Diamictite and oolitic ironstones, a sedimentary association at Ordovician–Silurian transition in the north Gondwana margin: New evidence from the inner nappe of Sardinia Variscides (Italy)

The contact between the Silurian black phyllite and the Cambro–Ordovician underlying rocks has been investigated over different tectonic units, affected by green-schist facies metamorphism, in the inner nappe zone of the Sardinia Variscides. In spite of strain and metamorphism, the field work highlighted the occurrence of diamictic sediments. In the Canaglia Tectonic Unit the diamictite consists of dark, massive metamorphic claystone bearing chamositic ooliths, chamositic nodules and millimetre to centimetre sized clasts, dispersed, or gathered in clusters, within the muddy matrix. In the Argentiera Tectonic Unit the diamictite consists of angular clasts, ranging in size from few millimetres to several decimetres, scattered within a finely laminated black sericitic meta-argillite. Field data, textural and compositional analyses suggest a glacio-marine environment for the formation of the diamictites.

The Canaglia diamictite deposited in a protected, glacial-influenced, shore. Compositionally it can be defined as ironstone; in the Upper part it hosts a horizon of clast-supported conglomeratic hard ironstone, mostly made of magnetite, which testifies for sub-aerial reworking. The source of the iron is to be related to local, glacio-eustatic driven, emergence of Upper Ordovician alkaline mafic volcanics. These are widespread in the uppermost Ordovician of the Canaglia Unit, possibly linked to the rifting stage that invested the north Gondwana margin, before the uppermost Ordovician–early Silurian sea level rise.

The Argentiera diamictite deposited beyond the iron-rich diamictite in the outer euxinic shelf that was reached by rain out of rafted debris.     

Deep groundwater flow and geochemical processes in limestone aquifers: evidence from thermal waters in Derbyshire, England, UK

Thermal waters potentially provide information on geochemical processes acting deep within aquifers. New isotopic data on groundwater sulphate, inorganic carbon and strontium in thermal and non-thermal waters of a major limestone aquifer system in Derbyshire, England, UK, are used to constrain sulphate sources and groundwater evolution. Shallow groundwaters gain sulphate from oxidation of sulphide minerals and have relatively ¹³C-depleted dissolved inorganic carbon (DIC). Thermal waters have relatively high Sr/Ca and more ¹³C-enriched DIC as a result of increased water–rock interaction. In other respects, the thermal waters define two distinct groups. Thermal waters rising at Buxton have higher Mg, Mn and ⁸⁷Sr/⁸⁶Sr and lower Ca and SO₄, indicating flow from deep sandstone aquifers via a high permeability pathway in the limestone. By contrast, Matlock-type waters (97% of the thermal flux) have elevated sulphate concentrations derived from interaction with buried evaporites, with no chemical evidence for flow below the limestone. About 5% of the limestone area's groundwater flows to the Matlock group springs via deep regional flow and the remainder flows via local shallow paths to many non-thermal springs. Gypsum dissolution has produced significant tertiary porosity and tertiary permeability in the carbonate aquifer and this is an essential precursor to the development of karstic drainage.

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Resumen Las aguas termales potencialmente proporcionan información sobre procesos geoquímicos que actúan a profundidad en acuíferos. Nuevos datos isotópicos de sulfatos, carbón inorgánico y estroncio en aguas termales y no-termales de un acuífero importante de caliza en Derbyshire, Inglaterra se utilizan para delinear las fuentes de sulfato y la evolución de aguas subterráneas.Las aguas subterráneas no muy profundas adquieren sulfato a través de la oxidación de minerales de sulfuro y poseen carbón inorgánico disuelto (DIC) relativamente deplatado de ¹³C. Las aguas termales muestran un ratio Sr/Ca relativamente alto y poseen (DIC) más enriquecido en¹³C, como resultado de la mayor interacción de roca-agua. En otros aspectos, los aguas termales definen dos grupos distintivos. Las aguas termales que ascienden en Buxton tienen mas Mg, Mn y ⁸⁷Sr/⁸⁶Sr y menos Ca and SO₄, indicando flujo de acuíferos de areniscas profundas por un sendero de alta permeabilidad en la caliza.En contraste el tipo de agua - Matlock (97% del flujo termal) posee altas concentraciones de sulfato, derivado por interacción con evaporitas enterradas, sin evidencia química de flujo debajo de la caliza. Aproximadamente 5% del agua del área de la caliza se fluye al grupo de manantiales de Matlock a través de un flujo regional profundo y el resto fluye por senderos locales poco profundos a muchos de los manantiales no-termales. La disolución de yeso ha producido porosidad terciaria importante así como permeabilidad en el acuífero de carbonato y este es un precursor esencial del desarrollo de drenaje kárstico.

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Résumé Les eaux thermales peuvent apporter des informations sur les processus géochimiques dans les aquifères profonds. De nouvelles données isotopiques sur les sulfates présents dans les eaux souterraines, le carbone inorganique et le strontium dans les eaux thermales et non thermales d'un système aquifère calcaire majeur dans le Derbyshire, Angleterre, Royaume Uni, sont utilisées pour comprendre les sources de sulfates et l'évolution des eaux souterraines. Les eaux souterraines phréatique s'enrichissent en sulfate via l'oxydation des minéraux sulfatés et ont un Carbone Inorganique Dissous (DIC) relativement appauvri en¹³C. Les eaux thermales ont un rapport Sr/Ca relativement plus élevé et un DIC plus enrichi en¹³C, du fait de l'interaction accrue des eaux avec les roches. En d'autres mots, les eaux thermales définissent deux groupes distincts. Les eaux thermales remontant à Buxton ont un Mg, un Mn et un rapport ⁸⁷Sr/⁸⁶Sr plus hauts, mais un Ca et SO₄ plus faible, indiquant un écoulement à travers les zones perméables des aquifers gréseux. Par contraste, les eaux du type de Matlock (97% du flux thermique), possèdent des concentrations élevées en sulfates, provenant de l'interaction des eaux avec les évaporites enfouies, tandis qu'il n'existe aucune évidence chimique d'un écoulement sous les calcaires. Sur environ 5% de la surface des calcaires, les eaux souterraines alimentent des sources non-thermales. La dissolution du Gypse a produit une porosité tertiaire significative et une perméabilité dans les aquifères calcaires, et ceci est un précurseur essentiel au développement du drainage karstique.

     

青藏高原东南缘腾冲后碰撞粗面安山岩形成的深部岩浆过程:来自辉长质物质的启示

位于青藏高原东南缘的腾冲火山区发育大量的后碰撞高钾钙碱性岩浆岩(8Ma至今),形成了近连续的玄武岩-玄武安山岩-粗面安山岩-英安岩系列。在晚更新世(0.3~0.4Ma)粗面安山岩中发现了大量的辉长岩包体与辉长质矿物聚晶。辉长岩包体分为两类：I类为辉长苏长岩(直径2~4cm),主要由斜长石(50%~60%)、单斜辉石(20%~30%)和斜方辉石(5%~10%)组成,矿物间呈高角度接触的开放结构并包含少量的基质玻璃;Ⅱ类辉长岩包体(直径2~12cm)主要由斜长石(40%~50%)与单斜辉石(30%~40%)组成,含少量铁钛氧化物(5%~10%),矿物间见少量的基质玻璃与微晶斜方辉石(粒径 < 50μm)。矿物聚晶(粒径2~5mm)由斜长石(40%~60%)、单斜辉石(20%~30%)和斜方辉石(5%~10%)组成,矿物间呈高角度接触的开放结构,其间贯入基质玻璃。辉长岩包体、矿物聚晶与寄主粗面安山岩斑晶具有相同的矿物组成(斜长石+单斜辉石+斜方辉石);单斜辉石具有相似的微量元素组成特征;辉长岩包体全岩主量元素成分落在玄武岩-玄武安山岩-粗面安山岩-英安岩的演化序列中,它们与寄主粗面安山岩具有一致的Sr-Nd-Pb同位素组成。上述观测结果表明,辉长岩包体和矿物聚晶与寄主岩浆同源,它们均来自粗面安山质岩浆房。矿物温度与压力计算结果表明,辉长岩包体与矿物聚晶的结晶温度低于斑晶,形成深度位于粗面安山质岩浆房的中上部。这些具有开放结构的矿物聚晶与辉长岩包体可能代表粗面安山质岩浆喷发前形成晶粥的"碎片"：矿物聚晶与I类辉长岩包体矿物间基质含量较高,矿物成分与寄主粗面安山质熔体(全岩成分)平衡,可能代表寄主粗面安山质岩浆形成的晶粥;Ⅱ类辉长岩包体矿物间紧密程度较高,矿物与较演化的岩浆平衡,可能代表早期较演化的岩浆形成的经历压实作用的晶粥。粗面安山质岩浆的快速上升将这些晶粥破碎并以辉长岩包体与矿物聚晶的方式运移至地表。岩浆房中基性岩浆的补给是导致安山质岩浆喷发的重要诱发机制。     

Thermotectonic evolution of a rifted continental margin: fission track evidence from the Kangerlussuaq area, SE Greenland

Fission track analyses of apatites from sediments, Precambrian gneisses and Caledonian to Tertiary intrusive rocks from the Kangerlussuaq region reveal its post-Caledonian thermotectonic history. Inland the history involves cooling to temperatures within the high temperature part of the apatite annealing interval and slow cooling (or reheating) continued in Cretaceous times through this interval. Apatites from coastal areas between Kangerlussuaq and Tasiilaq reveal only Tertiary cooling. In Tertiary times cooling accelerated after the main intrusive phase in the Tertiary. The evolution is taken as evidence for a general uplift/erosion since Caledonian times probably disturbed by basin formation and sedimentation and reheating due to magmatic activity. Thermal subsidence of the rift shoulders following the opening of the adjacent oceanic basin is not indicated. Annealing patterns inland of the plume centre in the Kangerlussuaq provide no evidence for the earlier movement of the plume from a westerly direction.     

Sedimentologic evidence for outburst floods from the Laurentide Ice Sheet margin in Wisconsin, USA: implications for tunnel-channel formation

Clast-supported boulder gravel in outwash-fans along the glacial-maximum margin of the Laurentide Ice Sheet in Wisconsin indicates the occurrence of outburst floods. These sediments, with clast intermediate axes of up to 2 m, are located downstream of tunnel channels and were deposited shortly before cessation of glaciofluvial activity on each fan. Since tunnel channels with fans are widespread along the ice-sheet margin in the western Great Lakes region, these outburst floods were probably common. Paleodischarge estimates derived from the boulder deposits are poorly constrained, but values of at least several hundred m³ s⁻¹ are likely. Four potential water sources for the floods exist: an extreme surface-melt event, an extreme precipitation event, drainage of supraglacial lakes, or drainage of stored subglacial meltwater. We focus on the storage of subglacial meltwater behind the ice-sheet margin, as proglacial permafrost was present as ice advanced to its maximum extent, and a frozen-bed zone upstream from the margin probably impeded drainage through groundwater aquifers. Decay of this permafrost ‘seal’ would have eventually allowed trapped water to drain through the tunnel channels. We suggest that the 2-m boulders were entrained in an outburst of subglacial water that enlarged a pre-existing channel cut by ablation-derived flows.     

Lower Oligocene thrust-system in the epi-Ligurian succession: evidence from the Enza Valley (northern Apennines,Italy)

Within the lower part (Upper Eocene-Oligocene) of the epi-Ligurian succession, outcropping in the Emilian side of the northern Apennines (Enza Valley), duplications by thrust tectonics were recognized through the systematic integration of field geology with calcareous nannofossil biostratigraphy. This thrust system, derived from the overthrusting of two thrust-sheets over a footwall, is unconformably overlain by a Rupelian succession. The thrust structure of the Enza Valley, affected by a subsequent wide overturned syncline together vith the unconformable succession, shows a remarkable Lower Oligocene contractional tectonics, previously not recognized in the northern Apennines.The comparison of this thrust system with other outcropping areas of the epi-Ligurian succession makes probable the wide-spread occurrence of the Lower Oligocene tectonics in the uppermost structural levels of the chain (epi-Ligurian domain).In a regional tectonic framework, the Rupelian thrust tectonics affecting the epi-Ligurian succession can be related to the Lower Oligocene closure of the innermost portion of the Subligurian basin (Aveto-Petrignacola Formation) due to the NE-verging overthrusting of the External Ligurian Units. In this context the unconformable succession of the Enza Valley seems correlable with the basal portion of the Subligurian Eratica Sandstone (Rupelian-Chattian) which unconformably overlies a deformed substratum (Mesoalpine Phase).     

青藏高原西北缘盆山过渡带陡坡地貌的形成时代与成因

平均海拔大于4500 m的青藏高原,是通过高原边缘的陡坡地貌与海拔低于1500 m的周缘盆地或平原相连接的,这些围绕高原的陡坡地貌是何时、如何形成的呢？本文通过对西昆仑山中段北缘主逆冲断层上盘陡坡地貌区9件磷灰石样品的裂变径迹年龄与长度分析表明：在海拔3900～4635 m的陡坡地貌中的裂变径迹样品年龄为6.2±1.4 Ma～0.9±0.3 Ma,呈现“上新下老”的反序分布特征; 而通过热历史模拟显示约5 Ma,约3～2 Ma,约2～1 Ma 和约1 Ma该地区出现多阶段的隆升与剥露。结合前人研究成果和野外地质的观察认为,现今青藏高原西北缘陡坡地貌的形成是中新世晚期以来高原边界叠瓦状断裂系经历了约8 Ma、约5 Ma、约3～2 Ma、约2～1 Ma和约1 Ma多阶段后展式逆冲运动的结果,这为青藏高原周缘陡坡地貌的形成和青藏高原的隆升时代与型式提供了关键的热年代学约束。     

Hyperpycnal-fed turbidite lobe architecture and recent sedimentary processes: A case study from the Al Batha turbidite system, Oman margin

The main sediment depocenter along the Oman margin is the Al Batha turbidite system that develops in the Gulf of Oman basin. It is directly connected to the wadi Al Batha, and forms a typical sand and mud rich point source system that acts as regional sediment conduit and feeds a ~ 1000 km² sandy lobe.The Al Batha lobe depositional architecture has been investigated in detail using very high-resolution seismic, multibeam echosounder data and sediment cores. Several scales of depositional architecture can be observed. The Al Batha lobe is composed of several depositional units, made of stacked elementary sediment bodies (thinner than 5 m) that are each related to a single flow event. The lobe is connected to the feeder system through a channel-lobe transition zone (CLTZ) that extends on more than 25 km. The lobe can be divided into proximal, middle and distal lobe areas. The proximal lobe is an area of erosion and by-pass with small axial feeder channels that rapidly splay into several small distributaries. They disappear in the mid-lobe area where deposits consist of vertically stacked tabular to lens-shaped sediment bodies, with a lateral continuity that can exceed 10 km. The distal lobe fringe shows a classical facies transition towards thin-bedded basin plain deposits.Sub-surface deposits consist of sandy turbidites and hyperpycnites, interbedded with fine-grained deposits (thin turbidites, hyperpycnites, or hemipelagites). Although these distal deposits are mainly related to flow transformations and concentration evolution, they highlight the importance of flooding of the wadi Al Batha on the sediment transfer to the deep basin. The thick sandy hyperpycnites recovered in such a distal area are also possibly related to the initial properties of gravity flows, in relation to the flooding characteristics of mountainous desert streams.Finally, the Al Batha lobe depositional architecture is typical of sand-rich lobes found within “small”, sand and mud rich turbidite systems fed by mountainous “dirty” rivers. Turbidite sedimentation in the Al Batha system appears to be primarily controlled by the strong climatic and geomorphic forcing parameters (i.e. semi-arid environment with ephemeral, mountainous rivers subjected to flash-flooding).     

青藏高原西北缘盆山过渡带陡坡地貌的形成时代与成因

平均海拔大于4500 m的青藏高原,是通过高原边缘的陡坡地貌与海拔低于1500 m的周缘盆地或平原相连接的,这些围绕高原的陡坡地貌是何时、如何形成的呢？本文通过对西昆仑山中段北缘主逆冲断层上盘陡坡地貌区9件磷灰石样品的裂变径迹年龄与长度分析表明：在海拔3900～4635 m的陡坡地貌中的裂变径迹样品年龄为6.2±1.4 Ma～0.9±0.3 Ma,呈现“上新下老”的反序分布特征; 而通过热历史模拟显示约5 Ma,约3～2 Ma,约2～1 Ma 和约1 Ma该地区出现多阶段的隆升与剥露。结合前人研究成果和野外地质的观察认为,现今青藏高原西北缘陡坡地貌的形成是中新世晚期以来高原边界叠瓦状断裂系经历了约8 Ma、约5 Ma、约3～2 Ma、约2～1 Ma和约1 Ma多阶段后展式逆冲运动的结果,这为青藏高原周缘陡坡地貌的形成和青藏高原的隆升时代与型式提供了关键的热年代学约束。     

The space problem of caldera resurgence: an example from Ischia Island, Italy

A space problem can arise in a resurgent caldera when the resurgent block is non-cylindrical, such as, for example, when it is bounded by inward- or outward-dipping faults. Ischia caldera (Italy) is an excellent case study because it is well exposed and resurgence is ongoing. On the western and eastern flanks of the Ischia resurgent horst, uplift occurred along NNW-striking normal faults with inclination from sub-vertical to vertical (>85°). The geometry of these faults suggests negligible extension within the horst. Along the northern flank, uplift was accomplished by ENE-striking normal faults that dip 60–85° outward; a few bear striae which indicate almost pure dip-slip. The southern flank of the horst is a monocline trending ENE associated with vertical faults. In a NNW–SSE section, the resulting resurgent horst has a wedge shape with an upward apex. The uplift of this wedge can be accommodated by contemporaneous regional extension along NE- to east–west striking normal faults whose motions create space for resurgence without deformation of the caldera floor. Similar interaction with regional tectonics could exist in other calderas, such as Yellowstone (USA) in an extensional setting, Los Azufres (Mexico) in a transtensional regime and Chalupas (Ecuador) in a transpressional one. At other calderas, resurgence was accommodated by caldera-floor arching as at Valles (USA) or by shortening deformations between the caldera rim and the uplifting block as at Latera (Italy).     

Relationships between glacier and rock glacier in the Maritime Alps, Schiantala Valley, Italy

In the Schiantala Valley of the Maritime Alps, the relationship between a till-like body and a contiguous rock glacier has been analyzed using geomorphologic, geoelectric and ice-petrographic methodologies. DC resistivity tomographies undertaken in the till and in the rock glacier show the presence of buried massive ice and ice-rich sediments, respectively. Ice samples from a massive ice outcrop show spherical gas inclusions and equidimensional ice crystals that are randomly orientated, confirming the typical petrographic characteristics of sedimentary ice. The rock glacier formation began after a phase of glacier expansion about 2550 ± 50 ¹⁴C yr BP. Further ice advance during the Little Ice Age (LIA) overrode the rock glacier root and caused partial shrinkage of the pre-existing permafrost. Finally, during the 19th and 20th centuries, the glacial surface became totally debris covered. Geomorphological and geophysical methods combined with analyses of ice structure and fabric can effectively interpret the genesis of landforms in an environment where glaciers and permafrost interact. Ice petrography proved especially useful for differentiating ice of past glaciers versus ice formed under permafrost conditions. These two mechanisms of ice formation are common in the Maritime Alps where many sites of modern rock glaciers were formerly occupied by LIA glaciers.     

Relationships between ultrapotassic and carbonate-rich volcanic rocks in central Italy: petrogenetic and geodynamic implications

The Pleistocene intra-Apennine volcanic (IAV) centres occurring east of the potassium-rich Roman comagmatic province show variable petrological and geochemical composition. Some rocks have a strongly undersaturated ultrapotassic kamafugitic affinity with K₂O/Na₂O=8–20, whereas the rocks from the southern center of Mt. Vulture are still strongly undersaturated in silica but are enriched in both Na₂O and K₂O with K/Na around unity. Carbonate-rich pyroclastic rocks, believed to represent carbonatitic magmas, are found in the IAV centers. Kamafugites have high abundances of LILE and high LILE/HFSE ratios, and their incompatible element patterns resemble closely those of ultrapotassic rocks from the adjoining Roman province. The Vulture volcanics also display high contents of LILE, but their LILE/HFSE ratios are intermediate between intraplate alkaline rocks and kamafugites. The carbonate-rich rocks exhibit an exotic mineralogy and high enrichments in LILE, which speaks for a carbonatitic affinity. However, they have similar incompatible element patterns but consistently lower abundances of almost all the elements than the associated silicate volcanics. These data favour the hypothesis that the IAV carbonate rocks may represent mixtures of silicate magmas and geochemically depleted carbonate material. The sedimentary carbonates that crop out extensively along the Apennine chain may be the source of barren carbonate material. Overall, geochemical data of IAV centres and of the rocks from the Roman province display strong geochemical and isotopic evidence of being generated in an upper mantle that was modified by addition of upper crustal material brought down by subduction processes. A possible exception is represented by Mt. Vulture which, however, occurs east of the main axis of the Apennines, on the western margin of the foreland Adria plate. The occurrence of strongly undersaturated alkaline rocks requires magma generation at high pressures and . This is in agreement with the hypothesis that subduction processes under the Apennines occurred by consumption of poorly hydrated thinned or delaminated continental crust.     

Volatile content and degassing processes in the AD 79 magma chamber at Vesuvius (Italy)

The evolution of volatiles in the AD 79 magma chamber at Vesuvius (Italy) was investigated through the study of melt inclusions (MI) in crystals of different origins. FTIR spectroscopy and EMPA were used to measure H₂O, CO₂, S and Cl of the different melts. This allowed us to define the volatile content of the most evolved, phonolitic portion of the magma chamber and of the mafic melts feeding the chamber. MI in sanidine from phonolitic and tephri-phonolitic pumices show systematic differences in composition and volatile content, which can be explained by resorption of the host mineral during syn-eruptive mixing. The pre-eruption content of phonolitic magma appears to have been dominated by H₂O and Cl (respectively 6.0 to 6.5 wt% and 6700 ppm), while magma chamber refilling occurred through the repeated injection of H₂O, CO₂ and S-rich tephritic magmas (respectively 3%, 1500 ppm and 1400 ppm). Strong CO₂ degassing probably occurred during the decompressional path of mafic batches towards the magma chamber, while sulphur was probably released by the magma following crystallization and mixing processes. Water and chlorine strongly accumulated in the magma and reached their solubility limits only during the eruption. Chlorine solubility appears to have been strongly compositionally controlled, and Cl release was inhibited by groundmass crystallization of leucite, which shifted the composition of the residual liquid towards higher Cl solubilities. Received: 28 October 1999 / Accepted: 21 April 2000     

Fracturing,fluid flow and shear zone development: Relationships between chemical and mechanical processes in Proterozoic mafic dykes from southwestern Montana,USA

Fluids can play an important role in the localization of deformation in the deep crust, yet the specific mechanisms active during the complex interactions between metasomatism, metamorphism and deformation remain elusive. Precambrian metagabbronorite dykes in southwest Montana contain fractures filled with Hbl±Grt and discrete cm‐scale shear zones with well‐preserved strain gradients. This system offers an ideal opportunity to constrain the chemical and mechanical processes that facilitated strain localization. An early M₁ assemblage of Grt₁+Cpx₁+Pl₁+Qz developed at conditions of 0.51–0.85 GPa and 500–700°C and is preserved largely as a static replacement of relict igneous phases (Opx, Pgt, Pl) in coronitic textures. An M₂ assemblage characterized by Grt₂+Pl₂±Cpx₂+Hbl+Scp+Qz developed at 0.86–1.00 GPa and 660–730°C coincided with fluid flow and deformation associated with shear zone development. Microstructural observations in marginal protomylonite/mylonite and laminated ultramylonite suggest a shear zone evolution that involved (1) nucleation from pre‐existing fractures that were sites for major fluid infiltration, (2) initial widening coincident with grain‐size reduction by microfracturing, dislocation creep, and synkinematic metamorphic reaction by solution transfer, and (3) a switch in the dominant deformation mechanisms active in the ultramylonite from grain‐size insensitive mechanisms to grain‐size sensitive granular flow accommodated by fluid‐assisted diffusion. Throughout this evolution, the effective bulk compositions of the rock volumes responding to metamorphism changed through a combination of mechanical and metasomatic processes.     

Crystallization processes in the Bjerkreim-Sokndal layered intrusion,south Norway: evidence from the boundary between two macrocyclic units

The Bjerkreim-Sokndal (BKSK) layered intrusion belongs to the Rogaland anorthosite province in southern Norway. The northwestern part of BKSK consists of a ca. 6 km-thick Layered Series, made up of macrocyclic units (MCU) arranged in a syncline. Each MCU, which resulted from the crystallization of a major-magma influx, can be subdivided into a series of cumulate zones. The MCU III/IV boundary has been studied in seven profiles across its strike length of 24 km. Massive piC¹ at the base of MCU IV overlies laminated and modally layered phimC in the central part of the chamber and phimacC towards the flanks; there is a discordance of between 2 and 6° between the base of MCU IV and phase layering in MCU III. The MCU IV piC is overlain by 75–100 m of massive poiC (the Svaalestad unit of Michot 1960; a similar olivine-bearing unit occurs near the base of MCU III) which has more primitive compositions than the underlying piC. This is followed by laminated and modally layered phiC, phimC and phimacC. The reversal to more primitive mineral assemblages across the MCU III/IV boundary is accompanied by a cryptic reversal; plagioclase and Ca-poor pyroxene have compositions of about An 44/Mg no. 71 at the top MCU III and about An 52/Mg no. 77 near the base of MCU IV. Olivines in the MCU IV poiC vary unsystematically from Fo 66 to 76. Macrocyclic units III and IV crystallized from monzonoritic parental magma. The BKSK magma chamber had a broad saucer-like shape with a small thickness to breadth ratio. The magma in the chamber during crystallization of MCU III was compositionally zoned and crystallized on an inward-sloping floor by down-dip accretion. Just before the major-magma influx at the base of MCU IV, phimC was crystallizing from the basal-magma layer at the centre of the chamber, while phimacC was crystallizing towards the flanks. The new, dense magma fountained into and mixed with the basal-magma layers already in the chamber. This hybrid magma crystallized during continued influx to produce massive piC at the base of MCU IV. This hybrid unit is thickest near the centre of the chamber and smoothed out the floor to an essentially horizontal surface. Continued influx resulted in the dense, primitive magma ponding on the floor; this crystallized fairly rapidly to produce the massive poiC unit. The return of normal fractional crystallization conditions is marked by the overlying sequence of modally and cryptically layered cumulates which duplicate the succession in MCU III. The variation in thickness of the upper part of MCU IV indicates that crystallization of the BKSK Layered Series was accompanied by sinking of the floor at a greater rate near the centre of the chamber than towards the flanks. This was accompanied by compaction of the underlying cumulates, promoting the development of lamination and the expulsion of intercumulus melt to encourage the development of adcumulates. ¹ p plagioclase - i ilmenite - h Ca-poor pyroxene - o olivine - m magnetite - a apatite - c Ca-rich pyroxene - C cumulate     

Relationships between Campi Flegrei and Mt. Somma volcanism: evidence from melt inclusions in clinopyroxene phenocrysts from volcanic breccia xenoliths

Summary We present compositions of reheated melt inclusions in clinopyroxene phenocrysts from three mafic xenoliths in Breccia Museo, Campi Flegrei, Italy. Melt inclusion compositions are remarkably different from the compositions of known contemporary Campi Flegrei lavas, being significantly enriched in K₂O and depleted in Na₂O. Some differences are also evident in FeO^* (total Fe as FeO) and TiO₂ contents. The clinopyroxene phenocrysts could not have crystallised from Campi Flegrei magmas. We suggest that they originated from a volcanic system genetically very similar to, and possibly linked with, the >14 ka volcanic system of Mt. Somma, another Campanian volcano ∼ 30 km east from Campi Flegrei, from which Vesuvius subsequently developed. This result indicates a close relationship (or link) between the two volcanic systems which have until now been considered separate. We speculate that the link was established prior to eruption of the Neapolitan Yellow Tuff (NYT) (∼ 12 ka). The xenoliths were derived from a volcanic system older than the host breccias themselves. We suggest that this older volcanism had close similarities with the volcanism of the older products of Mt. Somma (∼25 ka). Received March 20, 2000; accepted November 2, 2000     

Relationships between hydrodynamic parameters and grain size in two contrasting transitional environments: The Lagoons of Venice and Cabras, Italy

A comparison was made of shallow water sediments from the Lagoon of Venice (LV) and the Lagoon of Cabras (LC), comparing depositional environments and exploring the relationships between hydrodynamics and sedimentological parameters. The two water bodies are very different in size (LV: 360 km²; LC: 22 km²), and the sediments predominantly consist of silty-clay (LV: Mz ≈ 26 μm; LC: Mz ≈ 6 μm). However, there are large differences between the two lagoons with respect to sand (LV: mean 19%; LC: mean ~ 3%) and clay (LV: mean 20%; LC: mean 45%) contents. The Lagoon of Venice (mean depth ~ 1 m) can be considered a tidal basin, whereas the Lagoon of Cabras (mean depth ~ 2 m) has the character of a coastal lake in which wind is the main hydrodynamic forcing factor. A comparison of sediment grain-size distributions with water circulation patterns in different parts of the lagoons highlighted some interesting differences. Grain-size analyses of samples reveal a deficiency of particles around 8 μm in the LC, which is interpreted as reflecting the transition between cohesive flocs/aggregates and non-cohesive coarser silt particles, while the transition limit in the LV is ~ 20 μm. Thus, particles are cohesive below 8 μm in the LC and below ~ 20 μm in the LV. This is probably because of the differences in the clay/silt ratio, which is much lower in the LV (~ 0.3) than in LC (~ 1), conferring a “silt-dominated network structure” on most of the LV sediments.The hydrographical data used were root mean square velocity (RMSV) and water residence time (WRT), computed under the main forcing conditions. The results show a general correlation between RMSV and sortable silt in the LC, and between RMSV and coarser sediments (63–105 μm) in the LV. Some significant differences between the lagoons were detected in the degree of correlation between WRT and grain size. Root mean square velocity (~ 7 cm s^− 1 in the LV and ~ 3 cm s^− 1 in the LC) was a greater forcing factor in the LC than in the LV. Conversely, WRT, which is on average ~ 16 days in the LV and ~ 19 days in the LC, has more influence in the LV. This study highlights the usefulness of comparing environments with different hydrodynamic energies, e.g., tidal and/or wind-driven currents, to elucidate and thereby improve our understanding of the processes governing the spatial distribution of sedimentological features, the transport mechanisms of sediments, and the relationship between them. The results demonstrate that the approach outlined in this study has the potential to provide a universal hydro-sedimentological classification scheme.     

Diagenesis of oxyanions (V, U, Re, and Mo) in pore waters and sediments from a continental margin

This research tests the hypothesis that trace metals respond to the extent of reducing conditions in a predictable way. We describe pore water and sediment measurements of iron (Fe), manganese (Mn), vanadium (V), uranium (U), rhenium (Re), and molybdenum (Mo) along a transect off Washington State (USA). Sediments become less reducing away from the continent, and the stations have a range of oxygen penetration depths (depth to unmeasurable O₂ concentration) varying from a few millimeters to five centimeters. When oxygen penetrates ∼1 cm or less, Fe is reduced in the pore waters but reoxidized near the sediment-water interface, preventing a flux of Fe²⁺ to overlying waters, whereas Mn oxides are reduced and Mn²⁺ diffuses to overlying waters. Both Re and U authigenically accumulate in sediments. Only at the most reducing location, where the oxygen penetrates 0.3 cm below the sediment-water interface, does the surface 30 cm of sediments become reducing enough to authigenically accumulate Mo.Stations in close proximity to the Juan de Fuca Ridge crest are enriched in Mn and Fe from hydrothermal plume processes. Both V and Mo clearly associate with Mn cycling, whereas U may be associating with either Mn oxides and/or Fe oxyhydroxides. Rhenium is uncomplicated by adsorption to Mn oxides and/or Fe oxyhydroxides, and Re accumulation in sediments appears to be due solely to the extent of reducing conditions. Therefore, authigenic sediment Re enrichment appears to be the best indicator for intermediate reducing conditions, where oxygen penetrates less than ∼1 cm below the sediment-water interface, when coupled with negligible authigenic Mo enrichment.     

Calcium carbonate dissolution in deep sea sediments: reconciling microelectrode, pore water and benthic flux chamber results

We report the benthic fluxes of O₂, titration alkalinity (TA), Ca²⁺, NO₃⁻, PO₄³⁻, and Si(OH)₄ from in situ benthic flux chamber incubations on the Ceara Rise and Cape Verde Plateau and compare them to previously published results. We find within analytical uncertainty that the TA flux is twice the calcium flux, suggesting that dissolution/precipitation of CaCO₃ is the principal mechanism controlling benthic TA and Ca²⁺ fluxes. At sites where the sediments contain significant (>35%) CaCO₃ and the overlying waters are supersaturated with respect to CaCO₃, the ratios of the total dissolution rate to the remineralization rate are significantly less than at all other study sites. We propose that these observations can be explained by precipitation of fresh CaCO₃ at the supersaturated sediment surface followed by redissolution deeper in the sediments because of metabolically-produced CO₂. A numerical simulation is presented to demonstrate the feasibility of this explanation. In addition, surface exchange reactions in high-CaCO₃ sediments coupled with high rates of particle mixing may also impact rates of metabolic dissolution and depress chamber-derived estimates of carbonate alkalinity and calcium benthic fluxes. These results suggest that at supersaturated, high CaCO₃ locations, previous models of sediment diagenesis may have overestimated the impact of metabolic dissolution on the preservation of CaCO₃ deposited on the sea floor.     

Relationships between joint apparent separation, Schmidt hammer rebound value, and distance to faults, in rocky outcrops, Calabria, Southern Italy

In order to investigate how the apparent separation of jointing varies according to the distance from faults, and how the mechanical properties of rock masses depend on this distance and jointing density, a number of regression analysis were performed on the variables s (apparent joint separation), d (distance from major fault), and sh (rebound value of Schmidt hammer).The variables were measured at 380 stations distributed over a wide study area located in the Aspromonte range in Calabria, Southern Italy.The most significant results of simple regression analysis are expressed by the formulas:

s=c+Fd⁰⁵