Terrestrial Volcanic Rocks Research Articles

Tracing titanomagnetite alteration with magnetic measurements at cryogenic temperatures

SUMMARY Titanomagnetite containing up to 0.6–0.7 Ti atoms per formula unit is a primary magnetic mineral phase in submarine basalts and in some terrestrial volcanic rocks. On a geological timescale, it often undergoes alteration, forming new magnetic phases that may acquire (thermo)chemical remanent magnetization. The initial stage of this natural process can be modelled by prolonged laboratory annealing at moderately elevated temperatures. In this study, our goal is to characterize the alteration products resulting from annealing a submarine basalt containing homogeneous titanomagnetite Fe3−xTixO4 (x ≈ 0.46) at temperatures of 355, 500 and 550 °C for up to 375 hr, by examining their magnetic properties over a wide range of temperatures. The effect of extended annealing is most apparent in the low-temperature magnetic properties. In the fresh sample, a magnetic transition is observed at 58 K. Below the transition temperature, the field-cooled (FC) and zero-field-cooled (ZFC) saturation isothermal remanent magnetization (SIRM) curves are separated by a tell-tale triangular-shaped area, characteristic for titanomagnetites of intermediate composition. The room-temperature SIRM (RT-SIRM) cycle to 1.8 K in zero field has a characteristic concave-up shape and is nearly reversible. For the annealed samples, the magnetic transition temperature shifts to lower temperatures, and the shape of the curves above the transition changes from concave-up to concave-down. The shape of the RT-SIRM cycles also progressively changes with increasing annealing time. The SIRM loss after the cycle increases up to ∼30 per cent for the samples annealed for 375 hr at 355 °C, and for 110 hr at 500 and 550 °C. The Curie temperatures of the newly formed magnetic phases exceed the Curie temperature of the fresh sample (205 °C) by up to 350 °C. While this effect is most commonly attributed to extensive single-phase oxidation (maghemitization), the behaviour observed at cryogenic temperatures appears incompatible with the known properties of highly oxidized titanomaghemites. Therefore, we propose that, at least in the initial stage of the ‘dry’, that is, not involving hydrothermalism, alteration of titanomagnetite, temperature- and time-controlled cation reordering is the primary mechanism driving changes in both low- and high-temperature magnetic properties.

Phase equilibria of ultramafic compositions on Mercury and the origin of the compositional dichotomy

Measurements of major element ratios obtained by the MESSENGER spacecraft using x-ray fluorescence spectra are used to calculate absolute element abundances of lavas at the surface of Mercury. We discuss calculation methods and assumptions that take into account the distribution of major elements between silicate, metal, and sulfide components and the potential occurrence of sulfide minerals under reduced conditions. These first compositional data, which represent large areas of mixed high-reflectance volcanic plains and low-reflectance materials and do not include the northern volcanic plains, share common silica- and magnesium-rich characteristics. They are most similar to terrestrial volcanic rocks known as basaltic komatiites. Two compositional groups are distinguished by the presence or absence of a clinopyroxene component. Melting experiments at one atmosphere on the average compositions of each of the two groups constrain the potential mineralogy at Mercury's surface, which should be dominated by orthopyroxene (protoenstatite and orthoenstatite), plagioclase, minor olivine if any, clinopyroxene (augite), and tridymite. The two compositional groups cannot be related to each other by any fractional crystallization process, suggesting differentiated source compositions for the two components and implying multi-stage differentiation and remelting processes for Mercury. Comparison with high-pressure phase equilibria supports partial melting at pressure <10kbar, in agreement with last equilibration of the melts close to the crust–mantle boundary with two different mantle lithologies (harzburgite and lherzolite). Magma ocean crystallization followed by adiabatic decompression of mantle layers during cumulate overturn and/or convection would have produced adequate conditions to explain surface compositions. The surface of Mercury is not an unmodified quenched crust of primordial bulk planetary composition. Ultramafic lavas from Mercury have high liquidus temperatures (1450–1350°C) and very low viscosities, in accordance with the eruption style characterized by flooding of pre-existing impact craters by lava and absence of central volcanoes.

Magnetic anisotropy of HED and Martian meteorites and implications for the crust of Vesta and Mars

We investigate the petrofabric of crustal rocks from Mars and Vesta through the measurement of the anisotropy of the magnetic susceptibility (AMS) of achondrites. Previous data are integrated with new measurements to obtain a dataset that provide macroscopic information about the magnetic fabric of 41 meteorites of the howardite–eucrite–diogenite clan (HED, falls only) and 16 Martian meteorites. The interpretation takes into account the large contribution of paramagnetism to the magnetic susceptibility of these meteorites. We use a model that allows the computation of the anisotropy degree of the population of ferromagnetic grains and provides a quantitative proxy for the degree of shape preferential orientation of these grains in HED and Martian meteorites. The results also provide quantitative information about the shape of the magnetic fabric (prolate, oblate). In HED achondrites, the ductile FeNi grains are sensitive strain recorders and our magnetic fabric data provide the first quantitative insights to the strain history of the crustal rocks of Vesta. Most HED achondrites are breccias but display a strong and spatially coherent magnetic anisotropy, indicating that intense deformation of FeNi grains took place after brecciation. The average fabric of eucrites, howardites is oblate (i.e. the texture is foliated) whereas the fabric of diogenites is more neutral. The howardite results suggest the existence of an isotropic fraction of ferromagnetic minerals that can be ascribed to the presence of carbonaceous chondrite clasts that have preserved their original magnetic fabric. In this hypothesis, howardites have an intensity of petrofabric very similar to eucrites and diogenites. Thermal metamorphism (itself possibly impact-related) plus lithostatic compaction occurring after brecciation appears as the best candidate to explain the observed petrofabric in eucrites and diogenites, whereas compaction by hypervelocity impacts may be reponsible for the fabric of howardites. Martian meteorites may still possess their primary magmatic fabric. Among Martian meteorites, basaltic shergottites and nakhlites display an oblate fabric (foliated texture) with only limited variations among each group. Olivine–phyric shergottites have a neutral fabric that points to a different petrogenesis. Nakhlites have weaker fabric intensity than shergottites. The fabric intensity is comparable to what is classically observed in terrestrial volcanic and plutonic rocks.

Eocene adakitic volcanism in southern British Columbia: Remelting of arc basalt above a slab window

The Princeton Group is an assemblage of terrestrial volcanic and clastic sedimentary rocks in south-central British Columbia, and is part of the Challis–Kamloops belt that stretches from central British Columbia to the northwestern United States. The volcanic rocks were largely deposited as cinder cones and composite volcanoes, and are composed of basaltic andesite (olivine + clinopyroxene), andesite and dacite (hornblende +plagioclase + clinopyroxene), and rhyolite (biotite + quartz + K-feldspar), with calc–alkaline affinity. New 40Ar/ 39Ar dates on hornblende and groundmass separates, and whole rock indicate that magmatism took place during the Early to Middle Eocene, from 53–47 Ma. New neodymium isotopic measurements, in conjunction with previously published results, indicate that the Princeton Group has an ɛNd 50 = 1.2–6.4 and therefore represents primarily juvenile additions to the continental crust. The major and trace element abundances of Princeton Group rocks resemble those of many modern continental arcs. The compositions are notable, however, because they have an “adakitic” signature that extends throughout their entire compositional range, including high-Mg# basaltic andesite. Trace element modelling indicates that this signature was not derived from anatexis of normal oceanic crust, but from an “arc-like” source enriched in large-ion lithophile elements. This source may have been basaltic dykes that were emplaced into the lithospheric mantle during Mesozoic arc magmatism and subsequently partially melted during an event of lithospheric heating in the Eocene. The heating may have been caused by upwelling asthenosphere related to a slab window or slab tear.

Plagioclase compositions derived from thermal emission spectra of compositionally complex mixtures: Implications for Martian feldspar mineralogy

The compositions of plagioclase, the most abundant mineral in the Martian crust, reflect changing conditions during magmatic evolution. Plagioclase contains spectral features at thermal infrared wavelengths that permit its detection by thermal emission spectrometers (Thermal Emission Spectrometer (TES), Thermal Emission Imaging System (THEMIS), and Miniature TES (Mini‐TES)) on Mars spacecraft. Previous studies have determined the accuracy with which average plagioclase compositions can be modeled in simple two‐component sand mixtures and terrestrial volcanic rocks. Studies of terrestrial rock analogs suffer from difficulties in accurately determining the average plagioclase composition for comparison with the spectrally modeled composition. Sand mixtures, however, provide a means of controlling plagioclase compositions for direct comparison to those modeled by linear deconvolution. This has allowed us to address how compositional complexity may affect our ability to derive average plagioclase compositions from thermal emission data. In this study, we examine the accuracy with which average plagioclase compositions can be modeled from emission spectra of complex mixtures of three, four, and five compositions of coarse (500–850 μm) plagioclase sand. Additionally, we examine multiphase mixtures of plagioclase with pyroxene, olivine, magnetite, and ilmenite that are analogous to selected Martian surface materials. Increasing the number of plagioclase compositions or introducing additional mineral phases does not affect the accuracy previously reported for modeling average plagioclase compositions. Plagioclase can be modeled to within 6 An of measured compositions at laboratory, TES, THEMIS, and Mini‐TES resolution (within 1σ standard deviation).

Diffusion–reaction in a thermal gradient: Implications for the genesis of anorthitic plagioclase, high alumina basalt and igneous mineral layering

Piston-cylinder experiments investigating the interaction between basaltic andesite melt and partially molten gabbro in a thermal gradient provide insight into melt-rock reaction processes occurring during magma differentiation in the crust. In two experiments juxtaposing basaltic andesite and gabbro at 0.5 GPa pressure for durations of either 13 or 26 days, diffusive chemical exchange between the two materials results in mineral layering and notable mineral compositions such as anorthitic plagioclase. Specifically, the basaltic andesite gains Al 2O 3, MgO and CaO from the gabbro and loses Na 2O, K 2O, SiO 2 and FeO to it with a plagioclase-rich layer developing at the interface between the two materials in a process termed diffusion–reaction. The percent crystallinity of the basaltic andesite increases during the process and the plagioclase crystals within the interface region develop anorthitic cores (up to An 90) that abruptly shift in composition to thin rims that are in Na–Ca exchange equilibrium with the co-existing melt. Both the mineralogical layering and bulk compositional change occurring at the interface are reproduced in model simulations of diffusion–reaction. Isotopic tracers ( 45Ca, 6Li, 84Sr and 136Ba) initially deposited at the basaltic andesite–gabbro interface in the 13-day experiment were detected in the cores of the anorthitic plagioclase after the experiment, demonstrating that the melt chemically communicates with the plagioclase cores over the duration of the diffusion–reaction experiment. The formation of anorthitic plagioclase during diffusion–reaction may explain its widespread occurrence in terrestrial volcanic rocks without requiring the presence of ultra-calcic melts. Textures and mineralogical changes in the gabbro indicate that chemical transport occurs throughout the experiments despite temperatures at the cold end of the experimental capsule approaching 500 °C. For instance, apatite, FeNiS, olivine and almost pure albite occur at distinct, specific horizons in the gabbro within the 26-day experiment. Because the bulk element profiles indicating chemical transport reflect analyses of almost completely solid gabbro, equilibration between minerals and fluids/melts must be rapid. The overall effect of the diffusion–reaction process is to make an ascending magma more primitive in composition (and in this case, produce anorthitic plagioclase) while making surrounding crustal wall rocks more evolved. Several observations within igneous rocks support the occurrence of this process, suggesting that the genesis of porphyritic high alumina basalt, ubiquitously observed at convergent margins, could reflect a diffusion–reaction process in the crust.

Paleomagnetic results of the Cretaceous marine sediments in Tongyouluke, southwest Tarim

Paleomagnetic and rock magnetic studies on samples of 18 sites from the Cretaceous marine sedimentary rocks in the Tongyouluke section, Akto County, southwest Tarim, China show that the magnetic carriers of the Lower Cretaceous are dominated by hematite with some magnetite, while the magnetic carriers of the Upper Cretaceous are characterized by a combination of magnetite and titanomagnetite as well as hematite and goethite. Stepwise thermal demagnetization is performed and vector analysis is used to isolate magnetic components, which illustrates a single magnetic component or double magnetic components. The high temperature stable components are dual polarities and pass polarity test, reversal test and consistency test. The overall mean direction of the Lower Cretaceous is D = 27.0°, I = 42.0°, a95=6.5° with pole position at f = 190.3°, ?=63.1°, dp=4.9°, dm=8.0°. The overall mean direction of the Upper Cretaceous is D = 29.1°, I = 39.4°, a95=11.2° with pole position at f =190.9°, ?=60.3°, dp= 8.0°, dm=13.4°. Compared with the inclination of the Early Cretaceous from red beds of north Tarim, the contemporary inclination of southwest Tarim is 10.0° ± 7.8° sharper, but it is still 8.1°±8.9°, shallower than that of Early Cretaceous basalts in southwest Tarim. Although these pa-leomagnetic data show slightly larger confidence limit, the paleolatitude of the marine Cretaceous tends to lie between that of terrestrial red beds and volcanic rocks.

Accuracy of plagioclase compositions from laboratory and Mars spacecraft thermal emission spectra

Plagioclase, the most abundant mineral in many terrestrial and Martian igneous rocks, exhibits a range of compositions that reflects changing conditions during crystallization. Thermal emission spectrometers (TES, THEMIS, and Mini‐TES) on board Mars spacecraft provide a means of determining average plagioclase compositions directly. We demonstrate that spectrally modeled average plagioclase compositions in terrestrial basalts, andesites, and dacites correspond to measured values to within +10/−6 An (molar [Ca/(Ca + Na)] × 100) of weighted average values and +13/−14 An of normative compositions. Analyses of spectra from two‐component plagioclase mixtures (varied by volume and composition) provide additional insight into potential effects of plagioclase zoning. Considering the variability in types of observed plagioclase zoning patterns in terrestrial volcanic rocks, normal zoning (calcic cores and sodic rims) cannot be assumed and thus cannot account for an apparent “sodic bias” seen in previous spectrally modeled compositions. This bias is better explained by higher abundances of sodic groundmass plagioclase relative to calcic phenocrysts. Deconvolution modeling of laboratory spectra of physical mixtures (convolved to various instrument resolutions) predicts that TES, THEMIS, and Mini‐TES spectra may be used to extract plagioclase compositions to within +9/−12 An, +11/−12 An, and +8/12 An (respectively) of measured values in coarse sand plagioclase mixtures, with 1σ standard deviations of ±4 An for laboratory, TES, and Mini‐TES resolutions and ±6 An for THEMIS resolution.

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

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

Single grain (U–Th)/He ages from phosphates in Acapulco meteorite and implications for thermal history

The cooling history of the Acapulco meteorite for >400°C is well established using various chronometers suggesting extremely fast cooling (>1000°C/Ma). In contrast, the thermal history for low temperatures (<400°C) is poorly understood because of large uncertainties in the chronometers applicable to this temperature range. To better constrain the cooling history for the low-temperature range, we applied (U–Th)/He dating techniques to individual phosphate grains. One whitlockite and 11 apatite grains yielded (U–Th)/He ages ranging from 1272±22 (1σ, analytical error only) Ma to 4584±51 Ma, with tight clustering at ∼4.55 Ga. The weighted mean of the five oldest ages (4538±32 Ma, 1σ uncertainty including systematic error) is suggested to be the minimum age representing primary cooling of the Acapulco body passing through ∼120°C. Although it is impossible to precisely quantify the effects of energetic α particle ejection from the outermost ∼20 μm of the phosphates, petrographic evidence suggests that most dated samples are fragments likely derived from the interior of larger grains, thus greatly reducing this source of error. Indeed the five oldest samples cannot have suffered substantial ejection since the uncorrected ages are identical with the crystallization age of the Acapulco meteorite. The new (U–Th)/He data suggest rapid cooling of Acapulco down to ∼120°C. This evidence suggests that the younger 40Ar/ 39Ar age (4507±9 (1σ) Ma) obtained from Acapulco plagioclase, which should reflect cooling through ∼300°C, is spuriously young due to systematic errors (i.e., decay constants and/or standard data) in the 40Ar/ 39Ar method, as suggested by comparison between high-precision 40Ar/ 39Ar and U/Pb ages for terrestrial volcanic rocks. The scattered He age distribution <4.0 Ga implies very heterogeneous thermal disturbances after the primary cooling of the body.

Analysis of terrestrial and Martian volcanic compositions using thermal emission spectroscopy: 2. Application to Martian surface spectra from the Mars Global Surveyor Thermal Emission Spectrometer

Atmospherically corrected thermal infrared spectra of large regions of the Martian surface from the Mars Global Surveyor Thermal Emission Spectrometer (MGS TES) previously have been interpreted to represent two general spectral classes. One class represents a basalt to basaltic andesite composition, and the other class represents a basaltic andesite to andesite composition. We have performed new linear deconvolutions of the two Martian surface type spectra with an end‐member set tailored to represent volcanic rock types. Our preparatory study of laboratory spectra of terrestrial volcanic rocks (acquired at 2 cm−1 sampling), convolved to TES spectral sampling (10 cm−1), shows little degradation in deconvolution results when compared to results acquired using the higher spectral resolution data, indicating that the deconvolution technique is valid for analyzing data at TES resolution. Our spectral fits to the Martian data agree well with previous models and do not exhibit any notable deviations from the Martian spectra that would indicate the absence of any significant end‐members in our model. Modal mineralogies obtained with these new spectral fits also compare favorably (within the previously stated uncertainties) to prior results. The newly derived modal mineralogies are used with new and traditional classification schemes for volcanic igneous rocks (introduced in a companion paper [Wyatt et al., this issue]) to classify the Martian compositions. Our results substantiate the previously proposed hypothesis that these two spectral classes on the Martian surface represent volcanic compositions with distinguishable differences in silica content ranging from basalt to andesite.

Analysis of terrestrial and Martian volcanic compositions using thermal emission spectroscopy: 1. Determination of mineralogy, chemistry, and classification strategies

We have examined and applied existing classification schemes for volcanic rocks and developed new schemes using thermal emission spectra of terrestrial volcanic rocks. Laboratory thermal infrared spectra (5–25 μm, at 2 cm−1 spectral sampling), deconvolved modal mineralogies, and derived mineral and bulk rock chemistries were used to distinguish basalt, basaltic andesite, andesite, and dacite. Modal mineralogies derived from linear deconvolution of terrestrial volcanic rocks were compared to modes measured by an electron microprobe phase mapping technique to determine the accuracy of linear deconvolution in modeling specific mineral abundances. One σ standard deviations of the absolute differences between modeled and measured mineral abundances range from 2.4 to 12.2 vol %, with an average standard deviation of 4.8 vol % being in agreement with average uncertainties calculated in previous studies. Weighted average compositions of feldspars in the deconvolution generally overlap the measured ranges of plagioclase compositions and the presence of low‐calcium and high‐calcium pyroxenes was correctly identified. Bulk chemistries of volcanic rocks were derived with a relatively high degree of accuracy (1σ standard deviations ranging from 0.4 to 2.6 vol %) by combining the compositions of spectrally modeled phases in proportion to their relative abundances in a particular sample. These data were collectively used to examine existing and develop new volcanic rock classification schemes. However, no single classification scheme was effective in accurately classifying all samples. Multiple steps of classification were required to distinguish volcanic rocks, reflecting the mineralogic diversity and continuum of compositions that exists in volcanic rock types. In a companion paper [Hamilton et al., this issue] these schemes are applied to the classification of Martian surface compositions.

Vestiges of life in the oldest Greenland rocks? A review of early Archean geology in the Godthåbsfjord region, and reappraisal of field evidence for >3850 Ma life on Akilia

The Godthåbsfjord region of West Greenland contains the most extensive, best exposed and most intensely studied early Archean rocks on Earth. A geological record has been described of numerous magmatic events between ∼3.9 and 3.6 Ga, and evidence of life at >3.85 Ga and ∼3.8–3.7 Ga has been proposed from two widely-separated localities. Some of these claims have recently been questioned, and the nature of the best preserved remnants of the oldest known terrestrial volcanic and sedimentary rocks in the Isua greenstone belt are being reinvestigated and substantially reinterpreted. The first part of this article reviews the evolution of geological research and interpretations, outlining the techniques by which the geological history has been determined and the ensuing controversies. The second part re-examines crucial field evidence upon which the antiquity of the oldest terrestrial life is claimed from the island of Akilia.

Chemical, multispectral, and textural constraints on the composition and origin of rocks at the Mars Pathfinder landing site

Rocks at the Mars Pathfinder site are probably locally derived. Textures on rock surfaces may indicate volcanic, sedimentary, or impact‐generated rocks, but aeolian abration and dust coatings prevent unambiguous interpretation. Multispectral imaging has resolved four spectral classes of rocks: gray and red, which occur on different surfaces of the same rocks; pink, which is probably soil crusts; and maroon, which occurs as large boulders, mostly in the far field. Rocks are assigned to two spectral trends based on the position of peak reflectance: the primary spectral trend contains gray, red, and pink rocks; maroon rocks constitute the secondary spectral trend. The spatial pattern of spectral variations observed is oriented along the prevailing wind direction. The primary spectral trend arises from thin ferric coatings of aeolian dust on darker rocks. The secondary spectral trend is apparently due to coating by a different mineral, probably maghemite or ferrihydrite. A chronology based on rock spectra suggests that rounded maroon boulders constitute the oldest petrologic unit (a flood deposit), succeeded by smaller cobbles possibly deposited by impact, and followed by aeolian erosion and deposition. Nearly linear chemical trends in alpha proton X‐ray spectrometer rock compositions are interpreted as mixing lines between rock and adhering dust, a conclusion supported by a correlation between sulfur abundance and red/blue spectral ratio. Extrapolations of regression lines to zero sulfur give the composition of a presumed igneous rock. The chemistry and normative mineralogy of the sulfur‐free rock resemble common terrestrial volcanic rocks, and its classification corresponds to andesite. Igneous rocks of this composition may occur with clastic sedimentary rocks or impact melts and breccias. However, the spectral mottling expected on conglomerates or breccias is not observed in any APXS‐analyzed rocks. Interpretation of the rocks as andesites is complicated by absence of a “1 μm” pyroxene absorption band. Plausible explanations include impact glass, band masking by magnetite, or presence of calcium‐ and iron‐rich pyroxenes and olivine which push the absorption band minimum past the imager's spectral range. The inferred andesitic composition is most similar to terrestrial anorogenic icelandites, formed by fractionation of tholeiitic basaltic magmas. Early melting of a relatively primitive Martian mantle could produce an appropriate parent magma, supporting the ancient age of Pathfinder rocks inferred from their incorporation in Hesperian flood deposits. Although rocks of andesitic composition at the Pathfinder site may represent samples of ancient Martian crust, inferences drawn about a necessary role for water or plate tectonics in their petrogenesis are probably unwarranted.

Giant Impact and Fission Hypotheses for the Origin of the Moon: A Critical Review of Some Geochemical Evidence

Geochemical data for Ni, Co, Cr, V, and Mn have played an important role in theories for the Moon's origin. It has been argued that the data for these elements strongly support formation of the Moon as ejecta from the Earth, either as a result of one giant or numerous smaller impacts on the proto-Earth. These theories have come to be known as the “Giant Impact” and “Impact-triggered Fission” hypotheses, respectively, and the first of these has been the leading explanation for the origin of the Moon over the past decade. Data for these same “diagnostic” elements also have been used to argue for significant distinctions between the bulk compositions of the Moon and a eucrite (HED) parent body, which otherwise appear to be remarkably similar in their compositions. We review geochemical evidence pertaining to the origin of the Moon, focusing on the diagnostic elements, and find that there is no strong geochemical support for either the Giant Impact or Impact-triggered Fission hypotheses. We show that basalts produced in the Moon and a HED parent body (mare basalts and eucrites, respectively) were derived by the melting of source regions with similar compositions. Mare and eucrite basalts differ in Ni and Co abundances but lie on the same igneous fractionation trend. Chromium, Mg# (Mg/[Fe+Mg]), and V abundance systematics suggest a close similarity between mare and eucrite basalts, and a significant difference from terrestrial volcanic rocks, which are depleted in Cr. Mare and eucrite basalts differ in their Mn abundances and Fe/Mn ratios, but the same can be said for mare and terrestrial basalts. On the whole, the Moon appears to be more chemically similar to the HED parent body than to the Earth. This suggests that either: (1) the HED parent body (probably the asteroid Vesta) formed by an impact mechanism and is an escaped satellite, or (2) the Moon is a captured body that formed independently of Earth. Similar conclusions were reached long ago by Anders and colleagues (e.g., Anders, 1977), long before the Giant Impact Hypothesis attained popularity.

Helium isotope composition of the early Iceland mantle plume inferred from the Tertiary picrites of West Greenland

Picrites from the 61 million year old Vaigat Formation of the Nuussuaq Peninsula in West Greenland have 3He/ 4He ratios trapped in olivine phenocrysts which range up to 30 times the atmospheric ratio. These high values, measured during gas extraction by crushing in vacuum, are similar to the highest magmatic 3He/ 4He ratios found in young terrestrial volcanic rocks. By analogy with young basalts, in which crushing selectively extracts magmatic helium, any significant cosmogenic 3He appears to be absent in these picrites. Additional evidence for the absence of cosmogenic helium is provided by fusion results on the crushed olivine powders and by a single stepwise crushing experiment, in which only magmatic and radiogenic helium components are resolvable. The West Greenland picrites have Pb, Nd and Sr isotope compositions which overlap those found in picrites from Iceland and in basalts from Loihi Seamount, localities which today also have high 3He/ 4He ratios. Isotopic variations in He, Pb, Nd and Sr for the West Greenland picrites are interpreted to largely result from interaction of the early Iceland mantle plume with the upper mantle during plume ascent and dispersion beneath the continental lithosphere. The presence of high 3He/ 4He ratios in West Greenland, and the onset of magmatism across the North Atlantic Volcanic Province near 62 Ma, supports the hypothesis for very rapid dispersion (>1 m/year) of mantle plume head material during the earliest stages of plume impact, as predicted in recent numerical simulations of plume behavior during thermal mantle convection with non-Newtonian rheology.

Al-spinels in primitive arc volcanics

Al-rich spinels (100Cr/(Cr + Al) 50 wt%) are common in alpine peridotites, both terrestrial and lunar mafic and ultramafic cumulates, and in certain metamorphic rocks, but they are apparently rare in terrestrial volcanic rocks. Here we describe the occurrence of Al-rich spinel inclusions in olivine phenocrysts in island arc volcanic rocks from five new localities: Bukit Mapas (Sumatra) and eastern Bali in the Sunda arc, and Epi, Merelava, and Ambrym islands in the Vanuatu arc. More commonly, relatively Cr-rich spinels also occur as inclusions in the saine olivine phenocrysts, and it appears that the Cr-poor aluminous spinels must be in disequilibrium with the host basaltic melts. In the rocks studied, Al-rich spinels also coexist with trapped silicate glasses and highly aluminous clinopyroxene in melt inclusions in olivine. This paragenesis suggests an origin involving contamination by localized Al-rich melt pockets as opposed to a xenocrystic origin. Two mechanisms to produce this high-Al melt in basaltic magma chambers are suggested: (1) localized high-Al melt production by complete breakdown of assimilated lower crustal gabbroic rocks. In this model the high-Al melt may crystallise Al-rich spinels which are subsequently trapped as solid inclusions by phenocryst phases of the host basaltic melt or may be trapped as melt inclusions in which Al-rich spinels and Al-rich clinopyroxene crystallise as daughter phases, and (2) in congruent breakdown of amphibole in amphibole-rich cumulates in sub-arc, or sub-OIB volcano magma chambers. The latter reaction produces a melt with ∼ 20–22% of Al2O3, aluminous clinopyroxene, Al-rich spinel and olivine. Mixing between these amphibole breakdown products and host basaltic melt may occur throughout the evolution of a magmatic system, but particularly during recharge with hot magnesian basalt batches. Aluminous spinels and aluminous clinopyroxene produced during amphibole breakdown, or perhaps crystallised from aluminous melt produced in the saine reaction, are incorporated into the magma during recharge, and subsequently trapped, together with the coexisting Cr-spinels, by crystallising olivine and clinopyroxene.

Sedimentology, stratigraphy, and tectonic implications of a paleo-Proterozoic continental extensional basin: the El Sherana – Edith River groups, Northern Territory, Australia

The ca. 1.83 Ga El Sherana – Edith River basin of Northern Territory, Australia, contains terrestrial sedimentary and volcanic rocks deposited during continental extension or transtension. Braid-plain sandstones and conglomerates, turbiditic sediments, and interbedded mafic and felsic volcanics, including ignimbrites, filled the basin. Alluvial fans and rock avalanche breccias are locally developed. Prebasinal structure controlled antecedent topography and influenced drainage patterns, thickness changes, and facies distribution.Unconformities bound all formations of the El Sherana and Edith River groups, revealed by beveled and incised strata, reversals in paleocurrent trends, sharp discordance in juxtaposed facies, and paleovalleys filled with unique sediments. Scarp-derived sediments are preserved only in a small, transfer-related strike-slip basin within the larger basin environment. Unconformities are regional and are interpreted as time lines throughout the basin. These unconformities developed during regrading of slopes forced by active tectonism, accompanied by syntectonic sedimentation and basin depocenter migration.The El Sherana – Edith River strata overlie older sediments (~2.1–1.88 Ga) of the Pine Creek orogen, which formed during a period of global orogeny related to continental assembly. Development of the El Sherana – Edith River basin began substantially (40–50 Ma) after denudation of the Pine Creek orogen belt, and is related to regional postcollisional extension. This extension is recorded by basin formation, bimodal tholeiitic and alkalic magmatism, and elevated geotherms. Other paleo-Proterozoic basins of northern Australia show a similar history of rift deposition above a fossil compressional belt.

The Dog Bay Line: a major Silurian tectonic boundary in northeast Newfoundland

The Dog Bay Line separates different Silurian rock groups in northeast Newfoundland. West of the line, terrestrial volcanic rocks and sandstones (Botwood Group) overlie marine greywackes and conglomerates (Badger Group). East of the line, red sandstones overlie shallow marine shales and limestones (Indian Islands Group). Throughout Dog Bay, the line is marked by a disrupted zone of dark grey to black shales, volcanic rocks, and gabbros. Pervasive dextral, transpressive ductile deformation followed by successively more brittle extension with renewed dextral movements mark the northwest side of the line on the coast.The Dog Bay Line is traceable for 100 km and it is open-ended. Dextral offset is deduced to be many tens of kilometres. The line trends northeast, parallel to outcrop belts, and both the line and outcrop belts are curved eastward at the coast. The Mount Peyton Batholith, dated at 420 ± 8 Ma, apparently cuts the line.The Dog Bay Line occurs within the Dunnage Zone whose Cambrian–Ordovician rocks represent vestiges of the Iapetus Ocean. Northwest of the line, the Silurian rocks were deposited on Ordovician rocks already accreted to Laurentia. Southeast of the line, the Silurian rocks were deposited on Ordovician rocks already amalgamated with the continental Gander Zone. Timing of major movement and a Silurian marine to terrestrial depositional change recorded on both sides of the line agree within error with isotopic ages for the onset of plutonism, regional deformation, and metamorphism in central Newfoundland. The Dog Bay Line may mark the terminal Iapetus Ocean.

The development of Early Permian sequences and hydrothermal alteration in the Myall Syncline, central eastern New South Wales

The Myall Syncline, within the southern Tamworth Belt of the New England Fold Belt, contains a sequence of concentric meridionally folded Carboniferous to Early Permian shallow marine to fluvial‐lacustrine sedimentary and bimodal terrestrial volcanic rocks. The sequence spans a cycle of marine regression and transgression. The Late Carboniferous Koolanock Sandstone is overlain by the Muirs Creek Conglomerate, a possibly diachronous unit, which is succeeded apparently conformably by the Early Permian Markwell Coal Measures. Deposition of the coal measures took place during the intervals between extrusion of the members of the Alum Mountain Volcanics: the basal rhyolitic to dacitic predominantly ash flow Sams Road Rhyolite Member (new name); the Burdekins Gap Basalt Member; and the flow dominated Lakes Road Rhyolite Member. The Markwell Coal Measures had previously been interpreted as overlying the Alum Mountain Volcanics. The Late Permian marine Bulahdelah Formation disconformably overlies the Alum Mountai...