Basalt Clasts Research Articles

Activités volcaniques sous-marines à la limite Jurassique-Crétacé dans le Rif externe (Maroc). Âge et relation avec la sédimentation et la paléogéographie du sillon rifain externe

Following the recent stratigraphic works carried out on the Jurassic-Cretaceous boundary in the external Rif chain (Mesorif area and Bou Haddoud nappe), numerous submarine volcanism traces have been discovered in Upper Oxfordian to Upper Berriasian deposits. These traces display various aspects: volcaniclastic complexes incorporated within Upper Berriasian marls; volcanic lavas and basalt clasts included in the breccias with clay matrix of Upper Oxfordian to Lower Berriasian age, or in brecciated Lower Tothonian calcareous beds of the Early Tithonian. These submarine volcanic activities took place in a carbonate platform environment during the Kimmeridgian to Early Tithonian interval or in a pelagic basin from Late Tithonian onwards. They caused an intense brecciation of Upper Jurassic carbonate formations and a general dismantling of marly calcareous alternations of Upper Tithonian-Lower Berriasian. Therefore, the Upper Tithonian-Lower Berriasian deposits are marked by frequent stratigraphic gaps in many outcrops of Mesorif, Prerif areas and in the Bou Haddoud nappe.

Emplacement of the Jurassic Mirdita ophiolites (southern Albania): evidence from associated clastic and carbonate sediments

Sedimentology can shed light on the emplacement of oceanic lithosphere (i.e. ophiolites) onto continental crust and post-emplacement settings. An example chosen here is the well-exposed Jurassic Mirdita ophiolite in southern Albania. Successions studied in five different ophiolitic massifs (Voskopoja, Luniku, Shpati, Rehove and Morava) document variable depositional processes and palaeoenvironments in the light of evidence from comparable settings elsewhere (e.g. N Albania; N Greece). Ophiolitic extrusive rocks (pillow basalts and lava breccias) locally retain an intact cover of oceanic radiolarian chert (in the Shpati massif). Elsewhere, ophiolite-derived clastics typically overlie basaltic extrusives or ultramafic rocks directly. The oldest dated sediments are calpionellid- and ammonite-bearing pelagic carbonates of latest (?) Jurassic-Berrasian age. Similar calpionellid limestones elsewhere (N Albania; N Greece) post-date the regional ophiolite emplacement. At one locality in S Albania (Voskopoja), calpionellid limestones are gradationally underlain by thick ophiolite-derived breccias (containing both ultramafic and mafic clasts) that were derived by mass wasting of subaqueous fault scarps during or soon after the latest stages of ophiolite emplacement. An intercalation of serpentinite-rich debris flows at this locality is indicative of mobilisation of hydrated oceanic ultramafic rocks. Some of the ophiolite-derived conglomerates (e.g. Shpati massif) include well-rounded serpentinite and basalt clasts suggestive of a high-energy, shallow-water origin. The Berriasian pelagic limestones (at Voskopoja) experienced reworking and slumping probably related to shallowing and a switch to neritic deposition. Mixed ophiolite-derived clastic and neritic carbonate sediments accumulated later, during the Early Cretaceous (mainly Barremian-Aptian) in variable deltaic, lagoonal and shallow-marine settings. These sediments were influenced by local tectonics or eustatic sea-level change. Terrigenous sediment gradually encroached from neighbouring landmasses as the ophiolite was faulted or eroded. An Aptian transgression was followed by regression, creating a local unconformity (e.g. at Boboshtica). A Turonian marine transgression initiated widespread Upper Cretaceous shelf carbonate deposition. In the regional context, the southern Albania ophiolites appear to have been rapidly emplaced onto a continental margin in a subaqueous setting during the Late Jurassic (Late Oxfordian-Late Tithonian). This was followed by gradual emergence, probably in response to thinning of the ophiolite by erosion and/or exhumation. The sedimentary cover of the south Albanian ophiolites is consistent with rapid, relatively short-distance emplacement of a regional-scale ophiolite over a local Pelagonian-Korabi microcontinent.

Dating of volcanism and sedimentation in the Skelton Group, Transantarctic Mountains: Implications for the Rodinia-Gondwana transition in southern Victoria Land, Antarctica

The Ross orogen in southern Victoria Land, Antarctica, is composed of Skelton Group metasedimentary rocks and varied plutons of the Granite Harbor intrusives. In the Baronick Glacier area, metasediments are dominated by thick sequences of coarse conglomerate, interbedded with pisolitic limestones. Clasts within metaconglomerates are predominantly volcanic, ranging in composition from basanite to rhyolite. Baronick basaltic clast compositions are mildly alkalic and ocean-island basalt–like, compatible with eruption in a continental-margin rift environment. Sr-Nd isotope ratios suggest rhyolites and quartz-trachytes were produced by assimilation and fractional crystallization mechanisms. Two rhyolites yielded U-Pb zircon ages determined by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) of 650.1 ± 4.0 Ma and 648.5 ± 3.2 Ma (both 2σ). Studies of southern Victoria Land Skelton Group metasediments indicate that, although dominated by Grenville-aged detrital zircons, they contain an important component dated between 630 and 680 Ma, overlapping the age of volcanism in both the Baronick Glacier and central Transantarctic Mountain areas. The distribution of 650 Ma, rift-related volcanism in the southern Victoria Land segment of the Ross orogen, and the absence of significant older, but post-Grenville, extension-related zircon populations, is interpreted to reflect initiation of Rodinia fragmentation on the Antarctic margin at this time. It is proposed that rifting along the paleo-Pacific margin was diachronous, with the Antarctic segment of the rift opening substantially earlier than in Australia. Based on the dates of subduction-related calc-alkaline plutons formed during the Ross and Delamerian orogenies, the paleo–Pacific Ocean in Antarctica may also have closed earlier than in Australia, effectively defining the time of Gondwana assembly.

Silurian clastic sediments in the North Qilian Shan, NW China: Chemical and isotopic constraints on their forearc provenance with implications for the Paleozoic evolution of the Tibetan Plateau

The North Qilian Shan is an accretion–collision orogenic belt in the northeastern margin of the Tibetan Plateau that connects the North China plate to the north with the Tethyan orogenic assemblage to the south. Its Early Paleozoic temporal–spatial tectonic evolution has been extensively studied, but the nature and tectonic setting of its Silurian deposition is poorly understood. Our new petrological analysis suggests that the clast composition of fan conglomerates is complex. In the Danbanshan and Zhongdabanshan sections, abundant lenticular conglomerate beds contain mainly subrounded–rounded metamorphic clasts; whereas the Tongziba and Haichaoba sections contain volcanic, chert, limestone, ultramafic and granitoid clasts. However, in the Sunan area, clast types are relatively simple, dominated by basalt and granitoids, and at Tongziba gabbro and basalt clasts are subrounded and angular, indicating two different types of provenance. Ultramafic, limestone and abundant chert clasts in the Haichaoba and Tonghe conglomerates indicate an ophiolitic source, whereas subrounded–rounded quartzite and granitoid gneiss clasts were probably derived from the Central Qilian basement. Geochemical data from the volcanic and granitoid clasts suggest an arc-related source and their zircon U–Pb ages of 515–429 Ma suggest that the arc magmatism did not end in the Mid-Silurian. Sandstone petrology and detrital modes suggest that the Silurian sediments were deposited in a forearc basin. Paleogeographic reconstructions inferred from NNW–NNE paleocurrent data indicate that an island arc was present off the southern margin of the Silurian basin, and that southward subduction of oceanic crust between North China and the Qilian Shan was active or continued in the Silurian.

The spatial and temporal distribution of lunar mare basalts as deduced from analysis of data for lunar meteorites

In this work we analyze data for lunar meteorites with emphasis on the spatial and temporal distribution of lunar mare basalts. The data are mostly from the Lunar Meteorite Compendium ( http://www-curator.jsc.nasa.gov/antmet/lmc/contents.cfm cited hereafter as Compendium) compiled by Kevin Righter, NASA Johnson Space Center, and from the associated literature. Analysis of the data showed that (i) a significant part of the lunar meteorite source craters are not larger than hundreds of meters in diameter; (ii) cryptomaria seem to be rather abundant in lunar highlands; (iii) the ratios of lunar meteorites belonging to three broad petrologic groups (mare basalt/gabbro, feldspatic highland breccias, and mingled breccias which are a mixture of mare and highland components) seem to be roughly proportional to the areal distribution of these rocks on the lunar surface; and (iv) the meteorite mare basalt ages show a range from ∼2.5 to 4.3 Ga and fill the gaps in the Apollo/Luna basalt age distribution. The ages of mare basalt clasts from mingled breccias seem to be systematically higher than those of “normal” mare basalts, which supports the suggestion that mingled breccias originated mostly from cryptomaria.

Subglacial Hydrothermal Alteration Minerals in Jökulhlaup Deposits of Southern Iceland, with Implications for Detecting Past or Present Habitable Environments on Mars

Jökulhlaups are terrestrial catastrophic outfloods, often triggered by subglacial volcanic eruptions. Similar volcano-ice interactions were likely important on Mars where magma/lava may have interacted with the planet's cryosphere to produce catastrophic floods. As a potential analogue to sediments deposited during martian floods, the Holocene sandurs of Iceland are dominated by basaltic clasts derived from the subglacial environment and deposited during jökulhlaups. Palagonite tuffs and breccias, present within the deposits, represent the primary alteration lithology. The surface abundance of palagonite on the sandurs is 1-20%. X-ray diffraction (XRD) analysis of palagonite breccias confirms a mineral assemblage of zeolites, smectites, low-quartz, and kaolinite. Oriented powder X-ray diffractograms (< 2 microm fraction) for palagonite breccia clasts and coatings reveal randomly ordered smectite, mixed layer smectite/illite, zeolites, and quartz. Visible light-near infrared (VNIR) and shortwave infrared (SWIR) lab spectroscopic data of the same palagonite samples show H2O/OH(-) absorptions associated with clays and zeolites. SWIR spectra derived from Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) images of the sandurs reveal Al-OH(-) and Si-OH(-) absorption features. The identified alteration mineral assemblage is consistent with low temperature (100-140 degrees C) hydrothermal alteration of basaltic material within the subglacial environment. These results suggest that potential martian analog sites that contain a similar suite of hydrated minerals may be indicative of past hydrothermal activity and locations where past habitable environments for microbial life may be found.

The variety of lithologies in the Yamato-86032 lunar meteorite: Implications for formation processes of the lunar crust

We performed a petrologic, mineralogical, geochemical, and isotopic study of several lithologies in the Y-86032 feldspathic breccia. This study leads us to conclude that Y-86032 likely originated on the lunar farside. Y-86032 is composed of several types of feldspathic clasts, granulitic breccias, and minor basaltic clasts set in a clastic matrix. We identify an “An97 anorthosite” that has An contents similar to those of nearside FANs. Mg′ (= molar Mg/(Mg + Fe) × 100) values vary significantly from ∼45 to ∼80 covering the ranges of both nearside FANs and the Mg′ gap between FANs and the Mg-suite. A light-gray feldspathic (LG) breccia making up ∼20% of the investigated slab (5.2 × 3.6 cm 2) mainly consists of fragments of anorthosites (“An93 anorthosite”) more sodic than nearside FANs. LG also contains an augite–plagioclase clast which either could be genetically related to the An93 anorthosite or to slowly-cooled basaltic magma intruded into the precursor rock. The Na-rich nature of both An93 anorthosite and this clast indicates that the LG breccia was derived from a relatively Na-rich but incompatible-element-poor source. The Mg′ variation indicates that the “An97 anorthosite” is a genomict breccia of several types of primary anorthosites. Granulitic breccias in Y-86032 have relatively high Mg′ in mafic minerals. The highest Mg′ values in mafic minerals for the “An97 anorthosite” and granulitic breccias are similar to those of Mg-rich lithologies recently described in Dhofar 489. Basaltic clasts in the dark-gray matrix are aluminous, and the zoning trends of pyroxene are similar to those of VLT or LT basalts. The crystallization of these basaltic clasts pre-date the lithification age of the clastic matrix at ∼3.8 Ga. The low K contents of plagioclase in both the anorthositic and basaltic clasts and generally low incompatible element abundances in all the lithologies in Y-86032 indicate that KREEP was not involved during the formation of the precursor lithologies. This observation further suggests that urKREEP did not exist in the source regions of these igneous lithologies. All these facts support the idea that Y-86032 was derived from a region far distant from the PKT and that the lithic clasts and fragments are indigenous to that region. An An97 anorthositic clast studied here has distinct Sm–Nd isotopic systematics from those previously found for another An97 anorthositic clast and “An93 anorthosite”, and suggests either that An97 anorthosites come from isotopically diverse sources, or that the Sm–Nd isotopic systematics of this clast were reset ∼4.3 Ga ago. These lines of geochemical, isotopic, and petrologic evidence suggest that the lunar crust is geochemically more heterogeneous than previously thought.

Characterisation and prediction of thickness and material properties of periglacial cover beds, Tharandter Wald, Germany

Soils of Central European subdued mountain ranges are structured by various layers of relic periglacial slope deposits (Pleistocene cover beds). These have great impact on subsurface substrate properties. Therefore, gaining extensive spatial information on material composition and thickness of slope deposits is essential. Here we present a GIS-supported approach to predict layer properties which goes beyond existing, deterministic procedures. In a section of the Tharandter Wald, Saxony, Germany, we deduced subsurface structures from soil pits and auger cores. Substrates were analysed to clarify stratigraphical setting and relation to parent rock types. We assume cryoturbation, solifluction, slope wash and aeolian sediment admixture to be important processes of periglacial cover bed formation. These processes are mainly controlled by geological and geomorphological factors. Combining assumptions and field data, we determine properties of periglacial cover beds. Based on relief parameters, an integrated data set is generated, which provides information for a geostatistical interpolation of measured layer thicknesses. The composition of solifluidally dislocated substrate is approximated by a modified flow model. Modelling returned thicknesses of loess-bearing layers, i.e. upper layer and intermediate layer. Prediction of the upper layer involved an inverse distance weighted approach and yielded an inaccuracy of < 10 cm for 75% of the validation data (< 15 cm for 90%, i.e. RMSE of 10 cm and standard deviation of 10 cm). The lower unit (intermediate layer) was approximated using topographic parameters, which returned an inaccuracy of < 5 cm for 75% of the validation data (< 6 cm for 90%, i.e. RMSE of 8 cm, standard deviation of 8 cm). Due to data limitations, thicknesses of underlying, loess-free slope deposits (basal layers) were not calculated. The modified flow model is able to predict the distribution of rock types dislocated from their original outcrops very accurately, except for basalt clasts, which were transported farther than predicted.

Record of Late Miocene glacial deposits on Isla Marambio (Seymour Island), Antarctic Peninsula

AbstractWe report and describe two new small diamictite outcrops on Isla Marambio (Seymour Island), Antarctic Peninsula. These rocks rest on an erosional unconformity on top of the Eocene La Meseta Formation and are unconformably covered by glaciomarine rocks of the ?Pliocene–Pleistocene Weddell Sea Formation. The lithology, fossil content and isotopic ages obtained strongly suggest that the rocks belong to the Hobbs Glacier Formation and support a Late Miocene age for this unit. Additionally, the dated basalt clast provides the oldest age (12.4 Ma) for the James Ross Island Volcanic Group recorded up to now. The here described diamictite cannot be confidently correlated with a glaciomarine unit previously assigned to the Late Eocene–Lower Oligocene taken as proof that initial expansion of ice on Antarctica encompassed the entire continent synchronously in the earliest Oligocene. However, it is now evident that there are likely to be more, short but important, stratigraphic sequences of key regional and Antarctic wide interest preserved on the plateau of Isla Marambio.

Antarctic lunar meteorites Yamato-793169, Asuka-881757, MIL 05035, and MET 01210 (YAMM): Launch pairing and possible cryptomare origin

The Antarctic lunar meteorite Meteorite Hills (MET) 01210 is a polymict regolith breccia, dominantly composed of mare basalt components. One relatively large (2.7 × 4.7 mm) basalt clast in MET 01210 (MET basalt) shows remarkable mineralogical similarities to the lunar-meteorite crystalline mare basalts Yamato (Y)-793169, Asuka (A)-881757, and Miller Range (MIL) 05035. All four basalts have similar rock texture, mineral assemblage, mineral composition, pyroxene crystallization trend, and pyroxene exsolution lamellae. The estimated TiO 2 contents (∼2.0 wt%) of the MET basalt and MIL 05035 are close to the bulk-rock TiO 2 contents of Y-793169 and A-881757. These similarities suggest that Y-793169, A-881757, MIL 05035, and the MET basalt came from the same basalt flow, which we designate the YAMM basalt. The source-basalt pairing of the YAMM is also supported by their similar REE abundances, crystallization ages (approx. 3.8–3.9 Ga), and isotopic compositions (low U/Pb, low Rb/Sr, and high Sm/Nd). The pyroxene exsolution lamellae, which are unusually coarse (up to a few microns) by mare standards, imply a relatively slow cooling in an unusually thick lava and/or subsequent annealing within a cryptomare. Reported noble gas and CRE data with close launch ages (∼1 Ma) and ejection depths (deeper than several meters) among the four meteorites further indicate their simultaneous ejection from the moon. Despite the marginally close terrestrial ages, pairing in the conventional Earth-entry sense seems unlikely because of the remote recovery sites among the YAMM meteorites. The high abundance (68%) of mare components in MET 01210 estimated from a two-component mixing model calculation could have resulted from either lateral mixing at a mare-highland boundary or vertical mixing in a cryptomare. The proportion of mare materials in MET 01210 is greater than in Apollo core samples at the mare-highland boundary. The burial depth (>several meters deep) inferred from the lack of surface irradiation of MET 01210 exceeds the typical mare regolith thickness (a few meters). Thus, the source of the YAMM meteorites is likely a terrain of locally high mare-highland mixing within a cryptomare. We searched for a possible source crater of the YAMM meteorites within the well-defined cryptomare, based on the multiple constraints obtained from this study and published data. An unnamed 1.4 km-diameter crater (53°W, 44.5°S) on the floor of the Schickard crater is the most suitable source for the YAMM meteorites. The 238U/ 204Pb ( μ) value of the YAMM basalts is extremely low, relative to those of the Apollo mare basalts, but comparable to those of the Luna 24 very low-Ti basalts. The low- μ source indicates a derivation from a less differentiated mantle with a lack of KREEP components. Although the chemical sources of materials and heat source of melting might be independent, the heat source that generated the source magma of the YAMM and Luna 24 basalts may not be related to KREEP, unlike the case of the Apollo basalts. The distinct chemical and isotopic compositions of mantle sources between the Apollo basalts and the YAMM/Lunar 24 basalts imply differences in mantle composition and thermal evolution between the Procellarum KREEP Terrane (PKT) and non-PKT regions of the nearside.

Geochemistry of Palaeoproterozoic volcanic rocks of the Iricoumé Group, Pitinga Mining District, Amazonian craton, Brazil

The Iricoumé Group includes 1.88 Ga volcanic units of the Iricoumé–Mapuera volcano-plutonic association, part of the Uatumã magmatic series in the Guyana shield portion of the Amazonian craton. In the Pitinga Mining District, these rocks consist dominantly of felsic trachyte to rhyolite, associated with voluminous ignimbrite and minor ash-fall tuffs and surge deposits. Mafic rocks are present as basaltic clasts within volcanic breccias, and mostly as mafic microgranular enclaves in the associated Mapuera plutonic rocks. The felsic rocks have high contents of SiO2, FeOt, K2O, Rb, and alkalis; low TiO2, CaO, Sr, Ba, Nb, Ta, and Eu; and show metaluminous to weakly peraluminous bulk-rock compositions. They exhibit alkaline geochemical features, expressed by Na2O + K2O averages of 8.8 wt.%, FeOt/(FeOt + MgO) ≥ 0.8, and high Ga/Al ratios, compatible with A-type magmas. The studied samples plot in the field of within-plate or post-collisional rocks in a (Nb + Y) versus Rb diagram. Nb/Y ratios indicate that they are comparable to A2-type rocks which, allied with their high LREE/Nb ratios, suggest that they were produced from mantle sources modified by previous subduction in a post-collisional setting. Two compositional populations of Ca-amphibole, a Mg-rich (actinolite to Mg-hornblende) and a Fe-rich one (Fe-edenite to Fe-pargasite, Fe-hornblende and Fe-actinolite), characterize the Iricoumé Group volcanics. The Fe-rich amphiboles crystallized under lower fO2 and higher pressure conditions compared with the Mg-rich amphiboles, indicating different levels of crystallization or re-equilibration during ascent of the magmas. Zircons from rhyolites show trace-element compositions typical of magmatic crystals with high Th/U ratios, and REE patterns compatible with zircon-melt partition coefficients for silicic magma compositions. Their relatively lower zircon/rock partition coefficients are due to early apatite crystallization. Fractional crystallization mainly of plagioclase-hornblende and biotite-alkali feldspar with minor amounts of apatite explains the geochemical trends observed in the felsic Iricoumé volcanic rocks.

First evidence for Cambrian glaciation provided by sections in Avalonian New Brunswick and Ireland: Additional data for Avalon–Gondwana separation by the earliest Palaeozoic

The first evidence for Cambrian glaciation is provided by two successions on the Avalon microcontinent. The middle lowest Cambrian (middle Terreneuvian Series and Fortunian Stage–Stage 2 boundary interval) has an incised sequence boundary overlain by a fluvial lowstand facies and higher, olive green, marine mudstone on Hanford Brook, southern New Brunswick. This succession in the lower Mystery Lake Member of the Chapel Island Formation may be related to melting of an ice sheet in Avalon. The evidence for this interpretation is a muddy diamictite with outsized (up to 10 cm in diameter), Proterozoic marble and basalt clasts that penetrated overlying laminae in the marine mudstone. That eustatic rise was associated with the mudstone deposition is suggested by an approximately coeval rise that deposited sediments with Watsonella crosbyi Zone fossils 650 km away in Avalonian eastern Newfoundland. A sea-level rise within the Watsonella crosbyi Chron, at ca. 535 Ma, may correspond to a unnamed negative 13C excursion younger than the basal Cambrian excursion (BACE) and the ZHUCE excursion in Stage 2 of the upper Terreneuvian Series. Cambrian dropstones are now also recognized on the northern (Gander) margin of Avalon in continental slope–rise sedimentary rocks in southeast Ireland. Although their age (Early–Middle Cambrian) is poorly constrained, dropstones in the Booley Bay Formation provide additional evidence for Cambrian glaciation on the Avalon microcontinent. Besides providing the first evidence of Cambrian glaciation, these dropstone deposits emphasize that Avalon was not part of or even latitudinally close to the terminal Ediacaran–Cambrian, tropical carbonate platform successions of West Gondwana.

Vitriclastic lithofacies from Macquarie Island (Southern Ocean): compositional influence on abyssal eruption explosivity in a dying Miocene spreading ridge

Macquarie Island is composed of a complete section of oceanic crust that formed in a slow-spreading mid-ocean ridge 2.0 to 3.5 km below sea level. Vitriclastic facies preserved on the island have both pyroclastic and hyaloclastic characteristics. Monomict hyaloclastic breccia facies are widespread across the island and are predominantly composed of near-primitive (~7.9 wt% MgO) subalkaline/transitional (~0.7 wt% K2O) sideromelane shards and crystalline basalt clasts with low vesicularity (LV, < 15% vesicles). Breccias are thick bedded and structureless with matrix-supported angular pillow fragments, bomb-sized fluidal mini-pillows, and globular glass lapilli. Clasts are lithologically similar to interbedded pillow basalts and laterally grade into fine-grained sandstone facies. These sandstones are normal-graded, well-laminated, thin bedded, and interstratified with red pelagic mudstone. Lithofacies associations indicate that the hyaloclastic breccias were formed proximal to a source vent via quench-fragmentation, and subsequently reworked by ocean-bottom currents into distal epiclastic sandstone facies. During eruption, co-genetic pillow lava and hypabyssal intrusions mingled with the breccia, forming fluidal peperite. Rare polymict pyroclastic facies only occur in the highest stratigraphic levels and are mostly composed of highly vesicular (HV, 15–50% vesicles) sideromelane shards and crystalline basalt clasts with alkaline (~1.0 wt% K2O) fractionated (~6.8% MgO) compositions. Minor lithic grains are composed of subalkaline (~0.7 wt% K2O) to very highly alkaline (~1.7 wt% K2O) LV sideromelane shards, and amphibole-bearing diabase. The pyroclastic facies contains medium to thick beds of lapilli-tuff that exhibit both reverse and normal grading, diffuse lamination, and planar-grain fabric. These beds are locally overlain by thin fine-grained tuff beds entirely composed of cuspate to very thin elongate bubble-wall shards. These characteristics indicate that explosive deep-marine eruptions produced high-density coarse-grained gravity flows that were covered by slower suspension settle-out of delicate bubble-wall shards. Stratigraphic relationships suggest that explosive eruptions started during the waning stages of more alkaline volcanism along the proto-Macquarie spreading center.

Oxygen isotopic constraints on the origin and parent bodies of eucrites, diogenites, and howardites

A few eucrites have anomalous oxygen isotopic compositions. To help understand their origin and identify additional samples, we have analyzed the oxygen isotopic compositions of 18 eucrites and four diogenites. Except for five eucrites, these meteorites have Δ 17O values that lie within 2σ of their mean value viz., −0.242 ± 0.016‰, consistent with igneous isotopic homogenization of Vesta. The five exceptional eucrites—NWA 1240, Pasamonte (both clast and matrix samples), PCA 91007, A-881394, and Ibitira—have Δ 17O values that lie, respectively, 4σ, 5σ, 5σ, 15σ, and 21σ away from this mean value. NWA 1240 has a δ 18O value that is 5σ below the mean eucrite value. Four of the five outliers are unbrecciated and unshocked basaltic eucrites, like NWA 011, the first eucrite found to have an anomalous oxygen isotopic composition. The fifth outlier, Pasamonte, is composed almost entirely of unequilibrated basaltic clasts. Published chemical data for the six eucrites with anomalous oxygen isotopic compositions (including NWA 011) exclude contamination by chondritic projectiles as a source of the oxygen anomalies. Only NWA 011 has an anomalous Fe/Mn ratio, but several anomalous eucrites have exceptional Na, Ti, or Cr concentrations. We infer that the six anomalous eucrites are probably derived from five distinct Vesta-like parent bodies (Pasamonte and PCA 91007 could come from one body). These anomalous eucrites, like the isotopically normal, unbrecciated eucrites with 4.48 Gyr Ar-Ar ages, are probably deficient in brecciation and shock effects because they were sequestered in small asteroids (∼10 km diameter) during the Late Heavy Bombardment following ejection from Vesta-like bodies. The preservation of Vesta’s crust and the lack of deeply buried samples from the hypothesized Vesta-like bodies are consistent with the removal of these bodies from the asteroid belt by gravitational perturbations from planets and protoplanets, rather than by collisional grinding.

Evolution of porosity and diffusivity associated with chemical weathering of a basalt clast

Weathering of rocks as a result of exposure to water and the atmosphere can cause significant changes in their chemistry and porosity. In low‐porosity rocks, such as basalts, changes in porosity, resulting from chemical weathering, are likely to modify the rock's effective diffusivity and permeability, affecting the rate of solute transport and thus potentially the rate of overall weathering to the extent that transport is the rate limiting step. Changes in total porosity as a result of mineral dissolution and precipitation have typically been used to calculate effective diffusion coefficients through Archie's law for reactive transport simulations of chemical weathering, but this approach fails to account for unconnected porosity that does not contribute to transport. In this study, we combine synchrotron X‐ray microcomputed tomography (μCT) and laboratory and numerical diffusion experiments to examine changes in both total and effective porosity and effective diffusion coefficients across a weathering interface in a weathered basalt clast from Costa Rica. The μCT data indicate that below a critical value of ∼9%, the porosity is largely unconnected in the basalt clast. The μCT data were further used to construct a numerical pore network model to determine upscaled, effective diffusivities as a function of total porosity (ranging from 3 to 30%) for comparison with diffusivities determined in laboratory tracer experiments. By using effective porosity as the scaling parameter and accounting for critical porosity, a model is developed that accurately predicts continuum‐scale effective diffusivities across the weathering interface of the basalt clast.

Pyroclastic flow deposits from a kimberlite eruption: The Orapa South Crater, Botswana

The Orapa Diamond Mine (Republic of Botswana) exposes a bi-lobate kimberlite pipe that erupted during the Late-Cretaceous epoch (~ 93 Ma) through Archaean basement and volcano–sedimentary rocks of the Karoo Supergroup. Geological mapping of the crater zone of the South Pipe has revealed a 15–25 m thick in-situ kimberlite pyroclastic flow deposit. The pyroclastic deposit fills in the crater and completely drapes lower units, indicating that the parent flow originated from an adjacent kimberlite pipe. The unit comprises a basal coarse lithic concentration layer exhibiting imbricated clasts, which grades upwards into massive poorly sorted lapilli tuff. The tuff contains abundant sub-vertical degassing structures defined by lithic enrichment and depletion in fine-grained material. Degassing structures commonly emanate from blocks in the basal layer. The presence of degassing structures and a coarse basal layer distinguishes this deposit from pipe-filling massive volcaniclastic kimberlite, which is typically homogeneous in terms of texture and clast size over distances on the order of 100s of metres. Studies of the thermal remanent magnetism in basalt clasts from the deposit, together with serpentine–diopside assemblages, indicate that it was emplaced at elevated temperatures on the order of 200–440 °C, consistent with deposition from a pyroclastic flow. The lithofacies characteristics can be explained by the interaction of the pyroclastic flow with the complex topography of a pre-existing crater.

Glassy fragmental rocks of Macquarie Island (Southern Ocean): Mechanism of formation and deposition

Glassy fragmental rocks are interlayered with pillow basalt and tabular basalt on Macquarie Island (54°30′ S, 158°54′ E). These facies formed along the Proto-Macquarie Spreading Ridge between 6 and 12 Ma and have since been uplifted and exposed on the apex of the Macquarie Ridge Complex. Through a combination of field and microscopic analyses, we investigate the submarine production, transportation, deposition and lithification of basalt and sideromelane clasts within a spreading-ridge environment. The findings of this study indicate that these glassy grains form predominantly by cooling-contraction granulation of pillow lava rinds while crystalline basalt clasts are derived from the fragmentation of pillows along concentric and radial cooling joints. Hyaloclastite breccias consist of crystalline volcanic clasts in a matrix of glassy fragments, and are termed pillow-fragment breccias when clasts identifiable as pillows account for > 25% of the cobble-sized fraction. This glassy fragmental sediment was transported predominantly by short-lived grain flows and deposited as a result of syn-eruptive talus accumulation. The above interpretations culminate in the production of a depositional model: these glassy fragmental rocks formed on the slopes of pillow cones following gravitational collapse of a destabilised cone flank along the Proto-Macquarie Spreading Ridge. Scanning electron microscopy reveals that palagonite alteration rims on glassy grains lithify the sediment. The findings may be used as an analogue for the formation of glassy fragmental rocks along past and present mid-oceanic ridges.

Alteration textures in terrestrial volcanic glass and the associated bacterial community

Alteration textures were examined in subglacial (hyaloclastite) deposits at Valafell, Southern Iceland. Pitted and 'elongate' alteration features are observed in the glass similar to granular and tubular features reported previously in deep-ocean basaltic glasses, but elongate features generally did not have a length to width ratio greater than five. Elongate features were found in only 7% of surfaces. Crystalline basalt clasts, which are incorporated into the hyaloclastite, did not display elongate structures. Pitted alteration features were poorly defined in crystalline basalt, comprising only 4% of the surface compared to 47% in the case of basaltic glass. Examination of silica-rich glass (obsidian) and rhyolite similarly showed poorly defined pitted textures that comprised less than 15% of the surface and no elongate features were observed. These data highlight the differences in alteration textures between terrestrial basaltic glass and previously studied deep-ocean and subsurface basaltic glass, and the important role of mineralogy in controlling the type and abundance of alteration features. The hyaloclastite contains a diverse and abundant bacterial population, as determined by 16S rDNA analysis, which could be involved in weathering the glass. Despite the presence of phototrophs, we show that they were not involved in the production of most alteration textures in the basaltic glass materials we examined.

Uranium–thorium chronometry of weathering rinds: Rock alteration rate and paleo-isotopic record of weathering fluids

Abstract The potential of 238U–234U–230Th chronometry for constraining the formation rate of weathering rinds developed on fresh rocks is assessed by analyzing a weathering rind on a basaltic clast from a 125 kyr old Costa Rican alluvial terrace. Eighteen subsamples were collected from one section of the clast by drilling cores (4 mm in diameter and 5 mm depth) along two transects straddling the core–rind boundary. Variations of major and trace element concentrations along the two transects point out (a) intense loss of alkaline and alkaline-earth elements, (b) conservative behaviour of elements such as Zr, Hf and Th, and (c) external input of U into the rind without any evidence of U loss during basalt weathering. In addition, variations in U concentrations along the transects show that the main U–Th fractionation process associated with the weathering of the basaltic clast is an external input of U (without addition nor loss of Th) in the basalt rind transition zone, and that, once deposited U is immobile in the weathering rind. In the frame of this interpretation scheme, a weathering rate of 0.5 ± 0.2 mm/kyr can be calculated for the studied clast, which is consistent with geological and isotopic evidence constraining the depositional ages of the terraces. In addition, the variations in the (234U/238U) ratio along the analysed transects as well as the increase in Sr isotopic ratios within the weathering rind are best explained by temporal variation of the U activity ratios and Sr isotope ratios of the soil solutions brought into the rind. This work highlights how well detailed U–Th chronological studies of weathering rinds can (1) constrain the formation rates of weathering systems, and (2) record the time variation of isotopic composition of weathering fluids.

Shock brecciation around the Kidd Creek deposit, Abitibi belt, Canada

The Kidd–Munro assemblage, Abitibi belt, Canada, is an ultramafic–mafic–felsic volcanic sequence that contains the giant Kidd Creek volcanic-hosted massive sulfide (VMS) deposit. The Kidd basin, 1.6 km northeast of the deposit, contains pervasively brecciated pillowed and massive basalts. The breccia is distinctly different from most breccias in volcanic rocks, which form through volcanic processes or during later deformation or alteration. The Kidd Creek breccia occurs pervasively through otherwise undeformed pillow interiors and margins, and also in localized corridors of particularly intense brecciation. Clasts are angular, up to 4 cm wide, hosted in a very fine-grained matrix, and commonly show jig-saw fit texture. The chemical compositions of the breccia fragments and matrix are generally similar, although the matrix is slightly enriched in high field-strength elements (HFSE) and heavy rare-earth elements (HREE) and depleted in some mobile elements, such as Rb and Ba. The breccia contains altered basaltic clasts and fragments of in-filled amygdales and is crosscut by late-stage quartz–carbonate–sulfide veins. The observations imply that the breccia was formed in-situ, with minimal transport of material, and developed after solidification of the volcanic rocks. In-situ breccias, such as these, are known to form proximal to major fault zones, but no such structure occurs in the vicinity of the Kidd Basin. We suggest the brecciation was caused by the propagation of shock waves from explosive volcanic eruption, perhaps related to the emplacement of felsic volcanic rocks observed in the Kidd Creek Mine. The breccia was subject to enhanced hydrothermal fluid flow, perhaps linked to the formation of the ore deposit.