Clinopyroxene Research Articles

Dark mantles over domes and rilles on the Moon: Evidence for pyroclastic materials

A collection of dark mantles covering volcanic domes and surrounding rilles were examined. Remotely sensed information on FeO and TiO2 content, clinopyroxene (CPX) and olivine (where olivine is an assumed proxy for glass) abundance, band center positions near 1 and 2 μm, and a parameter related to OH and/or H2O content were assessed for these mantling deposits. Based on our examination of these dark mantles associated with domes and rilles we found that all of the examined dark mantles had higher FeO abundances than the surroundings. The Birt E pit of Rima Birt and the mantle over the Manilius-1 dome are higher in glass and lower in CPX than their surroundings and with deeper band depth near 3 μm: all these indicators suggest higher pyroclastic glass abundance. The mantle at Rima Calippus is also nominally glass-rich although its band depth near 3 μm is comparable to nearby mare basalts and thus is not taken to be unusually hydrated. The dark mantles associated with the Yangel-1 and Cauchy-5 domes are higher in CPX, lower in glass, and higher in TiO2 content with low band depth near 3 μm indicating they are glass-poor and nominally rich in ilmenite. The Hyginus crater of Rima Hyginus is distinct from the other studied mantles in that it has higher TiO2 content, lower CPX abundance, higher glass or olivine, and weak 3 μm band depth. Also, the band minimum near 1 μm deos not occur at a longer wavelength than surrounding mare suggesting that in this instance the higher olivine (as a proxy for glass) might actually be due to olivine rather than glass. Thus, in the study of dark mantles associated with domes and rilles each represents deposits and eruptions that need to be characterized on a case-by-case basis rather than as a collective group.

Melting of hydrous pyroxenites with alkali amphiboles in the continental mantle: 1. Melting relations and major element compositions of melts

Melting experiments on ultramafic rocks rich in the hydrous minerals phlogopite or phlogopite + K-richterite, some including 5% of accessory phases, have been conducted at 15 and 50 kbar. The assemblages represent probable source components that contribute to melts in cratonic regions, but whose melt compositions are poorly known. A main series of starting compositions based on MARID xenoliths consisted of a third each of clinopyroxene (CPX), phlogopite (PHL) and K-richterite (KR) with or without 5% ilmenite, rutile or apatite. Additional experiments were run without KR and with higher proportions of accessory phases. Melt traps were used at near-solidus temperatures to facilitate accurate analysis of well-quenched melts, for which reversal experiments demonstrate equilibrium.Results show that KR melts rapidly and completely within 50 °C of the solidus, so that melts reflect the composition of the amphibole and its melting reaction. Melts have high SiO2 and especially K2O but low CaO and Al2O3 relative to basaltic melts produced from peridotites at similar pressures. They have no counterparts amongst natural rocks, but most closely resemble leucite lamproites at 15 kbar. KR and PHL melt incongruently to form olivine (OL) and CPX at 15 kbar, promoting SiO2 contents of the melt, whereas orthopyroxene OPX is increasingly stable at lower lithosphere pressures, leading to an increase in MgO and decrease in SiO2 in melts, which resemble olivine lamproites. Melts of mica pyroxenites without KR are richer in CaO and Al2O3 and do not resemble lamproites. These experiments show that low CaO and Al2O3 in igneous rocks is not necessarily a sign of a depleted peridotite source. Accessory phases produce melts exceptionally rich in P2O5 or TiO2 depending on the phases present and are unlike any melts seen at the Earth’s surface, but may be important agents of metasomatism seen in xenoliths. The addition of the 5% accessory phases ilmenite, rutile or apatite result in melting temperatures a few ten of degrees lower; at least two of these appear essential to explain the compositions of many alkaline igneous rocks on cratons.Melting temperatures for CPX + PHL + KR mixtures are close to cratonic geotherms at depths > 130 km: minor perturbations of the stable geotherm at >150 km will rapidly lead to 20% melting. Melts of hydrous pyroxenites with a variety of accessory phases will be common initial melts at depth, but will change if reaction with wall-rocks occurs, leading to volcanism that contains chemical components of peridotite even though the temperature in the source region remains well below the melting point of peridotite. At higher temperatures, extensive melting of peridotite will dilute the initial alkaline melts: this is recognizable as alkaline components in basalts and, in extreme cases, alkali picrites. Hydrous pyroxenites are, therefore, components of most mantle-derived igneous rocks: basaltic rocks should not be oversimplified as being purely melts of peridotite or of mixtures of peridotite and dry pyroxenite without hydrous phases.

Mafic mineralogy assemblages at the Chang’e-4 landing site: A combined laboratory and lunar in situ spectroscopic study

Context.Chang’e-4 (CE-4) provides unprecedented information about lunar materials exposed by the South Pole-Aitken (SPA) basin. Diverse results have been obtained from previous interpretations of CE-4 visible and near-infrared (VNIR) spectra. Some studies suggest that materials at the CE-4 landing site are dominated by olivine and orthopyroxene, but others argue that only a small amount of olivine should be exposed at the CE-4 landing site.Aims.Laboratory spectroscopy studies using the Engineering Model of CE-4 Visible and Near-infrared Imaging Spectrometer (VNIS) are critical in constraining the accurate mineral proportions and composition of soils and boulders at the CE-4 landing site.Methods.VNIR spectra of nine lunar analogs – prepared by mixing orthopyroxene (OPX), clinopyroxene (CPX), olivine (OL), and plagioclase – were acquired using the CE-4 VNIS Engineering Model. The spectral indices model and modified Gaussian model were developed to estimate CPX/(CPX+OPX) and OL/(OL+CPX+OPX) and are applicable to the in situ spectra acquired by the Yutu-2 VNIS spectrometer.Results.The lunar rocks and regolith at the CE-4 landing site excavated by the Finsen impact are CPX-rich with limited OL (CPX:OPX:OL = 56:29:17). The mineral chemistries of the four lunar rocks show Mid-Ca, Fe pyroxene, and Mid-Mg OL (Fo60−79), providing critical constraints for mineral compositions in the SPA compositional anomaly. These rocks exhibit high M1 intensity ratios, indicating that they were crystallized at a high temperature (980–1300 °C) and a rapid-cooling magmatic system produced by impact melt differentiation or volcanic resurfacing events.

A new method for determining the petrologic type of unequilibrated ordinary chondrites that can be applied to asteroids

In order to find an additional quantitative way to estimate the petrographic type of unequilibrated ordinary chondrites (UOC), and one that can be used remotely in the study of asteroids, we have analyzed the near-infrared spectra of a suite of UOC observed falls. We obtained spectra from the RELAB database at Brown University and applied several methods for determining the amount of clinopyroxene (CPX) as a percentage of the total pyroxene in the meteorites. The presence of low-Ca CPX has long been known to be characteristic of little-metamorphosed ordinary chondrites. The methods we used were (1) naked-eye determination of the wavelength of the absorption features at ~1 μm and ~2 μm, (2) determination of the wavelengths of these features by fitting polynomial equations, and (3) determining the relative intensities of the CPX and OPX features after isolation by a curve fitting procedure. The measurements were then “calibrated” using data from the literature to obtain values for the amount of CPX in the total pyroxene. We find that there is an empirical relationship between the amount of CPX detected by these methods of spectrum analysis and the petrologic type.Petrologic type=+4.402–0.019×CPX%We explain this empirical relationship (1) as evidence that in pyroxene bearing rocks the spectrum of pyroxene dominates (this has been known in the 1970s), (2) that low-Ca CPX is so abundant in these meteorites (up to 40 vol%) that it is easily detected by reflectance spectroscopy, and (3) compositional effects caused by Ca and Fe in the pyroxenes partially cancel out or are small. We thus have a new method of quantitatively measuring the level of metamorphic alteration experienced by these important meteorites and of assigning them a petrologic type of 3.0 to 3.9. More importantly, unlike existing methods, this can be applied remotely so that chondritic asteroid surfaces (i.e. those of Q and S asteroids) can also be characterized in terms of their metamorphic history. As an example, (433) Eros and (25143) Itokawa were found to be types ~3.5 and ~3.4, respectively. We briefly discuss the implications of this for understanding the history of meteorites and asteroids.

Clinopyroxene from basaltic rocks of the Erzgebirge - Krušné hory Mts. - implications for modelling the magmatic plumbing system

*Corresponding author The Erzgebirge-Krusne hory area is part of the NW flank of the Eger-Ohře Rift. Within this rift-related area, basaltic rocks with low ( 3.5 wt. %) of TiO 2 occur. The distribution of these basaltic rock types shows strong spatial dependency. Ti-rich basalts are concentrated in the western Erzgebirge-Krusne hory Mts. with a sharp change at longitude 13.3 °E. Clinopyroxene and olivine are the most abundant phenocrysts. The zoning and chemistry of the clinopyroxene act as a recorder of the different stages of the magmatic plumbing systems. Seven different types of clinopyroxene phenocrysts are defined with four distinct chemical types: Cr-rich clinopyroxene, green-core py roxene, Mg-rich clinopyroxene and finely zoned Ti-rich clinopyroxene with sector zoning. Chemical and petrographic analysis of individual zones within clinopyroxenes allows the qualitative description of distinct steps of the multistage, regionally diverse processes yielding to the formation of both Ti-rich basalts in the W as well as Ti-poor basalts in the E. Two different types of asthenospheric melts are suggested, with increased activities of H 2 O and potassium in the western Erzgebirge Mts. During ascent they incorporated xenoliths and xenocrysts of both the mantle (peridotite, olivine and Cr-rich clinopyroxene) and the crustal (granitic and gneissic rocks, quartz and feldspar) origin. Furthermore, green cores of clinopyroxene were introduced into the magmas. Fractionation processes took place within lithospheric magma chambers, having been accompanied by reaction of xenocrysts and xenoliths with the melt. The residence time in the magma chambers associated with the eastern and western parts of the Erzgebirge appears to have been totally different: short in the eastern and long in the western parts. Within the latter, no mantle-derived xenoliths, xenocrysts, or magmatic olivines are found. It is inferred that these xenoliths and crystals settled to the bottom of the magma chambers forming cumulates and therefore did not come to the surface. In contrast, the basaltic rocks of the East still contain mantle-derived xenoliths and xenocrysts as well as magmatic olivine. The size and amount of clinopyroxene phenocrysts in the West significantly exceed that of the East. Moreover, the crystallization of giant clino pyroxenes was restricted to the western region. Mixing of the magmas with Ti-rich melts, presumably derived from an eclogitic source, took place in crustal magma chambers. In any case, the proportion of Ti-rich melts in the West significantly exceeded that of the East.

Ultrahigh-pressure metamorphism in the archean-proterozoic mobile belt (Gridino complex, Karelia, Russia)

412 Ultrahighpressure metamorphism is considered as an indicator of the modern style of plate tectonics, generally, and of subduction, in particular (1). Accord� ing to some data, the formation of this style started on the Earth in the Neoproterozoic (1); according to other data, it began 2 b. y. before, at the Meso- Neoarchean boundary (2). The absence of ultrahigh� pressure complexes with an older age than the Neoproterozoic is an important argument in favor of the first hypothesis. In this paper we describe the results of petrological investigation of a metagabbro dyke in the Gridino eclogite complex (Karelia) dem� onstrating that ultrahighpressure events were realized in the subducting lithospheric slab already in the Pale� oproterozoic. The Gridino complex is part of the Belomorian mobile belt of Karelia (3-5). It represents a tectonic plate of northwestern strike occurring in the coastal area and on the islands of the White Sea up to 60 km from Sukhaya Bay in the northwest to the Suprotivnye Islands in the southeast at a width of 6-7 km. The main volume of the complex is represented by migma� tized gneiss containing rock boudines of various sizes and compositions (3, 4). Rocks of the basic composi� tion (metabasites), namely eclogite and amphibolites, strongly prevail among boudines in gneiss; amphiboli� tized metaultramafites, marbles, zoisite, and scapolite rocks are rarely observed. The complex is cut by dykes and small massifs of metagabbronorites metamor� phosed under the conditions of the eclogite and upper amphibolite facies, as well as intrusions and veins of plagiogranites (3). Dating of eclogites from this com� plex demonstrates a wide range of ages from the Neoarchean (2.74 Ga) to the Paleoproterozoic (1.9 Ga) (3-5). This paper contains the results of study of metagab� bro dyke cutting migmatized amphibole-biotite gneiss on Bezymyannyi Island. Metagabbro has massive structure, with fineto mediumgrained, heterogranoblastic texture. The main rockforming minerals are garnet, clinopyrox� ene, plagioclase, orthopyroxene, biotite, amphibole, and quartz, which are related to different stages of metamorphic evolution of the rock, according to the

Genesis of ultramafite complexes of the basement of the West Siberian Platform

86 In recent years, the widespread occurrence of ultramafic rocks in the basement of the West Siberian Platform has been established [1–5, etc.]. Ultramafic complexes of the basement can be divided into two groups. The first group, which includes the vast major ity of the studied samples, is characterized by almost complete serpentinization with the presence of only Cr spinel as a primary mineral. The second small group is represented by peridotites containing chromites, olivines, orthopyroxenes, and clinopy roxenes. The compositions of the rocks were deter mined by the X ray spectral fluorescence (XRF) method at the Zavaritskii Institute of Geology and Geochemistry, Ural Branch, Russian Academy of Sci ences (Yekaterinburg). The compositions of minerals were studied with an X ray scanning electron micro probe analyzer CAMEBAX MICRO at the Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences (Novosibirsk).

Argon diffusion in pyroxenes: Implications for thermochronometry and mantle degassing

The rate at which argon diffuses in pyroxenes is relevant to several problems in the Earth and planetary sciences. Pyroxenes are among the most abundant minerals in the Earth's upper mantle and are important hosts for potassium and radiogenic 40Ar ( 40Ar*). Additionally, pyroxenes commonly contain > 25% of the total 40Ar* within meteorites, and therefore hold great potential for quantifying the timing, duration, and thermal conditions of events experienced by meteorites and their parent bodies. We conducted detailed, feedback-controlled laser heating diffusion experiments using synthetically produced 37Ar in gem-quality clinopyroxene (CPX) and orthopyroxene (OPX) crystals. Stepwise heating between 850 and 1350 °C yielded linear Arrhenius arrays from which activation energies ( E a ) of 379.2 ± 4.1 and 371.0 ± 6.0 kJ/mol and pre-exponential factors ( D o ) of (1.36 × 10 − 4 ) − 6.15 × 10 − 5 + 1.12 × 10 − 4 and (5.73 × 10 − 2 ) − 2.25 × 10 − 2 + 3.72 × 10 − 2 were determined for CPX and OPX, respectively; these E a 's are an order of magnitude higher than previously reported values obtained with a different experimental approach ( Watson et al., 2007) and indicate that Ar diffusion within pyroxenes is strongly temperature dependent. At high temperatures Ar diffusion in pyroxenes is rapid, which indicates that diffusive 40Ar distributions observed within meteoritic pyroxenes may provide information about brief, high-temperature shock heating events. In contrast, subsequent low-T conditions should minimally influence these concentration gradients. The experimental Ar diffusion kinetics in OPX and CPX correspond to closure temperatures of 600–700 and 700–800 °C, respectively (for 10 °C/Ma cooling). These results suggest that diffusive equilibration through 0.1–0.5 mm pyroxene grains occurs in minutes to hours at basaltic melt generation temperatures (~ 1300 °C), and is therefore unlikely to inhibit mantle degassing.

Quantifying 3D crystal populations, packing and layering in shallow intrusions: A case study from the Basement Sill, Dry Valleys, Antarctica

In many shallow level intrusions, complex crystal layering structures can be preserved due to pulses of intrusion and magma batches that arrive with a significant crystal payload. In this study, we investigate crystal layering within the Dais intrusion of the Basement Sill, Dry Valley, Antarctica. A single three-dimensional (3D) Rhythmic layer is imaged using X-ray computed tomography (CT) to successfully measure the phase distributions of pyroxene and plagioclase through the rock, and a combination of textural quantification in 2D and the 3D high-resolution data allows the texture to be further broken down into the key components: orthopyroxene (OPX), cumulate OPX, clinopyroxene (CPX), and plagioclase (PLAG). The texture was formed by an initial OPX cumulate framework of 37%–47%. The OPX in the cumulate overgrew by an additional 7%–10%; with the concomitant growth of CPX and PLAG in the interstices, a mixture of fine plagioclase microcrystals and melt were expelled into the upper layer. Textural development in the upper layer was arrested by the rapid growth of OPX and CPX dendrites forming oikiocrysts containing the initial microcrystal population of existing PLAG nuclei, along with plagioclase overgrowths in the areas where the pyroxenes are absent. Calculations of liquid and solidification development at the distance the sample is from the sill contact suggests that the textures in this part of the sill developed over ∼140 yr or less.

The nature and age of the Suroyam clinopyroxenite massif (Middle Urals)

The Suroyam alkaline ultrabasitesyenite (mainly pyroxenite) massif, including the Suroyam apatitetita� nomagnetite deposit, is a part of the Nyazepetrovsk structural-formational zone located on the western slope of the Middle Urals, within the Ufimskii promon� tory of the East European Platform. The Nyazepetrovsk complex tectonically overlaps the sedimentary shelf and bathyal deposits of the mar� gin of the Paleozoic continent (Fig. 1) and is the uppermost tectonic sheet developed here. The com� plex is made of volcanogenic-sedimentary and intru� sive rocks of Ordovician-Early Devonian age, approximately correlated with the Early and Late islandarc oceanic formations of the Tagil zone (1). According to the chemical composition and metallo� genic features, the Suroyam massif located in the area of these formations can be compared with massifs of a palladiumbearing "branch" of the Platinum belt (Ural PGEbearing belt) in the Urals. The Rb-Sr iso� chrone age of the massif is 415.2 ± 1.1 Ma. Recent data obtained provide confirmation of the allochthonous origin of the Suroyam massif and its positional con� nection with the Tagil zone. The stratigraphic section of the Nyazepetrovsk zone is represented by volcanic and volcanogenic- sedimentary formations with subordinate siliceous rocks, siltstone, and sandstones. The section is distin� guished into three formations (from ancient to young): basalt, basalt-andesitebasalt, and trachybasalttra� chyte. Based on rare findings of radiolarians, con� odonts, and macrofauna, the age of the formations is regarded as Ordovician-Early Devonian. The Suroyam massif is located in the southern area of the axial part of the Nyazepetrovsk zone, and it is mainly composed of apatitetitanomagnetite clinopy� roxenites, subjected to strong superimposed potassium and sodium metasomatism, connected with manifes� tations of alkaline magmatism (2). The nature of the massif is under discussion. There were ideas expressed in (3, 4) about its platform origin, but more convincing evidence is needed. We have identified, however, sim� ilarities of the Suroyam massif with some specific mas� sifs of the Platinum belt with titanomagnetite mineral� ization. In particular, among the most widespread types in the Suroyam massif (Fig. 2) are highferrugi� nous clinopyroxenites, green and dark green rocks, characterized by fine to coarse grained structure, as well as pegmatoid structure. The main rockforming minerals are clinopyroxene, titanomagnetite, apatite, and phlogopite. The quantitative ratios of these miner� als vary greatly. The central part of the Suroyam massif is composed of coarsegrained titanomagnetite cli� nopyroxenites (like kosvites) with a titanomagnetite content of 6-25%; the marginal part is characterized mainly by apatitetitanomagnetite types of rocks, con� taining 2.9-19.5% of apatite. The undeformed rocks have sideronitic and granoblastic texture. Clinopyroxene forms short and long prismatic grains. According to the chemical composition, cli� nopyroxene is represented by diopsidehedenbergite, containing 25-30% of the hedenbergite molecule. The iron content in pyroxene ranges from 14 to 26% (20 at. % on average). There was noted a high content of alumina (2.49-5.05%) with variations in the augite content from 6 to 11%. The titanomagnetite is lowtitanium, and it is rep� resented by xenomorphic grains with sideronitic tex� ture. In addition it forms finegrained, platy, den� dritic, and dropshaped inclusions in clinopyroxene. In general, titanomagnetite is distributed uniformly in the rock. However, there are sections with a banded distribution of ore minerals. In titanomagnetite there are inclusions of xenomorphic clinopyroxene and prismatic apatite. Sometimes, the ore mineral forms

Phase Relations and Liquid Lines of Descent in Hydrous Ferrobasalt--Implications for the Skaergaard Intrusion and Columbia River Flood Basalts

Crystallization experiments using a hydrous ferrobasalt as starting material, conducted at 200MPa, 940^12008C, at a wide range of water activities (0 1^1) and redox conditions (QFM 3 to QFMþ 4, where QFM is the quartz^fayalite^magnetite oxygen buffer), show that H2O influences significantly the differentiation history of ferrobasaltic magmas. A combination of our data with published experiments on dry ferrobasalt at 1atm provides an extensive experimental database for modeling and quantifying crystallization and differentiation processes within a typical Fe-rich tholeiitic system under both dry and hydrous conditions.The addition of H2O decreases liquidus temperatures and changes significantly the proportions, temperature range and sequence of crystallizing mineral phases. The dry liquidus is at about 11708C whereas the liquidus for H2O-saturated melts is at 10608C.The main phases crystallizing from H2O-bearing ferrobasalt at the investigated conditions are olivine (OL), clinopyroxene (CPX), plagioclase (PL), magnetite (MT), hematite (HM), ilmenite (ILM) and amphibole (AM). The phase assemblage is similar to that of the dry system except for the presence of HM at extremely oxidizing conditions and AM at low temperatures (59508C) and H2O-saturated conditions. The important observation made in this study is that the stability of Fe^Ti-oxides, and in particular MT, as well as the simultaneous coprecipitation of MTand ILM, are almost independent of the activity of H2O (aH2O) in the system, whereas the liquidus temperatures of the silicate minerals are dramatically depressed by increasing aH2O.The stabilities of oxides are controlled mainly by the redox conditions prevailing in the system.The most pronounced effect of aH2O on the liquidus temperatures of silicates is observed for PL, which shows a considerable delay in crystallization with progressive magma differentiation. Early crystallization of Fe^Ti-oxides in H2O-bearing ferrobasaltic compositions precludes any significant Fe enrichment during differentiation. As Fe enrichment is a characteristic feature of the Skaergaard intrusion, it implies that the Skaergaard parental magma did not contain considerable amounts of water. On the other hand, our experiments indicate that the differentiation of some ferrobasaltic series from the Columbia River flood basalt province might have occurred in magmatic systems containing significant amounts of volatiles ( 0 5^3 wt%H2O).

Analysis of OMEGA/Mars Express data hyperspectral data using a Multiple-Endmember Linear Spectral Unmixing Model (MELSUM): Methodology and first results

The mineralogical composition of the Martian surface is investigated by a Multiple-Endmember Linear Spectral Unmixing Model (MELSUM) of the Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) imaging spectrometer onboard Mars Express. OMEGA has fully covered the surface of the red planet at medium to low resolution (2–4 km per pixel). Several areas have been imaged at a resolution up to 300 m per pixel. One difficulty in the data processing is to extract the mineralogical composition, since rocks are mixtures of several components. MELSUM is an algorithm that selects the best linear combination of spectra among the families of minerals available in a reference library. The best fit of the observed spectrum on each pixel is calculated by the same unmixing equation used in the classical Spectral Mixture Analysis (SMA). This study shows the importance of the choice of the input library, which contains in our case 24 laboratory spectra (endmembers) of minerals that cover the diversity of the mineral families that may be found on the Martian surface. The analysis is restricted to the 1.0–2.5 μm wavelength range. Grain size variations and atmospheric scattering by aerosols induce changes in overall albedo level and continuum slopes. Synthetic flat and pure slope spectra have therefore been included in the input mineral spectral endmembers library in order to take these effects into account. The selection process for the endmembers is a systematic exploration of whole set of combinations of four components plus the straight line spectra. When negative coefficients occur, the results are discarded. This strategy is successfully tested on the terrestrial Cuprite site (Nevada, USA), for which extensive ground observations exist. It is then applied to different areas on Mars including Syrtis Major, Aram Chaos and Olympia Undae near the North Polar Cap. MELSUM on Syrtis Major reveals a region dominated by mafic minerals, with the oldest crustal regions composed of a mixing between low-calcium pyroxenes (LCPs) (orthopyroxenes (OPx)) and high-calcium pyroxenes (HCPs) (clinopyroxenes (CPx)). The Syrtis volcanic edifice appears depleted in LCP (OPx) and enriched in HCP (CPx), which is consistent with materials produced with a lower partial fusion degree at an age younger to the surrounding crust. Strong olivine signatures are found between the two calderas Nili Patera and Meroe Patera and in Nili Fossae. A strong signature of iron oxides is found within Aram Chaos, with a spatial distribution also consistent with thermal emission spectrometer (TES). Gypsum is unambiguously detected in the northern polar region, in agreement with the study of Langevin et al. [2005. Sulfates in the north polar region of Mars detected by OMEGA/Mars Express. Science 307(5715), 1584–1586]. Our results show that the linear spectral unmixing provides good first order results in a variety of mineralogical contexts, and can therefore confidently be used on a wider scale to analyze the complete archive of OMEGA data.

Lithium isotopic systematics of the mantle-derived ultramafic xenoliths: implications for EM1 origin

Isotopic signatures of mantle-derived xenoliths have provided much information on the evolution of their mantle source regions. A recently developed multiple-collector inductively coupled plasma mass spectrometry method allows precise and accurate lithium isotopic determinations of Li-poor samples such as peridotites. We present Li–Sr–Nd isotopic systematics of clinopyroxenes (CPXs) in mantle-derived ultramafic xenoliths. The results show that Ichino-megata (Northeastern Japan) and Bullenmerri (Southeastern Australia) samples have positive δ7Li values (δ7Li∼+4 to +7‰, δ7Li=[[7Li/6Li]sample/[7Li/6Li]L-SVEC standard−1]×1000) common to values previously reported for terrestrial volcanic rocks. By contrast, unusually low δ7Li values (δ7Li∼−17‰) are observed in many samples from the Far East region of Russia (Sveyagin, Ennokentiev, and Fevralsky) and southwestern Japan (Kurose and Takashima). The δ7Li values of Sikhote-Alin (Sveyagin and Ennokentiev) samples vary widely from −17.1‰ to −3.1‰, while the δ7Li values are positively correlated with 143Nd/144Nd, and negatively correlated with 87Sr/86Sr. On the other hand, the δ7Li values of the Bullenmerri samples are essentially constant (δ7Li=+5.0 to +6.0‰), while the 87Sr/86Sr (0.7027∼0.7098) and 143Nd/144Nd ratios (0.51224∼0.51297) vary widely. These features can be explained by the results of a binary mixing between a depleted component (low-87Sr/86Sr, and high-143Nd/144Nd) and an enriched component (high-87Sr/86Sr, and low-143Nd/144Nd). The enriched component (metasomatic agent) in the mantle beneath the Sikhote-Alin area has extraordinarily low δ7Li value (<−17‰), whereas the metasomatic agent in the mantle beneath the Bullenmerri area has positive δ7Li value (+6‰). Based on the Sr–Nd isotopic systematics and coexistent hydrous mineral, metasomatic agents of the Sikhote-Alin and Bullenmerri samples are classified into anhydrous EM1-type and hydrous EM2-type, respectively. From these features, we infer that anhydrous EM1-like metasomatic agent may have an extremely low δ7Li value, whereas hydrous EM2-like metasomatic agent may have a positive δ7Li value. It has been predicted that the δ7Li value of subducted highly altered mid-ocean ridge basalt (MORB) would be extremely low compared to that of fresh MORB due to the preferential loss of heavier Li (δ7Li>altered MORB) from the subducted slab during dehydration at low temperature. Consequently, it is deduced that Li of metasomatic agent with an extremely low δ7Li value is derived from subducted highly altered basalt. The enrichment of isotopically light Li (low δ7Li) may be a general property of EM1 mantle reservoir. The Li isotopic data suggest further that the EM1 and HIMU sources originate from different parts of a recycling oceanic crust. This is essentially the same as the models proposed previously based on the radiogenic isotopic data, but with the Li isotopic data requiring uppermost, highly altered basaltic crust as well as pelagic sediment in the EM1 source, but not so in the HIMU end-member. Because of the apparent sensitivity of Li isotopic composition to the alteration profile of subducted MORB, it may provide complementary information to Sr, Nd, and Pb isotopic compositions regarding the mantle source.

Helium isotope ratios in mafic phenocrysts and geothermal fluids from La Palma, the Canary Islands (Spain): implications for HIMU mantle sources

We report a comprehensive study of He isotope ( 3He/ 4He) variations on well-characterized lavas from La Palma, the current locus of activity of the Canary hotspot. Cogenetic olivine (OL) and clinopyroxene (CPX) phenocrysts from 11 basaltic lava flows representing all stages of the island’s evolution were analyzed by crushing in vacuo. Additionally, the sample set is supplemented by a CO 2-rich bubbling cold spring in Taburiente caldera. Phenocryst 3He/ 4He ratios vary between 6.3 to 8.9R A; both extremes occur in the 2 to 0.6 Ma shield-building Taburiente lavas. Historic flows vary between 7.0 to 7.8R A, whereas phenocrysts from a single submarine basement sill (4.0–2.9 Ma) have 3He/ 4He ratios of 8.3R A (OL) and 8.4R A (CPX). He isotopic equilibrium characterizes all the phenocryst pairs except one Taburiente lava. The absence of a correlation between 3He/ 4He and [He] contrasts with other ocean island basalts, e.g., Heard Island (Hilton et al., 1995) and, together with equilibrium in phenocrystic 3He/ 4He, suggests that the La Palma sample suite has 3He/ 4He ratios that have suffered minimal crustal contamination. The cold spring (geothermal) sample gives a 3He/ 4He ratio of 9.5R A, which is higher than any of the phenocrysts and in excellent agreement with a previously reported value for Taburiente caldera (Perez et al., 1996). The geothermal 3He/ 4He values indicate that La Palma is the first HIMU-like ocean island (where HIMU implies a distinctive mantle source with high time-integrated 238U/ 204Pb (μ) ratio) with reported 3He/ 4He ratios higher than the canonical value (8 ± 1R A) characteristic of depleted mid-ocean ridge basalt (MORB) mantle. We present two simple mixing scenarios that are compatible with the He–Pb isotope systematics of La Palma: a) a high- 3He mantle plume, extensively but variably depleted in its original He content, mixes with a HIMU source characterized by both radiogenic He and lead isotope compositions; and b) mixing between a composite plume, with high 3He/ 4He and radiogenic lead, and depleted mid-ocean ridge basalt mantle. The latter model is favored as it appears to account more readily for the mid-ocean ridge MORB-like He isotope characteristics of the lavas. Addition of radiogenic He—most likely produced in mantle melts frozen into the sub-Canarian mantle lithosphere—to resultant mixtures may subsequently occur in either model. Such a process can explain the absence of correlations between He and Pb isotope ratios in the lavas, and the lower 3He/ 4He values of the phenocrysts with respect to the geothermal fluids. We find no need to invoke a radiogenic growth/diffusion model (Hanyu and Kaneoka, 1998) to explain the He–Pb isotope relationships for La Palma. These mixing models may also be applicable to other HIMU islands: however, any high- 3He plume signature of St. Helena and Mangaia may be obscured due to the high proportion of the HIMU component at these localities and/or by addition of radiogenic He.

M1 site splitting due to Next Nearest Neighbor effects and ferric iron in tetrahedral site in clinopyroxene megacrysts

It is well known that in pyroxene structure, there are two metal sites, M1 and M2. Generally speaking, ferrous iron in each of these sites would normally be expected to give rise to a doublet. However, anomalies have been found in the relative areas of the peaks in the room temperature spectra of some clinopyroxene (CPX) when the above assignment is followed. According to the calculation of Next Nearest Neighbor configurations of divalent cations in M1, we found that the four configurations of M1 can be divided into two groups. One group is 3Ca configuration that increases with the content of Ca (p. f. u); the other group is made up of three No-3Ca configurations that decrease with the content of Ca. The two groups contribute to the spectrum structure of M1, so in this study we fit two doublets for ferrous iron in M1. Though there were several reports on Fe3+ in tetrahedral site previously, it was not sure that Fe3+ occupies the T site is a universal fact in CPX, despite of the content of Al. We found that the Fe3+ in the T site fitted by Mossbauer spectroscopy is negatively correlated to the Si content in the T site and positively correlated to the Fe3+ in the T site estimated on the supposition that Fe3+ and Al occupy the T site randomly. If it is true, it is important in the modeling of ion exchange geobarometries and geothermometries.

The composition of back‐arc basin lower crust and upper mantle in the Mariana Trough: A first report

Abstract The Mariana Trough is an active back‐arc basin, with the rift propagating northward ahead of spreading. The northern part of the Trough is now rifting, with extension accommodated by combined stretching and igneous intrusion. Deep structural graben are found in a region of low heat flow, and we interpret these to manifest a low‐angle normal fault system that defines the extension axis between 19°45′ and 21°10′N. A single dredge haul from the deepest (∼5.5 km deep) of these graben recovered a heterogeneous suite of volcanic and plutonic crustal rocks and upper mantle peridotites, providing the first report of the deeper levels of back‐arc basin lithosphere. Several lines of evidence indicate that these rocks are similar to typical back‐arc basin lithosphere and are not fragments of rifted older arc lithosphere. Hornblende yielded an 40Ar/39Ar age of 1.8 ± 0.6 Ma, which is interpreted to approximate the time of crust formation. Harzburgite spinels have moderate Cr# (&lt;40) and coexisting compositions of clinopyroxene (CPX) and plagioclase (PLAB) fall in the field of mid‐ocean ridge basalt (MORB) gabbros. Crustal rocks include felsic rocks (70‐80% SiO2) and plutonic rocks that are rich in amphibole. Chemical compositions of crustal rocks show little evidence for a ‘subduction component’, and radiogenic isotopic compositions correspond to that expected for back‐arc basin crust of the Mariana Trough. These data indicate that mechanical extension in this part of the Mariana Trough involves lithosphere that originally formed magmatically. These unique exposures of back‐arc basin lithosphere call for careful study using ROVs and manned submersibles, and consideration as an ocean drilling program (ODP) drilling site.

UPb isotopic geochemistry of komatiites and pyroxenes from the southern Abitibi greenstone belt, Canada

New U and Pb concentrations and Pb isotopic data are reported for several komatiitic flows and for primary magmatic clinopyroxenes (CPX) from flows and sills of basaltic compositions from the southern Abitibi greenstone belt, for which RbSr, SmNd and ReOs isotope data were previously published. A suite of komatiites from Pyke Hill displays a narrow range of 206Ph/ 204Pb (17.91–19.20) when compared to previously reported values of ∼ 16–30 for komatiites in the same area. Whole-rock samples leached with HCl underwent up to 40% weight loss. The leachates display essentially the same isotopic composition as the bulk samples but the residues have higher 206Pb/ 204Pb (19.9–24.0). The CPX display 206Pb/ 204Pb (13.96–17.98) lower than whole rocks and leaching resulted in either higher or lower Pb ratios for the residues compared to bulk samples. For both whole rocks and mineral separates, leaching induced a significant decrease of Pb concentrations in the residues, which contained < 0.04 ppm compared to 1–2 ppm for the bulk samples. The U concentrations did not vary systematically with the degree of leaching but the residues always had higher 238U/ 204Pb than the bulk samples. Whole rocks and CPX exhibit U loss relative to Pb. All the samples define a ∼ 2.73-Ga PbPb isochron, identical to that previously reported for komatiites in the same area. However, our isochron is displaced toward slightly more radiogenic 207Pb/ 204Pb compared to the previously reported isochrons and this is interpreted as reflecting mantle source heterogeneities for komatiites. These mantle sources are depleted and the most radiogenic one is similar in composition to that of carbonatites and other alkaline complexes in the Superior Province. We also show that the Pb concentration in komatiites from Pyke Hill displays a coherent variation with respect to MgO, FeO, Re and Os, and that Pb behaves like Os in all the samples, suggesting a common bearing phase controlling the distribution of these two elements. The “hydrothermal Pb removing” inferred by B. Dupré and N.T. Arndt to account for large variations of the 206Pb/ 204Pb ratios of some komatiite flows is not universal. If our samples were affected by any hydrothermal remobilisation, it did not mask the primary magmatic distribution of Pb.

Refertilization in a Convecting Mantle: Effects on Primary MORB Compositions

The widely held view is that mid-ocean ridge basalts (MORBs) are produced by polybaric fractional melting of the sub-oceanic mantle in which the composite primary MORBs are derived from a relatively fertile mantle by 10-25% total melting (McKenzie, 1985; Johnson et al., 1990). The differences in primary MORB compositions are inferred to result principally from differences in the pressure and extent of melting (Klein and Langrnuir, 1987; Kinzler and Grove, 1992). The compositional variations between abyssal peridorites, in this scenario, reflect different degrees of fractional melting in which the least depleted peridotites represent the residues of ~10% melting and the most depleted peridotites represent the residues of ~25% melting (Dick and Fisher, 1984; Michael and Bonatti, 1985). It has been suggested, however, that the abundances of moderately incompatible elements (e.g., Na) in abyssal peridotites are not compatible with these rocks having been formed as residues of variable degrees of partial melting (Elthon, 1992). It was suggested instead that abyssal peridotites are a mixture of two distinct components. One component is a depleted harzburgite that has either no clinopyroxene (CPX) or that has < 2% CPX with very low abundances of Na20 (0.01 wt. %), Ti (300 ppm), Zr (0.1 ppm), and Ce (0.02 • chondrites). The other component is a basaltic liquid, which is potentially highly variable in composition. If the chemical variations in abyssal peridotites reflect some type of melt addition (or refertilization) process into depleted harzburgite mantle, there are profound implications for how the source regions of MORBs have evolved. Most importantly, if the mantle melts to produce a harzburgite (< 2% CPX) on a routine basis, the differences in primary composite MORBs will reflect most strongly the differences in the local mantle composition as the mantle enters the midocean ridge melting regime. In light of the variable compositions of mantle xenoliths and peridotite massifs as well as the strong evidence for melt addition processes in these samples, it might also be expected that the sub-oceanic mantle is both variable in composition and is influenced by meltaddition (refertilization) processes. It is proposed here that the MORB source region has been through the mid-ocean ridge melting regime at least 1 to 5 times during normal mantle evolution and consists of a highly depleted harzburgite mantle (similar to that found in peridotite massifs) mixed with basaltic/komatiitic material that has refertilized the mantle. In this scenario, the ascending mantle beneath midocean ridges varies significantly in composition, depending on the amount and type of basaltic/ komatiitic mantle added. Within the melting regime, the mantle melts to whatever extent is necessary to deplete the mantle to harzburgite (+ a few percent clinopyroxene) where thermal considerations stop further melting. It is proposed, therefore, that differences in primary MORB compositions are principally due to differences in the amount and composition of basaltic/komatiitic material added into the source region.

Compositional trends of minerals in oceanic cumulates

Five differences are noted between the compositions of minerals in oceanic cumulates and minerals that crystallize from mid‐ocean ridge basalts (MORBs) at 1 atm. These differences are (1) Mg numbers of clinopyroxene (CPX) in oceanic cumulates are often higher (&gt;88) than found in experimental studies of MORBs at 1 atm; (2) Mg numbers of orthopyroxene (OPX) in oceanic cumulates are often higher (&gt;81) than found in experimental studies of MORBs at 1 atm; (3) the low‐Ca pyroxene in oceanic cumulates (with some notable exceptions) is OPX rather than (inverted) pigeonite; (4) Na2O contents of CPX in oceanic cumulates are often higher (0.3 to 0.8% Na2O) than in 1‐atm experiments or phenocrysts in MORBs (typically 0.15 to 0.35% Na2O); and (5) TiO2 contents of CPX in oceanic cumulates are often higher than in 1‐atm experiments. These characteristics are consistent with crystallization of many oceanic gabbros at moderate pressures (3–10 kbar). The Na2O and TiO2 contents of liquids calculated to be in equilibrium with CPXs from oceanic cumulates generally are comparable to the abundances of these oxides in MORBs. The notable exception is the suite of cumulates from Deep Sea Drilling Project site 334, which appears to have crystallized at low pressures from strongly depleted Na2O‐poor, TiO2‐poor, SiO2‐rich basalts. We suggest that such strongly depleted basaltic liquids are a widespread, but minor, component in mid‐ocean ridge petrogenesis even though such liquids have never been directly sampled.

Chemical trends in abyssal peridotites: Refertilization of depleted suboceanic mantle

It is suggested that the chemical trends in abyssal peridotites result dominantly from two processes. The first process is fractional melting of the suboceanic mantle (Johnson et al., 1990) to the point at which clinopyroxene (CPX) is essentially eliminated from the residual mantle. The residue at this stage is depleted harzburgite with either no CPX or &lt;2% CPX that has very low abundances of Na2O (0.01 wt %), Ti (300 ppm), Zr (0.1 ppm) and Ce (0.02 × chondrites). The composite liquids produced during melting are comparable to primitive mid‐ocean ridge basalt glasses (1.5–2.6% Na2O, 5500–7500 ppm Ti, 50–80 ppm Zr). The second process is low‐pressure (&lt;5 kbar) refertilization of the depleted harzburgite with the addition of up to 10% depleted basaltic liquids; following recrystallization, mantle peridotites at this stage are CPX‐bearing harzburgites and Iherzolites. The chemical trends in abyssal peridotites are suggested to result dominantly from these refertilization processes rather than from partial melting. The rejection of fractional melting as the dominant process controlling the chemical trends in abyssal peridotites is based principally on the Na2O versus MgO variations in abyssal peridotites; the Na2O abundances in abyssal peridotites at 42% MgO (0.055 wt %) are 30 times higher than the Na2O abundances (0.002 wt %) in residual mantle formed by fractional melting of primitive upper mantle.