Valentina Batanova

Melt percolation monitored by Os isotopes and HSE abundances: a case study from the mantle section of the Troodos Ophiolite

Combined siderophile and lithophile element systematics in mantle rocks can be used to monitor melt percolation processes in the Earth’s mantle. Here we present a coherent dataset from a single melt channel from the mantle section of the Troodos Ophiolite Complex on Cyprus. The melt channel is composed of a dunite vein that is surrounded by harzburgite. Dunite and harzburgite both have refractory Cr-spinel (Cr/(Cr+Al) of 0.58–0.60). Likewise, clinopyroxenes in both the dunites and harzburgites have strongly depleted REE patterns with (Gd/Yb)N values varying from 0.03 to 0.07. Such consistent lithophile element patterns suggest that the harzburgite and dunite interacted with the same melt during the melt percolation process. The distribution of the highly siderophile elements (HSEs) (Os, Ir, Ru, Pt, Pd and Re) in the melt channel cannot be explained by conventional partial melting models, but can be explained by melt-peridotite reaction. The harzburgites have slightly suprachondritic Os isotope ratios (187Os/188Ost=90 Ma=0.1288–0.1311) compared to the 187Os/188Ost=90 Ma of the carbonaceous chondrite reference (0.1264), and their HSE concentrations overlap with the range observed for lherzolites and harzburgites world-wide. In contrast, the dunites are significantly enriched in 187Os (187Os/188Os90 Ma=0.1335–0.1374), like volcanic rocks from island arcs world-wide. HSE patterns in the dunites are also typical for mantle melts, in that they are enriched in Pd, Pt and Re relative to Ir, Os and Ru, which are lower than in the primitive mantle. Hence, the harzburgites and dunites have complementary HSE concentrations and ratios. In addition, HSE ratios such as Ir/Os, Re/Os, systematically increase from the harzburgite towards the dunite ((Ir/Os)N: 0.36–1.8; (Re/Os)N: 0.14–9.5). This implies that Ir, Os and Ru behave incompatibly and become fractionated from each other during the melt percolation process. These features are interpreted to reflect the progressive reaction of a mantle melt with spinel–lherzolite to form harzburgite and eventually dunite. We suggest that an upper mantle peridotite was infiltrated by a radiogenic mantle melt typical for subduction-related volcanism. At low melt/rock ratios a harzburgite residue is left behind and its HSE distribution and the REE pattern of cpx can be explained by open-system melting if one assumes the HSEs to behave incompatibly. Continued melt percolation eventually produces dunites, and all mantle sulfides are removed from the peridotite. Thus, the sulfides and the HSE distribution in the dunites are not of residual origin but are dominated by sulfides that segregated from a sulfide-saturated melt with a radiogenic Os signature. The HSE variation in harzburgites and dunites from the melt channel can be interpreted as a mixing line that has HSE-bearing sulfides from the melt and from the residual mantle as end members. We conclude that HSEs become significantly mobilized and fractionated during melt percolation processes, thus providing useful proxies for melting and enrichment processes in the Earth’s mantle.

Compositional heterogeneity in subduction-related mantle peridotites, Troodos massif, Cyprus

Geochemical mapping has revealed the presence of two distinct units within the mantle section of the Troodos massif, Cyprus. The first unit (U1) is composed of spinel lherzolite (Cr# [Cr/(Cr+Al)] of chrome spinels = 0.22‐0.28), which contains numerous dunite bodies accompanied by zones of clinopyroxene-bearing harzburgite (U1-harzburgite). The second unit (U2) mainly consists of refractory clinopyroxene-poor harzburgite (U2-harzburgite; Cr# of chrome spinels = 0.51‐0.70). The U1 spinel lherzolite is interpreted as residue after mid-ocean ridge basalt extraction, which was tectonically juxtaposed with U2. It was not involved in production of suprasubduction-type Troodos magmas, but has subsequently been modified by their percolation. The dunite bodies and related zones of U1-harzburgite might represent pathways of these melts in the mantle. The U2-harzburgite could have resulted from reaction between host rock and melts.

Origin of Geochemical Heterogeneity in the Mantle Peridotites from the Bay of Islands Ophiolite, Newfoundland, Canada: Ion Probe Study of Clinopyroxenes

Abstract Representative samples from a 6 km thick ophiolitic mantle section were selected for ion probe analysis of clinopyroxene (cpx). The purpose was to evaluate the trace element heterogeneity occurring on a kilometer-scale in the geologically well constrained uppermost mantle beneath the ancient Bay of Islands ophiolite spreading center. The range of REE patterns in the mantle section can be explained by 7–20% near-fractional melting of a weakly depleted mantle source followed by varying degrees of trace element chromatographic exchange between an interstitial liquid and the refractory residue and/or precipitation of phases from the interstitial liquid. Peridotites from the basal mantle section are the least depleted ones. They require little to no overprint by melt circulation and can be modeled as residues of fractional melting with a low (0.1%) residual porosity. Their presence in the mantle section is possibly related to ductile accretion of less depleted peridotite during ophiolite obduction. Peridotites from the central, but mainly uppermost mantle section require stronger interaction with percolating melts. Uppermost peridotites also show the highest spatial variability in the degree of depletion. A preferred, but nonunique model for their formation is refertilization of initially more refractory residues due to cpx ± plagioclase addition from liquids depleted relative to N-MORB. Such depleted liquids are also inferred from wehrlitic rocks occurring in the crust-mantle transition zone. They may have originated as melts emerging from mantle columns or mixtures of such melts, i.e., melts whose compositions were affected by reaction with a large volume of mantle rocks.

A quantitative link between recycling and osmium isotopes

Recycled subducted ocean crust has been traced by elevated 187Os/188Os in some studies and by high nickel and low manganese contents in others. Here, we show that these tracers are linked for Quaternary lavas of Iceland, strengthening the recycling model. An estimate of the osmium isotopic composition of both the recycled crust and the mantle peridotite implies that Icelandic Quaternary lavas are derived in part from an ancient crustal component with model ages between 1.1 × 109 and 1.8 × 109 years and from a peridotitic end-member close to present-day oceanic mantle.

Komatiites reveal a hydrous Archaean deep-mantle reservoir

Archaean komatiites (ultramafic lavas) result from melting under extreme conditions of the Earth’s mantle. Their chemical compositions evoke very high eruption temperatures, up to 1,600 degrees Celsius, which suggests even higher temperatures in their mantle source. This message is clouded, however, by uncertainty about the water content in komatiite magmas. One school of thought holds that komatiites were essentially dry and originated in mantle plumes while another argues that these magmas contained several per cent water, which drastically reduced their eruption temperature and links them to subduction processes. Here we report measurements of the content of water and other volatile components, and of major and trace elements in melt inclusions in exceptionally magnesian olivine (up to 94.5 mole per cent forsterite). This information provides direct estimates of the composition and crystallization temperature of the parental melts of Archaean komatiites. We show that the parental melt for 2.7-billion-year-old komatiites from the Abitibi greenstone belt in Canada contained 30 per cent magnesium oxide and 0.6 per cent water by weight, and was depleted in highly incompatible elements. This melt began to crystallize at around 1,530 degrees Celsius at shallow depth and under reducing conditions, and it evolved via fractional crystallization of olivine, accompanied by minor crustal assimilation. As its major- and trace-element composition and low oxygen fugacities are inconsistent with a subduction setting, we propose that its high H2O/Ce ratio (over 6,000) resulted from entrainment into the komatiite source of hydrous material from the mantle transition zone. These results confirm a plume origin for komatiites and high Archaean mantle temperatures, and evoke a hydrous reservoir in the deep mantle early in Earth’s history.

Structure of melt flow channels in the mantle

Structural events during the formation of the mantle peridotite section in the Voikar-Syn’ya massif of the Polar Urals are considered. The structural units of the mantle section were formed during several deformation stages. Dunite bodies in restitic peridotites were formed in the course of deformation that completed the formation of large-scale folds of high-temperature plastic flow of mantle material. The final stage of deformation accompanied by migration of melt through harzburgite occurred in the shallow mantle in the setting of suprasubduction spreading related to the ascent of a mantle diapir. The rate of plastic deformation was relatively low. As a result, the intracrystal translation gliding of dislocations in olivine was the main mechanism in both harzburgite and dunite. The paths of focused melt flow are marked by dunite veins and associated pyroxenite and chromitite. It is suggested that stress concentration in fold hinges and their abrupt relaxation with formation of orthogonal network of weakened zones with high permeability was one of the possible mechanisms of the formation of melt conduits. The dispersed melt ascending from a great depth spontaneously migrated toward these zones. The distribution and structure of chromitite bodies reflect multistage formation of dunite, nonstationary dynamics of melt flow through restite, and abrupt variations of local stress fields in the areas adjacent to melt conduits.

Unique Compositional Peculiarities of Olivine Phenocrysts from the Post Flood Basalt Diamondiferous Malokuonapskaya Kimberlite Pipe, Yakutia

ISSN 1028334X, Doklady Earth Sciences , 2015, Vol. 463, Part 2, pp. 828–832.

Evidence for the Suprasubduction Origin of Mantle Section Rocks of Voykar Ophiolite, Polar Urals

1394 Reconstruction of geodynamic environments of formation of mantle section rocks of ophiolite com� plexes is an important problem in geology. Solution of this problem cannot be a simple consequence of reconstruction of environment of formation of crustal parts of ophiolite sequences because of their possible tectonic combination with the mantle basement. Analysis of concentrations of incompatible trace ele� ments in clinopyroxene and/or amphibole from man� tle section rocks is a direct method to establish their origin. Such an approach allows us to estimate the geochemical peculiarities of mantle melts migrating through peridotites and determine the geodynamic environments of their formation. The first data on the composition of clinopyroxene and amphibole from pyroxenite veins from the mantle section of Voykar ophiolite, Polar Urals, proving their suprasubduction origin are reported in this paper.

Polychronous formation of mantle complexes in ophiolites

The paper presents new determinations of the U-Pb zircon age of high-Al chromitite from dunite of the mantle section of the Voikar-Synya massif at the Kershor site in the boundary zone with rocks of the dunite-wehrlite-clinopyroxenite complex. The high-Cr chromitite from dunite in the central part of the same massif contains zircon dated at ca. 0.6 Ga [10]. It is suggested that Paleoproterozoic (2.0−1.9 Ga) zircons from chromitites of the mantle section near the petrological Moho boundary were formed in the course of partial melting of peridotites and/or their interaction with migrating MORB-type melts. The occurrence of Vendian and Paleoproterozoic zircons in chromitites from different parts of the mantle section, as well as previously published petrological, geochemical, and geological data [2, 11, 22] allow us to suggest a complex multistage evolution of the mantle section in ophiolites. The arguments stated below show that chromitites and host dunites could have been formed at different times and were probably related to different processes. Thus, not only various complexes of the pre-Paleozoic oceanic crust reworked in the suprasubduction setting differ in age, but also the mantle rock of similar petrography, vary in the time of their formation.

Experimental testing of olivine–melt equilibrium models at high temperatures

Data are presented on the equilibrium compositions of olivine and melts in the products of 101 experiments performed at 1300–1600°C, atmospheric pressure, and controlled oxygen fugacity by means of new equipment at the Vernadsky Institute. It was shown that the available models of the olivine–melt equilibrium describe with insufficient adequacy the natural systems at temperatures over 1400°C. The most adequate is the model by Ford et al. (1983). However, this model overestimates systematically the equilibrium temperature with underestimating by 20–40°C at 1450–1600°C. These data point to the need for developing a new, improved quantitative model of the olivine–melt equilibrium for high-temperature magnesian melts, as well as to the possibility of these studies on the basis of the equipment presented.