RADON DEPTH MIGRATION1

A depth migration method is presented that uses Radon-transformed common-source seismograms as input. It is shown that the Radon depth migration method can be extended to spatially varying velocity depth models by using asymptotic ray theory (ART) to construct wavefield continuation operators. These operators downward continue an incident receiver-array plane wave and an assumed point-source wavefield into the subsurface. The migration velocity model is constrained to have longer characteristic wavelengths than the dominant source wavelength such that the ART approximations for the continuation operators are valid. This method is used successfully to migrate two synthetic data examples:

• 1 a point diffractor, and
• 2 a dipping layer and syncline interface model.

It is shown that the Radon migration method has a computational advantage over the standard Kirchhoff migration method in that fewer rays are computed in a main memory implementation.     

The “Zero-order Model” revisited: Paul Schindler's influence on the development of trace metal scavenging models

In 1975 Paul Schindler produced the first oceanic trace metal scavenging model to explicitly include the role of surface chemistry as a control on trace metal water column residence times. The eighteen years that have elapsed since the publication of Schindler's seminal paper have seen the development of a variety of oceanic scavenging models; yet, the fundamental insight of his Zero-order Model remains the benchmark. This paper describes the role of Paul Schindler's work on surface chemistry in providing a framework for the current generation of trace element scavenging models.     

Fossil carotenoids and paleolimnology of meromictic Mahoney Lake,British Columbia,Canada

Vertical distribution of fossil carotenoids in a sediment core from meromictic Mahoney Lake was studied. Besides okenone and demethylated okenone, lutein and zeaxanthin and-carotene isomers were identified. No carotenoids typical for purple nonsulfur or green sulfur bacteria were detected. The ratio of zeaxanthin to lutein (above 1:1 in all samples) indicates a dominance of cyanobacteria over green algae in the phytoplankton assemblages of the past. Okenone, which is found exclusively in Chromatiaceae, was the dominating carotenoid in all sediment zones.The oldest sediment layers containing okenone were deposited 11 000 years ago. Between 9000 and 7000 and since 3000 years b.p., Chromatiaceae reached a considerable biomass in the lake. Vertical changes in okenone concentration were not related to changes of paleotemperatures. In contrast, okenone concentrations decreased during periods of volcanic ash input. During most of the lake history, however, mean okenone concentrations were positively correlated with sedimentation rates. This indicates that vertical changes of okenone concentration in the sediment reflect past changes of purple sulfur bacterial biomass in the lake.According to these results, the past limnology of Mahoney Lake resembled that of the present with a sulfide-containing monimolimnion and a well-developed population of okenone-bearing purple sulfur bacteria.     

Origin and differentiation of picritic arc magmas,Ambae (Aoba), Vanuatu

Key aspects of magma generation and magma evolution in subduction zones are addressed in a study of Ambae (Aoba) volcano, Vanuatu. Two major lava suites (a low-Ti suite and high-Ti suite) are recognised on the basis of phenocryst mineralogy, geochemistry, and stratigraphy. Phenocryst assemblages in the more primitive low-Ti suite are dominated by magnesian olivine (mg 80 to 93.4) and clinopyroxene (mg 80 to 92), and include accessory Cr-rich spinel (cr 50 to 84). Calcic plagioclase and titanomagnetite are important additional phenocryst phases in the high-Ti suite lavas and the most evolved low-Ti suite lavas. The low-Ti suite lavas span a continuous compositional range, from picritic (up to 20 wt% MgO) to high-alumina basalts (<5 wt% MgO), and are consistent with differentiation involving observed phenocrysts. Melt compositions (aphyric lavas and groundmasses) in the low-Ti suite form a liquid-line of descent which corresponds with the petrographically-determined order of crystallisation: olivine + Cr-spinel, followed by clinopyroxene + olivine + titanomagnetite, and then plagioclase + clinopyroxene + olivine + titanomagnetite. A primary melt for the low-Ti suite has been estimated by correcting the most magnesian melt composition (an aphyric lava with 10.5 wt% MgO) for crystal fractionation, at the oxidising conditions determined from olivine-spinel pairs (fo₂ FMQ + 2.5 log units), until in equilibrium with the most magnesian olivine phenocrysts. The resultant composition has 15 wt% MgO and an mg ^Fe2 value of 81. It requires deep (3 GPa) melting of the peridotitic mantle wedge at a potential temperature consistent with current estimates for the convecting upper mantle (T _p 1300°C). At least three geochemically-distinct source components are necessary to account for geochemical differences between, and geochemical heterogeneity within, the major lava suites. Two components, one LILE-rich and the other LILE- and LREE-rich, may both derive from the subducting ocean crust, possibly as an aqueous fluid and a silicate melt respeetively. A third component is attributed to either differnt degrees of melting, or extents of incompatible-element depletion, of the peridotitic mantle wedge.     

Primitive island arc and oceanic lavas from the hunter ridge-hunter fracture zone. Evidence from glass,olivine and spinel compositions

Summary At its southernmost end, the main spreading centre of the North Fiji Basin is propagating into arc crust of the poorly-known Hunter Ridge. We define nine magmatic groups from major element glass chemistry and olivine and spinel compositions in samples dredged from twenty six sites in this area by the R/V Academician A. Nesmeyanov in 1990. These include groups of boninites, island arc tholeiites (IAT), mid-ocean ridge basalts (MORB), enriched mid-ocean ridge basalts (E-MORB), olivine porphyritic andesites and basaltic andesite and Na-rhyolites. Primitive lavas containing highly forsteritic olivine phenocrysts are common in all the groups, except for the rhyolites.We report over 100 glass analyses for dredged rocks from this region, and about 300 olivine-spinel pairs for representatives of all the magmatic groups identified, except the Na-rhyolites.The MORB in this region are probably produced at the propagating spreading centre in the southern part of the North Fiji Basin. Juxtaposition of shallow, hot MORB-source diapirs supplying the MORB in this area, and the sub-arc damp, refractory upper mantle beneath the Hunter Ridge, provides suitable petrogenetic conditions to produce a range of magma types, from island arc tholeiites through to high-Ca boninites. The latter were recovered in eleven dredges.The E-MORB lavas recovered from the extreme southern margin of the North Fiji Basin are shown to be essentially identical to those dredged from adjacent older South Fiji Basin crust. It is hypothesized that the former were either scraped off the South Fiji Basin crust during an episode of oblique subduction that may have generated the Hunter Ridge during the last 5 Myr, or alternatively, that slices of the South Fiji Basin crust were trapped and incorporated into the North Fiji Basin as the subduction zones fronting the Vanuatu arc stepped or propagated southward.

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Primitive Inselbogen- und ozeanische Laven von Hunter Rücken und der Hunter BruchZone: die Bedeutung der Zusammensetzung von Glas, Olivin und Spinell

Zusammenfassung Das spreading centre des Nord-Fidschi-Beckens setzt sich an seinem südlichsten Ende in die Inselbogen-Kruste des noch wenig bekannten Hunter-Rückens fort.Wir definieren 9 magmatische Gruppen auf der Basis der Hauptelement-Zusammensetzung von Gläsern und der Zusammensetzung von Olivin und Spinell in Proben die das Forschungsschiff R/V Akademiker A. Nesmeyanov von 26 Stellen im Jahr 1990 aufgesammelt hat. Zu diesen gehören Gruppen von Boniniten, Inselbogentholeiiten (IAT), Basalten zentralozeanischer Rücken (MORB), angereicherte zentralozeanische Rücken (E-MORB), Olivin-porphyritische Andesite und basaltische Andesite, sowie Na-Rhyolite. Primitive Laven mit Olivinkristallen, die reich an Forsterit-Komponenten sind, kommen in allen diesen Gruppen, mit Ausnahme der Rhyolite, vor.Wir legen über 100 Glas-Analysen von Gesteinen aus diesem Gebiet vor und über 300 Olivin-Spinell-Paare für Vertreter aller der hier identifizierten magmatischen Gruppen, mit Ausnahme der Natriumrhyolite.Die MORB in diesem Gebiet sind wahrscheinlich an dem aktiven Spreading Center im Südteil des Nord-Fidschi-Beckens entstanden. Das nebeneinander Vorkommen von seichten heißen MORB-Quellen Diapiren, die MORB in diesem Gebiet erzeugen, und der Sub-Inselbogen, wasserhaltige, refraktäre obere Mantel unter dem Hunter Rücken führen zu geeigneten petrogenetischen Bedingungen für die Entstehung einer Vielfalt von Magmatypen, von Inselbogentholeiiten bis zu sehr Kalziumreichen Boniniten. Die letzteren wurden in 11 Dredge-Proben gefunden.Die E-MORS Laven, die aus dem extremen Südteil des Nord-Fidschi-Beckens entnommen wurden, sind im wesentlichen mit jenen identisch, die aus der benachbarten älteren Kruste des Süd-Fidschi-Beckens stammen. Es wird vermutet, daß die letzteren entweder aus dem Süd-Fidschi-Becken während einer Episode obliger Subduktion, die in den letzten 5 Millionen Jahren zur Entstehung des Hunter Ridge geführt hat, abgeschert wurden, oder auch daß Teile der Kruste des Süd-Fidschi-Beckens in das Nord-Fidschi-Becken inkorporiert wurden als Subduktionszonen gegenüber dem Vanuato-Bogen sich nach Süden fortbewegten.

     

Synthetic zeolites formed from expanded perlite: Type,formation conditions and properties

Summary The starting material used was expanded perlite with a grain size < 40 m (74.5 wt.% SiO₂; 12.5 wt.% Al₂O₃). This material is a waste product obtained during the production of expanded perlite. The experiments were carried out with KOH solutions, mixtures of KOH and NaOH solutions (1:1) as well as NaOH solutions in the concentration range 0.5 N to 6.0 N at temperatures of between 100° and 140°C and with reaction periods of 2 hours to 13 days in closed system. In the experiments with KOH containing solutions zeolite ZK-19 (phillipsite), W (merlinoite), G (chabazite) and F (edingtonite) formed. Without addition of aluminium high percentages of zeolite ZK-19 (80–100 wt.%) and zeolite W (90–100 wt.%) were obtained. The addition of aluminium rendered possibly the formation of 90 to 100 wt.% of zeolite G and 85 to 100 wt.% of zeolite F, respectively. In the experiments with NaOH solutions analcime, zeolite Na-Pc (gismondine), zeolite HS (sodalite hydrate) and zeolite A formed. High percentages of zeolite Na-Pc (90–100 wt.%), zeolite HS (up to 100 wt.%) and analcime (up to 100 wt.%) were synthesized without addition of aluminium. The formation of high percentages of zeolite A (95–100 wt.%), however, needs the addition of aluminium, NaCI and seed crystals. The temperature stability of the zeolites decreases in the following sequence: K-F > K-W K-ZK-19 (Na), K-W Na, K-F G_si-rich (Na), K-ZK-19 >> Na-Pc G_si-poor. Zeolite A has a very good temperature stability up to temperatures of } 550 °C similar to that of zeolite K-W. At higher temperatures, however, its stability is very poor. The NH₄ ⁺-exchange capacities (meq/g) of the different zeolites amount to the following values: ZK-19:2.8 - 3.2; W:3.0 - 3.2; G:2.3 - 3.6; A:3.1 - 3.2; Na-Pc:3.5 - 3.6; F : 3.9 - 4.8.

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Zeolithsynthese aus Blähperlit—Art, Bildungsbedingungen und Eigenschaften

Zusammenfassung Ausgangsmaterial der experimentellen Untersuchungen war Blähperlit mit einer Korngröße < 40 ,m (74,5 Gew.-% SiO₂; 12,5 Gew.-% Al₂O₃). Dieses Material ist ein Abfallprodukt, das bei der Produktion von Blähperlit anfällt. Die Experimente wurden mit KOH-Lösungen, Lösungsgemischen aus KOH und NaOH (1:1) sowie mit NaOH-Losungen im Konzentrationsbereich 0,5 n-6,0 n bei Temperaturen von 100° – 140°C und über Reaktionszeiten von 2 Stunden bis zu 13 Tagen im geschlossenen System durchgeführt. In den Experimenten mit KOH-hältigen Lösungen bildeten sich die Zeolithe ZK-19 (Phillipsit), W (Merlinoit), G (Chabasit) und F (Edingtonit). Hohe Prozentgehalte an Zeolith ZK-19 (80 – 100 Gew.-%) und Zeolith W (90–100 Gew.-%) entstehen nur ohne Zugabe von Aluminium. Die Bildung von 90–100 Gew.-% Zeolith G bzw. 85–100 Gew. % Zeolith F ist dagegen durch die Zugabe von Aluminium möglich. In den Experimenten mit NaOH-Lösungen bildeten sich die Zeolithe Analcim, Na-Pc (Gismondin), HS (Sodalithhydrat) und Zeolith A. Hohe Prozentanteile an Zeolith Na-Pc (90–100 Gew.-%), HS (bis zu 100 Gew. %) und Analcim (bis zu 100 Gew.-%) wurden ohne Aluminium-Zugabe synthetisiert. Die Bildung von hohen Gehalten an Zeolith A (95–100 Gew. %) ist jedoch nur unter Zugabe von Aluminium, NaCl und Kristallkeimen möglich.Die Temperaturbeständigkeit der Zeolithe nimmt in der folgenden Reihenfolge ab: K-F > K-W - K-ZK-19 (Na), K-W Na, K-F G_si-reich (Na), K-ZK-19 >> Na-Pc G_si-am. Zeolith A weist bis zu Temperaturen von etwa 550°C eine gute Temperaturbeständigkeit auf, die in etwa der von Zeolith K-W entspricht. Bei höheren Temperaturen ist die Beständigkeit jedoch sehr gering.Die NH⁴⁺-Austauschkapazitäten (mÄqu/g) der verschiedenen Zeolithe erreichen folgende Werte: ZK-19:2,8 - 3,2; W:3,0 - 3,2; G:2,3 - 3,6; A:3,1 - 3,2; Na-Pc:3,5 -3,6; F:3,9 - 4,8.

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With 2 Figures     

Cesstibtantite—a geologic introduction to the inverse pyrochlores

Summary The crystal structure of cesstibtantite has been solved from diffractometer data collected on samples from Leshaia, Russia and the Tanco pegmatite, Manitoba. Cesstibtantite from the Leshaia pegmatite (type locality) hasa 10.515(2) Å, space groupFd3m, composition Cs_0.31(Sb_0.57Na_0.31Pb_0.02Bi_0.01)_O.91(Ta_1.88Nb_0.12)₂(O_5.69[OH, F]_0.31)₆(OH, F)_0.69, Z 8; its structure was refined toR 3.8,wR 4.3% using 96 observed (F > 3[F]) reflections (MoK). Cesstibtantite from the Tanco pegmatite hasa 10.496(1) Å, space groupFd3m, composition (Cs_0.22K_0.01)_0.23(Na_0.45Sb_0.39Pb_0.14· Ca_0.06Bi_0.02)_1.06(Ta_1.95Nb_0.05)₂(O_5.78[OH,F]_0.22)₆(OH,F)_0.55,Z 8; its structure was refined toR 3.9w R 3.7% using 104 observed reflections. Cesstibtantite differs from the normal pyrochlores in that it contains significant amounts of very large cations such as Cs. As these cations are too large (^VIII[r] > 1.60 Å) for the conventional [8]-coordinated A site, they occupy the [18]-coordinated site, which normally contains monovalent anions. Natural cesstibtantite samples are non-ideal in that both Cs and monovalent anions occur at the site; thus cesstibtantite is intermediate to thenormal pyrochlores (with only monovalent anions at the site) and theinverse pyrochlores (with only large cations at the site).

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Cesstibtantit—eine geologische Einfiihrung in die inversen Pyrochlore

Zusammenfassung Die Kristallstruktur von Cesstibtantit wurde auf der Basis von Diffraktometerdaten von Proben von Leshaia, Russland and dem Tanco Pegmatit, Manitoba, gelöst. Cesstibtantit aus dem Leshaia Pegmatit (Typlokalität) hat a 10.515(2) Å, RaumgruppeFd3m, die Zusammensetzung CS_0.31(Sb_0.57Na_0.31Pb_0.02Bi_0.01)_0.91(Ta_1.88Nb_0.12)₂· (O_5.69OH, F_0.31)₆(OH, F)_0.69 Z 8; die Struktur wurde aufR 3.8,wR 4.3% verfeinert unter Benützung von 96 beobachteten Reflexen. Cesstibtantit vom Tanco Pegmatit hat a 10.496(1) Å, RaumgruppeFd3m, die Zusammensetzung (Cs_0.22K_0.01)_0.23(Na_0.45· Sb_0.39Pb_0.14Ca_0.06Bi_0.02)_1.06(Ta_1.95Nb_0.05)₂(O_5.78OH,F_0.22)₆(OH,F)_0.55,Z 8; seine Struktur wurde aufR 3.9wR 3.7% auf der Basis von 104 beobachteten Rettexen verfeinert. Cesstibtantit unterscheidet sich von normalen Pyrochloren insofern, als er signifikante Mengen von sehr großen Kationen, wie z.B. Cs enthält. Da these Kationen zu groß sind (^VIII r 1.60 Å) für eine konventionelle [8]-koordinierteA Stelle, nehmen she die [18]-koordinierten Positionen ein, welche normalerweise monovalente Anionen enthalten. Natürliche Cesstibtantitproben sind nicht ideal insofern als sowohl Cs als auch monovalente Anionen in der Position vorkommen. Somit ist Cesstibtantit intermediär zu den normalen Pyrochloren (mit nur monovalenten Anionen auf der Position) and den inversen Pyrochloren (mit ausschließlichen großen Kationen an der Position).

     

An occurrence of ardennite in quartz veins in piemontite schist,Western Otago,New Zealand

Summary Ardennite of complex composition: (Mn²⁺ _3.488Ca_0.509Ba_0.002)₌₄(Mg_0.916916 Fe³⁺ _0.165 Mn³⁺ _0.099Cu_0.033Ni_0.009Zn_0.006 Ti_0.008Al_4.764)₌₆(As⁵⁺ _0.823V⁵⁺ _0.022P_0.005B_0.069Al_0.042Si_5.039)₌₆O_21.81(OH)_6.17 occurs in crack-seal quartz veins in quartz-albite-piemontite-spessartine-phengitehematite-chlorite-rutile-tourmaline ± calcite schist of the Haast Schist Group near Arrow Junction, western Otago, New Zealand. The Mn²⁺/Mn³⁺-ratio is sensitive to calculations and to accuracy of analyses. Boron is detected in ardennite for the first time. Other properties include = 1.734(3), = 1.735(3), = 1.751(3), 2V_Z = 30(2)°;a = 8.721(1),b = 5.816(1),c = 18.545(3) Å,V = 940.7(2) ų. Associated mineral phases are spessartine, hematite, piemontite containing 0.7% SrO and 0.06% PbO, and phengite. Later-stage vein minerals comprise chlorite, albite, and manganoan calcite which were deposited under less highly oxidizing conditions. Digenite with minor intergrown covellite occurs in small amount with manganoan calcite and quartz in a cross-cutting late-stage veina chalcopyrite and native copper occur in other late-stage veins. Arsenic and other components of the ardennite and associated minerals are derived from highly oxidized ferromanganese oxide- and hydroxide-bearing siliceous pelagic sediments that formed the protolith for the piemontite schist. The veins formed at a relatively early stage after metamorphism peaked in the chlorite zone of the greenschist facies under conditions that have been estimated at about 4.5 kbar, 390 °C.

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Vorkommen von Ardennit in Quarzgängen aus Piemontit-Schiefern, West-Otago, Neuseeland

Zusammenfassung Ardennit mit der Zusammensetzung (Mn²⁺ _3.488Ca_0.509Ba_0.002)₌₄(Mg_0.916Fe³⁺ _0.165Mn³⁺ _0.099Cu_0.033Ni_0.009Zn_0.006 Ti_0.008Al_4.764)₌₆(As⁵⁺ _0.823V⁵⁺ _0.022P_0.005B_0.069Al_0.042Si_5.039)₌₆O_21.81(OH)_6.17 tritt in Crack-seal-Quarzgängen in Quarz-Albit-Piemontit-Spessartin-Phengit-Hämatit-Chlorit-Rutil-Turmalin ± Calcit-Schiefern der Haast Schiefer-Gruppe nahe der Arrow Junction, West-Otago, Neuseeland, auf. Die Proportionen von Mn²⁺/Mn³⁺ hängen von der Kalkulation und der Genauigkeit der Analyse ab. Bor wird zum ersten Mal im Ardennit bestimmt. Andere Eigenschaften sind: = 1.734(3), = 1.735(3), = 1.751(3), 2Vz = 30(2)°; a = 8.721(1), b = 5.816(1), c = 18.545(3) Å, V = 940.7(2) Å3. Assoziierte Mineralphasen sind Spessartin, Hämatit, Piemontit, der 0.7% SrO und 0.06% PbO enthält und Phengit. Spät gebildete Gangmineralien, wie Chlorit, Albit und Mn-Calcit, sind unter geringer oxidierenden Bedingungen entstanden. Digenit mit etwas Covellin tritt in kleinen Mengen zusammen mit Mn-Calcit und Quartz in einem querschlägigen Gang auf, Chalcopyrit und gediegenes Kupfer kommen in anderen späten Gängen vor. Arsen und andere Komponenten des Ardennites and der assoziierten Minerale können von hochoxidierten, Fe-Mn-Oxid- und Hydroxyd-führenden, Sireichen, pelagischen Sedimenten hergeleitet werden, die das Ausgangsgestein für den Piemontit darstellen. Die Gänge wurden in einem relativ frühen Stadium, nach dem Metamorphosehöhepunkt, innerhalb der Chloritzone der Grünschiefer-Fazies, unter ungefähr 4.5 kbar und 390°C, gebildet.

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With 4 Figures     

Die Rheingrabenrandverwerfung im Wattkopftunnel bei Ettlingen (Nordschwarzwald)

The Wattkopftunnel, near Ettlingen (Nordschwarzwald), drives through the eastern margin of the Rheingraben. The tunnel passes cenozoic and mesozoic sediments. Early quarternary and tertiary beds are situated west of the main thrust of the Rheingraben. Fossil record indicates upper Oligocene age (Chatt) for parts of the tertiary sediments. At the eastern border of the Rheingraben, wedges of jurassic and middle triassic series are squeezed. East of the Rheingraben the tunnel drives in the lower triassic Bausandstein. The eastern margin of the Rheingraben was investigated in detail during tunneling. Faults of the Rheingraben margin are distributed in an 130 meter wide fault zone in the tunnel. Total stratigraphic separation by the normal faults reaches more than 2 000 meters. The cenozoic sequence suffered synsedimentary to early diagenetic deformation, while the mesozoic series are characterized by ruptural deformation. The fault- and joint system is directed in the rheinische Richtung (SSW-NNE). East of the Rheingraben a second direction occur, running parallel to the lower Albtal (W-E).

     

Boron isotope variations in nature: a synthesis

The large relative mass difference between the two stable isotopes of boron, ¹⁰B and ¹¹B, and the high geochemical reactivity of boron lead to significant isotope fractionation by natural processes. Published ¹¹B values (relative to the NBS SRM-951 standard) span a wide range of 90. The lowest ¹¹B values around — 30 are reported for non-marine evaporite minerals and certain tourmalines. The most ¹¹B-enriched reservoir known to date are brines from Australian salt lakes and the Dead Sea of Israel with ¹¹B values up to +59. Dissolved boron in present-day seawater has a constant world-wide ¹¹B value of + 39.5. In this paper, available ¹¹B data of a variety of natural fluid and solid samples from different geological environments are compiled and some of the most relevant aspects, including possible tracer applications of boron-isotope geochemistry, are summarized.

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Résumé La grande différence relative de masse entre les isotopes stables du bore, ¹⁰B et ¹¹B, et la grande réactivité geochimique du bore ont pour conséquence un fractionnement isotopique naturel important. Les valeurs de ¹¹B publiées (par rapport au standard NBS SRM-951) varient de 90. Les valeurs de ¹¹B les plus basses (–30) correspondent aux evaporites non-marines et à certaines tourmalines. Le réservoir le plus enrichi en ¹¹B est représenté par les saumures des lacs salés d' Australie et par la Mer Morte en Israël, qui ont des valuers de ¹¹B allent jusqu'à + 59. L'eau de mer a une valeur de ¹¹B mondialement constante de + 39.5. Des valeurs de ¹¹B des solutions naturelles ainsi que des roches et minéraux de différentes origines, publiées jusqu'à présent, sont présentées ici. En outre quelques aspects importants concernant la géochimie des isotopes du bore y compris quelques applications sont exposés.