Presence Of Olivine Research Articles

Geologic sources and geographic distribution of sand tempers in prehistoric potsherds from the Mariana Islands

AbstractTemper sands in prehistoric potsherds of the Mariana Islands include terrigenous detritus derived from Paleogene volcanic bedrock and calcareous grains derived chiefly from modern fringing reefs, but also in part from uplifted Neogene limestones overlying volcanic bedrock. Calcareous sands are nondiagnostic of island of origin, but volcanic sands and the terrigenous component of hybrid sands composed of mixed terrigenous and calcareous grain types can be traced to geologic sources on Saipan and Guam, the only occupied islands where volcanic bedrock is extensively exposed. Quartzose tempers of several types were derived exclusively from dacitic volcanic rocks on Saipan. Nonquartzose tempers of andesitic parentage derive from both Saipan and Guam, but abundance of orthopyroxene as well as clinopyroxene is diagnostic of Saipan andesitic tempers, the presence of olivine is diagnostic of selected tempers from Guam, and placer temper sands rich in heavy ferromagnesian minerals occur only in sherds on Guam. Temper analysis documents widespread ceramic transfer from Saipan to other islands throughout Mariana prehistory, and more restricted ceramic transfer from Guam to nearby Rota, although the origin of some andesitic temper types is petrographically indeterminate. © 2001 John Wiley & Sons, Inc.

Petrography and metamorphism of the metasedimentary country-rocks of the Jacurici Valley Chromitite-Hosting mafic-ultramafic complexes, Bahia, Northeastern Brazil

This paper deal with on the metasedimentary country-rocks of the chromite-bearing ultramafic rocks that occur in the"Jacurici River Valley Chromium District" northeastern Bahia, Brazil. This region presents a complex geologic-petrologic framework of rocks that were intensely deformed, metamorphosed and transformed by metasomatic processes, making it difficult to interpret their evolutionary/metamorphic record. Although the metasedimentary country rocks have also been affected by such processes, it is possible to distinguish evidence of a previous high-grade metamorphism that affected them. Thermobarometric data for the observed mineralogical associations indicate P-T conditions around 750-800ºC and 7-8 kb for the metamorphic peak, based mainly on the presence of olivine in marbles and the cordierite-garnet-sillimanite-spinel association in aluminous gneisses.

A reassessment of the potential for reduction of dissolved CO2 to hydrocarbons during serpentinization of olivine

The concept that aqueous CO2 can be reduced to hydrocarbons abiotically during serpentinization of olivine has become widespread in the earth and planetary sciences. This process has been invoked to explain the occurrence of hydrocarbons in crystalline igneous rocks and proposed as a source of prebiotic organic compounds for the origin of life. We reevaluate this scenario through an experimental study of the reaction of dissolved CO2 in the presence of olivine under hydrothermal conditions (300°C, 350 bar). Reduction of CO2 to formate (HCOO−) was found to proceed rapidly, with H2 generated from hydrothermal alteration of olivine serving as the reductant. The reverse reaction, decomposition of formic acid to CO2 and H2, was also found to proceed rapidly. Although dissolved hydrocarbon concentrations increased throughout the experiments, isotopic labeling of dissolved CO2 with 13C indicated that these compounds were primarily generated from reduced carbon compounds already present in olivine at the beginning of the experiment rather than by reduction of CO2. The only hydrocarbon product from reduction of CO2 observed in the experiments was a small amount of methane (<0.04% conversion of dissolved CO2 in more than 2500 h of heating). Comparison of the reaction products with thermodynamic data indicates that reactions between dissolved CO2 and formate rapidly achieved metastable equilibrium at the experimental conditions, suggesting that similar reactions could control the concentration of formate in geologic fluids. The results indicate that the potential for abiotic formation of hydrocarbons during serpentinization may be much more limited than previously believed, and other mineral catalysts or vapor phase reactions may be required to explain many occurrences of abiotic hydrocarbons in serpentinites and igneous rocks.

The effect of titanium on the silica content and on mineral-liquid partitioning of mantle-equilibrated melts

High pressure, high temperature phase equilibria experiments were performed to investigate the effect of TiO2 on mineral-liquid equilibria for near natural olivine + orthopyroxene-saturated mafic liquids. At 1.2 GPa and 1360°C, and at 2.8 GPa and 1530°C, near-natural picritic basaltic liquids with TiO2 concentrations ranging from 0.4 to 22 wt.% were forced to be in equilibrium with olivine (ol) + orthopyroxene (opx) by adding appropriate quantities of San Carlos olivine with increasing TiO2. Titanium enrichment of the ol + opx saturated mafic liquids results in nearly pressure-independent linear decreases in SiO2 amounting to 0.60 ± 0.04 and 0.56 ± 0.05 wt.% SiO2 per wt.% TiO2 at 1.2 and 2.8 GPa, respectively. Olivine-liquid Fe-Mg KD decreases at 1.2 GPa from 0.33 at 0.4 wt.% TiO2 down to 0.22 at 19 wt.% TiO2. A similar trend, shifted to slightly higher KD values, is observed at 2.8 GPa.With increasing TiO2, the activity coefficient of SiO2 in the liquid increases owing to association of networking-modifying cations with Ti4+. Consequently, a smaller mole fraction of SiO2, XSiO2Liq, is required to maintain the silica activity imposed by the coexistence of olivine and orthopyroxene. Strong association between Ti and Fe in the liquid reduces aFeOLiq and γFeOLiq, thereby causing olivine-liquid Fe-Mg KD to decrease. These effects are expected to result in small decreases in SiO2 and very small increases in FeO in TiO2-enriched mantle-derived sources, such as those in OIB source regions derived from small degrees of melting of peridotite at high pressure. Stronger effects on the compositions of low-degree partial melts of more titanium-enriched rocks, such as pyroxenites, may be expected.Solubility of TiO2 in olivine under magmatic conditions and at modest (1.2–2.8 GPa) pressure is near 0.2 wt.%. For TiO2, partitioning between olivine and liquid is Henrian over the range of compositions studied, indicating constant activity coefficients for liquids between 0–20 wt.% TiO2 and for olivine between 0–0.23 wt.%. Orthopyroxene/liquid partitioning of TiO2 is variable, presumably owing to complex interactions between Al and Ti in orthopyroxene. Olivine/liquid partitioning of Al2O3 is nearly constant, but partitioning of Al2O3 between orthopyroxene and liquid is variable, owing to the aforementioned interactions between Al and Ti. Olivine/liquid and orthopyroxene/liquid partitioning of CaO diminish with increasing TiO2, suggesting stabilization of CaO in silicate liquids by formation of CaO-TiO2 complexes.At 1.2 GPa, saturation in aluminous armalcolite is found for liquids at 19.2 wt.% TiO2, in contrast, at 2.8 GPa, Ti-rich oxide saturation is not encountered for liquids with as much as 21.8 wt.% TiO2. The large TiO2 enrichment in the liquid required for oxide saturation in the presence of olivine + orthopyroxene at these pressures suggests that near-solidus liquids generated by melting of Ti-rich oxide-bearing peridotite would have ≥20 wt.% TiO2. The general absence of such Ti-rich primitive terrestrial lavas either precludes magmas from such source assemblages or indicates that they are strongly diluted by other magmas. Our results also suggest that armalcolite may be a significant phase in the Ti-enriched portions of the lunar mantle.

東北日本弧，青麻火山の形成史と主成分化学組成

The Aoso volcano is one of the representative stratovolcanoes of the Aoso-Osore volcanic zone in the northeast Japan arc. The volcanic activity of the Aoso volcano can be classified into two stages; the stages are separated by a caldera collapse event. The volumes of the products are 3.6 km3 (earlier stage) and 0.8 km3 (later stage). During the initial part of the earlier stage, thin lava flows (olivine orthopyroxene clinopyroxene basaltic andesite, SiO2=54.7-55.3%), block and ash flows (olivine bearing orthopyroxene clinopyroxene andesite, SiO2=57.9-58.5%), and thick lava (orthopyroxene clinopyroxene andesite, SiO2=59.0-59.9%) were extruded. The products construct either a conical shaped stratovolcano or series of stratovolcano. After that, scoria flows (orthopyroxene clinopyroxene andesite, SiO2=60.2%) and pumice flows (quartz orthopyroxene clinopyroxene hornblende andesite, SiO2=62.1-62.3%) spewed out, which might have caused the caldera at the top of this volcano. All the rocks during this stage can be plotted as linear lines on the co-variant diagrams. During the initial part of the later stage, pyroclastic rocks (olivine bearing hornblende orthopyroxene quartz dacite, SiO2=68.2-69.8%) were erupted, and lava domes grew. The lava domes can be separated into two types. One consists of clinopyroxene bearing orthopyroxene quartz dacite (SiO2=66.6-67.0%), whilst the other is clinopyroxene orthopyroxene dacite (SiO2=64.4-66.8%). All the eruptive rocks of this stage characteristically include mafic inclusions (quartz olivine bearing basaltic andesite and andesite, SiO2=53.8-57.5%). Each type of lava dome displays a distinct chemical trend in the covariant diagrams. All of the later stage products have higher SiO2 contents than the earlier stage products. Some of the later stage products, especially the pyroclastic rocks, were not plotted on the extrapolated lines of the trend drawn by the earlier stage products.

The Evolution of Troctolitic and High Al Basaltic Magmas inProterozoic Anorthosite Plutonic Suites and Implications for the Voisey's BayMassive Ni-Cu Sulfide Deposit

The Voisey's Bay massive Ni-Cu sulfide deposit is contained within troctolitic (plagioclase + olivine) intrusions of the 1.34 to 1.29 Ga Nain Plutonic Suite, coastal Labrador, Canada, and represents a new setting for magmatic sulfide deposits. Proterozoic anorthosite plutonic suites consist of vast quantities (as much as 20,000 km 2) of cumulate anorthosite-leucotroctolite-leuconorite, intrusions of troctolite and Fe-enriched dioritic rocks, and large granitic batholiths. Crystallization ages span nearly 1,200 m.y. within the Middle Proterozoic (2.12-0.92 Ga). Many Proterozoic anorthosite plutonic suites were emplaced into relatively young Proterozoic crust and some were intruded along Archean-Proterozoic boundaries (e.g., Nain, Labrador; Laramie, Wyoming; Korosten, Ukraine). Nearly all of the largest plutonic suites contain significant areas (thousands of km 2) of plagioclase-rich cumulate rocks that are olivine bearing (olivine anorthosite-leucotroctolite). Troctolitic rocks (20-30% modal olivine) occur in Proterozoic anorthosite plutonic suites, principally as cumulates in layered or massive intrusions (e.g., Kiglapait, Hettasch, Jonathon-Nain) intruded into anorthositic rocks. With the notable exception of the Voisey's Bay deposit, nearly all troctolitic rocks crystallized late in the evolution of individual plutonic suites. Many of the troctolitic intrusions have fine-grained olivine-rich marginal rocks that closely represent parental magma compositions. Numerous fine-grained olivine-normative dikes with high Al basaltic compositions also occur within many plutonic suites (Nain, Harp Lake, Laramie, Adirondack) and share compositional characteristics with the troctolitic magmas. These olivine-normative high Al basaltic magmas record the least evolved compositions in Proterozoic anorthosite plutonic suites (Mg = 0.43-0.64; An = 45-60), have high Al 2O 3 (15-19 wt %) and FeO(total) (9-15 wt %) contents, and typically show the least contaminated Sr and Nd isotope compositions in a given plutonic suite. Many dikes and marginal rocks have strong positive Eu anomalies, a feature of the melt composition that is not related to plagioclase accumulation. Except at Laramie, the least evolved troctolitic magmas appear to have about 8 wt percent MgO and corresponding Ni and S concentrations between 100 to 150 ppm and 0.02 to 0.05 wt percent, respectively. Based on closed-system fractionation in the Kiglapait intrusion, sulfide saturation occurs late in the crystallization history of troctolitic magmas, after Ni concentrations have been severely depleted by protracted olivine fractionation. The optimal conditions for formation of a Ni-Cu sulfide deposit associated with a Proterozoic anorthosite plutonic suite require a relatively high MgO parental magma that has not crystallized much prior olivine and an external source of S to promote early sulfide saturation, probably through desulfurization reactions as hot troctolitic magmas ascend through the crust. An economically important massive Ni-Cu sulfide deposit on the scale of Voisey's Bay clearly requires large volumes of magma, probably involving continuous replenishment of relatively Ni rich, unfractionated parental magma in staging chambers or mixing between magmas in conduits during ascent. A model for the formation of troctolite-associated massive Ni-Cu sulfide deposits is proposed. Depending on the relative degrees of sulfide saturation and fractionation of the mixed magmas, mixing between troctolitic magmas in a conduit or magma chamber may either poison the system, stopping sulfide segregation, or enhance sulfide segregation.

Petrology and chemistry of the new shergottite Dar al Gani 476

Abstract— In 1998, Dar al Gani (DaG) 476 was found in the Libyan desert. The meteorite is classified as a basaltic shergottite and is only the 13th martian meteorite known to date. It has a porphyritic texture consisting of a fine‐grained groundmass and larger olivines. The groundmass consists of pyroxene and feldspathic glass. Minor phases are oxides and sulfides as well as phosphates. The presence of olivine, orthopyroxene, and chromite is a feature that DaG 476 has in common with lithology A of Elephant Moraine (EET) A79001. However, in DaG 476, these phases appear to be early phenocrysts rather than xenocrysts. Shock features, such as twinning, mosaicism, and impact‐melt pockets, are ubiquitous. Terrestrial weathering was severe and led to formation of carbonate veins following grain boundaries and cracks.With a molar MgO/(MgO + FeO) of 0.68, DaG 476 is the most magnesian member among the basaltic shergottites. Compositions of augite and pigeonite and some of the bulk element concentrations are intermediate between those of lherzolitic and basaltic shergottites. However, major elements, such as Fe and Ti, as well as LREE concentrations are considerably lower than in other shergottites.Noble gas concentrations are low and dominated by the mantle component previously found in Chassigny. A component, similar to that representing martian atmosphere, is virtually absent. The ejection age of 1.35 ± 0.10 Ma is older than that of EETA79001 and could possibly mark a distinct ejection.Dar al Gani 476 is classified as a basaltic shergottite based on its mineralogy. It has a fine‐grained groundmass consisting of clinopyroxene, pigeonite and augite, feldspathic glass and chromite, Ti‐chromite, ilmenite, sulfides, and whitlockite. Isolated olivine and single chromite grains occur in the groundmass. Orthopyroxene forms cores of some pigeonite grains. Shock‐features, such as shock‐twinning, mosaicism, cracks, and impact‐melt pockets, are abundant. Severe weathering in the Sahara led to significant formation of carbonate veins crosscutting the entire meteorite.Dar al Gani 476 is distinct from other known shergottites. Chemically, it is the most magnesian member among known basaltic shergottites and intermediate in composition for most trace and major elements between Iherzolitic and basaltic shergottites. Unique are the very low bulk REE element abundances. The CI‐normalized abundances of LREEs are even lower than those of Iherzolitic shergottites. The overall abundance pattern, however, is similar to that of QUE 94201.Textural evidence indicates that orthopyroxene, as well as olivine and chromite, crystallized as phenocrysts from a magma similar in composition to that of bulk DaG 476. Whether such a magma composition can be a shergottite parent melt or was formed by impact melting needs to be explored further. At this time, it cannot entirely be ruled out that these phases represent relics of disaggregated xenoliths that were incorporated and partially assimilated by a basaltic melt, although the texture does not support this possibility.Trapped noble gas concentrations are low and dominated by a Chassigny‐like mantle component. Virtually no martian atmosphere was trapped in DaG 476 whole‐rock splits. The exposure age of 1.26 ± 0.09 Ma is younger than that of most shergottites and closer to that of EETA79001. The ejection age of 1.35 ± 0.1 Ma could mark another distinct impact event.

Petrogenesis of Tertiary Andesite Lava Flows Interlayered with Large-Volume FelsicAsh-Flow Tuffs of the Western USA

The San Juan volcanic field (Colorado), the Indian Peak volcanic field (Utah–Nevada) and the central Nevada volcanic field formed during Oligocene and Miocene times. Each field is characterized by >3000 km3 of rhyolitic and dacitic ash-flow tuff sheets and smaller volumes (<300 km3) of interlayered andesitic lavas. In each field, andesite lavas erupted from vents within and peripheral to calderas formed by approximately contemporaneous felsic ash-flow eruptions. Olivine andesites occur only peripheral to calderas, whereas hornblende andesites occur only within calderas. Pyroxene andesites occur at both locations. The parental magmas of the andesites formed by partial melting of mantle material, as shown by the presence of olivine or Cr-spinel in some units. Compositional evolution of andesites appears to have been controlled in part by crystal fractionation at 0.2–0.8 GPa, based on compositional and mineralogic comparisons with liquid lines of multiple saturation from experiments involving data for andesitic compositions. Modeling of bulk compositional variations suggests crystal fractionation may have been accompanied by mixing with dacitic or rhyolitic magmas. At locations beneath present-day calderas, upward-migrating basaltic to andesitic magmas encountered dacitic or rhyolitic magmas. The more felsic magmas gravitationally trapped the rising mafic magmas. These trapped magmas evolved from basaltic or olivine andesitic compositions to hornblende andesitic compositions by crystal fractionation combined with mixing with dacitic and rhyolitic magmas. Subsequent eruption or crystallization of dacitic and rhyolitic magmas removed the density contrasts and allowed hornblende andesites to erupt within calderas. In contrast, parental mafic magmas at locations peripheral to present-day calderas ascended without encountering dacitic or rhyolitic magmas, and these erupted as olivine andesites. Thus andesites initially evolved separately from the region's voluminous dacite and rhyolite magmas, but later mixing with the felsic magmas influenced compositions of many andesites.

Dynamic crystallization experiments on the Angra dos Reis achondritic meteorite

With the recent discovery of angrite meteorites in the Antarctic collection which clearly have a melt origin the hypothesis that Angra dos Reis (ADOR) itself is a cumulate is being reconsidered. A recent model suggests that ADOR is a porphyry, not a cumulate. Dynamic crystallization experiments have been conducted to determine whether a melt of ADOR composition can crystallize with porphyritic textures at reasonable cooling rates under appropriate heterogeneous nucleation conditions. The nucleation conditions were varied by melting the starting material at different degrees of superheat. Porphyritic textures were produced if nuclei are absent from the melt and embryos are few in number. If some degree of supercooling is developed while the embryos are growing to critical size, the phenocrysts grow with skeletal shapes. Fassaitic pyroxene grows over a large temperature interval before the appearance of kirschsteinite, spinel, and a pyroxferroite-like phase. Alternatively, if the nucleation density is high and irregularity distributed at the beginning of cooling, a granular texture very similar to ADOR is produced. In this case the large fassaites would grow later and poikilitic enclose the early formed granular fassaite. The phases present and their composition depend on the oxygen fugacity, and a complex redox history for ADOR can be inferred. Olivine is not present in quantities compatible with ADOR and kirschsteinite appears to have been stabilized kinetically in the experiments. While a volcanic or hypabyssal history for ADOR is possible based on the experiments, the presence of olivine, particularly as a phenocryst, remains unexplained.

The petrology of Poás volcano lavas: basalt-andesite relationship and their petrogenesis within the magmatic arc of Costa Rica

Basalt, andesite and dacite from Poás volcano exhibit a distinct petrogenetic evolution. The basalts (Al 2O 3 = 16–17.4%, K 2 O = 1.43–1.61%) are hypersthene-normative tholeiites with phenocrysts of labradorite, olivine, and two pyroxenes. Orthopyroxene (En 67-56) is generally rimmed by clinopyroxene (Fs 10–18 Wo 39–48). In all basalts and in a few andesites, pyroxene phenocrysts have Al- and Ti-rich salite rims that surround cores of orthopyroxene or augite having lower values. The Al z values (% of tetrahedral site occupied by Al) for these augites are above 6%, thus defining on the Al z vs. TiO 2 diagram an intermediate trend between arc-related cumulates and rift-related tholeiites and ophiolite-layered cumulates. Moreover, the presence of olivines with coronae of wormy intergrowths of orthopyroxene and Ti-magnetite (similar to those occurring in metagabbros of the granulite facies and characteristic of Arenal gabbroic xenoliths) together with anorthite megacrysts (An 94-91) suggest a possible contamination origin for these basalts. Sr-isotope ratios for the basalts are between 0.703816 and 0.703849 whereas andesites and dacites have lower ratios (0.703676–0.703715): these relationships together with the mineral compositions suggest that the basalts may have formed from basaltic andesites by contamination with olivine gabbros and anorthosites of the lower crust. Pearce element-ratio analysis confirms that the basalts had a different origin from the rest of the suite and that the dacite may be derived by crystal fractionation of the less silicic basaltic andesite with removal mainly of clinopyroxene and plagioclase.

Iron Mössbauer spectra of lava from Jeju island and its similarities to Moon basalts

We report the result of measurements of the57Fe Mossbauer spectrum for powder samples of lava taken from four widely separated sites on Jeju island. The spectra consist of well defined quadrupole doublets having a 2+ charge state and indicate the presence of olivine, clinopyroxene, and ilmenite. In addition we find a six line magnetically split hyperfine pattern characteristic of impure hematite, with considerable line broadening and reduced internal field. The observed spectra have a remarkable similarity to those found for moon basalts. A recently popularized “collision ejection” model for the moon formation would be consistent with our observations. The data is a particularly simple and direct illustration of the well-known fact that the iron silicates and ilmenite found on the moon are very similar in composition and relative abundance to that found on earth.

The Binningup H5 Chondrite: A New Fall from Western Australia

Abstract— Following a brilliant daylight fireball at 10:10 a.m. (local time) on 30 September 1984, a single stone weighing 488.1 grams was recovered from Binningup beach (33°09′23″S, 115°40′35″E), Western Australia. Data from 23 reported sightings of the fireball indicate an angle of trajectory 20–40° from the horizontal, a flight‐path bearing N210°E and an end‐point (ca. 32°39′S, 115°54.5′E) at a height of ∼20–30 km.A recrystallized chondritic texture and the presence of olivine and low‐Ca orthopyroxene with compositions of Fa18.4 (PMD 1.1)and Fs16.1 (PMD 1.1), respectively, show that Binningup is a typical member of the H‐group of ordinary chondrites. Uniform mineral compositions and the presence of generally microcrystalline plagioclase feldspar indicate that the meteorite belongs to petrologic type 5. Pervasive fracturing of silicates suggests mild pre‐terrestrial shock loading.Measurements (dpm kg−1) of cosmogenic radionuclides including 22Na (61 ± 5), 26Al (49 ± 3) and 54Mn (66 ± 10) indicate a normal history of irradiation.

Comparative Re [sbnd]Os, Sm [sbnd]Nd and Rb [sbnd]Sr isotope and trace element systematics for Archean komatiite flows from Munro Township, Abitibi Belt, Ontario

A series of komatiite flows from Munro Township, Ontario, have Re and Os concentrations that range from 0.545 to 1.27 ppb and 1.00 to 2.20 ppb, respectively. The 187Re/ 186Os in the suite range from 14.05 to 52.37 and the 187Os/ 186Os range from 1.49 to 3.22, giving a crystallization age of2726 ± 93Ma and an of initial 187Os/ 186Os of0.873 ± 0.035. This age is in good agreement with the crystallization ages that have been reported for these rocks using other isotopic systems. The Re Os isochron indicates that the system was resistant to the low grade metamorphism that altered the rocks. The initial Os isotopic composition is consistent with the derivation of the melts from a mantle source(s) that originated with a chrondritic ( 187Os/ 186Os) i and evolved with an approximately chondriticRe/Os. Sm Nd data for the same suite cluster about anε Nd value at 2.72 Ga of +2 to +3. Rb Sr isotope data indicate significant mobilization of these elements by the greenschist-grade metamorphism that has altered the rocks. Comparisons of Re and Os concentrations with those of MgO, FeO, Ni, Cr and S suggests that Re was incompatible ( D < 0.01) with the mantle residues and also with the predominantly olivine assemblages that crystallized from the komatiite melts. A lack of correlation between Os and MgO indicates that Os concentrations were not controlled by the presence of olivine in the mantle residue or olivine crystallization within the flows. Os was, however, moderately compatible with the mantle residues ( D > 1). Os concentrations were likely controlled by the presence or precipitation of trace metallic or sulfide phases both during melting and crystallization, and not by the modally predominant silicate minerals.

Mineralogy of the type kajanite from Kalimantan. Similarities and differences with typical lamproites

The type kajanite is a leucite-bearing rock. Its chemical composition is close to that of lamproites or lamprophyres on the basis of the Al2O3, TiO2 and K2O contents as well as Mg/(Mg+Fe) and (Na+K)/Al ratios. Nevertheless kajanite is not peralkaline in composition, and K2O/Na2O ratio is lower than generally found in lamproites. Similarities and differences with typical lamproites are also apparent in the mineralogicai composition : the presence of olivine (with Cr-spinel inclusions), phlogopite, leucite and richterite is in agreement with lamproite definition, but the composition of richterite and Cr-spinels is more aluminous. Groundmass oxides are Ti-magnetites unlike typical lamproites which contain Ti-rich oxides (ilmenite and/or pseudobrookite). In spite of apparent similarities, this detailed study indicates that kajanite is indeed not a typical lamproite but its character places it close to the lamproite group of rocks.

Mineralogy and composition of the upper mantle

Seismic velocities are calculated for two petrological models of the upper mantle, an olivine‐rich assemblage, pyrolite, and a garnet‐clinopyroxene rich, olivine bearing, assemblage, piclogite. These are compared with recent seismic profiles for various tectonic provinces. The shield data is most consistent with a cold olivine and orthopyroxene‐rich LID (the seismic lithosphere) extending to 150 km followed by a high temperature gradient and/or a change in mineralogy that serves to decrease the velocity. From 200 to 400 km the velocities follow a 1400°C adiabat. The rise‐tectonic mantle is much slower, presumably hotter and is likely to be above the solidus to depths of at least 300 km. The high Vp/Vs ratio of the lower oceanic lithosphere in the western Pacific is most consistent with eclogite.

Dunites and serpentinites in the Sanbagawa metamorphic belt, central Shikoku and Kii Peninsula, Japan.

Large dunite-serpentinite masses of the Sanbagawa metamorphic belt occur in the axial zone of large scale recumbent fold. The texture and chemistry of olivines from the Higashi-Akaishi and Shiraga bodies (central Shikoku) and the Ryumon body (Kii peninsula) demonstrate the two types of dunite to be present in this belt. Dunites of the Higashi-Akaishi body had experienced two stages of metamorphism, resulting in slight compositional change of their olivines, but the dunites remained dunite, whereas those of the Shiraga and Ryumon bodies consist of olivines newly formed by the metamorphism of serpentinite. The newly formed olivines contain the minerals of serpentinite, such as magnetite, antigorite, chlorite, brucite and magnesite, as inclusions, and some of them have a positive correlation between XFe and NiO content being the reverse to the compositional trend formed by igneous crystallization. The presence of olivine+antigorite in the Shiraga and Ryumon bodies is in harmony with the thermal structure of the Sanbagawa terrain. This harmony and the widespread occurrence of newly formed olivine indicate that this type of dunite was formed by the metamorphism of serpentinite during the Sanbagawa metamorphism. From the view point of thermal history, not all the peridotites in the Sanbagawa metamorphic belt are simple dismembered masses of the Higashi-Akaishi-type peridotite.

Partitioning of Ni 2+ between basaltic and synthetic melts and olivines—an experimental study

We present experimental measurements of olivine/liquid distribution coefficients for Ni (K Ni) in basaltic and simple synthetic melts over a temperature range of 1400° to 1070°C. Our results suggest that 1. (1) K Niand activity coefficients for NiO in basaltic melts are strongly dependent on temperature (e.g., K Ni =wt. %NiO ol/wt. %NiO liq ranges from 5.9 at 1400° to 29 at 1100°C); 2. (2) at constant temperature K Ni is dependent upon melt composition and structure, but this dependence is small over the composition range of common basaltic magmas; 3. (3) K Ni can be used as a geothermometer to predict olivine crystallization temperatures of basaltic magmas to within ±50°C; 4. (4) K Ni is significantly greater than one at all natural magmatic temperatures, thus the presence of olivine as a cumulus phase during fractional crystallization or as a residual phase during partial melting has a significant effect on the content of Ni in the coexisting melt phase; 5. (5)K Ni does not vary significantly as a function of Ni content of the melt up to at least one mole percent (i.e., Henry's law behavior is obtained for olivine and melt solutions of Ni).

Pallasite meteorites—mineralogy, petrology and geochemistry

Pallasites are highly differentiated meteorites and provide a unique sample from the deep interiors of solar system parent bodies. They contain evidence of the former existence of one or more residual melts. Olivine is a major phase. Its primary shape is rounded; the angular crystals in many pallasites are secondary. Tubular inclusions are widespread. They perhaps are the residence of CO 2, released during laboratory heating experiments. Phosphoran olivine, a new variety of olivine containing 4–5 wt% P 2O 5, occurs in a few pallasites. Its Fe/Mg ratio is apparently independent of the host olivine composition. Pyroxene (not previously described from pallasites) occurs in symplectic intergrowths in seven meteorites. Compositionally, it lies in the gap between pyroxenes in chondrites and most irons. There are two groups: Fs 11.6 ± 0.2 and Fs 16.7 ± 0.2 The pyroxene contains exceptionally low Ca (< 0.1–0.2 wt%) and there is an indication of an inverse relation between Fe and Ca. Modal analyses and density measurements were made on all available specimens and bulk compositions were calculated. The ‘average’ pallasite contains 65 vol. % olivine and 50.5 wt % total Fe. Many of the densities of pallasites cluster around that calculated for close-packed olivine. Pallasites are exotic cumulates. Their textures resemble terrestrial cumulates, as does the presence of olivine and chromite. The metal texture resembles a solidified intercumulus liquid. Those pallasites containing olivine in excess of close-packing were subjected to adcumulus growth, thereby also explaining the widespread mutual borders. There is abundant evidence of deformation. For olivines this includes their fragmental shape and kink banding. Troilite formed a eutectic-like melt with kamacite: pieces of spalled olivine and schreibersite were injected into and captured by this melt. Troilite polycrystallinity resulted from the deformation. This deformation occurred while the pallasites were still deeply buried, resulting in incipient spheroidization of olivine fragments, including the formation of elongate, rounded crystals. A later, lower temperature deformation disrupted plessite. Pallasites formed in multiple parent bodies by processes that recurred in several places within the solar system, as shown by the mineralogical and textural similarities between pallasites that differ in their isotopic and trace element compositions. Type IIIB irons still seem the most likely associated meteorites. Two new pallasites, Dora and Rawlinna, are described briefly.

Chromite-chlorite intergrowths in peridotite at Chimwadzulu Hill, Malawi

SummaryChromiferous spinel and chlorite are associated and sometimes intergrown in serpentinized peridotites at Chimwadzulu Hill. The peridotites probably crystallized from a melt under the conditions of the spinel-lherzolite facies (O'Hara, 1967) in which the stable aluminous phase is spinel. The spinel that formed was probably a chromite with some Al2O3 and MgO. At lower temperatures aluminous spinel is unstable in the presence of olivine and orthopyroxene, chlorite being the stable aluminous mineral. It is thought that the chlorite crystallized under the conditions of medium-grade metamorphism that prevailed when the rocks were emplaced, and at the same time the spinel recrystallized, depleted in Al2O3 and MgO and correspondingly enriched in FeO, Fe2O3, and Cr2O3, giving rise to the observed chromite-chlorite association.

Variations de teneurs en Cu, Ni, Cr, Co et V dans quelques roches volcaniques des Causses et du Bas-Languedoc

Abstract Fourteen samples from the Causses and Bas-Languedoc volcanics, including a peridotite nodule, basaltic lavas, and pegmatoids, have been submitted to geochemical analysis for the first time, in order to determine the distribution of Cu, Ni, Co, and V. In the rocks studied, these elements decrease in percentage during the course of differentiation with the exception of Cu, whose percentage is not affected. The decrease of the Ni/Co ratio to below 1 suggests that Ni was removed more quickly from the magma than was Co. Median percentages calculated for the basalts indicate that the presence of olivines is accompanied by a marked enrichment of Ni, Co, Cr, and Cu.