Nature and origin of cone-forming volcanic breccias in the Te Herenga Formation, Ruapehu, New Zealand

 Volcanic breccias form large parts of composite volcanoes and are commonly viewed as containing pyroclastic fragments emplaced by pyroclastic processes or redistributed as laharic deposits. Field study of cone-forming breccias of the andesitic middle Pleistocene Te Herenga Formation on Ruapehu volcano, New Zealand, was complemented by paleomagnetic laboratory investigation permitting estimation of emplacement temperatures of constituent breccia clasts. The observations and data collected suggest that most breccias are autoclastic deposits. Five breccia types and subordinate, coherent lava-flow cores constitute nine, unconformity-bounded constructional units. Two types of breccia are gradational with lava-flow cores. Red breccias gradational with irregularly shaped lava-flow cores were emplaced at temperatures in excess of 580  °C and are interpreted as aa flow breccias. Clasts in gray breccia gradational with tabular lava-flow cores, and in some places forming down-slope-dipping avalanche bedding beneath flows, were emplaced at varying temperatures between 200 and 550  °C and are interpreted as forming part of block lava flows. Three textural types of breccia are found in less intimate association with lava-flow cores. Matrix-poor, well-sorted breccia can be traced upslope to lava-flow cores encased in autoclastic breccia. Unsorted boulder breccia comprises constructional units lacking significant exposed lava-flow cores. Clasts in both of these breccia types have paleomagnetic properties generally similar to those of the gray breccias gradational with lava-flow cores; they indicate reorientation after acquisition of some, or all, magnetization and ultimate emplacement over a range of temperatures between 100 and 550  °C. These breccias are interpreted as autoclastic breccias associated with block lava flows. Matrix-poor, well-sorted breccia formed by disintegration of lava flows on steep slopes and unsorted boulder breccia is interpreted to represent channel-floor and levee breccias for block lava flows that continued down slope. Less common, matrix-rich, stratified tuff breccias consisting of angular blocks, minor scoria, and a conspicuously well-sorted ash matrix were generally emplaced at ambient temperature, although some deposits contain clasts possibly emplaced at temperatures as high as 525  °C. These breccias are interpreted as debris-flow and sheetwash deposits with a dominant pyroclastic matrix and containing clasts likely of mixed autoclastic and pyroclastic origin. Pyroclastic deposits have limited preservation potential on the steep, proximal slopes of composite volcanoes. Likewise, these steep slopes are more likely sites of erosion and transport by channeled or unconfined runoff rather than depositional sites for reworked volcaniclastic debris. Autoclastic breccias need not be intimately associated with coherent lava flows in single outcrops, and fine matrix can be of autoclastic rather than pyroclastic origin. In these cases, and likely many other cases, the alternation of coherent lava flows and fragmental deposits defining composite volcanoes is better described as interlayered lava-flow cores and cogenetic autoclastic breccias, rather than as interlayered lava flows and pyroclastic beds. Reworked deposits are probably insignificant components of most proximal cone-forming sequences. Received: 1 October 1998 / Accepted: 28 December 1998     

Field and paleomagnetic characterization of lithic and scoriaceous breccias at Pleistocene Broken Top volcano, Oregon Cascades

Cirque-wall exposures of cone-forming deposits of Pleistocene Broken Top volcano, Oregon Cascade Range, reveal that the volcano is composed of unconformity-bounded constructional units of coherent lava (lava-flow cores) and breccia. Coarse-grained autoclastic breccias are found above and below lava-flow cores and may extend downslope from coherent lava outcrops where they may or may not be associated with thin lava stringers. Mantle-bedded scoria-fall breccias are recognized by generally good sorting, mantle bedding, and presence of aerodynamically shaped bombs. These breccias vary considerably in thermal oxidation coloration (black, red, orange, purple). Many breccia layers are unsorted mixtures of scoria and lithic (nonvesicular) fragments that grade laterally to unambiguous autoclastic breccia or lava-flow cores. These layers are interpreted as hybrid pyroclastic–autoclastic deposits produced by incorporation of falling or fallen tephra into advancing lava-flow fronts. This latter breccia type is common at Broken Top and offers particular challenges for clast or deposit classification.Progressive thermal demagnetization results for selected examples of different breccia types show that most scoria-fall and autoclastic breccias are emplaced at elevated temperatures (averaging 100–300°C). Clasts within single deposits record different emplacement temperatures ranging, in some cases, from 100 to over 580°C indicating a lack of thermal equilibration within deposits. Magnetization directions for single breccia deposits are more dispersed than data typically reported for lava flows. Settling and rotation of clasts after cooling or incorporation of colder clasts that are not significantly reheated probably accounts for the relatively high dispersion and suggests that paleomagnetic studies demanding low within-site dispersion (e.g., for determining paleomagnetic poles or evaluating tectonic rotation) should avoid volcanic breccias.     

Lateral variations within coarse co-ignimbrite lithic breccias of the Kos Plateau Tuff, Greece

 Coarse, co-ignimbrite lithic breccia, Ebx, occurs at the base of ignimbrite E, the most voluminous and widespread unit of the Kos Plateau Tuff (KPT) in Greece. Similar but generally less coarse-grained basal lithic breccias (Dbx) are also associated with the ignimbrites in the underlying D unit. Ebx shows considerable lateral variations in texture, geometry and contact relationships but is generally less than a few metres thick and comprises lithic clasts that are centimetres to a few metres in diameter in a matrix ranging from fines bearing (F2: 10 wt.%) to fines poor (F2: 0.1 wt.%). Lithic clasts are predominantly vent-derived andesite, although clasts derived locally from the underlying sedimentary formations are also present. There are no proximal exposures of KPT. There is a highly irregular lower erosional contact at the base of ignimbrite E at the closest exposures to the inferred vent, 10–14 km from the centre of the inferred source, but no Ebx was deposited. From 14 to <20 km from source, Ebx is present over a planar erosional contact. At 16 km Ebx is a 3-m-thick, coarse, fines-poor lithic breccia separated from the overlying fines-bearing, pumiceous ignimbrite by a sharp contact. This grades downcurrent into a lithic breccia that comprises a mixture of coarse lithic clasts, pumice and ash, or into a thinner one-clast-thick lithic breccia that grades upward into relatively lithic-poor, pumiceous ignimbrite. Distally, 27 to <36 km from source Ebx is a finer one-clast-thick lithic breccia that overlies a non-erosional base. A downcurrent change from strongly erosional to depositional basal contacts of Ebx dominantly reflects a depletive pyroclastic density current. Initially, the front of the flow was highly energetic and scoured tens of metres into the underlying deposits. Once deposition of the lithic clasts began, local topography influenced the geometry and distribution of Ebx, and in some cases Ebx was deposited only on topographic crests and slopes on the lee-side of ridges. The KPT ignimbrites also contain discontinuous lithic-rich layers within texturally uniform pumiceous ignimbrite. These intra-ignimbrite lithic breccias are finer grained and thinner than the basal lithic breccias and overlie non-erosional basal contacts. The proportion of fine ash within the KPT lithic breccias is heterogeneous and is attributed to a combination of fluidisation within the leading part of the flow, turbulence induced locally by interaction with topography, flushing by steam generated by passage of pyroclastic density currents over and deposition onto wet mud, and to self-fluidisation accompanying the settling of coarse, dense lithic clasts. There are problems in interpreting the KPT lithic breccias as conventional co-ignimbrite lithic breccias. These problems arise in part from the inherent assumption in conventional models that pyroclastic flows are highly concentrated, non-turbulent systems that deposit en masse. The KPT coarse basal lithic breccias are more readily interpreted in terms of aggradation from stratified, waning pyroclastic density currents and from variations in lithic clast supply from source. Received: 21 April 1997 / Accepted: 4 October 1997     

Submarine eruption-fed and resedimented pumice-rich facies: the Dogashima Formation (Izu Peninsula,Japan)

In the Izu Peninsula (Japan), the Pliocene pumice-rich Dogashima Formation (4.55?±?0.87 Ma) displays exceptional preservation of volcaniclastic facies that were erupted and deposited in a below wave-base marine setting. It includes high-concentration density current deposits that contain clasts that were emplaced hot, indicating an eruption-fed origin. The lower part of the Dogashima 2 unit consists of a very thick sequence (<12 m) of massive grey andesite breccia restricted to the base of a submarine channel, gradationally overlain by pumice breccia, which is widespread but much thinner and finer in the overbank setting. These two breccias share similar mineralogy and crystal composition and are considered to be co-magmatic and derived from the destruction of a submarine dome by an explosive, pumice-forming eruption. The two breccias were deposited from a single, explosive eruption-fed, sustained, sea floor-hugging, water-supported, high-concentration density current in which the clasts were sorted according to their density. At the rim of the channel, localised good hydraulic sorting of clasts and stratification in the pumice breccia are interpreted to reflect local current expansion and unsteadiness rather than to be the result of hydraulic sorting of clasts during fall from a submarine eruption column and/or umbrella plume. A bimodal coarse (>1 m) pumice- and ash-rich bed overlying the breccias may be derived from delayed settling of pyroclasts from suspension. In Dogashima 1 and 2, thick cross- and planar-bedded facies composed of sub-rounded pumice clasts are intercalated with eruption-fed facies, implying inter-eruptive mass-wasting on the flank of a submarine volcano, and reworking and resedimentation by high-energy tractional currents in a below wave-base environment.     

Magnetic susceptibility logging of Chicxulub proximal impact breccias in the Santa Elena borehole: implications for emplacement mode

Magnetic susceptibility logging is used to study the impact breccias in the Chicxulub crater. The basic premise is that the high contrasts in magnetic properties can be used to characterize the breccias. The Santa Elena borehole was drilled 110 km radial distance from crater center and sampled a 172 m thick sequence of impact breccias, between 332 and 504 m depth. Breccia units are distinguished from differences in composition, size, and relative contents of clasts, type of matrix and textural and lithological assemblages, which can be resolved in the susceptibility logs. The whole-core log shows characteristic variation patterns with high, intermediate and low susceptibilities. High resolution logging of matrix and clasts records the heterogeneous nature of impactites, with higher variability at smaller spatial scales. Measurements confirm that diamagnetic susceptibilities characterize the carbonate clasts, high susceptibilities the basement granitic clasts and intermediate values the silicate melt-rich and silicate-poor matrix. Intermediate variable susceptibilities characterize breccias rich in melt particles. Correlation of matrix and clast logs with whole-core log shows that signal is controlled by the matrix. Logs for clast shows a discrete distribution with peaks of intermediate to high values, which correlate with large clast distributions. The ejecta blanket includes the fallback suevites rich in silicate melt particles and shocked minerals, the high temperature vapor deposits from ejecta curtain collapse and high velocity basal flows, and the carbonate rich deposits from lateral basal flows and secondary cratering. Late fallback suevites record minor turbulent conditions resulting from progressive cooling of the ejecta plume.     

Mineralogy and petrology of the Abee enstatite chondrite breccia and its dark inclusions

The Abee E4 enstatite chondrite breccia consists of clasts (many rimmed by metallic Fe, Ni), dark inclusions and matrix. The clasts and matrix were well equilibrated by thermal metamorphism, as evidenced by uniform mineral compositions, recrystallized chondrules, low MnO content of enstatite and high abundance of orthoenstatite. The clasts acquired their metal-rich rims prior to this metamorphic episode. The occurrence in Abee of relatively unmetamorphosed dark inclusions, clasts with nearly random magnetic orientations and a matrix with a uniform magnetic orientation [18,19] indicates that clast and matrix metamorphism occurred prior to the agglomeration of the breccia.The dark inclusions are an unusual kind of enstatite chondritic material, distinguished from the clasts and matrix by their relative enrichments in REE [21–23], low relative abundances of kamacite, total metallic Fe, Ni and silica, lower niningerite/(total sulfide) ratios, high relative abundances of oldhamite and martensite, smaller euhedral enstatite, more heterogeneous enstatite and metallic Fe, Ni, more calcic enstatite and more nickeliferous schreibersite.We propose the following model for the petrogenesis of the Abee breccia: The maximum metamorphic temperature of breccia parent material was?- 840°C (the minimum temperature of formation of Abee niningerite) and perhaps near 950–1000°C (the Fe-Ni-S eutectic temperature). Euhedral enstatite crystals in metallic Fe, Ni- and sulfide-rich areas grew at these metamorphic temperatures into pliable metal and sulfide. Breccia parent material was impact-excavated from depth, admixed with dark inclusions and rapidly cooled (700 to 200°C in about 2 hours) [15]. During this cooling, clast and matrix material acquired thermal remanent magnetization. Random conglomeration of clasts and unconsolidated matrix materials caused the clasts to have random magnetic orientations and the matrix areas to have net magnetic intensities of zero (due to the cancellation of numerous randomly oriented magnetic vectors of equal intensity in the matrix). A subsequent ambient magnetic field imparted a uniform net magnetic orientation to the matrix and caused the magnetic orientations of the clasts to be somewhat less random. The Abee breccia was later consolidated, possibly by shock or by shallow burial and very long-period/low-temperature (< 215°C) metamorphism.     

Early lunar differentiation: 4.42-AE-old plagioclase clasts in Apollo 16 breccia 67435

We report on a⁴⁰Ar-³⁹Ar study of the Apollo 16 breccia 67435 and present ages of five samples representing matrix, lithic clasts and plagioclase clasts. While the matrix age spectrum does not have a well-defined plateau, the two lithic clasts gave plateau ages of 3.96 and 4.04 AE. Since all samples had apparent ages of ～1 AE in the fractions ≤600°C extraction temperature, the breccia might have been assembled in a rather mild process at about that time or even more recently out of material with different metamorphic ages. The two plagioclase samples, of which one was a single 9-mg mineral clast and the other a 15-mg composite of several clasts, also have ages of ～1 AE in the low-temperature release fractions, but are apparently undisturbed by any ～4-AE events since they both have well-defined plateaux at 4.42 AE. The age of these strongly calcic plagioclase clasts, believed to be remnants of the anorthositic lunar crust, establishes a lower age limit to the end of the early lunar differentiation and thus places a strong constraint to the lunar evolution.     

Petrology and chemistry of two “large” granite clasts from the moon

Pristine granite clasts in Apollo-14 breccias 14321 and 14303 have estimated masses of 1.8 and 0.17 g, respectively. The 14321 clast is ～ 60% K-feldspar and 40% quartz, with traces of extremely Mg-poor mafic silicates and ilmenite. The 14303 clast is roughly 33% plagioclase, 32% K-feldspar, 23% quartz, 11% pyroxene, and 1% ilmenite; pyroxene and ilmenite are moderately Mg-rich; plagioclase and pyroxene are strongly zoned. Both clasts are severely brecciated, but monomict (pristine). Both have abundant graphic intergrowths of K-feldspar with quartz. Unlike the majority of similar Earth rocks, both clasts are devoid of hydrous phases. The bulk composition of the 14321 clast is similar to those of several other lunar granitic samples, but the 14303 clast is unique: it bears as close a resemblance to KREEP as it does to other lunar granites. Silicate liquid immiscibility may explain why the granites are low in REE relative to KREEP.     

Downslope movement of in situ clasts on forested hillslopes,Virginia

Downslope movements of 109 clasts ranging in intermediate diameter from 30 to 870 mm were monitored over a five-year period on hillslopes in the Valley and Ridge province of southwest Virginia. Gradients ranged from 8° to 42°. Regression on slope tangent, depth of clast base below ground surface, clast size, and clast shape explained 70 per cent of clastmovement variance. A substantial part of the unexplained variance appears due to variation in substrate and slope aspect. Additional measurements made on two steep boulder streams showed extremely low rates.     

Interaction between caldera collapse and eruptive dynamics during the Campanian Ignimbrite eruption,Phlegraean Fields,Italy

The Campanian Ignimbrite (36000 years B.P.) was produced by the explosive eruption of at least 80 km³ DRE of trachytic ash and pumice which covered most of the southern Italian peninsula and the eastern Mediterranean region. The eruption has been related to the 12-x15-km-diameter caldera located in the Phlegraean Fields, west of Naples. Proximal deposits on the periphery of the Phlegraean Fields comprise the following pyroclastic sequence from base to top: densely welded ignimbrite and lithic-rich breccias (unit A); sintered ignimbrite, low-grade ignimbrite and lithic-rich breccia (unit B); lithic-rich breccia and spatter agglutinate (unit C); and low-grade ignimbrite (unit D). Stratigraphic and componentry data, as well as distribution of accidental lithic types and the composition of pumice clasts of different units, indicate that coarse, lithic-rich breccias were emplaced at different stages during the eruption. Lower breccias are associated with fines-rich ignimbrites and are interpreted as co-ignimbrite lag breccia deposits. The main breccia unit (C) does not grade into a fines-rich ignimbrite, and therefore is interpreted as formed from a distinct lithic-rich flow. Units A and B exhibit a similar pattern of accidental lithic types, indicating that they were erupted from the same area, probably in the E of the caldera. Units C and D display a distinct pattern of lithics indicating expulsion from vent(s) that cut different areas. We suggest that unit C was ejected from several vents during the main stage of caldera collapse. Field relationships between spatter agglutinate and the breccia support the possibility that these deposits were erupted contemporaneously from vents with different eruptive style. The breccia may have resulted from a combination of magmatic and hydrothermal explosive activity that accompanied extensive fracturing and subsidence of the magma-chamber roof. The spatter rags probably derived from sustained and vigorous pyroclastic fountains. We propose that the association lithic-rich breccia and spatter agglutinate records the occurrence of catastrophic piecemeal collapse.     

Some observations concerning the morphology and sedimentology of a protalus rampart,Okstindan, Norway

A protaius rampart up to 4.75 m high and consisting largely of angular clasts up to boulder size is described. Bulk samples contained up to 35 per cent fines by weight, compared with up to 68 per cent fines in adjacent till. Although the rampart material was clast dominated, fines filled much of the void space, in contrast with most other protalus ramparts described in the literature.     

Kimberlite wall-rock fragmentation processes: Venetia K08 pipe development

Current kimberlite pipe development models strongly advocate a downward growth process with the pipe cutting down onto its feeder dyke by means of volcanic explosions. Evidence is presented from the K08 kimberlite pipe in Venetia Mine, South Africa, which suggests that some pipes or sub-components of pipes develop upwards. The K08 pipe in pit exposure comprises >90 vol.% chaotic mega-breccia of country rock clasts (gneiss and schist) and <10 vol.% coherent kimberlite. Sub-horizontal breccia layers, tens of metres thick, are defined by lithic clast size variations and contain zones of shearing and secondary fragmentation. Textural studies of the breccias and fractal statistics on clast size distributions are used to characterize sheared and non-sheared breccia zones and to deduce a fragmentation mechanism. Breccia statistics are compared directly with the statistics of fragmented rock produced from mining processes in order to support interpretations. Results are consistent with an initial stage of brecciation formed by upward-moving collapse of an explosively pre-conditioned hanging wall into a sub-terranean volcanic excavation. Our analysis suggests that the pre-conditioning is most likely to have been caused by explosions, either phreatic or phreatomagmatic in nature, with a total energy output of 2.7 × 10⁹ kJ (656 t of TNT). A second stage of fragmentation is interpreted as shearing of the breccia caused by multiple late kimberlite intrusions and possible bulk movement of material in the pipe conduit related to adjacent volcanism in the K02 pipe.     

Major,minor and trace element abundances in samples from the Apollo 17 station 7 boulder: Implications for the origin of early lunar crustal rocks

Apollo 17 station 7 boulder consortia samples were analyzed for major and minor elements by a combined semimicro atomic absorption spectrophotometric and colorimetric procedure. Lithophile trace element abundances were determined by stable isotope dilution mass spectrometry. Three matrix types samples (77135, 77115, and 77075) were found to be KREEP-rich fragment-laden melts with analogues throughout the Apollo 17 landing site. Of the five clasts analyzed, at least one (77115,19, troctolite) is thought to be a cumulate; 77135, 77115, and 77075 are thought to have originated by impact fusion of material with similar composition. This original material may represent a partial melt of a parent material of the composition of an included, shocked norite breccia (77215).     

A Pliocene shoaling basaltic seamount: Ba Volcanic Group at Rakiraki, Fiji

At Rakiraki in northeastern Viti Levu, the Pliocene Ba Volcanic Group comprises gently dipping, pyroxene-phyric basaltic lavas, including pillow lava, and texturally diverse volcanic breccia interbedded with conglomerate and sandstone. Three main facies associations have been identified: (1) The primary volcanic facies association includes massive basalt (flows and sills), pillow lava and related in-situ breccia (pillow-fragment breccia, autobreccia, in-situ hyaloclastite, peperite). (2) The resedimented volcaniclastic facies association consists of bedded, monomict volcanic breccia and scoria lapilli-rich breccia. (3) The volcanogenic sedimentary facies association is composed of bedded, polymict conglomerate and breccia, together with volcanic sandstone and siltstone-mudstone facies. Pillow lava and coarse hyaloclastite breccia indicate a submarine depositional setting for most of the sequence. Thick, massive to graded beds of polymict breccia and conglomerate are interpreted as volcaniclastic mass-flow deposits emplaced below wave base. Well-rounded clasts in conglomerate were reworked during subaerial transport and/or temporary storage in shoreline or shallow water environments prior to redeposition. Red, oxidised lava and scoria clasts in bedded breccia and conglomerate also imply that the source was partly subaerial. The facies assemblage is consistent with a setting on the submerged flanks of a shoaling basaltic seamount. The coarse grade and large volume of conglomerate and breccia reflect the high supply rate of clasts, and the propensity for collapse and redeposition on steep palaeoslopes. The clast supply may have been boosted by vigorous fragmentation processes accompanying transition of lava from subaerial to submarine settings. The greater proportion of primary volcanic facies compared with resedimented volcaniclastic and volcanogenic sedimentary facies in central and northwestern exposures (near Rakiraki) indicates they are more proximal than those in the southeast (towards Viti Levu Bay). The proximal area coincides with one of two zones where NW-SE-trending mafic dykes are especially abundant, and it is close to several, small, dome-like intrusions of intermediate and felsic igneous rocks. The original surface morphology of the volcano is no longer preserved, though the partial fan of bedding dip azimuths in the south and east and the wide diameter (exceeding 20 km) are consistent with a broad shield.     

Characteristics of submarine pumice-rich density current deposits sourced from turbulent mixing of subaerial pyroclastic flows at the shoreline: field and experimental assessment

This study investigates the types of subaqueous deposits that occur when hot pyroclastic flows turbulently mix with water at the shoreline through field studies of the Znp marine tephra in Japan and flume experiments where hot tephra sample interacted with water. The Znp is a very thick, pumice-rich density current deposit that was sourced from subaerial pyroclastic flows entering the Japan Sea in the Pliocene. Notable characteristics are well-developed grain size and density grading (lithic-rich base, pumice-rich middle, and ash-rich top), preponderance of sedimentary lithic clasts picked up from the seafloor during transport, fine ash depletion in coarse facies, and presence of curviplanar pumice clasts. Flume experiments provide a framework for interpreting the origin and proximity to source of the Znp tephra. On contact of hot tephra sample with water, steam explosions produced a gas-supported pyroclastic density current that advanced over the water while a water-supported density current was produced on the tank floor from the base of a turbulent mixing zone. Experimental deposits comprise proximal lithic breccia, medial pumice breccia, and distal fine ash. Experiments undertaken with cold, water-saturated slurries of tephra sample and water did not produce proximal lithic breccias but a medial basal lithic breccia beneath an upper pumice breccia. Results suggest the characteristics and variations in Znp facies were strongly controlled by turbulent mixing and quenching, proximity to the shoreline, and depositional setting within the basin. Presence of abundant curviplanar pumice clasts in submarine breccias reflects brittle fracture and dismembering that can occur during fragmentation at the vent or during quenching. Subsequent transport in water-supported pumiceous density currents preserves the fragmental textures. Careful study is needed to distinguish the products of subaerial versus subaqueous eruptions.     

Theoretical cosmogenic nuclide concentration in river bed load clasts: Does it depend on clast size?

Terrestrial Cosmogenic Nuclides (TCNs) have been widely used to date the exposure of alluvial surfaces and to estimate catchment-scale erosion rates. However, TCN concentration differences in samples of different grain sizes remain to be fully understood. In order to explore the possibility that river processes generate such differences, we develop a numerical model to calculate along-stream clast-scale TCN concentrations. Using the hillslope model, there is a progressive detachment of successive clasts of specific sizes followed by their instantaneous fall into the river. In the river, transport velocity and TCN concentration evolution in a clast depend 1) on the probability of being trapped in the sediment mixing layer of the river or within an adjacent terrace; 2) on its size which decreases downstream by attrition. The size-dependent transport law corresponds to the partial transport state in a river. We model the distribution of TCN concentrations in different clast size fractions in the 0–5 cm radius range for catchments in steady-state erosion, and for catchments experiencing sedimentation.We propose that clast attrition tends to increase the variance of TCN concentrations of the small clast size fractions because these fractions incorporate initially big clasts that travelled a long distance in addition to small clasts contributed near the outlet. We obtained numerous clast size–TCN concentration correlations, positive or negative, the significance of which depends on the initial clast size distribution, hillslope erosion rate, river length and lithology. For an equilibrium catchment, even large, we found that the addition of TCN concentration acquired during river transport is negligible compared to TCN concentration acquired on a hillslope, although a clast size–TCN concentration relationship can result from or be modified by clast attrition. On the contrary, aggrading catchments may show a significant clast size-dependent TCN concentration increase during river transport. This may introduce a small bias in the TCN-derived catchment erosion rate, but it could be used positively to quantify the mean transport velocity of clasts of different sizes over thousands of years. In addition, the lack of correlation between TCN concentration and clast size does not imply that the mean transport velocity is the same for all clast size fractions. Overall, our study provides an alternative explanation for observed clast size-dependent TCN concentrations and brings to the fore the need for measuring TCN concentration in larger clast size fractions than is usually done. To see if the byproducts of abrasion dilute or increase the TCN concentration of sand, all products should be included in a future study.     

Noble gases in the St. Mesmin chondrite: Implications to the irradiation history of a brecciated meteorite

Helium, neon and argon were analysed in matrix samples and in different clasts of the polymict-brecciated LL-chondrite St. Mesmin. All clasts have high K-Ar ages with a mean value of4.40 ± 0.26Ga. One exotic H-group xenolith, however, has a K-Ar age of only1.36 ± 0.05Ga. The low age indicates that the St. Mesmin breccia was compacted to its present structure relatively late in its history and that the St. Mesmin meteorite developed from regolith material on the meteorite's parent body. This is further demonstrated by the high concentrations of solar noble gases in the matrix and the cosmic ray pre-exposure of one individual clast.     

Vesiculation of high fountaining Hawaiian eruptions: episodes 15 and 16 of 1959 Kīlauea Iki

The 1959 summit eruption of Kīlauea volcano produced the highest recorded Hawaiian fountain in Hawai‘i. Quantitative analysis of closely spaced samples from the final two high-fountaining episodes of the eruption result in a fine-scale textural study of pyroclasts and provide a record of postfragmentation processes. As clast vesicularity increases, the vesicle number density decreases and vesicle morphology shifts from small and round to larger and more irregular. The shift in microtexture corresponds to greater degrees of postfragmentation expansion of clasts with higher vesicularity. We suggest the range of clast morphologies in the deposit is related to thermal zonation within a Hawaiian fountain where the highest vesicularity clasts traveled in the center and lowest traveled along the margins. Vesicle number densities are greatest in the highest fountaining episode and therefore scale with intensity of activity. Major element chemical analyses and fasciculate crystal textures indicate microlite-rich zones within individual clasts are portions of recycled lava lake material that were incorporated into newly vesiculating primary melt.     

Reliability of geomagnetic paleointensity data: the effects of the NRM fraction and concave-up behavior on paleointensity determinations by the Thellier method

To test the reliability of the Thellier method for paleointensity determinations, we studied six historic lavas from Hawaii and two Gauss-age lava flows from Raiatea Island (French Polynesia). Our aim is to investigate the effects of the NRM fraction and concave-up behavior of NRM–thermal remanent magnetization (TRM) diagrams on paleointensity determinations. For the Hawaiian samples, the paleointensity results were investigated at both sample and site levels. For consistency and confidence in the paleointensity results, it is important to measure multiple samples from each cooling unit. The results from the Raiatea Island samples confirm that reliable paleointensities can be obtained from NRM–TRM diagrams with concave-up curvature, provided the data are accompanied by successful partial TRM (pTRM) checks and no significant chemical remanent magnetization (CRM) production. We conclude that reliable determinations of the paleofield strength require analyses of linear segments representing at least 40–50% of the total NRM. This new criterion has to be considered for future studies and for evaluating published paleointensities for calculating average geomagnetic field models. Using this condition together with other commonly employed selection criteria, the observed mean site paleointensities are typically within 10% of the Definitive Geomagnetic Reference Field (DGRF). Our new results for the Hawaii 1960 lava flow are in excellent agreement with the expected value, in contrast to significant discrepancies observed in some earlier studies.

Overestimates of paleointensity determinations can arise from cooling-rate dependence of TRM acquisition, viscous remanent magnetization (VRM) at elevated temperatures, and TRM properties of multidomain (MD) particles. These outcomes are exaggerated at lower temperature ranges. Therefore, we suggest that, provided the pTRM checks are successful and there is no significant CRM production, it is better to increase the NRM fraction used in paleointensity analyses rather than to maximize correlation coefficients of line segments on the NRM–TRM diagrams.

We introduce the factor, Q = Nq, to assess the quality of the weighted mean paleointensity, H_w, for each cooling unit.     

Processes at the margins of supraglacial debris cover: Quantifying dirty ice ablation and debris redistribution

Current glacier ablation models have difficulty simulating the high-melt transition zone between clean and debris-covered ice. In this zone, thin debris cover is thought to increase ablation compared to clean ice, but often this cover is patchy rather than continuous. There is a need to understand ablation and debris dynamics in this transition zone to improve the accuracy of ablation models and the predictions of future debris cover extent. To quantify the ablation of partially debris-covered ice (or ‘dirty ice’), a high-resolution, spatially continuous ablation map was created from repeat unmanned aerial systems surveys, corrected for glacier flow in a novel way using on-glacier ablation stakes. Surprisingly, ablation is similar (range ~ 5 mm w.e. per day) across a wide range of percentage debris covers (~ 30–80%) due to the opposing effects of a positive correlation between percentage debris cover and clast size, countered by a negative correlation with albedo. Once debris cover becomes continuous, ablation is significantly reduced (by 61.6% compared to a partial debris cover), and there is some evidence that the cleanest ice (<~ 15% debris cover) has a lower ablation than dirty ice (by 3.7%). High-resolution feature tracking of clast movement revealed a strong modal clast velocity where debris was continuous, indicating that debris moves by creep down moraine slopes, in turn promoting debris cover growth at the slope toe. However, not all slope margins gain debris due to the removal of clasts by supraglacial streams. Clast velocities in the dirty ice area were twice as fast as clasts within the continuously debris-covered area, as clasts moved by sliding off their boulder tables. These new quantitative insights into the interplay between debris cover characteristics and ablation can be used to improve the treatment of dirty ice in ablation models, in turn improving estimates of glacial meltwater production. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd