospected resources of nickel, copper, and cobalt make the development of the Ioko-Dovyrensk massif a quite promising undertaking.

The peridotites of the Kukës ophiolite (Albania): structure and kinematics

Structural study of the Kukës ultramafic massif which belongs to the Albanian Mirdita zone is of great interest for reconstruction of Alpine tectonic environments. From bottom to top, the following lithological succession is observed in the ultramafic-mafic section: harzburgites, harzburgites with dunitic lenses, dunites, an intermediate zone and gabbros. The ultramafic rocks are apparently weakly deformed. The textures indicate a high-temperature deformation and slight locally superimposed low-temperature deformation. This is confirmed by olivine lattice-preferred orientation which indicates that (010)[100] was the activated glide system in olivine (i.e., at high temperature). The kinematics of deformation have been determined from the lattice-preferred orientation of olivine; an inversion of the shear sense occurs at the base of the dunite zone and is interpreted as being due to asthenospheric flow. A simple geometric reconstruction indicates that the Kukës ultramafic massif could correspond to the eastern flank of a NNW-SSE-orientated paleo-ridge.

Asteroid Differentiation: Pyroclastic Volcanism to Magma Oceans

Abstract— Asteroid differentiation was driven by a complex array of magmatic processes. This paper summarizes theoretical and somewhat speculative research on the physics of these processes. Partial melts in asteroids migrate rapidly, taking &lt; 106 years to reach surface regions. On relatively small (&lt;100 km) asteroids with sufficient volatiles in partial melts (&lt;3000 ppm), explosive volcanism accelerated melts to greater than escape velocity, explaining the apparent lack of basaltic components on the parent asteroids of some differentiated meteorites. Partial melting products include the melts (some eucrites, angrites), residues (lodranites, ureilites), and unfractionated residues (acapulcoites). The high liquidus temperatures of magmatic iron meteorites, the existence of pallasites with only olivine, and the fact that enstatite achondrites formed from ultramafic magmas argue for the existence of magma oceans on some asteroids. Asteroidal magma oceans would have been turbulently convective. This would have prevented crystals nucleated at the upper cooling surface (the only place for crystal nucleation in a low‐pressure body) from settling until the magma became choked with crystals. After turbulent convection slowed, crystals and magma would have segregated, leaving a body stratified from center to surface as follows: a metallic core, a small pallasite zone, a dunite region, a feldspathic pyroxenite, and basaltic intrusions and lava flows (if the basaltic components had not been lost by explosive volcanism). The pallasite and dunite zones probably formed from coarse (0.5–1 cm) residual olivine left after formation of the magma ocean at &gt;50% partial melting of the silicate assemblage. Iron cores crystallized dendritically from the outside to the inside. The rapid melt migration rate of silicate melts suggests that 26Al could not be responsible for forming asteroidal magma oceans because it would leave the interior before a sufficient amount of melting occurred. Other heat sources are more likely candidates. Our analysis suggests that if Earth‐forming planetesimals had differentiated they were either small (&lt;100 km) and poor in volatiles (&lt;1000 ppm) or they were rich in volatiles and large enough (&gt;300 km) to retain the products of pyroclastic eruptions; if these conditions were not met, Earth would not have a basaltic component.

Evidence for melt segregation towards fractures in the Horoman mantle peridotite complex

MELT segregation from a solid matrix in the upper mantle is the first step in the formation of magmas. The mechanisms of melt segregation proposed so far have been based on two different physical models: the percolation of interstitial melt driven by a density difference between melt and matrix, associated with compaction and deformation of the matrix1; or the suction of interstitial melt into fractures from a surrounding porous matrix2–4. Although the latter process was originally invoked to explain the occurrence of dunite and pyroxenite–gabbro dykes with zones depleted in melt component on both sides2,5–7, most of these zones are discordant, small in scale and clearly formed by reaction between peridotite and basaltic melt8. I have suggested9 that thick, concordant dunite zones in the most depleted peridotite layers of the Horoman peridotite complex formed by suction of partial melt towards fractures later filled by dunite. Here I report a detailed mineralogical variation across the dunite which, when set in its geological and petrographic context, provides clearer evidence for melt segregation by dynamic forcing3, rather than passive percolation. This mechanism of melt segregation may play an important role in the formation of primary magmas with low production rate and large geochemical variability.

Structural classification of chromite pods in southern New Caledonia

A structural classification of podiform chromite orebodies from southern New Caledonia results in a division of deposits into three major types: discordant, subconcordant, and concordant, with penetrative structures (foliation and lineation) in the enclosing peridotite. Discordant deposits are very irregular in shape and clearly crosscut banding and foliation. Subconcordant deposits are generally tabular in shape and lie within 10 degrees to 25 degrees in strike and/or dip to the foliation. Concordant deposits are also tabular and lie parallel to foliation in peridotite; pyroxene lineation in host peridotite always indicates the elongation direction of these deposits. Within subconcordant and discordant deposits, chromite lineations are often oblique to those in the surrounding rocks; they follow local variations in the orebody shape and indicate possible deposit extensions at depth. The classification of chromite bodies corresponds to three stages of increasing deformation, as evidenced by chromite ore textures. Discordant deposits which are the least deformed are characterized by primary textures such as the nodular one, the foam texture, the chromite net, and the occluded silicate texture. Within subconcordant and mainly concordant deposits, massive, disseminated, and antinodular ores show evidence of strong deformation. The lack of geochemical distinctions between the different deposit types supports such an hypothesis.These chromite deposits in ophiolitic harzburgites are thought to have been formed beneath an oceanic spreading ridge. If the discordant pods are considered as representative of the original situation, it is proposed that the chromite has crystallized and has been dynamically concentrated along steep conduits traversing the enclosing harzburgite and feeding a magma chamber. Next the chromite-enriched pipes are caught up by plastic deformation in the mantle and tectonically reoriented toward the foliation.The chromite deposits in the Massif du Sud are located within a domain about 1.5 km thick in the harzburgites and dunite zones. This domain is limited upward by the first cumulates and gabbros and downward by the transition to a different plastic flow regime in the harzburgites.Finally, some guides to chromite prospecting and exploration are cited, applicable at different geologic scales and based on lithologic and structural criteria.

Geology of the Greenhills Ultramafic Complex, Bluff Peninsula, Southland, New Zealand

Extensive coastal outcrops and moderate relief combine to provide an excellent exposure of the 14-km 2 Greenhills Complex. This complex consists of a deformed layered series of rocks, including dunite, wehrlite, and gabbro, of Permian age intruded into Lower Permian metasedimentary rocks of the Southland syncline. Widespread slumping in the layered series and the emplacement of the complex as a plug of hot fluid mush indicate recurring conditions of instability during evolution of the complex. Emplacement of the complex was accompanied by the intrusion of innumerable dikes and by folding and faulting; the material of many of the dikes was derived from the complex. Country rock, dominantly volcanic microbreccia with intercalated bands of impure marble, increases from regional prehnite-pumpellyite facies through greenschist facies to a marginal gabbroic rock, itself the result of contact metamorphism equivalent to pyroxene-hornfels adjacent to the complex. The layered series of the Greenhills Complex is stratigraphically divisible into: (1) an upper gabbroic portion dominated by an approximately 640-m-thick gabbro zone which includes a 15-m-thick allivalite unit; (2) a lower ultramafic or peridotitic portion comprised of a 150-m-thick wehrlite zone (which includes near its base a poikilitic wehrlite unit less than 30 m thick), and a basal dunite zone exceeding 600 m in thickness. Feldspathic wehrlite occurs in a narrow transition zone between the upper and lower portions of the complex. The layered series shows well-developed accumulate structures and textures resembling cumulates in classic stratiform intrusions. Cryptic layering is shown by the range in composition of the primary minerals. These minerals in order of separation from the Greenhills magma are: olivine (Fo 90–65 ), clinopyroxene (Ca 41 Mg 54 Fe 5 to Ca 44 Mg 47 Fe 9 ), and plagioclase (An 92–88 ). The Greenhills Complex is essentially the result of crystal settling during fractional crystallization of a basaltic magma, modified by fluid mush flow and mixing of rocks along mutual contact zones. The magma is believed to have had affinities transitional between alkaline and tholeiitic magma types. Differentiation took place in a hydrous open system connected with the surface of the earth.