A 3.5 Ga record of water-limited, acidic weathering conditions on Mars

The secondary mineral budget on Earth is dominated by clay minerals, Al-hydroxides, and Fe-oxides, which are formed under the moderate pH, high water-to-rock ratio conditions typical of Earth's near-surface environment. In contrast, geochemical analyses of rocks and soils from landed missions to Mars indicate that secondary mineralogy is dominated by Mg (± Fe, Ca)-sulfates and Fe-oxides. This discrepancy can be explained as resulting from differences in the chemical weathering environment of Earth and Mars. We suggest that chemical weathering processes on Mars are dominated by: (1) a low-pH, sulfuric acid-rich environment in which the stoichiometric dissolution of labile mineral phases such as olivine and apatite (± Fe–Ti oxides) is promoted; and (2) relatively low water-to-rock ratio, such that other silicate phases with slower dissolution rates (e.g., plagioclase, pyroxene) do not contribute substantially to the secondary mineral budget at the Martian surface. Under these conditions, Al-mobilization is limited, and the formation of significant Al-bearing secondary phases (e.g., clays, Al-hydroxides, Al-sulfates) is inhibited. The antiquity of rock samples analyzed in-situ on Mars suggest that water-limited acidic weathering conditions have more than likely been the defining characteristic of the Martian aqueous environment for billions of years.     

Spectral and petrologic analyses of basaltic sands in Ka'u Desert (Hawaii) – implications for the dark dunes on Mars

Dark aeolian deposits on Mars are thought to consist of volcanic materials due to their mineral assemblages, which are common to basalts. However, the sediment source is still debated. Basaltic dunes on Earth are promising analogs for providing further insights into the assumed basaltic sand dunes on Mars. In our study we characterize basaltic dunes from the Ka'u Desert in Hawaii using optical microscopes, electron microprobe, and spectral analyses. We compare the spectra of terrestrial and Martian dune sands to determine possible origins of the Martian dark sediments. Our results show that the terrestrial sands consist primarily of medium to coarse sand‐sized volcanic glass and rock fragments as well as olivine, pyroxene, and plagioclase minerals. Grain shapes range from angular to subrounded. The sample composition indicates that the material was derived from phreatomagmatic eruptions partially with additional proportions of rock fragments from local lava flows. Grain shape and size indicate the materials were transported by aeolian processes rather than by fluvial processes. Spectral analyses reveal an initial hydration of all terrestrial samples. A spectral mineralogical correlation between the terrestrial and Martian aeolian sands shows a similarity consistent with an origin from volcanic ash and lava. We suggest that the Martian deposits may contain similar abundances of volcanic glass, which has not yet been distinguished in Martian spectral data. Copyright © 2011 John Wiley & Sons, Ltd.     

Investigating the ancient Martian magnetic field using microwaves

The new microwave palaeointensity technique has been used to investigate samples from the Martian meteorite Nakhla. This technique is a promising new way to obtain absolute palaeointensity information regarding the ancient Martian magnetic field as recorded by the Martian meteorites. Assuming that a part of the magnetic remanence is of thermal origin and originating on Mars the two samples studied yield estimates of 4 μT for the Martian magnetic field at 1.35 Ga.     

Comment on “the troodos ophiolitic complex was probably formed in an island arc”, by A. Miyashiro

The use of bulk chemical analyses from the Troodos complex to determine the chemical affinity of the ophiolites is unjustified since these rocks exhibit conspicuous alteration in thin section. Very similar rocks on mainland Greece can be shown, on mineralogical grounds, to have been mainly tholeiitic originally, with a subordinate alkaline trend. This is compatible with their origin at an oceanic ridge. Regional geological evidence provides strong support for this origin. However, bulk chemical analyses from them could be used, like those from Troodos, to argue against a mid-ocean ridge origin. This suggests that the use of bulk chemical analyses from these rocks is very unreliable. In the presence of other, more reliable data, inferences from bulk chemistry can be rejected. The Troodos ophiolites are very similar, in both petrology and geological position, to those in Greece and probably originated in the same way.     

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.     

Aqueous alteration in the Northwest Africa 817 (NWA 817) Martian meteorite

Samples of a new Martian meteorite of the nakhlite family (NWA 817) contain traces of an iron-rich alteration product. Textural arguments indicate that this alteration product has been formed on the parent body of the meteorite (Mars). The chemical composition and structural data (X-ray diffraction, transmission electron microscopy and vibrational spectroscopy) show that the alteration mineral is a hydrous phase from the smectite family. Major elements and rare earth elements suggest that the formation of the alteration phase is related to the circulation of an aqueous fluid which composition is controlled by the dissolution of feldspars to account for a positive Eu anomaly, olivine and possibly apatite. Hydrogen isotope data display negative δD values ranging from −60 to −280‰ in olivine and pyroxenes and from −140 to −181‰ in the alteration phase. The values of δD for the alteration product show a small scatter with a mean value of −170±14‰. These values are lower than those previously obtained on other Martian meteorites, which give mainly positive δD values. These positive values have been interpreted as resulting from the interaction of the Martian meteorites with water from the Martian atmosphere. Ruling out the effect of terrestrial alteration, it is suggested that alteration in the NWA 817 meteorite was likely produced on Mars by the circulation of an aqueous fluid originating from a chemical reservoir, such as the Martian mantle, which has not equilibrated with a fractionated Martian atmosphere.     

Interlaboratory comparison of some chemical analyses of Hawaiian Volcanic Rocks

The comparability of chemical analyses of Hawaiian volcanic rocks by different laboratories is investigated. The comparability is reasonably good for most purposes, but small differences in compositions of rocks determined by different laboratories should not be used as a basis of classification or to support petrogenic deductions.     

Bacteria in the Tatahouine meteorite: nanometric-scale life in rocks.

We present a study of the textural signature of terrestrial weathering and related biological activity in the Tatahouine meteorite. Scanning and transmission electron microscopy images obtained on the weathered samples of the Tatahouine meteorite and surrounding soil show two types of bacteria-like forms lying on mineral surfaces: (1) rod-shaped forms (RSF) about 70-80 nm wide and ranging from 100 nm to 600 nm in length; (2) ovoid forms (OVF) with diameters between 70 and 300 nm. They look like single cells surrounded by a cell wall. Only Na, K, C, O and N with traces of P and S are observed in the bulk of these objects. The chemical analyses and electron diffraction patterns confirm that the RSF and OVF cannot be magnetite or other iron oxides, iron hydroxides, silicates or carbonates. The sizes of the RSF and OVF are below those commonly observed for bacteria but are very similar to some bacteria-like forms described in the Martian meteorite ALH84001. All the previous observations strongly suggest that they are bacteria or their remnants. This conclusion is further supported by microbiological experiments in which pleomorphic bacteria with morphology similar to the OVF and RSF objects are obtained from biological culture of the soil surrounding the meteorite pieces. The present results show that bacteriomorphs of diameter less than 100 nm may in fact represent real bacteria or their remnants.     

Chemical divides and evaporite assemblages on Mars

Assemblages of evaporite minerals record detailed physical and chemical characteristics of ancient surficial environments. Accordingly, newly discovered regions of saline minerals on Mars are high priority targets for exploration. The chemical divide concept of evaporite mineral formation is used successfully to predict evaporite mineralogy and brine evolution on Earth. However, basaltic weathering largely controls fluid compositions on Mars and the robust predictive capabilities of terrestrial chemical divides cannot be used to interpret Martian evaporites. Here we present a new chemical divide system that predicts evaporite assemblages identified in SNC-type meteorites, ancient evaporites discovered on Meridiani Planum by the Opportunity rover, and Mars Express OMEGA data. We suggest that a common fluid type that has been buffered to different pH levels by basaltic weathering controls the variability among Martian evaporite assemblages and that evaporite mineralogy and brine evolution is essentially established by the initial composition of the dilute evaporating fluid.     

RbSr isotopic characteristics and chemistry of the 3.6-b.y. Hebron gneiss,Labrador

RbSr isotopic analyses of the intensely deformed Hebron gneiss, Labrador, yield an isochron of approximately 3.6 b.y. with an initial⁸⁷Sr/⁸⁶Sr ratio of 0.7044, and chemical analyses show these rocks to be granodioritic in composition. It is believed that the isochron reflects a metamorphic event and that the Hebron gneiss was either derived from a compositionally anomalous zone in the mantle or from previously existing sialic crustal material. The Hebron gneiss is compositionally similar to some of the oldest rocks in the Archean cratons of Labrador, West Greenland, Rhodesia, South Africa and Minnesota (U.S.A.).     

Magnetic parameters and variations in the composition of igneous rocks of the Franz Josef Land archipelago

A combined study of magnetic parameters of basalt and andesite samples is performed in the framework of geological investigations of the Franz Josef Land at the paleomagnetic laboratory of Munich University. The study included the determination of the coercivity, saturation magnetization, Curie points, natural remanent magnetization (NRM), and magnetic susceptibility and the examination of ferromagnetic minerals with a microscope. Data on the chemical composition of rocks are obtained for all samples, and radiological ages are determined for the majority of rocks.Thermomagnetic curves of samples are subdivided into four types depending on the composition of ferromagnetic NRM carriers.The data obtained point to multiple changes in the predominant composition of igneous rocks. Each stage of magmatism is characterized by a specific type of the ferromagnetic component in the rocks and, therefore, magnetomineralogical investigations can be used for differentiation and correlation of the igneous rocks.     

Some data on the comendite type area of S. Pietro and S. Antioco islands,Sardinia

A summary of the available data on the peralkaline rocks of S. Pietro and S. Antioco islands, together with, new chemical analyses and some preliminary K-Ar ages are reported. Peralkaline rocks occur as ignimbrites, lava flows and domes usually deeply affected by hydrothermal alteration. Pantelleritic varieties are found within the dominantly comenditic association, which display K₂O contents higher than Na₂O ones. K-Ar data indicate that these peralkaline rocks have a middle Miocene age (? 15 m.y.). They occur in close field association with coheval andesitic and subalkaline acid volcanics belonging to the final products of the Tertiary calc-alkaline volcanic cicle of Sardinia.     

Transitional basalts of alkaline or tholeiitic affinity in the somali trap series (southeastern margin of the main Ethiopian rift from 8° 10′ to 8° 70′ Lat. N)

The Trap Series outcropping in the southeastern margin of the Main Ethiopian Rift has a total thickness of not less than 500–600 m. In this series, mainly composed of flood lavas and rare scoriaceous beds, basaltic rocks — which on a serial viewpoint are transitional basalts — are the best represented. Alkali-olivine basalts (with modal and normative nepheline) are interbedded within the lower and middle members of the series, while in the upper members andesine basalts and dark phonolitic tephrites are present. Among the transitional basalts, rocks of alkaline and of tholeiitic affinity are present as shown by petrographic and chemical analyses. On the basis of petrochemical and geovolcanological data, it is probable that these rocks and the phonolitic-tephrites originated from different primary magmas.     

Magma characterization of the Mexican volcanic belt (Mexico)

Over 330 bulk chemical analyses of rocks (Oligocene or Miocene to Recent) from the Mexican Volcanic Belt (MVB) have been compiled. These analyses all together represent a calc-alkaline sequence of continental margin type. The North-Eastern part of the Belt seems to overlap with another volcanic province, namely. Eastern Cordillera in which magmas are mostly of alkaline sequence. A model involving fractional crystallization as the principal mechanism is proposed for magma genesis in the MVB.     

Carbonatite-alkalic complex of Panwad-Kawant,Gujarat, and its bearing on the structural characteristics of the area

Carbonatite-alkalic rocks occur in the form of dykes and small volcanic plugs in the area, with major central type volcanic activity restricted to Amba Dongar. The trappean flows of Blanford and Bose are identified as plagioclasecalcite rocks. An attempt is made to explain the origin of these rocks which are extensively cut by dykes of alkaline rocks, carbonatites and dolerites. By far the dominant lavas are «fissure phonolites» (Wright, 1963) and tinguaites. The chemical analyses of these rocks show that the magma is mainly of continental sodic alkaline suite, probably turning sodi-potassic, a suggestion drawn from the occurrence of lamprophyres and pseudoleucite tinguaites, and the higher potassium contents of some rocks. Bagh sediments are mainly represented by sandstones which show mild contact effects with carbonatite, especially in the south.     

Geochemistry of mafic cainozoic volcanic rocks from Sardinia (western mediterranean)

The Cainozoic volcanism of Sardinia (Italy) can be divided into two main cycles with different magmatic and geodynamic significance. The early cycle — Oligo-Miocenic in age (29-13 My ago) — shows the calc-alkaline character typical of converging plate areas. The later activity, ranging from Lower Pliocene (about 5 My ago) to recent Pleistocene, produced mostly basic lavas extruded onto a continental plate («within plate basalts»). It was related to a period of tensional tectonics which had affected the western Mediterranean area prior to, and during, the volcanic activity. Intermediate and acid volcanic products were associated with the mafic rocks of the latest magmatic episodes. The main groups of rocks — the basic ones, already classified from their petrographic features and geological setting — can be characterized very well when a statistical elaboration of their chemical analyses is used. In fact, from chemical data it is possible to distinguish the Oligo-Miocene volcanic products from those of Plio-Pleistocene age. Moreover, within this latter group basanites, alkalic and subalkalic basaltic rocks can clearly be distinguished. Samples that had not clearly been defined on the basis of their petrographic characteristics (anonymous samples) have been attributed to one or the other of the main groups by means of discriminant functions. Chemical variations in the Plio-Pleistocenic rocks are due to fractionation episodes at shallow depths superimposed on primary magmatic variations. A model of partial fusion of the mantle accounts for many but not all the observed original chemical variations. Different physical melting conditions, the effects of minor mineral phases in the mantle and, probably, crustal contamination were also effective in creating the observed chemical variations.     

Origin of the troodos and other ophiolites: A reply to moores

Advocates of the hypothesis of mid-oceanic ridge origin for ophiolites endeavored to show similarities between ophiolites and mid-oceanic ridges as evidence for their hypothesis. Some of the similarities claimed to exist appear to be true. However, this does not prove their hypothesis, because analogous similarities exist between ophiolites and some island arcs as well. Such advocates claim that rocks of the Troodos ophiolitic complex were subjected to such intense metasomatic changes that my hypothesis of island-arc origin for Troodos based on the bulk chemical analyses of rocks is not justified. However, a detailed examination of compositional variation in Troodos volcanic rocks has revealed that, though some components (e.g., K) were strongly mobile during secondary processes, other components were not, and the presently observed compositional variation resulted mainly from crystallization differentiation. Their assumption of large-scale silica metasomatism for Troodos cannot explain the compositional features of rocks and is contradictory to the hypothesis of mid-oceanic ridge origin, because such metasomatism does not appear to take place in mid-oceanic ridges.     

西藏金沙江缝合带一些岩石的初步研究(Ⅱ):基性、超基性岩

对采自藏北无人区西藏金沙江缝合带的基性、超基性岩样品进行了岩石学、岩石化学、稀土和微量元素、同位素等分析测试 ,以及主要矿物成分的电子探针分析。研究结果表明 ,这些样品为辉长岩和辉石岩 ,属于钙碱性至碱性系列 ,其矿物组成没有斜方辉石 ,未见变形显微构造特征。其化学成分以高Al2 O3,FeO ,Na2 O +K2 O和低SiO2 ,MgO含量为特征。稀土元素配分型式以斜率较大的微弱负铕异常的右倾曲线为特征 ,LREE富集。它们具有较高的Sr丰度和87Sr/ 86 Sr比值 (>0 70 6 9)以及高的2 0 7Pb/ 2 0 4 Pb ,2 0 8Pb/ 2 0 4 Pb和2 0 6 Pb/ 2 0 4 Pb比值。上述特征有别于藏北蛇绿岩的基性、超基性岩 ,表明它们不是蛇绿岩套组合 ,其形成可能与金沙江缝合带的弧后扩张有关     

火星磁场和行星发电机理论

火星磁场是火星主要观测物理场之一,火星磁场研究对火星探索具有重要的科学意义.本文侧重介绍火星磁场探测所取得的主要成果,在介绍火星行星磁场起源的行星发电机理论的基础上,重点讨论了动力学机制、起始时间、停止的原因等关键性问题,并指出了研究中存在的一些问题.     

High magnetic susceptibility produced by thermal decomposition of core samples from the Chelungpu fault in Taiwan

We carried out thermomagnetic susceptibility analyses of fault rocks from core samples from Hole B of the Taiwan Chelungpu Fault Drilling Project (TCDP) to investigate the cause of high magnetic susceptibilities in the fault core. Test samples were thermally and mechanically treated by heating to different maximum temperatures of up to 900 °C and by high-velocity frictional tests before magnetic analyses. Thermomagnetic susceptibility analyses of natural fault rocks revealed that magnetization increased at maximum heating temperatures above 400 °C in the heating cycle, and showed three step increases, at 600 to 550 °C and at 300 °C during the cooling cycle. These behaviors are consistent with the presence of pyrite, siderite and chlorite, suggesting that TCDP gouge originally included these minerals, which contributed to the generation the magnetic susceptibility by thermomechanical reactions. The change in magnetic susceptibility due to heating of siderite was 20 times that obtained by heating pyrite and chlorite, so that only a small fraction of siderite decomposition is enough to cause the slight increase of the susceptibility observed in the fault core. Color measurement results indicate that thermal decomposition by frictional heating took place under low-oxygen conditions at depth, which prevented the minerals from oxidizing to reddish hematite. This finding supports the inference that a mechanically driven chemical reaction partly accounts for the high magnetic susceptibility. A kinetic model analysis confirmed that frictional heating can cause thermal decomposition of siderite and pyrite. Our results show that decomposition of pyrite to pyrrhotite, siderite and, to some extent, chlorite to magnetite is the probable mechanism explaining the magnetic anomaly within the Chelungpu fault zone.