Blanca Bauluz

Geochemistry of Precambrian and Paleozoic siliciclastic rocks from the Iberian Range (NE Spain): implications for source-area weathering, sorting, provenance, and tectonic setting.

Abstract The major and trace element chemical composition of Precambrian and Paleozoic shales and sandstones from the Iberian Range (Spain) has been investigated to determine the provenance and tectonic setting of these rocks, as well as to appraise the influence of the weathering, hydraulic sorting and recycling processes upon source rock signature. The samples studied belong to a prograde sequence from diagenesis to anchizonal grade, and are mainly composed of quartz, illite/mica phases and/or kaolinite, with chlorite, feldspars, and carbonates as minor components. The major element distribution (Chemical Index of Alteration (CIA) indices) reflects that recycling processes have been important in homogenizing the composition of shales and sandstones. The higher CIA indices observed in the Silurian and Devonian shales (80 and 85, respectively) compared with those of Precambrian to Ordovician and the Carboniferous age (about 70) indicate that their source area underwent more intense chemical weathering processes, possibly due to climatic and/or tectonic variations. Alternatively, the source area of the Silurian and Devonian shales may have been composed of recycled sedimentary materials. Mineral fractionation is mainly observed in coarser rocks through zircon, apatite and xenotime accumulation, although the shales are not free of these phases. The zircon content of the fine-grained rocks is not high enough to affect rare earth element (REE) contents, but phosphate minerals (apatite and xenotime) at least partially control the REE distribution. The influence of apatite and xenotime are slightly higher in the Precambrian shales as the phosphate concentration produces a decrease in the La/Sm ratios. Chondrite-normalized REE patterns and negative Eu anomaly size of the studied rocks are similar to that of Post-Archean Australian shales (PAAS) indicating that they originally come from a differentiated silicic source. The higher REE contents in the studied shales in relation to PAAS indicate that recycling processes in the Iberian Range sources were probably more intense than that of PAAS. The slight differences among the REE patterns of the different groups of shales probably do not reflect changes in source-area composition, but instead suggest variations in mineral sorting, chemical weathering and/or sediment recycling. The Th/Sc, Co/Th, Cr/Th, Cr/V, V/Ni ratios support a primitive silicic source for these rocks, with the higher Th/Sc and lower Co/Th in Cambrian and Ordovician shales indicating a higher proportion of felsic material in their primitive source area. The compositional maturity of analyzed sandstones is typical of cratonic environments and their La, Sc, Th and Zr contents reflect their passive continental margin setting.

Retrograde diagenesis, a widespread process on a regional scale

Abstract Pelitic and basic rocks occurring within prograde sequences in Portugal, Spain and Hungary have been studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The minerals formed in typical prograde reactions define the general sequences, but smectite, chlorite-smectite (corrensite) and/or berthierine were found to have replaced chlorite, whereas kaolinite and mixed-layer illite-smectite replaced illite-muscovite. Alteration occurred under conditions normally associated with diagenesis, subsequent to regional metamorphism, and we therefore refer to such processes with the term “retrograde diagenesis”. In the cases studied and in other cited examples, reactions occurred on a regional basis via pervasive fluids under open-system conditions inferred to be related to tectonic stress. The observed alterations could generally not be inferred from XRD data, although the presence of pure smectite in sediments other than bentonite is suggestive of retrograde relations, especially where other minerals are consistent with a higher grade of diagenesis. Retrograde diagenesis is readily observed through imaging of textures by TEM, however. Textural features show that retrograde reactions are more common than generally assumed, and that care should be used in interpreting geological events where appropriate textural relations are not seen.

Coexisting altered glass and Fe–Ni oxides at the Cretaceous–Tertiary boundary, Stevns Klint (Denmark): direct evidence of meteorite impact

Abstract The Cretaceous–Tertiary (K–T) boundary at Stevns Klint, Denmark, is noteworthy for its large Ir anomaly that is taken as evidence of extraterrestrial components, but the origin of the smectite in this marl has been variously interpreted to have a detrital, meteorite impact, or volcanic origin. We have carried out scanning electron microscopy and transmission electron microscopy (TEM)/analytical electron microscopy observations of the impact and contiguous layers within the K–T marl at Stevns Klint. TEM images show abundant smectite, much of which occurs with layers curving around and grading into cores of nanometer-scale glass shards. The smectite composition is unusual in having both significant octahedral Al and Mg. The glass and smectite major element compositions are similar and unique relative to glasses of terrestrial and extraterrestrial origin with the exception for one kind of glass at the K–T boundary in Haiti. Abundant 10–20-nm diameter iron oxides having as much as 10% Ni and minor Zn are intergrown with smectite. We interpret these domains to be altered meteorite fragments, which formed when impact glass was transformed to smectite. The direct association of unique glass and meteorite fragments is unambiguous evidence for meteorite impact. These data may imply fall-out of globally distributed impact-derived particles over an extended time period. The relations imply that TEM observations may be a powerful tool in detecting other impact events in the geological record.

TEM study of mineral transformations in fired carbonated clays: relevance to brick making

Abstract This study uses transmission electron microscopy (TEM) and analytical electron microscopy (AEM) supported by X-ray diffraction (XRD) and scanning electron microscopy (SEM) to investigate the mineralogical and textural changes produced in carbonated clays by firing. Sample bars were prepared using raw clays composed of quartz, illite and carbonates with minor amounts of smectite, chlorite, feldspars and Fe oxides. The raw samples were then fired at temperatures of between 800 and 1050°C. The XRD data show that increases in firing temperature result in dehydroxylation of clay minerals, carbonate decomposition and the formation of Ca-bearing silicates (e.g. gehlenite, wollastonite, pyroxenes and anorthite). The sizes of the Ca-silicate crystals make the use of the SEM inappropriate since they lie below the resolution threshold. However, TEM/AEM do provide the required textural and compositional characterization, revealing that there is a broad range of pyroxene compositions, some of which resemble fassaite, and that Ca/Mg ratios increase with temperature. The TEM also shows significant dehydroxylation and vitrification of the clay-rich matrix at T of ~800°C. Observed mineralogical and textural changes probably occurred in a system with a local disequilibrium much like small-scale, high-temperature metamorphic reactions (i.e. pyrometamorphism). The importance of these results is that they enable the selection of more appropriate raw clay composition and firing dynamics (temperature, firing duration and cooling rate) for both the brickmaking industry and brick conservation in the field of cultural heritage.

Transmission electron microscopy study of illitization in pelites from the Iberian range, Spain : Layer-by-layer replacement?

A sequence of interstratified illite-smectite (I-S) and illite in Paleozoic pelites and metapelites from the Iberian Range, Spain, was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The fine-grained matrix of diagenetic pelites is composed of I-S with sequences of illite- and smectite-like layers in a given sample. The Reichweite (R) values as determined by XRD and averaged over heterogenous I-S layer sequences increase with increasing grade, apparently continuously, in sharp contrast with TEM observations of other published sequences. Changes in I-S sequences along layers are rarely observed. In the higher-grade diagenetic pelites, I-S coexists with illite. Each I-S phase has a composition similar to that of illite, implying unique Al-Si distributions in contrast to smectite and muscovite. Selected area electron diffraction (SAED) patterns of I-S and illite are diagnostic of 1Md polytypism. Anchizonal metapelites consist of larger packets of well-crystallized muscovite, with SAED patterns corresponding to a two-layer polytype.The continuous sequence of changes studied by TEM in I-S sequences and lateral transitions among these units is consistent with illitization by layer-by-layer replacement, although other processes are possible also. Replacement of individual layers probably occurs via fluids at reaction interfaces, in contrast to solid-state reactions, sensu strictu. The transition from the diagenetic to anchizonal rocks (transition in textures and formation of muscovite-2M1) occurred via dissolution/crystallization, however, presumably by tectonic stress. XRD and TEM data imply a consistent prograde trend in the sequence, the XRD data denning the average, long-range Reichweite ordering sequence, whereas the TEM data define the short-range layer sequences.

TRANSMISSION ELECTRON MICROSCOPY STUDY OF SMECTITE ILLITIZATION DURING HYDROTHERMAL ALTERATION OF A RHYOLITIC HYALOCLASTITE FROM PONZA, ITALY

Dioctahedral phyllosilicates from an altered rhyolitic hyaloclastite located at Ponza Island, Italy, were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The samples are from a sequence previously characterized by X-ray diffraction (XRD) methods, indicating that a complete range of illitization accompanies alteration. Backscattered electron (BSE) images, obtained from ion-milled samples, show that samples partly retain the original textures since clay minerals pseudomorph lapilli fragments and preserve vesicular texture. The lowest-grade sample studied contains obsidian clasts partially replaced by smectite. As the alteration grade increases, illitization proceeds with formation of interstratified illite-smectite (I-S), zeolites, illitic phases, feldspars and quartz. The most altered sample contains illite, mica and quartz. Lattice-fringe images show that following the formation of smectite, illitization takes place through the formation of (R=1) I-S, highly illitic I-S and illite with mica; (R=1) I-S is the only ordered interstratified I-S. The BSE and TEM images of Ponza samples show irregular cavities filled with euhedral dioctahedral clay minerals and the zeolite mordenite, providing direct evidence for neocrystallization from a fluid. Chemical compositions by analytical electron microscopy (AEM) support the sequence described. Selected area electron diffraction (SAED) patterns indicate the predominance of 1 Md polytypism both in I-S and illitic phases, and the coexistence in the more altered samples of 1Md illite and a 2-layer mica polytype (probably 2M1), without the intermediate 1M polytype generally assumed to exist in prograde sequences. Previous XRD studies indicated progressive change from cis-vacant, turbostratically stacked smectite, to interstratified cis- and trans-vacant, 1Md I-S, to trans-vacant, 1M illite, and then to 2M1 illite in Ponza Island samples. We observed a clear correlation between the chemical compositions as determined by AEM and the proportion of cis-vacant determined by XRD, suggesting that the octahedral cation distributions change in the studied samples with increasing degree of illitization.

A re-evaluation of aff. Megaloolithidae eggshell fragments from the uppermost Cretaceous of the Pyrenees and implications for crocodylomorph eggshell structure

The Upper Cretaceous outcrops of the Pyrenees yield one of the most extensive and continuous records of paleoological remains anywhere in the world. Most of eggs and eggshells have been referred to the oofamily Megaloolithidae. In this study, we present a revision of eggshell fragments from the Blasi 2 locality, lattermost Maastrichtian in age, previously assigned to aff. Megaloolithidae. The presence of a blocky extinction pattern and basal knobs supports a crocodilian affinity of these materials. We classify them as Krokolithidae indet. Three structural layers can be recognised in the Blasi 2 eggshells, a feature that is shared with other recent eggshells (e.g. Crocodylus porosus and Crocodylus niloticus) and fossil crocodylomorph eggshells (Krokolitheswilsoni), which were previously described as single layered. The new proposed affinity of the Blasi 2 eggshells reduces the Megaloolithidae oodiversity of the last few million years of the Cretaceous in the Pyrenees to only two valid ootaxa, Megaloolithusmamillare and Megaloolithusbaghensis. The lack of more complete material precludes the erection of new ootaxa based on the Blasi 2 material.

Genesis of kaolinite from Albian sedimentary deposits of the Iberian Range (NE Spain): analysis by XRD, SEM and TEM

Abstract The kaolinite from Albian sedimentary deposits (Escucha and Utrillas Formations) of the Iberian Range (Spain) have been investigated. This research has shown the presence of different types of kaolinites (detrital and diagenetic) along with micaceous phases in these deposits. Detrital kaolinites show anhedral morphology, low crystallinity and a degree of ordering as well as the presence of interstratified smectite layers. They constitute the matrix of the claystones and siltstones and were probably formed as a consequence of intense weathering processes in the source area during the warm period of the early Cretaceous. Diagenetic kaolinites have been recognized in the sandstones and siltstones, with kaolinite growing between ‘expanded’ mica flakes and vermiform and euhedral kaolinite forming the matrix. They have euhedral morphologies, high crystallinity and a high degree of ordering. They grew in situ as a response to incipient diagenesis by K-feldspar dissolution and/or organic acid-rich fluids derived from the maturation of organic matter in shales.

TRANSMISSION AND ANALYTICAL ELECTRON MICROSCOPY EVIDENCE FOR HIGH Mg CONTENTS OF 1M ILLITE: ABSENCE OF 1M POLYTYPISM IN NORMAL PROGRADE DIAGENETIC SEQUENCES OF PELITIC ROCKS

The normal prograde diagenetic and low-grade metamorphic sequence of dioctahedral clay minerals including illite-rich I-S and illite, as observed by TEM, proceeds from a partially disordered 1Md stacking sequence to 2M1; i.e. 1M does not normally occur as an intermediate polytype. Examples of 1M illite stacking sequences have been studied, however, from the Golden Cross gold deposit, New Zealand, the Broadlands-Ohaaki geothermal system, New Zealand, the Potsdam Sandstone, New York, and the Silverton Caldera, Colorado. Specific clay-mineral packets identified by TEM techniques as 1M illite were found to have anomalously high Mg contents. The Broadlands illite provides the most definitive data, as separate packets of 1M and 2M1 illite coexist. Average compositions for 1M and 2M1 illite are (K1.66Ca0.04)Σ1.70(Al3.32Fe0.31Mg0.57Mn0.06)Σ4.26(Si6.43Al1.57)Σ8O20(OH)4 and (K1.57Na0.31Ca0.03)Σ1.91(Al3.58Fe0.05Mg0.29Mn0.01)Σ3.93(Si6.70Al1.30)Σ8O20(OH)4, respectively. In addition, 1Mdillite, which is the polytype occurring in the common 1Mdto 2M1 prograde sequence, is relatively Mg poor, but coexists with Mg-rich illite in the Silverton Caldera sample.These data confirm that 1M stacking is caused by compositional anomalies, and thus explain the lack of the 1M stacking sequence in normal diagenetic sequences in pelitic rocks, as most illite in such environments has a relatively small phengitic component. The parameter Δz, a measure of the corrugation of the oxygen sheets, may be the key parameter reflecting the polytypic state of dioctahedral and trioctahedral micaceous minerals. Such composition-determined relations may be related to the occurrence of 1M polytypism in glauconite and celadonite, both dioctahedral 2:1 clay minerals having large Mg or Fe octahedral-cation components, and in trioctahedral micas. Insofar as the 1M stacking sequence does not have the same composition as 2M1 material, these data confirm that the different varieties of illite are not polytypes, sensu stricto.

TEM study of meteorite impact glass at New Zealand Cretaceous–Tertiary sites: evidence for multiple impacts or differentiation during global circulation?

Abstract Study by transmission electron microscopy of samples from the Cretaceous–Tertiary (K–T) boundary clay at Flaxbourne River and Woodside Creek, New Zealand, has revealed the occurrence of nanometer-sized meteorite impact-derived glass. The average glass composition is exceptionally Ca-rich and is distinct from other glass found on Earth, apart from glass inferred to be of impact origin at Mexican and Haitian K–T sites. The glass shards are partially altered to montmorillonite-like smectite, with the dominant interlayer cation, Ca, reflecting the composition of the parent glass. The data imply a heterogeneous global distribution in composition of K–T boundary impact glass: Si-rich and Ca-rich in Mexico and Haiti, Si-rich in Denmark, and Ca-rich in New Zealand. This heterogeneous distribution may relate to dispersal processes similar to those used to account for the asymmetric distribution of clastic debris from the Chicxulub impact site. However, recent discovery of an impact crater of K–T boundary age in Ukraine raises the possibility of impact clusters which produce material of heterogeneous composition.