ON THE UPWARD CONTINUATION OF POTENTIAL FIELD DATA BETWEEN IRREGULAR SURFACES*

The potential field and its derivatives at points above an irregular surface can be approximately obtained from the sampled potential field data acquired on that surface. A method of minimizing the truncation effect, which appears when gravity and magnetic maps are processed with the aid of surface integrals, is derived. The results are compared with those of the most relevant similar methods by using a theoretical, but realistic, model.     

Sedimentation in a disequilibrium river-dominated estuary: the Raša River Estuary (Adriatic Sea, Croatia)

This paper examines sediment transport, sedimentation and properties of suspended matter and sediments in the Ra?a River estuary, a small, rock-bounded, microtidal, low-wave-energy karstic estuary in the north-eastern Adriatic. The Ra?a River is characterized by large variation in water flow and variable load of mineral particles. More than 90% of this load is brought into the estuary as fine-grained suspended matter, consisting of only 24–36% of carbonates, the rest being clays. Sedimentation occurs at the salt wedge, resulting in a prograding estuarine delta. Salt-induced flocculation is the predominant process of sediment deposition. The Ra?a estuary is infilling with sediment, and classifies as a disequilibrium estuary. We propose a modification of Cooper's (1993) classification scheme to include river-dominated, disequilibrium estuaries, with the Ra?a River as an example.     

High-density nitrogen inclusions in barite from a giant siderite vein: implications for Alpine evolution of the Variscan basement of Western Carpathians, Slovakia

Contrasting compositions and densities of fluid inclusions were revealed in siderite–barite intergrowths of the Dro?diak polymetallic vein hosted in Variscan basement of the Gemeric unit (Central European Carpathians). Primary two‐phase aqueous inclusions in siderite homogenized between 101 and 165 °C, total salinity ranged between 18 and 27 wt%, and CaCl₂/(NaCl + CaCl₂) weight ratios were fixed at 0.1–0.3. By contrast, mono‐ and two‐phase aqueous inclusions in barite exhibited total salinities between 2 and 22 wt%, and the CaCl₂/NaCl ratios ranged from NaCl‐ to CaCl₂‐dominated compositions. The aqueous inclusions in barite were closely associated with very high‐density (0.55–0.745 g cm^?3) nitrogen inclusions, in some cases containing up to 16 mol.% CO₂. Crystallization P–T conditions of siderite (175–210 °C, 1.2–1.7 kbar) constrained by the vertical oxygen isotope gradient along the studied vein, isochores of fluid inclusions and the K/Na exchange thermometer corresponded to minimal palaeodepths between 4.3 and 6.3 km, assuming lithostatic load and average crust density of 2.75 g cm^?3. Maximum fluid pressure during barite crystallization attained 3.6–4.4 kbar at 200–300 °C, and the most dense nitrogen inclusions maintained without decrepitation the residual internal pressure of 2.2 kbar at 25 °C. Contrasting fluid compositions, increasing depths of burial (～4–14 km) and decreasing thermal gradients (～40–15 °C km^?1) during initial mineralization stages of the Dro?diak vein reflect Alpine orogenic processes, rather than an incipient Permian rifting suggested in previous metallogenetic models. Siderite crystallized at rising P–T in a closed, rock‐buffered hydrothermal system developed in the Variscan basement during the north‐vergent Cretaceous thrusting and thickening of the Gemeric crustal wedge. Variable salinities of the barite‐hosted inclusions reflect a fluid mixing in open hydrothermal system, and re‐equilibration textures (lengths of decrepitation cracks proportional to fluid inclusion sizes) correspond to retrograde crystallization trajectory coincidental with transpression or unroofing. Maximum recorded fluid pressures indicate ～12‐km‐thick pile of imbricated nappe units accumulated over the Gemeric basement during the Cretaceous collision.     

Petrology and Geochemistry of West Philippine Basin Basalts and Early Palau-Kyushu Arc Volcanic Clasts from ODP Leg 195, Site 1201D: Implications for the Early History of the Izu-Bonin-Mariana Arc

Site 1201D of Ocean Drilling Program Leg 195 recovered basalticand volcaniclastic units from the West Philippine Basin thatdocument the earliest history of the Izu–Bonin–Marianaconvergent margin. The stratigraphic section recovered at Site1201D includes 90 m of pillow basalts, representing the WestPhilippine Basin basement, overlain by 459 m of volcaniclasticturbidites that formed from detritus shed from the Eocene–Oligoceneproto-Izu–Bonin–Mariana island arc. Basement basaltsare normal mid-ocean ridge basalt (N-MORB), based on their abundancesof immobile trace elements, although fluid-mobile elements areenriched, similar to back-arc basin basalts (BABB). Sr, Nd,Pb and Hf isotopic compositions of the basement basalts aresimilar to those of basalts from other West Philippine Basinlocations, and show an overall Indian Ocean MORB signature,marked by high ²⁰⁸Pb/²⁰⁴Pb for a given ²⁰⁶Pb/²⁰⁴Pb and high¹⁷⁶Hf/¹⁷⁷Hf for a given ¹⁴³Nd/¹⁴⁴Nd. Trace element and isotopicdifferences between the basement and overlying arc-derived volcaniclasticsare best explained by the addition of subducted sediment orsediment melt, together with hydrous fluids from subducted oceaniccrust, into the mantle source of the arc lavas. In contrastto tectonic models suggesting that a mantle hotspot was a sourceof heat for the early Izu–Bonin–Mariana arc magmatism,the geochemical data do not support an enriched, ocean islandbasalt (OIB)-like source for either the basement basalts orthe arc volcanic section. KEY WORDS: back-arc basalts; Izu–Bonin–Marianas; Philippine Sea; subduction initiation; Ocean Drilling Program Leg 195     

Whiting events and the formation of aragonite in Mediterranean Karstic Marine Lakes: new evidence on its biologically induced inorganic origin

This paper discusses the formation of whiting events, drifting milky clouds of water, and their role in the formation of recent aragonite sediments in a semi-enclosed, karstic, marine lake on the island of Mljet (Adriatic Sea). This study is based on detailed structural, morphological and sedimentological characterization of, and strontium distribution in, particles originating from suspended matter and sediments. The particles were examined by X-ray diffraction and electron diffraction analyses, scanning and transmission electron microscopy, as well as electron microprobe energy dispersive X-ray analyses and atomic absorption spectroscopy. Morphological features, granulometric characteristics, strontium enrichment and characteristic details of the aragonite structure in the needle-like particles were identical in both the suspended matter collected during whiting events and in the bottom sediments. The whiting events, which occasionally occurred in surface waters, were found to be sites of short-term active authigenic aragonite precipitation and the main source of fine-grained aragonite mud. This study exemplifies the role of biologically induced inorganic precipitation processes in the formation of recent aragonite mud in a restricted Mediterranean environment.