Lherzolite - carbonatite melt interaction in the presence of additive CO2 and H2O: Experimental data at 5.5 GPa and 1200-1450°C

<p>We study the reaction of garnet lherzolite with carbonatitic melt rich in molecular CO<sub>2</sub> and/or H<sub>2</sub>O in experiments at 5.5 GPa and 1200-1450°C. The experimental results show that carbonation of olivine with formation of orthopyroxene and magnesite can buffer the CO<sub>2</sub> contents in the melt, which impedes immediate separation of CO<sub>2</sub> fluid from melt equilibrated with the peridotite source. The solubility of molecular CO<sub>2</sub> in melt decreases from 20-25 wt.% at 4.5-6.8 wt.% SiO<sub>2</sub> typical of carbonatite to 7-12 wt.% in more silicic kimberlite-like melts with 26-32 wt.% SiO<sub>2</sub>. Interaction of garnet lherzolite with carbonatitic melt (2:1) in the presence of 2-3 wt.% H<sub>2</sub>O and 9-13 wt.% molecular CO<sub>2</sub> at 1200-1450°С yields low SiO<sub>2</sub> (<10 wt.%) alkali‐carbonatite melts, which shows multiphase saturation with magnesite-bearing garnet harzburgite. Thus, carbonatitic melts rich in volatiles can originate in a harzburgite source at moderate temperatures common to continental lithospheric mantle (CLM).</p><p>Having separated from the source, carbonatitic magma enriched in molecular CO<sub>2</sub> and H<sub>2</sub>O can rapidly acquire a kimberlitic composition with >25 wt.% SiO<sub>2 </sub>by dissolution and carbonation of entrapped peridotite. Furthermore, interaction of garnet lherzolite with carbonatitic melt rich in K, CO<sub>2</sub>, and H<sub>2</sub>O at 1350°С produces immiscible kimberlite-like carbonate-silicate and K-rich silicate melts. Quenched silicate melt develops lamelli of foam-like vesicular glass. Differentiation of immiscible melts early during ascent may equalize the compositions of kimberlite magmas generated in different CLM sources. The fluid phase can release explosively from ascending magma at lower pressures as a result of SiO<sub>2</sub> increase which reduces the solubility of CO<sub>2</sub> due to decarbonation reaction of magnesite and orthopyroxene.</p><p>The research was performed by a grant of the Russian Science Foundation (19-77-10023).</p>

Chapter 5.2b Erebus Volcanic Province: petrology

Igneous rocks of the Erebus Volcanic Province have been investigated for more than a century but many aspects of petrogenesis remain problematic. Current interpretations are assessed and summarized using a comprehensive dataset of previously published and new geochemical and geochronological data. Igneous rocks, ranging in age from 25 Ma to the present day, are mainly nepheline normative. Compositional variation is largely controlled by fractionation of olivine + clinopyroxene + magnetite/ilmenite + titanite ± kaersutite ± feldspar, with relatively undifferentiated melts being generated by <10% partial melting of a mixed spinel + garnet lherzolite source. Equilibration of radiogenic Sr, Nd, Pb and Hf is consistent with a high time-integrated HIMU sensu stricto source component and this is unlikely to be related to subduction of the palaeo-Pacific Plate around 0.5 Ga. Relatively undifferentiated whole-rock chemistry can be modelled to infer complex sources comprising depleted and enriched peridotite, HIMU, eclogite-like and carbonatite-like components. Spatial (west–east) variations in Sr, Nd and Pb isotopic compositions and Ba/Rb and Nb/Ta ratios can be interpreted to indicate increasing involvement of an eclogitic crustal component eastwards. Melting in the region is related to decompression, possibly from edge-driven mantle convection or a mantle plume.

Experimental recalibration of the Cr-in-clinopyroxene geobarometer: improved precision and reliability above 4.5 GPa

The pressure dependence of the exchange of Cr between clinopyroxene and garnet in peridotite is applicable as a geobarometer for mantle-derived Cr-diopside xenocrysts and xenoliths. The most widely used calibration (Nimis and Taylor Contrib Miner Petrol 139: 541–554, 2000; herein NT00) performs well at pressures below 4.5 GPa, but has been shown to consistently underestimate pressures above 4.5 GPa. We have experimentally re-examined this exchange reaction over an extended pressure, temperature, and compositional range using multi-anvil, belt, and piston cylinder apparatuses. Twenty-nine experiments were completed between 3–7 GPa, and 1100–1400 °C in a variety of compositionally complex lherzolitic systems. These experiments are used in conjunction with several published experimental datasets to present a modified calibration of the widely-used NT00 Cr-in-clinopyroxene (Cr-in-cpx) single crystal geobarometer. Our updated calibration calculates P (GPa) as a function of T (K), CaCr Tschermak activity in clinopyroxene $$\left( {a_{{{\text{CaCrTs}}}}^{{{\text{cpx}}}} } \right)$$ a CaCrTs cpx , and Cr/(Cr + Al) (Cr#) in clinopyroxene. Rearranging experimental results into a 2n polynomial using multiple linear regression found the following expression for pressure:$$P\left( {{\text{GPa}}} \right) = 11.03 + \left( { - T{ }\left( {\text{K}} \right){\text{ ln}}(a_{{{\text{CaCrTs}}}}^{{{\text{cpx}}}} ) \times 0.001088{ }} \right) + \left( {1.526 \times {\text{ln}}\left( {\frac{{{\text{Cr}}\#^{{{\text{cpx}}}} }}{{T{ }\left( {\text{K}} \right)}}} \right)} \right){ }$$ P GPa = 11.03 + - T K ln ( a CaCrTs cpx ) × 0.001088 + 1.526 × ln Cr # cpx T K where $${\text{Cr}}\#^{{{\text{cpx}}}} = \left( {\frac{{{\text{Cr}}}}{{{\text{Cr}} + {\text{Al}}}}} \right)$$ Cr # cpx = Cr Cr + Al , $$a_{{{\text{CaCrTs}}}}^{{{\text{cpx}}}} = {\text{Cr}} - 0.81 \cdot {\text{Cr}}\#^{{{\text{cpx}}}} \cdot \left( {{\text{Na}} + {\text{K}}} \right),$$ a CaCrTs cpx = Cr - 0.81 · Cr # cpx · Na + K , with all mineral components calculated assuming six oxygen anions per formula unit in clinopyroxene.Temperature (K) may be calculated through a variety of geothermometers, however, we recommend the NT00 single crystal, enstatite-in-clinopyroxene (en-in-cpx) geothermometer. The pressure uncertainty of our updated calibration has been propagated by incorporating all analytical and experimental uncertainties. We have found that pressure estimates below 4 GPa, between 4–6 GPa and above 6 GPa have associated uncertainties of 0.31, 0.35, and 0.41 GPa, respectively. Pressures calculated using our calibration of the Cr-in-cpx geobarometer are in good agreement between 2–7 GPa, and 900–1400 °C with those estimated from widely-used two-phase geobarometers based on the solubility of alumina in orthopyroxene coexisting with garnet. Application of our updated calibration to suites of well-equilibrated garnet lherzolite and garnet pyroxenite xenoliths and xenocrysts from the Diavik-Ekati kimberlite and the Argyle lamproite pipes confirm the accuracy and precision of our modified geobarometer, and show that PT estimates using our revised geobarometer result in systematically steeper paleogeotherms and higher estimates of the lithosphere‒asthenosphere boundary compared with the original NT00 calibration.

The role of lithospheric thickness in the formation of ocean islands and seamounts: contrasts between the Louisville and Emperor-Hawaiian hotspot trails

The Hawaii-Emperor and Louisville seamounts form the two most prominent time-progressive hotspot trails on Earth. Both formed over a similar time interval on lithosphere with a similar range of ages and thickness. The Hawaii-Emperor seamounts are large and magma productivity appears to be increasing at present. The Louisville seamounts, by contrast, are smaller and the trail appears to be waning. We present new major- and trace element data from five of the older (74–50 Ma) Louisville seamounts drilled during International Ocean Drilling Program (IODP) Expedition 330 and compare these to published data from the Emperor seamounts of the same age. Despite drilling deep into the shield-forming volcanic rocks at three of the Louisville seamounts, our data confirm the results of earlier studies based on dredge samples that the Louisville seamounts are composed of remarkably uniform alkali basalt. The basalt composition can be modelled by ∼1.5–3% partial melting of a dominantly garnet lherzolite mantle with a composition similar to that of the Ontong Java Plateau mantle source. Rock samples recovered by dredging and drilling on the Emperor Seamounts range in composition from tholeiitic to alkali basalt and require larger degrees of melting (2–10%) and spinel- to garnet lherzolite mantle sources. We use a simple decompression melting model to show that melting of mantle with a potential temperature of 1500ºC under lithosphere of varying thickness can account for the composition of the shield-forming tholeiitic basalts from the Emperor seamounts, while post-shield alkali basalt requires a lower temperature (1300–1400ºC). This is consistent with the derivation of Hawaii-Emperor shield-forming magmas from the hotter axis of a mantle plume and the post-shield magmas from the cooler plume sheath as the seamount drifts away from the plume axis. The composition of basalt from the Louisville seamounts shows no significant variation with lithosphere thickness at the time of seamount formation, contrary to the predictions of our decompression melting model. This lack of influence of lithospheric thickness is characteristic of basalt from most ocean islands. The problem can be resolved if the Louisville seamounts were formed by dehydration melting of mantle containing a small amount of water in a cooler plume. Hydrous melting in a relatively cool mantle plume (Tp = 1350–1400 °C) could produce a small amount of melt and then be inhibited by increasing viscosity from reaching the dry mantle solidus and melting further. The failure of the plume to reach the dry mantle solidus or the base of the lithosphere means that the resulting magmas would have the same composition irrespective of lithosphere thickness. A hotter mantle plume (Tp ≈ 1500 °C) beneath the Emperor seamounts and the Hawaiian Islands would have lower viscosity before the onset of melting, melt to a larger extent, and decompress to the base of the lithosphere. Thus our decompression melting model could potentially explain the composition of both the Emperor and Louisville seamounts. The absence of a significant lithospheric control on the composition of basalt from nearly all ocean islands suggests that dehydration melting is the rule and the Hawaiian islands the exception. Alternatively, many ocean islands may not be the product of mantle plumes but instead be formed by decompression melting of heterogeneous mantle sources composed of peridotite containing discrete bodies of carbonated and silica-oversaturated eclogite within the general upper mantle convective flow.

Geochemical characteristics of the Late Devonian basaltoids of the Kanin Peninsula and the Middle Timan

The petrochemistry and geochemistry of dolerites and basalts of the Late Devonian Kanin-Timan complex of the Kanin Peninsula and the Middle Timan are considered. Petrochemically, the rocks of the Kanin-Timan complex of the Kanin Peninsula and the Tsilma river area of the Middle Timan correspond to basaltoids of the normal range of alkalinity and partially to moderately alkaline varieties, and belong to the tholeiitic series. The least differentiated varieties are dolerites of the southeastern Kanin Peninsula, the most differentiated are the basalts of the river Tsilma of the Middle Timan. The lowest REE concentrations were found in the rocks of the central part of the Kanin Peninsula (36.5-56.8 g/t); in the same samples, the lowest LaN/YbN values were recorded (1.85 and 2.4, respectively), which indicates an increased degree of melting of the source. The highest REE concentrations were found in basalts from the river Tsilma (77.13-88.33 g/t), LaN/YbN values (2.49-2.7, respectively). The influence of the crustal component in the formation of melts from which rocks of the Kanin-Timan complex were formed, was established. The source of the melt was spinel-garnet lherzolite, the degree of melting varied from 10 to 30%. The maximum degree of melting was 30%, at which melts were formed, that gave rise to the least differentiated rocks of the Northern Timan and the central part of the Kanin Peninsula. The mantle source, that gave rise to the melts from which the rocks of the Kanin-Timan complex were formed, was enriched with subduction and crustal components, a similar type of source is characteristic of the basaltoids of the No-rilsk trough.

Zircon U–Pb Geochronology, Geochemistry and Geological Significance of the Anisian Alkaline Basalts in Gejiu District, Yunnan Province

The Gejiu Anisian alkaline basalts (GAAB), distributed in the southern part of the Emeishan large igneous province (ELIP), are crucial to understand the tectonomagmatic activity during the Triassic. Geochronological, geochemical, and Sr-Nd-Pb isotopic analyses were systematically applied to explore the origin, petrogenesis, and tectonic setting of the GAAB, and how they relate to the ELIP. Zircon U-Pb dating set the eruption date at 244 Ma. Most of the samples belonged to alkaline basalts and had high TiO2 (2.14–3.23 wt.%) and MgO (4.43–19.58 wt.%) contents. Large ion lithophile elements (LILEs) were enriched relative to high field strength elements (HFSEs). The rare earth elements (REEs) and trace element signatures in the normalized diagrams were similar to oceanic island basalts (OIB) and Emeishan high-Ti basalts. These samples had consistent Sr-Nd isotope compositions: the initial 87Sr/86Sr values ranged from 0.7044 to 0.7048 and εNd(t) = 3.25–4.92. The Pb isotopes were more complex, the (206Pb/204Pb)t, (207Pb/204Pb)t, (208Pb/204Pb)t ratios were 17.493–18.197, 15.530–15.722, and 37.713–38.853, respectively. Our results indicate that the GAAB originated from the deeper enriched mantle with 5% to 15% partial melting of garnet lherzolite and a segregation depth of 2 to 4 GPa (60–120 km). During the formation of the GAAB, clinopyroxene and Ti-Fe oxides were fractionally crystallized with insignificant crustal contamination. The GAAB were formed in a extensional regime that was related to the Gejiu-Napo rift event in the Triassic.

New data on the genesis and evolution of the primitive magmas of Mount Cameroon: contribution of melt inclusions

Mount Cameroon is a Plio-Quaternary volcanic massif, without a central crater, made up ofmore than 140 pyroclastic cones. It is one of the active volcanoes of the Cameroon Line. Mount Cameroon magmatic inclusions are found in microdroplets trapped in the early minerals (olivines) from the pyroclastic products. The analysis of these magmatic inclusions allowed us to find primitive liquids compared to lavas. Major elements study of the magmatic inclusions, trapped in the most magnesian olivines (Mg#84-86) of Mount Cameroon revealed "primitive" liquids of basanite and alkaline basalt type with variable composition compared to the much more uniform basalts of the magmatic series of Mount Cameroon. The study of these trapped liquids shows that: (i)- the original primitive lavas did not undergo the process of evolution by FC, but rather underwent fundamentally (or exclusively) the process of partial melting; (ii) the emitted lavas, evolved essentially by FC; (iii) the variations in the trace element contents of the primitive liquids directly reflect a variation in the rate of partial melting of a homogeneous mantelic source. The very high La/Yb ratios of the Mount Cameroon inclusions (> 20) characterize a garnet lherzolite source. Spectra of the magmatic inclusions show a negative anomaly or depletion in K, Rb and Ba as those of HIMU. The "primitive" liquids and lavas of Mount Cameroon represent a co-genetic sequence formed by varying degrees of partial melting of a source considered as homogeneous.

The effects of solid-solid phase equilibria on the oxygen fugacity of the upper mantle

Decades of study have documented several orders of magnitude variation in the oxygen fugacity (fO2) of terrestrial magmas and of mantle peridotites. This variability has commonly been attributed either to differences in the redox state of multivalent elements (e.g., Fe3+/Fe2+) in mantle sources or to processes acting on melts after segregation from their sources (e.g., crystallization or degassing). We show here that the phase equilibria of plagioclase, spinel, and garnet lherzolites of constant bulk composition (including whole-rock Fe3+/Fe2+) can also lead to systematic variations in fO2 in the shallowest ~100 km of the mantle. Two different thermodynamic models were used to calculate fO2 vs. pressure and temperature for a representative, slightly depleted peridotite of constant composition (including total oxygen). Under subsolidus conditions, increasing pressure in the plagioclase-lherzolite facies from 1 bar up to the disappearance of plagioclase at the lower pressure limit of the spinel-lherzolite facies leads to an fO2 decrease (normalized to a metastable plagioclase-free peridotite of the same composition at the same pressure and temperature) of ~1.25 orders of magnitude. The spinel-lherzolite facies defines a minimum in fO2 and increasing pressure in this facies has little influence on fO2 (normalized to a metastable spinel-free peridotite of the same composition at the same pressure and temperature) up to the appearance of garnet in the stable assemblage. Increasing pressure across the garnet-lherzolite facies leads to increases in fO2 (normalized to a metastable garnet-free peridotite of the same composition at the same pressure and temperature) of ~1 order of magnitude from the low values of the spinel-lherzolite facies. These changes in normalized fO2 reflect primarily the indirect effects of reactions involving aluminous phases in the peridotite that either produce or consume pyroxene with increasing pressure: Reactions that produce pyroxene with increasing pressure (e.g., forsterite + anorthite ⇄ Mg-Tschermak + diopside in plagioclase lherzolite) lead to dilution of Fe3+-bearing components in pyroxene and therefore to decreases in normalized fO2, whereas pyroxene-consuming reactions (e.g., in the garnet stability field) lead initially to enrichment of Fe3+-bearing components in pyroxene and to increases in normalized fO2 (although this is counteracted to some degree by progressive partitioning of Fe3+ from the pyroxene into the garnet with increasing pressure). Thus, the variations in normalized fO2 inferred from thermodynamic modeling of upper mantle peridotite of constant composition are primarily passive consequences of the same phase changes that produce the transitions from plagioclase → spinel → garnet lherzolite and the variations in Al content in pyroxenes within each of these facies. Because these variations are largely driven by phase changes among Al-rich phases, they are predicted to diminish with the decrease in bulk Al content that results from melt extraction from peridotite, and this is consistent with our calculations. Observed variations in FMQ-normalized fO2 of primitive mantle-derived basalts and peridotites within and across different tectonic environments probably mostly reflect variations in the chemical compositions (e.g., Fe3+/Fe2+ or bulk O2 content) of their sources (e.g., produced by subduction of oxidizing fluids, sediments, and altered oceanic crust or of reducing organic material; by equilibration with graphite- or diamond-saturated fluids; or by the effects of partial melting). However, we conclude that in nature the predicted effects of pressure- and temperature-dependent phase equilibria on the fO2 of peridotites of constant composition are likely to be superimposed on variations in fO2 that reflect differences in the whole-rock Fe3+/Fe2+ ratios of peridotites and therefore that the effects of phase equilibria should also be considered in efforts to understand observed variations in the oxygen fugacities of magmas and their mantle sources.

PETROLOGY AND PETROGENESIS OF SIAH KOOH VOLCANIC ROCKS IN THE EASTERN ALBORZ

Siah Kooh area is northeast of Shahroud city and is located in eastern Alborz. The lithologic composition of the volcanic rocks in the area consists of andesite, basalt, trachyandesite and quartztrachite. Plagioclase, olivine, and augite phenocrysts as the main minerals and apatite and magnetite, sericite, chlorite and apacite minerals are sub-minerals of volcanic rocks that are located in the glass slabs. Quartz is also found in fine-grained rock pulp and sometimes in phenocrysts. The dominant texture in these rocks are porphyritic, amygdaloidal and microlithic. According to geochemical studies of basaltic magmatic volcanic rocks, calc-alkaline potassium is high and negative Nb anomaly, Ce / Pb ratio and enrichment of rocks of light rare earth elements (LRRE) and high LREE / HREE ratio indicate contamination. The crust is an indicator of the presence of the garnet phase in the mantle source. On the other hand, the similarity of their trace elements to Oceanic Basalts (OIB) is a clear evidence of their relevance to this environment. Early basaltic magma originated from a mantle with a garnet-lherzolite composition with a partial melting rate of 15–12%. FeOtotal values in basalts and other structural evidence indicate the formation of these rocks in the early stages of intra-continental rifting which can be attributed to the pressure drop caused by intra-continental tidal phases associated with deep faults during the orogenic phases. Alpine attributed to Eocene time.