TiO2 Abundances Research Articles

Geology of mare deposits in South Pole‐Aitken basin as seen by Clementine UV/VIS data

A multispectral analysis of 21 discrete lunar mare deposits within the South Pole‐Aitken basin was undertaken in order to model the general mineralogical composition of farside mare deposits so that the near‐ and farside may be compared in terms of magma generation and emplacement. Determination of each deposit's mineralogy was based upon albedo, the 0.75/0.95 μm ratio indicating the presence of a 1.0 μm absorption band, and the strength and shape of that band. A combination of improved spatial resolution and the availability of the general multispectral signature for each pond allowed deposits to be examined for homogeneity in order to assess the nature and number of flows represented. FeO and TiO2 abundances were also estimated. Based upon the resultant data, 19 out of 21 ponds were determined to be mare deposits. Of these 19, 15 had spectra well‐defined enough to classify general mineralogy, nine having high‐Ca pyroxene signatures indicative of basalt and six showing low‐Ca signatures suggesting a substantial noritic component. We believe these noritic signatures can plausibly be attributed to vertical/lateral soil mixing processes for the smallest ponds, although a unique lithology cannot be ruled out for the largest pond. A majority of ponds in this study region have homogeneous spectral signatures, interpreted to represent individual eruptive phases. This indicates that very large volumes for these regions may be typical. Also, because such volumes are comparable to the largest terrestrial eruptions, they may have formed by a similar mechanism of emplacement. The vast majority of mature soils are spectrally redder (and thus have a lower estimated FeO abundance) than the Mare Serenitatis soil standard (MS‐2) in particular, and nearside basalts in general. These soils also average low to medium estimated TiO2 abundance, similar to Apollo 12 and 15, and Luna 24. The observations made here are consistent with a scenario in which basalts with low FeO and low to medium TiO2 contents were deposited during the lunar peak volcanic period that occurred in the Late Imbrian (3.80–3.20 Ga) and because of their small areal extent were more susceptible to mixing processes than nearside basalts.

Sector-zoned augite megacrysts in Aleutian high alumina basalts: implications for the conditions of basalt crystallization and the generation of calc-alkaline series magmas

Several high alumina basalts from the Aleutian volcanic centers of Cold Bay and Kanaga Island contain large (up to 1.5 cm diameter) megacrysts of sector-zoned augite. The megacrysts are invariably euhedral with well developed {001}, {010} and {111} forms. All crystals display concentric bands that are rich in mineral and glass inclusions. The sector zonation typically occurs as well developed (010), (100), (111) and (110) sectors which grew at different rates. A comparison of the width of synchronous growth bands indicates that following relative growth rates: (111) ≫ (100) ∼ (110) > (010). Compositionally, SiO2 and MgO abundances decrease, and TiO2, Al2O3, FeO and Na2O abundances increase in the different sectors in the order (111), (100) ∼ (110), (010). This order is identical to that deduced for the relative growth rates, implying that growth rate clearly had a role in the development of the sector zonation. Calculated pre-eruption H2O contents of the basalts range from 1 to 3 wt% but actual (measured) post-eruption H2O contents range from 0.01 to 0.3 wt%. Deteurium isotopic values are heavily depleted and range from −110 to −141‰ . Together these indicate significant vapor (H2O) exsolution prior to eruption. Maximum H2O abundances in primitive glass inclusions, thought to be most representative of the host liquid reservoir at the time of melt entrapment, systematically decrease from the core to the rim of one augite megacryst studied in detail. We conclude that the presence of sector-zoned augite is due to augite supersaturation and rapid crystallization brought about by magma decompression and volatile (H2O) exsolution. The calculated pre-eruption H2O contents of 1–3 wt% limit vapor exsolution and basalt crystallization to depths of less than 3 and more likely 1.5 km. Very rapid crystallization at very shallow depths makes it unlikely that the time scales between initial crystallization and final eruption are sufficient to permit appreciable amounts of fractional crystallization. Given that high alumina basalt fractionation is the dominant process for generating more evolved andesite, dacite and rhyolite magmas of the calc-alkaline suite, the inability of parental high alumina basalt to yield such derivative magmas in the low pressure environment places the likely site of fractionation in the high pressure environment, at or near the base of the crust.

THE COMPOSITION OF MESOZOIC PERIDOTITE TECTONITES OF THE EASTERN ALPS

Numerous small ultramafic bodies are exposed in Mesozoic cover units of the central Eastern Alps. The major occurrences are restricted to the tectonic windows of the Penninic zone and their surroundings. In the Lower Engadin window, three different mantle peridotite groups have been investigated. The Idalp ophiolite, situated at the northern rim of the Engadin window, is believed to be of south Penninic origin (Trumpy, 1972), whereas the Ramosch ophiolite at the southwestern margin is assigned to the north Penninic area (Vuichard, 1984). The tectonic position of the ultramafics in the vicinity of Nauders, at the southern margin, is unknown. Ophiolite remnants of south Penninic affinity are also present in the Glockner nappe of the Tauern Window. At the eastern end of the Alpine orogen similar peridotites appear in the small Rechnitz window. Numerous, usually small and highly serpentinized bodies of ultramafic rocks can be traced in the Matrei zone along the southern margin of the Tauern window; the Matrei Zone comprises both, Penninic and Lower Austroalpine elements. The Reckner ophiolite complex, near the NW corner of the Tauern window, is part of the tectonically higher units of the Lower Austroalpine nappe. Geochemical and petrological investigations reveal considerable differences between these mantle slices. The different ultramafic bodies are influenced by a pervasive regional metamorphism locally reaching amphibolite facies. Primary mineral assemblages of the peridotites have been mostly replaced by metamorphic parageneses. The products of metamorphism are dominantly serpentine minerals accompanied by various combinations of diopside, tremolite, chlorite, hydrogarnet and magnetite. In contrast to all other occurences, the samples from Nauders are characterized by a well preserved primary assemblage. Relic clinopyroxene and spinel can be found in peridotites of the Reckner and in the Matrei zone. Olivine and orthopyroxene pseudomorphed by serpentine minerals are present in less deformed areas only. Preserved clinopyroxenes of these mantle peridotites have up to 7 wt% Al2O3 and 1.5-2.0 wt% Na2O, which corresponds to a jadeite component of more than 10 %. The Cr2O3 content is generally high (up to 1 wt%). Clinopyroxene is frequently zoned with progressive depletion of Al, Na, Ti and Cr towards pure diopside. In samples from Nauders the Fo-content of olivine is about 90% and the NiO concentration varies between 0.4-0.5 wt%. The orthopyroxenes range in XMg from 91-92 and Al2O3 reaches up to 5 wt%. Titanian pargasite (XMg 0.88, about 3.5 wt% TiO2) is a secondary phase in Nauders peridotites only. Spinels from Nauders are different in composition to those of the Reckner complex. They are typically poor in Cr2O3 (about 7 wt%), high in Al2O3 (about 60 wt%) and develop rims which usually have lower Cr contents. Relics of spinel with significantly higher Cr values (Cr# 40-50, Cr2O3 35-40 wt%) are preserved in samples of the Reckner complex. The dominant rock types of all Mesozoic ultramafic bodies are residual lherzolites and harzburgites (XMg 87-94). The serpentinized ultramafics of the southern Penninic realm (Idalpe, Tauern and Rechnitz windows) are generally harzburgites with low Al2O3 (<2 wt%) and CaO contents (Fig. 1). The HREE concentrations (0.3-0.5 times chondrite) display patterns of moderately depleted restitic mantle (Fig. 2). Lherzolites and cumulate rocks (XMg = 0.80) are restricted to a few occurrences within the main mass of harzburgites. In contrast, more fertile lherzolites and harzburgites form ultramafic bodies of the Matrei zone, the Reckner complex, the Ramosch ophiolite and Nauders.The high abundances of Al2O3 (up to 4.6 wt%) and TiO2 (up to 0.22 wt%) are consistent with the estimated composition of a primitive upper mantle. All samples display higher HREE concentrations (up to 2.5 times chondrite) and a significant LREE depletion trend (Figs. 1 and 2). The Cr-Yb projection (Pearce and Parkinson, 1993) has been used to investigate the degree of mantle depletion. This diagram provides further evidence that the mantle material of the south Penninic region represents a more depleted source (about 15% melt depletion) than those of the Matrei zone and the Lower Austroalpine region (up to 5% melt depletion). These data support the model of a slow spreading oceanic environment (Hock and Koller, 1992) for the ophiolites of the Idalpe, Tauern and Rechnitz window, whereas the ultramafics of the Reckner, Matrei Zone and Nauders were probably generated in a pre-oceanic stage.

Barian-titanian micas from Ilha da Trindade, South Atlantic

AbstractExtremely high abundances of TiO2 (up to 12.57 wt.%) and BaO (up to 7.11 wt.%) are reported from dark reddish-brown phlogopite-biotite rims around paler phlogopite cores in a lamprophyric dyke from Ilha da Trindade, South Atlantic. Both TiO2 and BaO contents increase towards the rims of the micas and in groundmass crystals. This is accompanied by corresponding increases in FeO*, MnO and Mg number; whereas SiO2, Al2O3, Cr2O3, MgO and K2O abundances all decrease. The substitution of Ba into the biotite structure can be explained by the scheme Ba + Al = K + Si. Ti-substitution schemes however are much more complex, as no general equation can account for all the Ti incorporated into the structure and therefore several schemes are proposed.

Mapping the FeO and TiO2 content of the lunar surface with multispectral imagery

The derivation of quantitative elemental concentrations from multispectral imaging of the Moon has long been a goal of lunar remote sensing. Concentration maps at the spatial resolutions available from the recent Clementine mission would provide a revolutionary new tool for understanding the origin and evolution of the lunar crust. Lucey et al. [1995] presented a method for extracting the concentration of Fe from multispectral imaging of the Moon. This paper examines and quantifies important aspects ofthat technique left unexamined by Lucey et al. which had the potential to severely limit its utility. These aspects include the effects of maturity, grain size, mineralogy, shading due to topography, and the presence of glass. We also present a new algorithm for derivation of TiO2 from multispectral imaging of both mare and highland units. We find that both techniques are only weakly sensitive to maturity and that they have about 1 wt % accuracy based on examination of the spectral properties and compositions of resolved lunar sampling stations presented by Blewett et al. [1997]. We also discuss these findings in the context of two contrasting views of the effect of composition on lunar spectral properties presented by Pieters and coworkers and Hapke and coworkers. We find the view of Hapke and coworkers to be more consistent with our observations. Using a global mosaic of Clementine multispectral data and these element derivation algorithms, we find that the global modal abundance of FeO is 4.5 wt %±1 wt % and the global modal abundance of TiO2 is 0.45 wt %±1 wt. %.

FeO and TiO2 concentrations in the South Pole‐Aitken basin: Implications for mantle composition and basin formation

We use newly developed spectroscopic methods for determining FeO and TiO2 abundances to study the huge (2500 km diameter) South Pole‐Aitken basin. The floor of this basin shows iron and titanium values which are weakly correlated and largely range from 7 to 14 wt % FeO and from 0.5 to 1.5 wt % TiO2. This composition is consistent with an approximate 1:1 mixture of lower crustal material like that found elsewhere on the Moon and mantle rock containing 10–16 wt % FeO and &lt;0.1 wt % TiO2. Although mantle rock appears to compose about 50% of the floor deposits, models of the formation of large impact basins predict that virtually all the floor materials ought to be derived from the mantle.

Regolith composition and structure in the lunar maria: Results of long‐wavelength radar studies

Radar measurements at 70‐cm and 7.5‐m wavelengths provide insight into the structure and composition of the upper 5–100 m of the lunar regolith and crust. We combine high‐resolution (3–5 km) 70‐cm radar data for the nearside with earlier calibrated full‐disk observations at the same wavelength to provide a reasonable estimate of the lunar backscatter coefficient. These data are tested against models for echoes from a buried substrate and Mie scattering from surface and buried rocks. These mechanisms are expected to dominate the 70‐cm radar echo, with their relative importance determined by the rock population, regolith depth, substrate roughness, and the loss tangent of the soil. Results indicate that the 70‐cm radar echo for the maria comes largely from Mie scattering by rocks buried within the fine soil. Radar scattering from a buried substrate is not likely to greatly affect the observed return. We also compared the 70‐cm and 7.5‐m radar images to infrared eclipse temperature maps, crater‐population age estimates for the maria, and to TiO2 and FeO abundances inferred from Earth‐based telescopic and Clementine multispectral observations. These data imply that (1) the TiO2 (ilmenite) content of the regolith controls variations in 70‐cm depolarized echo strength among mare units, with higher titanium abundance leading to lower echoes; (2) changes in the average 70‐cm return for a given TiO2 abundance between maria of different ages do occur, but uncertainties in the current radar data do not allow us to uniquely distinguish between variations in rock population with age and calibration effects; (3) the 7.5‐m radar echoes are controlled by the age of the mare basalt flows, with older deposits having a greater degree of fracturing and higher backscatter. Future mapping at 12.6‐cm and 70‐cm wavelengths will help to resolve some of the issues raised here.

Petrogenesis of sediments in the absence of chemical weathering: effects of abrasion and sorting on bulk composition and mineralogy

ABSTRACTComminution in the glaciers that debouch into Guys Bight Basin, followed by selective sorting in the fluvial system, has had little effect on the bulk composition, or on the mineralogy, of the basin sands and muds. Most striking are the feldspar contents, and the feldspar‐quartz ratios in sands and muds, both of which remain similar to those of average bedrock. The feldspar contents of sands and muds range from 48 to 52% feldspar whereas average bedrock contains 51·7% feldspar. Feldspar‐quartz ratios average 1·58:1 in bedrock and 1·54:1, 1·66:1 and 1·69:1 in the medium to coarse sands, fine sands and muds, respectively, indicating minimal feldspar enrichment in the fine‐grained sediments. In the absence of appreciable chemical weathering, extreme abrasion followed by hydraulic sorting has not produced mature sediments such as quartz arenites and clay‐mineral‐rich muds. There is, however, some chemical differentiation. Preferential accumulation of mafic minerals in fine sands and muds is reflected in bulk compositions by higher abundances of MgO, FeO and TiO2, and in the mineralogy by enrichment of biotite in the fine grades. Bulk compositional studies focused solely on muds and mudstones will result in an overestimate of the mafic contribution from source rocks. This work shows that bulk compositional studies of sediments and sedimentary rocks should include all available granulometric grades.

Compositional Variability of the Serenitatis/Tranquillitatis Region of the Moon from Telescopic Multispectral Imaging and Spectroscopy

We present results from a combined CCD multispectral imaging and near-IR spectroscopic study of a region near the border between the Mare Serenitatis and Mare Tranquillitatis lunar impact basins. We have detected and spatially mapped variations in mare TiO2 abundance, in the degree of spectral maturity of impact craters and their ejecta blankets, and in the level of fresh highlands contamination within mare units. Techniques employed include color ratio images, image-oriented linear mixing models, and spectral band analyses. Specific findings include the detection of relatively fresh highlands materials in the craters Tacquet and Sulpicius Gallus and in the central peak and floor of the crater Plinius, the detection of low-TiO2 Serenitatis-like mare materials in a confined region of the ejecta from the crater Dawes, and the determination of immaturity rather than composition as the origin of the high albedo in the rectangular deposit surrounding Dawes. The combination of high spatial resolution imaging and high spectral resolution spectroscopy and the use of synthesizing techniques such as mixture modeling will provide valuable tools to optimize the scientific return from telescopic, Galileo, Clementine, and other future lunar data sets.

Potassium and incompatible-element enrichment in shoshonitic lavas from the Tavua volcano, Fiji

Shoshonitic lavas from the Tavua volcano on the island of Viti Levu, Fiji, display a range of compositions from mafic absarokite through shoshonite to evolved banakite. They were erupted during the early stages of the breakup of the once-continuous Vanuatu-Fiji-Tonga arc, yet their geochemical characteristics are similar to those of subduction-related volcanism. They have low TiO2 (0.5–0.7%) and HFSE abundances (e.g., 0.03–0.05 ppm Ta in the absarokites), very high LILE/HFSE ratios and slight LREE enrichment (CenYbn 2.7–4.5). Unusually for potassic rocks, they do not have anomalous radiogenic isotopes, Sr, Nd and Pb isotope ratios being similar to other circum-Pacific arcs, and Pb in particular being similar to MORB (143Nd144Nd ≈ 0.51300, 87Sr86Sr ≈ 0.7035–0.7040, 206Pb204Pb = 18.5–19.0). Simple mass-balance calculations restrict the amount of subducted sediment in the shoshonite source region to <4%.Variations in HFSE abundances and ratios are attributable to variable degrees of melting of a depleted mantle source region whereas major-element abundances indicate that the absarokites result from high degrees of melting. This is consistent with the slight enrichment of the LREE relative to the HREE. Incompatible-element enrichment is selective, the shoshonites being particularly enriched in Rb, Ba, K, Sr, Pb and P relative to island arc tholeiites, and this is reflected in, for example, anomalously high P/Nd ratios of between 120 and 180. These features are interpreted to be the result of source enrichment by hydrous fluids released from subducted oceanic lithosphere that scavenged material from both the mantle wedge and possibly subducted altered oceanic crust. High oxygen fugacities are indicated by small but significant negative Ce anomalies, the lack of Eu anomalies even in the most evolved samples despite plagioclase dominated fractionation, and the early appearance of titanomagnetite in the fractionating assemblage.Comparison with more typical island arc tholeiites indicates that the source enrichment of the Tavua shoshonites was an extreme example of that operating beneath arcs in general. Two alternatives are presented to explain the unusual composition of the shoshonites. The first implies an unusually large fluid input from the slab to the magma source region although the cause of this is unclear. The second appeals to the effects of tectonics on the convective regime in the mantle wedge. A change in the ratio of the fluid velocity to that of the convecting mantle modifies the range of elements that are transported by the fluid into the magma source region. This type of model is encouraged by the relationship between element enrichment and ionic radius and the association of shoshonites with changes in arc tectonics both in Fiji and other localities.

Petrologic and geochemical characteristics and origin of Gusui cherts, Guangdong Province, China

The characteristic structures of the Precambrian cherts from the Gusui section, Guangdong, China, include bedded structure, laminated structure, massive structure and pseudobrecciated structure. The chert is characterized by consistently low abundance of TiO2, Al2O3 and most trace elements. Howevver, it is enriched in Ba, As, Sb, Hg and Se. In Al-Fe-Mn ternary diagrams, it fells into the “hydrothermal field”. Correspondence analysis and factor analysis show that many elements show up in the factor that represents the leaching of country rocks by hydrothermal solutions, and are the very characteristic element association of the geochemically anomalous South China basement. Petrologic and geochemical evidence suggests a hydrothermal origin for the chert. The chert may have been formed in a Precambrian rift or an extension zone developed within the Yunkai marginal geosyncline. with a fault system linking it to an unknown heat source at depth.

Remote sensing of potential lunar resources: 2. High spatial resolution mapping of spectral reflectance ratios and implications for nearside mare TiO2 content

High spatial resolution maps illustrating variations in spectral reflectance 400/560 nm ratio values have been generated for the following mare regions: (1) the border between southern Mare Serenitatis and northern Mare Tranquillitatis (including the MS‐2 standard area and Apollo 17 landing site), (2) central Mare Tranquillitatis, (3) Oceanus Procellarum near Seleucus, and (4) southern Oceanus Procellarum around Flamsteed. We have also obtained 320–1000 nm reflectance spectra of several sites relative to MS‐2 to facilitate scaling of the images and provide additional information on surface composition. Inferred TiO2 abundances for these mare regions have been determined using an empirical calibration which relates the weight percent TiO2 in mature mare regolith to the observed 400/560 nm ratio. Mare areas with high TiO2 abundances are probably rich in ilmenite (FeTiO3) a potential lunar resource. The highest potential TiO2 concentrations we have identified in the nearside maria occur in central Mare Tranquillitatis. Inferred TiO2 contents for these areas are &gt;9 wt % and are spatially consistent with the highest‐TiO2 regions mapped previously at lower spatial resolution. We note that the morphology of surface units with high 400/560 nm ratio values increases in complexity at higher spatial resolutions. New telescopic spectra of landing sites successfully reproduce the Charette relation, although we find that the 400/560 nm value is strongly a fimction of the sample area size. The increased spectral contrast of the 400/730 nm ratio over the 400/560 nm ratio demonstrates the potential increased precision with which the 400/730 nm ratio might be used to estimate TiO2 abundances. Comparisons have been made with previously published geologic maps, Lunar Orbiter IV, and ground‐based images, and some possible morphologic correlations have been found between our mapped 400/560 nm ratio values and volcanic landforms such as lava flows, mare domes, and collapse pits.

時間指標テフラ阿蘇３によって示唆される最終間氷期直後５ｄ期の海面低下

We have discovered a distal tephra with similar petrographic and chemical properties to the Aso-3 pyroclastic flow deposits of the Aso caldera origin at several localities of Honshu, Japan, and adjacent seas. It is concluded to be a fallout ash associated with the Aso-3 pyroclastic flow, The tephra, named Aso-3, is a cry tal vitric fine-grained ash, consisting of dacitic pumiceous and bubble-walled glass shards, and orthopyroxene and crynopyroxene as mafic phertocrysts. It is identified with the aid of combined petrographic and stratigraphic parameters : relatively high index of glass (n = 1.5131.519), low index of orthopyroxene (γ =1.7021.705) and dacitic glass composition (rich in alkali, K2O> Na2O, rich in TiO2, FeO* and CaO low abundance in light rare earth elements and characteristic low degree of angle in chondritenormalized pattern).This tephra was dated by several radiometric methods : 103±4.2 (FT), 110±3 (TL) and 123±6 (K-Ar) in ka. The stratigraphic position of this tephra is above the last interglacial culmination in Kyushu and Honshu, and below such widespread time-marker tephras as Toya, Ata, On-Pml, SK, etc. In addition, the tephra occurs in the marine isotope stage (MIS) 5d in the abyssal sediment in the northwest Pacific.The occurrence of this tephra in the Quaternary sequence in northern coastal areas of Miyazaki, south Kyushu, clearly indicates that the sea level was more than c. 70 m lower than the preceding high stand of sea level of the MIS 5e. At the coastal cliff of Kawaminami, the Aso-3 tephra occurs 21 m above the present sea level in the basal part of the valley-filling deposits (Tohriyamahama formation) which followed the prominent marine terrace, Sanzaibaru of the MIS 5e. Former shoreline of this stage was uplifted to the elevation of c. 90 m. This tephra was also recognized in the brackish to fresh water layer sandwiched between marine sediments, called Shibikawa formation in Oga Peninsula, northern Honshu. It clearly suggests significant lowering of sea level at the time of Aso-3 eruption as well.

Mariner 10 multispectral images of the eastern limb and farside of the Moon

We present the first analysis of Mariner 10 multispectral image data for the eastern limb and farside of the Moon. A new, rigorous calibration of these data is used that provides results that are consistent with independently derived data sets. From these newly calibrated data, a color ratio image (0.48/0.58 μm) is produced to show relative TiO2 abundances in mare units on the eastern limb and farside of the Moon. Our results indicate that the farside lunar mare deposits examined have TiO2 contents that are intermediate to low (2–5% TiO2) relative to mare deposits on the nearside. We find no evidence for areally extensive deposits of either very low titanium (VLT) or high to very high titanium (VHT) basalts in these portions of the Moon. Our findings support previous interpretations that the source regions for lunar mare basalts are laterally heterogeneous. Additionally, examination of highland units reveals a relatively blue region that corresponds with a previously proposed cryptomare, suggesting enhanced TiO2 for this unit. Finally, based on these Mariner 10 data, telescopic spectral data, Galileo multispectral image data, and lunar meteorite analyses we speculate that volcanism in farside mare deposits did not include high titanium basalts (&gt; 8 wt %).

Mid-Pleistocene lavas from the Seguam volcanic center, central Aleutian arc: closed-system fractional crystallization of a basalt to rhyodacite eruptive suite

In contrast to adjacent volcanic centers of the modern central Aleutian arc, Seguam Island developed on strongly extended arc crust. K-Ar dates indicate that mid-Pleistocene, late-Pleistocene, and Holocene eruptive phases constitute Seguam. This study focuses on the petrology of the mid-Pleistocene, 1.07–07 Ma, Turf Point Formation (TPF) which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst-poor andesite to rhyodacite lavas. Increasing whole-rock FeO*/MgO from basalt to dacite, the anhydrous Plag+Ol+Cpx±Opx±Mt phenocryst assemblage, groundmass pigeonite, and the reaction Ol+Liq=Opx preserved in the mafic lavas indicate a tholeiitic affinity. Thermometry and comparison to published phase equilibria suggests that most TPF basalts crystallized Plag+Ol+Cpx±Mt at ≥1160°C between about 3–5 kb (±1–2% H2O), andesites crystallized Plag+Cpx+Opx±Mt at ≥1000°C between 3–4 kb with 3–5% H2O, and dacites crystallized Plag +Cpx±Opx±Mt at 1000°C between 1–2 kb with 2–3% H2O. All lavas crystallized at f o 2 close to the NNO buffer. Mineral compositions and textures indicate equilibrium crystallization of the evolved lavas; petrographic evidence of open-system mixing or assimilation is rare. MgO, CaO, Al2O3, Cr, Ni, and Sr abundances decrease and K2O, Na2O, Rb, Ba, Zr, and Pb increase with increasing SiO2 (50–71%). LREE enrichment [(Ce/Yb)n=1.7±0.2] characterizes most TPF lavas; total REE contents increase and Eu anomalies become more negative with increasing SiO2. Relative to other Aleutian volcanic centers, TPF basalts and basaltic andesites have lower K2O, Na2O, TiO2, Rb, Ba, Sr, Zr, Y, and LREE abundances. 87Sr/86Sr ratios (0.70361–0.70375) and ratios of 206Pb/204Pb (18.88–18.97), 207Pb/204Pb (15.58–15.62), 208Pb/204Pb (38.46–38.55) are the highest measured for any suite of lavas in the oceanic portion of the Aleutian arc. Conversely, eNd values (+5.8 to+6.7) are among the lowest from the Aleutians. Sr, Nd, and Pb ratios are virtually constant from basalt through rhyodacite, whereas detectable isotopic heterogenity is observed at most other Aleutian volcanic centers. Major and trace element, REE, and Sr, Nd, and Pb isotopic compositions are consistent with the basaltic andesitic, andesitic, dacitic, and rhyodacitic liquids evolving from TPF basaltic magma via closed-system fractional crystallization alone. Fractionation models suggest that removal of ∼80 wt% cumulate (61% Plag, 17% Cpx, 12% Opx, 7% Ol, and 3% Mt) can produce 20 wt% rhyodacitic residual liquid per unit mass of parental basaltic liquid. Petrologic and physical constraints favor segregation of small batches of basalt from a larger mid-crustal reservoir trapped below a low-density upper crustal lid. In these small magma batches, the degree of cooling, crystallization, and fractionation are functions of the initial mass of basaltic magma segregated, the thermal state of the upper crust, and the magnitude of extension. Tholeiitic magmas erupted at Seguam evolved by substantially different mechanisms than did calc-alkaline lavas erupted at the adjacent volcanic centers of Kanaga and Adak on unextended arc crust. These variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew. At Seguam, intra-arc extension promoted eruption of voluminous basalt and its differentiates, unmodified by interaction with lower crustal or upper mantle wallrocks.

C-type magmas: igneous charnockites and their extrusive equivalents

ABSTRACTIgneous charnockites are characterised by distinctively high abundances of K2O, TiO2, P2O5 and LIL elements and low CaO at a given SiO2 level compared to metamorphic charnockites, and I-, S- and A-type granites. They form a distinctive type of intrusive igneous rocks, the Charnockite Magma Type (CMT or C-type), which generally lack hornblende and consist of pyroxene, alkali feldspar, plagioclase, quartz, biotite, apatite, ilmenite and titanomagnetite. Although this mineral assemblage superficially resembles that of metamorphic charnockites, magmatic charnockites are characterised by inverted pigeonite, exceptionally calcic alkali feldspar, potassic plagioclase, and coexisting opaque oxides, all with crystallisation temperatures of 950-1050°C. Apatite is a ubiquitous phase which, together with the very high concentrations of Zr and TiO2 over a wide silica range, is consistent with the derivation of the Charnockite Magma Type by very high temperature partial melting and fractionation.The credibility of intrusive charnockites as a magmatic type has historically foundered because of their apparent restriction to granulite belts and the absence of any reported extrusive equivalents. We report examples of volcanic rocks, of various ages, with the same distinctive major and trace element compositions, mineral assemblages and high temperatures of crystallisation as intrusive chamockites.The Charnockite Magma Type is considered to be derived by melting of a hornblende-free or poor, LILE-enriched fertile granulite source which had not been geochemically depleted by a previous partial melting event but which was dehydrated in an earlier metamorphism. Whereas H2O-saturated melting produces migmatites or "failed" granites, and vapour-absent melting of an amphibolite can produce I-type granites, according to this model the vapour-absent melting of a hornblende-free or hornblende-poor granulite at even higher temperatures produces charnockites.

Remote sensing of potential lunar resources: 1. Near‐side compositional properties

New telescopic CCD multispectral imaging of the lunar near side and 330–870 nm spectroscopy of selected regions are presented as aids in estimating compositional differences relevant to locating potential lunar resources such as ilmenite (FeTiO3) and solar wind‐implanted 3He and H. Conversion of 400/560 nm CCD ratio images to weight percent TiO2 using a version of the Charette et al. (1974) empirical relation allows construction of a new TiO2 abundance map for the lunar maria accurate to ±2 wt % HO2. This map provides a consistent view of TiO2 distribution among mature mare soils and can be used to estimate areas potentially rich in ilmenite. Western Mare Tranquillitatis exhibits the highest TiO2 abundances (&gt;8 wt %), followed by regions near Flamsteed P and in northern Oceanus Procellarum. A 950/560 nm CCD ratio mosaic of the full Moon provides estimates of relative surface maturity on local scales as defined by the degree of agglutinate production due to micrometeorite impacts. Since high 3He concentrations are correlated with mature ilmenite‐rich soils, a combination of relative surface maturity maps and TiO2 abundance maps may be useful for estimating 3He (and possibly H) distribution on local scales. While dark mantle materials are also potential sources of ilmenite, 3He, and H, their compositional differences from mare soils prevent accurate remote mapping of their TiO2 abundance and relative maturity. Reflectance spectra (relative to a more rigorously defined MS‐2 standard region) of dark mantle materials and high‐TiO2 mare areas exhibit previously undocumented near‐UV (&lt;350 nm) changes in spectral slope apparently related to compositional variations. Determination of the primary lunar sites for resource utilization will be dictated by the future goals and priorities of the lunar resource program.

Geology and geochemistry of the Burnthill Granite and related W – Sn – Mo – F mineral deposits, central New Brunswick

Several significant W – Sn – Mo – F-bearing hydrothermal mineral deposits occur in the Miramichi Highlands of central New Brunswick. These deposits are spatially and temporally related to a group of five subcircular, undeformed, high-level, posttectonic granite plutons (Burnthill, Dungarvon, Rocky Brook, Sisters Brook, Trout Brook) of Middle Devonian age. The largest of the plutons is the Burnthill Granite (area = 180 km2), which comprises a texturally variable, polyphase intrusion composed of biotite monzogranites and alkali feldspar granites (International Union of Geological Sciences (IUGS) classification); apatite and zircon are the ubiquitous accessory minerals. Field and petrographic data indicate that the Burnthill Granite and two small (&lt; 7 km2) adjacent intrusions (Buttermilk Brook and Burnthill Mine granites), located 1 km from the southeast contact of the main intrusion, are genetically related and are epizonal in character. Classic epizonal features include sharp, discordant intrusive contacts with the host metasediments; a narrow contact metamorphic aureole with local zones of intense hydrothermal alteration and mineralization; an isotropic internal fabric; evidence of multiple intrusion; and the presence of porphyries.Geochemically the pluton is composed of mildly peraluminous, high-silica granites (SiO2 &gt; 74 wt. %), with low abundances of TiO2, MgO, CaO, P2O5, Sr, Ba, and Eu, and high concentrations of Y, Nb, Sn, Cs, Ta, W, Th, and U; high FeO/MgO ratios and Na2O + K2O contents are typical. Covariation among many of the major and trace elements within the volumetrically important phases of the Burnthill Granite can be explained by the process of fractional crystallization, and has resulted in an enrichment of those elements of economic interest (viz., Sn and W) in the more evolved granites. However, the anomalous behaviour of certain elements in these most evolved phases requires the operation of an additional process; this was most likely aqueous phase saturation followed by fluid separation, a process whose activity is corroborated by field and petrographic evidence (e.g., the development of miarolitic cavities and myrmekite). Normative Ab–Or–Q–An contents for the Burnthill Granite have a mean compostion of Ab31Or28Q38An3. This average value is almost identical to that of the piercing point assemblage in the "haplogranodiorite" system with 3 % normative anorthite, and is consistent with crystallization under water-saturated conditions at about 1 kbar (100 MPa) and 730–800 °C.Structural and petrochemical data demonstrate that the W – Sn – Mo – F-bearing mineral deposits were deposited in fracture and fault systems that developed after crystallization was completed, from hydrothermal fluids that were compositionally similar (high concentrations of F, Na, K, Sn, W) to the more evolved silicate melt phases of the Burnthill Granite. In addition to generating the wolframite–cassiterite–fluorite mineral deposits that occur in both endo- and exo-granitic settings, these fluids produced zones of intense, fracture-controlled, aluminosilicate alteration (viz., K-feldspathization, albitization, greisenization, and topaz alteration) that envelop the zones of mineralization.

Geochemistry of Late Proterozoic amphibolites and ultramafic rocks, southeastern Canadian Cordillera

Late Proterozoic amphibolites and ultramafic rocks from the southeastern Canadian Cordillera have been analysed for major and trace elements in order to determine the nature and origin of the protoliths. Geologic relations indicate that these rocks were produced during an episode of continental rifting in the Precambrian. Based on rare-earth-element (REE) patterns, immobile-incompatible-element ratios, and characteristic elemental abundances, amphibolites are subdivided into alkaline and tholeiitic metabasalts. Alkaline basalts are recognized by their steep REE patterns, high Zr/Y, high TiO2 and P2O5 abundances, and low Y/Nb and Ti/Zr. Tholeiitic basalts are subdivided into three groups: (I) high-Mg#, high-field-strength-element (HFSE)-depleted, light-REE (LREE)-enriched tholeiites with flat heavy REE (HREE) patterns; (II) LREE-enriched tholeiites depleted in HREE; and (III) low-Mg# tholeiites with flat REE patterns. Ultramafic rocks occur as boudins of partially recrystallized Cr-spinel-bearing harzburgite or therzolite, enriched in LREE (Ce/Sm = 1.7–1.9), HFSE, CaO, Al2O3, and TiO2 relative to depleted mantle.Geochemical data suggest that the basalts were derived from a heterogeneous mantle source that underwent different degrees of partial melting with variable amounts of subsequent crystal fractionation of the melts. High Mg#, high Cr and Ni abundances, low HFSE abundances, and high olivine saturation temperatures suggest that group I tholeiites are primary mantle melts produced by a relatively high degree of partial melting of a LREE-enriched, HFSE-depleted source. Group II and III basalts have undergone moderate olivine and pryoxene and limited plagioclase fractionation. Mass-balance calculations suggest that the ultramafic rocks represent a crustally contaminated primary-mantle-derived melt.Les éléments majeurs et traces des amphibolites et des ultramafites, d'âge protérozoïque tardif, du sud-est de la Cordillère canadienne ont été analysés dans le but de déterminer la nature et l'origine des protolithes. Les relations géologiques indiquent que ces roches se sont formées durant un épisode de rifting continental dans le Précambrien. Les diagrammes des terres rares, les rapports des éléments immobiles et incompatibles et les compositions chimiques caractéristiques ont permis de subdiviser les amphibolites en métabasaltes tholéiitiques et alcalins. Les basaltes alcalins sont reconnus par les courbes abruptes dans les diagrammes des terres rares, les rapports Zr/Y élevés et les fortes teneurs en TiO2 et P2O5 et les rapports Y/Nb et Ti/Zr faibles. Les basaltes tholéiitiques sont subdivisés en trois groupes : (I) avec Mg# élevé, appauvrissement en éléments de force de champ élevée, tholéiites enrichies en terres rares légères avec courbe horizontale des terres rares lourdes; (II) tholéiites enrichies en terres rares légères et appauvries en terres rares lourdes; et (III) tholéiites avec Mg# faible et avec courbe horizontale des variations des terres rares. Les ultramafites se présentent en boudins formés d'harzburgite incluant un spinelle chromifère partiellement recristallisé ou de therzolite qui sont enrichies en terres rares légères (Ce/Sm = 1,7–1,9), en éléments à force de champ élevée, en CaO, Al2O3 et TiO2, comparativement à un manteau appauvri.

Origin of ultramafic xenoliths containing exsolved pyroxenes from Hualalai Volcano, Hawaii

Hualalai Volcano, Hawaii, is best known for the abundant and varied xenoliths included in the historic 1800 Kaupulehu alkalic basalt flow. Xenoliths, which range in composition from dunite to anorthosite, are concentrated at 915-m elevation in the flow. Rare cumulate ultramafic xenoliths, which include websterite, olivine websterite, wehrlite, and clinopyroxenite, display complex pyroxene exsolution textures that indicate slow cooling. Websterite, olivine websterite, and one wehrlite are spinel-bearing orthopyroxene +olivine cumulates with intercumulus clinopyroxene +plagioclase. Two wehrlite samples and clinopyroxenite are spinel-bearing olivine cumulates with intercumulus clinopyroxene+orthopyroxene + plagioclase. Two-pyroxene geothermometry calculations, based on reconstructed pyroxene compositions, indicate that crystallization temperatures range from 1225° to 1350° C. Migration or unmixing of clinopyroxene and orthopyroxene stopped between 1045° and 1090° C. Comparisons of the abundance of K2O in plagioclase and the abundances of TiO2 and Fe2O3in spinel of xenoliths and mid-ocean ridge basalt, and a single 87Sr/ 86Sr determination, indicate that these Hualalai xenoliths are unrelated to mid-ocean ridge basalt. Similarity between the crystallization sequence of these xenoliths and the experimental crystallization sequence of a Hawaiian olivine tholeiite suggest that the parental magma of the xenoliths is Hualalai tholeiitic basalt. Xenoliths probably crystallized between about 4.5 and 9 kb. The 155°–230° C of cooling which took place over about 120 ka — the age of the youngest Hualalai tholeiitic basalt — yield maximum cooling rates of 1.3×10−3–1.91×10−3 °C/yr. Hualalai ultramafic xenoliths with exsolved pyroxenes crystallized from Hualalai tholeiitic basalt and accumulated in a magma reservoir located between 13 and 28 km below sealevel. We suspect that this reservoir occurs just below the base of the oceanic crust at about 19 km below sealevel.