Abundant Olivine Research Articles

Thermal infrared (vibrational) spectroscopy of Mg–Fe olivines: A review and applications to determining the composition of planetary surfaces

The reststrahlen features in thermal infrared, or vibrational, spectra of Mg-Fe olivines ((Mg,Fe) 2SiO 4) exhibit trends in position, strength, and number of features that are diagnostic of the relative proportions of the Mg and Fe cations in the minerals. Although band positions move to lower wavenumbers (longer wavelengths) across the forsterite–fayalite compositional binary in a generally linear manner, specific feature shifts in transmittance data are described best by two linear fits with a break in slope near Fo 70. The break in slope may be accompanied by an offset as well; both traits are attributed to structural changes in olivine brought about by distortion of the crystal lattice by Fe. Reflectance and emissivity spectra exhibit similar trends in band position with composition, and all three types of data demonstrate that some olivine band strengths change across the Mg–Fe solid solution series and also are diagnostic of composition. Olivines have been identified in a wide array of thermal infrared spectra of planetary materials and have been interpreted as being present on the surfaces of Mercury, the Moon, Mars, and a number of asteroids based on the analysis of thermal infrared spectra. New linear least squares models of the emissivity spectra of olivine-bearing Martian meteorites enable a preliminary estimation of the accuracy with which quantitative estimates of olivine abundance and solid solution composition can be derived from the spectra of mixtures.

SURFACE TEMPERATURE OF PROTOPLANETARY DISKS PROBED BY ANNEALING EXPERIMENTS REFLECTING SPITZER OBSERVATIONS

Pyroxenes and olivines are the dominant crystalline silicates observed in protoplanetary disks. Recent spectral observations from the Spitzer Space Telescope indicate that the abundance of olivine, generally associated with silica polymorphs, relative to pyroxene is higher in the outer cold part of the disk than in the inner warmer part. The interpretation of these unexpected results requires a comprehensive knowledge of the thermal processing of Mg-rich silicate dust. In this respect, amorphous analogs were thermally annealed to identify microscopic crystallization mechanisms. We show that pyroxenes are not produced in significant proportions below the glass transition temperature of the amorphous precursor. The annealing of amorphous enstatite leads to a mineralogical assemblage dominated by forsterite, with only minute amounts of pyroxenes at temperatures as high as the glass transition temperature of enstatite (1050 K). The decoupling of cation mobility in amorphous silicates, favors the crystallization of the most Mg-enriched silicates. These results are consistent with Spitzer observations of silicate dust and also with the documented mineralogy of presolar silicates, making the low-temperature annealing a likely formation process for these objects. Based on these laboratory experiments and Spitzer observations, it appears that the reported zoned mineralogy may directly records and calibrates the thermal gradient at the scale of protoplanetary disks.

Combined U–Th/He and 40Ar/ 39Ar geochronology of post-shield lavas from the Mauna Kea and Kohala volcanoes, Hawaii

Late Quaternary, post-shield lavas from the Mauna Kea and Kohala volcanoes on the Big Island of Hawaii have been dated using the 40Ar/ 39Ar and U–Th/He methods. The objective of the study is to compare the recently demonstrated U–Th/He age method, which uses basaltic olivine phenocrysts, with 40Ar/ 39Ar ages measured on groundmass from the same samples. As a corollary, the age data also increase the precision of the chronology of volcanism on the Big Island. For the U–Th/He ages, U, Th and He concentrations and isotopes were measured to account for U-series disequilibrium and initial He. Single analyses U–Th/He ages for Hamakua lavas from Mauna Kea are 87 ± 40 to 119 ± 23 ka (2 σ uncertainties), which are in general equal to or younger than 40Ar/ 39Ar ages. Basalt from the Polulu sequence on Kohala gives a U–Th/He age of 354 ± 54 ka and a 40Ar/ 39Ar age of 450 ± 40 ka. All of the U–Th/He ages, and all but one spurious 40Ar/ 39Ar ages conform to the previously proposed stratigraphy and published 14C and K–Ar ages. The ages also compare favorably to U–Th whole rock–olivine ages calculated from 238U– 230Th disequilibria. The U–Th/He and 40Ar/ 39Ar results agree best where there is a relatively large amount of radiogenic 40Ar (>10%), and where the 40Ar/ 36Ar intercept calculated from the Ar isochron diagram is close to the atmospheric value. In two cases, it is not clear why U–Th/He and 40Ar/ 39Ar ages do not agree within uncertainty. U–Th/He and 40Ar/ 39Ar results diverge the most on a low-K transitional tholeiitic basalt with abundant olivine. For the most alkalic basalts with negligible olivine phenocrysts, U–Th/He ages were unattainable while 40Ar/ 39Ar results provide good precision even on ages as low as 19 ± 4 ka. Hence, the strengths and weaknesses of the U–Th/He and 40Ar/ 39Ar methods are complimentary for basalts with ages of order 100–500 ka.

OIB-like, heterogeneous mantle sources of Permian basaltic magmatism in the western Tarim Basin, NW China: Implications for a possible Permian large igneous province

Permian basalts are widely distributed in the Tarim Basin and surrounding areas of NW China. The magmatism is represented by basaltic flows in Keping and mafic and ultramafic dykes in Silurian–Devonian strata in Bachu, southwestern Tarim Basin. The basalts in Keping have SiO2 (44.1%–55.5 wt.%) and total alkalis (Na2O+K2O=3.20–7.79 wt.%) similar to the mafic dykes in Bachu, but with much higher TiO2 (3.53–4.33 wt.%). An ultramafic dyke has relatively low SiO2 (44.6–43.2 wt.%) and high MgO (19.0–20.2 wt.%), reflecting the abundance of cumulate olivine. All the rocks, including both lavas and dykes, have parallel, mantle-normalized trace element patterns enriched in Rb, Ba, Th, Nb, Ta, Zr, Hf, and light rare earth elements (LREE). The basalts have higher initial 87Sr/86Sr ratios (0.7064 to 0.7080) and lower εNd(t) values (−2.66 to −9.27) than the dykes (initial 87Sr/86Sr ratios range from 0.7048 to 0.7052 and εNd(t) values from +1.64 to +5.16). Both the basalts and dykes show a narrow range of 206Pb/204Pb (17.87–18.77), 207Pb/204Pb (15.52–15.58) and 208Pb/204Pb (38.38–39.04) ratios. The dykes do not show significant crustal contamination and were derived from an OIB-like, asthenospheric mantle source. In contrast, the basaltic flows show variable degrees (up to 10%) of crustal contamination and were derived from an OIB-like, but isotopically more enriched, asthenospheric mantle source. Olivine from the ultramafic dyke has Fo values up to 85, corresponding to a melt temperature of 1300 °C and a melt Mg# of 63. The Tarim Basin magmatism reflects partial melting of heterogeneous mantle sources related to a major mantle plume. Spatially and temporally associated mafic–ultramafic and syenitic intrusions and volcanic rocks form the ~275 Ma Tarim large igneous province.

Modal mineralogy of CM2 chondrites by X-ray diffraction (PSD-XRD). Part 1: Total phyllosilicate abundance and the degree of aqueous alteration

CM carbonaceous chondrites are samples of incompletely serpentinized primitive asteroids. Using position sensitive detector X-ray diffraction (PSD-XRD) and a pattern stripping technique, we quantify the modal mineralogy of CM2 chondrites: Mighei; Murray; Murchison; Nogoya and Cold Bokkeveld. There is a narrow range in the combined modal volume (vol%) of the most abundant phases Mg-serpentine (25–33%) and Fe-cronstedtite (43–50%). Cold Bokkeveld is anomalous in containing more Mg-serpentine (49–59%) than Fe-cronstedtite (19–27%). Even including Cold Bokkeveld, the range in modal total phyllosilicate is 73–79% (average = 75%). Total phyllosilicate abundance provides a non-ambiguous measure of the degree of aqueous alteration and indicates that these meteorites have all experienced essentially the same degree of aqueous alteration. This reflects pervasive hydration of matrix across CM2 samples. Apparent differences in the alteration of chondrules observed in petrographic studies represent various stages in the progression towards complete hydration of all components but are not manifest in significant differences in modal mineralogy. For all samples there is a limited range in olivine (6.9%) and pyroxene (5%) abundances. Modal abundances of the remaining identified phases also show a limited range: calcite (0–1.3%); gypsum (0–1.6%); magnetite (1.1–2.4%); pentlandite (0–2.1%) and pyrrhotite (1–3.8%). As expected, we observe a strong negative correlation in the modal abundance of anhydrous Fe–Mg silicates (olivine + pyroxene) and total phyllosilicate (Mg-serpentine + Fe-cronstedtite) consistent with the idea that phyllosilicate is forming by aqueous alteration of the anhydrous components. The negative correlation in the modal abundance between Mg–serpentine and Fe-cronstedtite indicates: (a) mineralogic transformation of Fe-cronstedtite to Mg-serpentine by fluid driven recrystallisation or (b) that these meteorites had different initial abundances of olivine and pyroxene. The observed positive correlation in the relative proportion of Mg-serpentine with increasing total phyllosilicate abundance reflects the evolution of increasingly Mg-rich phyllosilicate during aqueous alteration. Fe-cronstedtite is the dominant phyllosilicate, while CM chondrule olivines are forsteritic and will form Mg-serpentine during aqueous alteration. This implies that matrix olivine was more Fe-rich than chondrule olivine prior to aqueous alteration.

Variations of olivine abundance and grain size in the Snap Lake kimberlite intrusion, Northwest Territories, Canada: A possible proxy for diamonds

The Snap Lake hypabyssal kimberlite is a 15° inclined intrusion that intrudes Archaean rocks of the Slave craton. Mapping has identified two main lithofacies within the intrusion, olivine-rich kimberlite (ORK) and olivine-poor kimberlite (OPK). Extensive bulk sampling has demonstrated that diamonds vary in both abundance and size across the intrusion. Since fresh forsteritic olivine and diamond possess similar densities they can be expected to be hydraulically equivalent. This study investigates the local scale variation of olivine grain size and content using image analysis, and whether olivine can be used as a proxy for diamond concentration and size. Direct measurement of diamonds is made difficult by extremely low concentrations of around 0.4 ppm and by the inability of the sampling methodology to sample selectively. Olivines from the two kimberlite varieties possess distinctly different crystal size distributions and this, together with contrasting groundmass textures, mineralogy and phlogopite compositions, indicates that the OPK and ORK represent two distinct batches of kimberlite magma. The general lack of sorting of olivine grains and the presence of decussate phlogopite textures in the ORK indicate that this magma crystallized rapidly, but was probably preceded by the OPK which had a more complex crystallization history as revealed by complex zoning patterns of its phlogopite grains. A methodology for comparing olivine and diamond size distributions has been developed, and a strong correlation between diamond and olivine size distributions is demonstrated. The correlation is used to demonstrate that diamond size distributions can be predicted from the proportions of OPK and ORK that comprise each bulk sample. It can be concluded that the two textural varieties of kimberlite possess distinctly different olivine (and diamond?) populations and that olivines are a good proxy for understanding diamond distributions at a local scale in the Snap Lake intrusion.

Geochemistry and Os–Nd–Sr isotopes of the Gaositai Alaskan-type ultramafic complex from the northern North China craton: implications for mantle–crust interaction

We report petrological, chemical and Os–Nd–Sr isotopic data for the Gaositai ultramafic complex from northern North China craton (NCC) to reveal its petrogenesis. The complex shows features of Alaskan-type intrusions, including (1) the concentric zoning from dunite core, to clinopyroxenite and hornblendite in the rim, and the common cumulative textures; (2) the abundance of olivine, clinopyroxene and hornblende, and the scarcity of orthopyroxene and plagioclase, and (3) the systematic decrease in Mg# of ferromagnesian phases from core to rim, accompanied by the Fe-enrichment trend of accessory spinel. The different rock types show highly varied, radiogenic Os isotopic ratios (0.129–5.2), and unradiogenic Nd isotopic composition (eNd(t) = −8 to −15), but are homogeneous in ISr ratios (0.7054–0.7066). The (187Os/188Os)i ratios are found to be anti-correlated with eNd(t) values and whole-rock Mg# as well. These data suggest significant crustal contamination during magma evolution. The crustal contaminants are dominantly Archean mafic rocks in the lower crust, and subordinate TTG gneisses at shallower crustal levels. The parental magma was hydrous picritic in composition, derived from an enriched lithospheric mantle source above a subduction zone. The zoned pattern of the complex formed probably through “flow differentiation” of a rapidly rising crystal mush along a fracture zone that was developed as a result of lithospheric extension in a back-arc setting in the northern margin of the NCC at ca. 280 Ma.

Solar wind as the origin of rapid reddening of asteroid surfaces

A comparison of the laboratory reflectance spectra of meteorites with observations of asteroids revealed that the latter are much 'redder', with the spectral difference explained by 'space weathering', though the actual processes and timescales involved have remained controversial. A recent study of young asteroid families concluded that they suffered only minimal space weathering. Here we report additional observations of those families, revealing that space weathering must be a very rapid process-the final colour of a silicate-rich asteroid is acquired shortly after its 'birth' (within 10(6) years of undergoing a catastrophic collision). This rapid timescale favours solar wind implantation as the main mechanism of space weathering, as laboratory experiments have shown that it is the most rapid of several competing processes. We further demonstrate the necessity to take account of composition when evaluating weathering effectiveness, as both laboratory and asteroid data show an apparent dependence of weathering on olivine abundance. The rapid colour change that we find implies that colour trends seen among asteroids are most probably due to compositional or surface-particle-size properties, rather than to different relative ages. Apparently fresh surfaces most frequently seen among small near-Earth asteroids may be the result of tidal shaking that rejuvenates their surfaces during planetary encounters.

Magma transport and storage at Piton de La Fournaise (La Réunion) between 1972 and 2007: A review of geophysical and geochemical data

Since the middle of the 20th century, improvement in volcano monitoring techniques has provided large data sets that can be used to evaluate the evolution of magma plumbing systems. At Piton de La Fournaise volcano, abundant eruptive activity and a dense monitoring network are especially conducive to such research. Analysis of the extensive set of geophysical and geochemical data associated with the last 35 years of activity provides new insights into magma transport and storage at Piton de La Fournaise. Two periods of frequent eruptions separated by 6 years of inactivity can be distinguished: 1972–1992 and 1998–2007. Considering these two periods, we show evidence of major changes in the shallow plumbing system in 2000. During 1972–1992 and 1998–2000, lava compositions and weak long term eruptive precursors (lasting few days to three weeks), characterized by no significant summit inflation and about ten volcano-tectonic earthquakes per day, suggest that eruptions were fed by the progressive drainage of an occasionally recharged shallow magma reservoir. Geophysical evidence of shallow magma reservoir recharge was only recorded in 1986 and 1998. In contrast, from 2000 to 2007, geophysical data highlight the appearance of long-term precursors (1 to 5 months of summit inflation and strong seismicity, up to one hundred events per day) preceding cycles of successive eruptions. Each cycle of eruptions was characterized by a sequence (3 to 10 months in duration) of summit and near-summit, proximal, eruptions, and ended with a distal, low-altitude oceanite (olivine abundances > 20 vol.%) eruption on the eastern flank of the volcano. Within each of these eruptive cycles, lavas became progressively more magnesian over time. The preferential motion of the eastern flank caused by continuous recharging of the shallow reservoir since 2000 would favour the occurrence of distal eruptions towards this flank at the end of a cycle.

Evaluation of contribution sources for the sediments of the La Paz Lagoon, based on statistical treatment of the mineralogy of their heavy fraction and surrounding rock and drainage basin characteristics

To identify the main sources of terrigenous sediments of the La Paz Lagoon, a description of the hydrology of the drainage basins and the mineralogy of the heavy fraction of the lagoon sediments is needed. In this work, 14 terrigenous sediment samples from the arroyos (dry streams) and 55 superficial marine sediment samples were collected from the study area. The drainage basins of the principal arroyos discharging into the lagoon were defined using geomorphologic analyses on Geographic Information Systems (GIS). The sediment samples’ grain size fraction of 3.25 – 3.00 phi was obtained by sieving. The heavy minerals were separated using bromoform and the mineralogical composition was determined using a petrographic polarization microscope. Three main sedimentary provinces in the lagoon were distinguished based on principal component analysis of the heavy mineral data and the study of the surrounding regional streams. The eastern sedimentary province is characterized by the predominance of orthopyroxene, especially hypersthene, as well as a high hornblende presence. This is due to the erosion of volcanoclastic sequences of the Comondu Formation and intrusive granitic complexes of Sierra de Las Cruces. The southeastern province displays a high abundance of hornblendes (up to 81% in some stations), micas, apatite, sphene, anatase, tourmaline, chlorite, clinopyroxene and orthopyroxene. Lower contents of amphibole (up to 27.2% in some stations) and resistant garnet and zircon minerals were found, as were some soft-pink piedmontite and pale-green hypersthene. These minerals were supplied by the El Cajoncito, Los Bledales, La Palma, Cardonal and El Novillo arroyos eroding the intrusive rocks of “Granito Sierra de Las Cruces”, “Tonalita La Buena Mujer” and in a smaller extent the extrusive and non-marine rocks (tuffs, riodacites, conglomerates and sandstones) of the Comondu Formation. The predominant minerals of the northwestern province are clinopyroxene, orthopyroxene and phosphatic ooids. Relatively high abundance of olivine, hornblende, zircon, epidote and garnet were also found. These minerals are presumably supplied by the arroyos La Ardilla and other streams to the north of the El Centenario and El Comitan villages, as well as by eolian and littoral transport of the El Mogote sandbar dunes, which accumulates eroded sediments from the marine sedimentary rocks of El Cien Formation.

Elemental and isotopic abundances of lithium in chondrule constituents in the Allende meteorite

Elemental and isotopic abundances of lithium in chondrule constituents in the Allende CV3 meteorite were determined using secondary ion mass spectrometry. Olivines and mesostasis dominated by a feldspathic phase are depleted in Li ( < 1 μ g g - 1 and 0.1 – 0.6 μ g g - 1 , respectively). In contrast, low-Ca pyroxenes and mesostasis dominated by a Na-rich phase are enriched in Li ( ∼ 1 – 8 μ g g - 1 and 0.4 – 3.5 μ g g - 1 , respectively) and the interchondrule matrix is generally enriched in Li ( ∼ 2.0 μ g g - 1 on average). The Li isotopic abundance of olivine ranges from δ 7 Li ∼ - 32 to 21. The spatial distributions of elemental and isotopic abundances of Li in olivines within individual chondrules exhibit no systematic pattern. This suggests that the distribution of Li in olivine was not disturbed during aqueous alteration or thermal metamorphism on the Allende meteorite parent body. Although mesostasis is the last crystallizing phase from a chondrule melt and is expected to be enriched in Li, in the Allende meteorite it is generally depleted in Li. We suggest that during aqueous alteration on the CV asteroid, Li in mesostasis was leached out by aqueous fluids. The Li-enriched Na-rich mesostasis was probably produced later by infiltration of Na-rich fluids. It seems likely that aqueous fluids sequestered alkali elements from the Allende–chondrite region in the CV parent asteroid, although significant amounts of Li are preserved in ferrous olivine in the interchondrule matrix.

Rock spectral classes observed by the Spirit Rover's Pancam on the Gusev Crater Plains and in the Columbia Hills

This paper examines the ferrous and ferric iron mineralogy of rocks inferred from 246 visible/near‐infrared (430–1010 nm) multispectral observations made by the Mars Exploration Rover Spirit's Pancam on its traverse from its landing site to its second Winter Haven location. Principal component, correspondence analyses, and a sequential maximum angle convex cone technique were used to identify 14 candidate classes. Spectra from the West Spur of Husband Hill and the Watchtower area had the highest 535 and 601 nm band depths indicating that these areas were more oxidized. Differences in the depth and band center of a near infrared (NIR) absorption feature were observed using 904 nm band depth and 803:904 nm ratio and parameters gauging the 754–864 and 754–1009 nm slopes. Spectra of rocks from the southern flank of Husband Hill had negative 754–1009 nm slopes and a broad NIR absorption consistent with high olivine abundances. Rocks observed on the lower West Spur, at the Cumberland Ridge locale, at the Husband Hill summit, and at the Haskin Ridge locale had deep 904 nm band depths and steep 754–864 nm slopes consistent with greater pyroxene abundances. These observations are consistent with results on iron‐bearing mineralogy from Spirit's Mössbauer spectrometer. Comparisons of these rock spectral classes with a set of terrestrial analog samples found similarities between the West Spur and Watchtower classes and red hematite‐bearing impact melts. Fewer similarities were found in comparisons of the Columbia Hills classes with basaltic hydrovolcanic tephras.

Evidence for extensive olivine‐rich basalt bedrock outcrops in Ganges and Eos chasmas, Mars

Several localized outcrops of olivine‐enriched bedrock have been previously identified in the Ganges and Eos Chasma area on the eastern end of Valles Marineris with the Thermal Emission Imaging System multispectral images. These outcrops form a layer in the walls of Ganges Chasma and appear to be the remnants of a once continuous unit, which was mapped over ∼100 km. In this study we further characterize the composition (forsterite content of ∼0.68), olivine abundance (10 to &gt;15%), thermal inertia (&gt;600 JK−1 m−2 s−1/2, consistent with in‐place rocky material), vertical dimension (∼60 to ∼220 m), extent (&gt;1100 km laterally), volume (∼9.9 × 104 km3), dip (∼0.013°NE), and continuity of this layer utilizing Thermal Emission Spectrometer hyperspectral, Thermal Emission Imaging System multispectral, and Mars Orbiter Laser Altimeter elevation data. Morphologic data from high‐resolution imagery display a relatively unmantled, rough, and pitted surface associated with the olivine‐enriched material, consistent with thermal inertia data. Four possibilities for the origin of the olivine‐enriched unit are (1) volcanism associated with tectonic rifting of the Valles Marineris system, (2) a volcaniclastic flow deposit, (3) an intrusive mafic sill, or (4) a discrete episode in Martian history during which flood lavas were erupted onto the surface. The most likely origin is an eruptive event consisting of compositionally uniform flood lavas originating from a primitive mantle source region, possibly associated with the initiation of Tharsis volcanism. This unit is one of the largest continuous compositional units found on Mars and is strikingly similar to other olivine‐enriched deposits identified in previous studies where compositional, morphologic, and thermophysical similarities are observed. These similarities may indicate that there was a period in early Martian history, where compositionally uniform and extensive olivine‐enriched flood basalts were erupted on the Martian surface.

Control of the Products of Serpentinization by the Fe2+Mg-1 Exchange Potential of Olivine and Orthopyroxene

It is argued that the high-Mg content (mg-number1⁄4 95 3) of the serpentine minerals in serpentinized peridotite is a consequence of the environmental FeMg 1 exchange potential imposed on the system by the abundance of olivine and orthopyroxene. Mass balance in the serpentinization reaction then requires the precipitation of an ironrich mineral that in most cases is magnetite.This causes hydrogen to be evolved in an oxygen-conserved reaction.The low-variance mineral assemblage Olþ SrpþBrcþMag sets the chemical potentials of H2O, SiO2 and O2 internally at an early stage in the process, but the paragenetic assessment of serpentinites is rendered difficult by the variable and usually unknown Fe3þ content of the serpentine minerals, particularly lizardite. Whole-rock analyses of highly to completely serpentinized peridotites reveal Fe/SFe ratios40 4, with an average value (0 69) similar to that of magnetite (0 67). This feature may be attributed to the presence of high-Fe lizardite, as has been found in Mo« ssbauer spectroscopy studies. Electron microprobe and scanning electron microcope analyses in the literature exhibit element trends (e.g. decreasing Si vs SFe a.p.f.u.) for olivinepseudomorph lizardite and, with some exceptions, for bastite lizardite, that show a substitution of the cronstedtite component (Fe charge-balanced onTand M sites). Cronstedtite substitution will be favoured at low temperature and/or low hydrogen fugacity, and in these circumstances less magnetite will be evolved during serpentinization, in some cases none at all. Some bastite lizardites from sea-floor settings show evidence of M-site vacancy substitution of Fe for Fe. In the course of progressive serpentinization, micrometer to millimeter-scale variations in SiO2 potential may well be present, but their influence on Fe in lizardite seems to be limited to a few cases of lizardite associated with orthopyroxene. Chrysotile is on average more Mg-rich and less variable in Fe/Mg ratio than lizardite, facts that may be attributed to the greater Fe content of lizardite. Chrysotile veins provide the best record available to us of the environmental FeMg 1 exchange potential in the pore fluid attending serpentinization.This potential serves as a robust control on serpentine and brucite compositions, although it may fail after olivine and orthopyroxene have been armoured or eliminated, and in more open-system environments (high water/rock ratio) such as on the sea floor or at serpentinite host-rock contacts. The default assumption in microprobe analyses that measured iron is all Fe can lead to inappropriate petrological conclusions in the case of serpentinites.

The Trinity ophiolite (California): the strange association of fertile mantle peridotite with ultra-depleted crustal cumulates

The Trinity ophiolite (California): the strange association of fertile mantle peridotite with ultra-depleted crustal cumulates

Mineralogy of volcanic rocks in Gusev Crater, Mars: Reconciling Mössbauer, Alpha Particle X‐Ray Spectrometer, and Miniature Thermal Emission Spectrometer spectra

Complete sets of mineral abundances for relatively unaltered volcanic or volcaniclastic rocks in Gusev Crater have been determined by modeling Mössbauer subspectral areas as mineral weight percentages, and combining those percentages with the proportions of iron‐free minerals not detected by Mössbauer (normative plagioclase, apatite, and chromite, as calculated from Alpha Particle X‐Ray Spectrometer (APXS) chemical analyses). Comparisons of synthetic thermal emission spectra calculated for these mineral modes with measured Miniature Thermal Emission Spectrometer (Mini‐TES) spectra for the same rock classes show either good agreements or discrepancies that we attribute to sodic plagioclase compositions or unmodeled sulfate, glass, or pigeonite. The normative compositions of olivine, pyroxene, and feldspar calculated from APXS data are consistent with spectroscopic constraints on mineral compositions. Systematic variations between olivine abundances in APXS norms (which sample tens of micrometers depth) and olivine proportions measured by Mössbauer (which sample hundreds of micrometers depth) support the hypothesis that dissolution of olivine by acidic fluids has occurred on weathered rock surfaces.

Interaction of magma with sedimentary wall rock and magnetite ore genesis in the Panzhihua mafic intrusion, SW China

In southwestern China, several large magmatic Fe–Ti–V oxide ore deposits are hosted by gabbroic intrusions associated with the Emeishan flood basalts. The Panzhihua gabbroic intrusion, a little deformed sill that contains a large titanomagnetite deposit at its base, concordantly intrudes late-Proterozoic dolostones. Mineralogical and chemical studies of the contact aureole in the footwall dolostones demonstrate that the metamorphism was largely isochemical but released large quantities of CO2 as the rocks were converted to marble and skarns during intrusion of the gabbroic magma. Petrological modelling of the crystallization of the intrusion, using H2O-poor Emeishan basalt as parent magma, shows that under normal conditions, Fe–Ti oxides crystallize at a late stage, after the crystallization of abundant olivine, clinopyroxene and plagioclase. In order for titanomagnetite to separate efficiently to form the ore deposit, this mineral must have crystallized earlier and close to the liquidus. We propose that CO2-rich fluids released during decarbonatization of sedimentary floor rocks passed up through the magma. Redox equilibria calculations show that when magma with the composition of Emeishan basalt is fluxed by a CO2-rich gas phase, its equilibrium oxygen fugacity (fO2) increases from the fayalite–magnetite–quartz buffer (FMQ) to FMQ + 1.5. From experimental constraints on magnetite saturation in basaltic magma under controlled fO2, such an oxidizing event would allow magnetite to crystallize near to the liquidus, leading to the formation of the deposit.

Mineralogical composition of sands in Meridiani Planum determined from Mars Exploration Rover data and comparison to orbital measurements

Constraints on the mineralogical composition of low‐albedo, low‐sulfur sands at Meridiani Planum are determined from Mars Exploration Rover (MER) Opportunity Miniature Thermal Emission Spectrometer (Mini‐TES), Mössbauer, and Alpha Proton X–Ray Spectrometer measurements. Results of this work show that the sand is olivine basaltic in composition, with minor amounts of sulfate and a high‐silica phase (glass or secondary amorphous silica). Measurements from all three instruments indicate that pyroxene is twice as abundant as olivine, and that the pyroxene composition is dominated by the low‐calcium variety. The volume abundance of olivine is constrained to be 10–15%. Results from detailed analyses of MER data are used to ground truth the spectral emissivity and mineral abundances derived from orbit with Mars Global Surveyor TES data. TES‐derived mineral abundances are within 5% of those derived from MER data, which is generally within the statistical errors associated with TES‐derived phase abundances. The agreement lends support to global‐ and regional‐scale variations in mineralogical composition determined from TES data in previous studies. An alternative method of least squares minimization is used for modeling the TES and Mini‐TES data; the benefits of this method are demonstrated by comparison with conventional least squares techniques previously used by TES data users.

Global distribution, composition, and abundance of olivine on the surface of Mars from thermal infrared data

We used spectral indexing and linear deconvolution to compare thermal infrared emission spectra of Fo91, Fo68, Fo53, Fo39, Fo18, and Fo1 olivine samples to Mars Global Surveyor Thermal Emission Spectrometer (TES) data over low‐albedo regions of Mars. The Fo91, Fo68, Fo53, and Fo39 spectral end‐members were confidently identified on Mars, a range of compositions wider than inferred from Martian meteorites. Small (less than hundreds of square kilometers) occurrences of the Fo91 spectral end‐member are present in the rims of the Argyre and Hellas impact basins and may represent Martian mantle materials. The Fo68 spectral end‐member is common throughout the highlands, chasmata, outflow channels, and Nili Fossae region. The Fo53 spectral end‐member occurs in eastern Syrtis Major, the Nili Fossae region, and smooth‐floored craters of the highlands. Although less abundant than Fo68 and Fo53, the distribution of the Fo39 spectral end‐member suggests that some olivine on Mars is more Fe‐rich than olivine in Martian meteorites. Global maps of olivine show that (1) materials containing 10–20% of olivine are common in the southern highlands of Mars, (2) olivine is most common near the topographic dichotomy boundary, and (3) olivine becomes uncommon near the poles suggesting that it may be influenced by topography and/or latitude (climate). Olivine is found in early Noachian to Amazonian terrains, some of which may be coeval with phyllosilicate and sulfate deposits detected by OMEGA implying that any early Noachian wet period of Mars' climate history may have been globally inhomogeneous or insufficient to weather the olivine that remains today.

On the forcing mechanism for the H2-driven deep biosphere

AbstractHeat produced in the mantle and core of the Earth by the decay of radioactive elements and mineral fusion results in large-scale mantle convection. The outer shell of the Earth that floats on the convective mantle is divided into rigid lithospheric plates. Subduction of dense cold plates into the mantle leads to plate tectonics. At divergent plate margins, heat is dissipated through hydrothermal convection cells. As ocean water is entrained into hydrothermal cells it interacts with fresh magmatic rocks and liberates ferrous iron. This iron reduces the ocean water to such an extent that it decomposes and forms hydrogen. Molecular hydrogen, as the most reduced component in the system, forms a basal component to a deep dark biosphere powered by metastable redox gradients. In this paper we review the driving force behind a hydrogen-driven deep biosphere. We present abundant observations of hydrogen produced at natural hydrothermal settings as well as in laboratory experiments. The key mineral reactions responsible for the bulk of this hydrogen production are then presented. A division of the reaction progression into an oxidized state and a reduced state is suggested. The amount of hydrogen produced is insignificant in the oxidized state whereas it becomes proportional to the amount of ferrous iron oxidized in the reduced state. The bulk of basalt-hosted aquifers are expected to reside in the oxidized state because of the low content of ferrous minerals, whereas abundant olivine in ultramafic-hosted systems is responsible for large-scale hydrogen production. Today the majority of the seafloor is basaltic. The Archean seafloor on the other hand consisted of fewer ultramafic exposures, but was dominated by ultramafic magnesium-rich lavas with a higher potential for hydrogen production than the present seafloor.