Magma fractionation systems beneath the mid-Atlantic ridge at 36–37 ° N

Variation of major and trace elements in drilled basalts from the Mid-Atlantic Ridge (DSDP Leg 37) reflects distinct cycles of low pressure fractionation operating independently within a complex network of magma storage reservoirs beneath the crustal spreading axis. Low pressure phase relations are determined by parental magma composition, which varies from An-rich (An/Di > ca. 1.4) to Di-rich (An/Di < ca. 1.4). High An/Di magmas probably formed under slightly hydrous conditions in the mantle. They have low LIL element contents, low P/Y and high Mg/(Mg + Fe) ratios. Zr, P and Y abundance and inter-element ratios are highly diagnostic of primary magma type, and are used to quantify fractional crystallization models.Low pressure fractionation hypotheses were tested by least-squares modelling of whole-rock and phenocryst chemistry, which indicated removal or addition of phenocryst assemblages: ol; pl; ol + pl; ol + pl + cpx; pl + cpx, (± sp). Accumulation of plagioclase or olivine is an important mechanism for generating highly porphyritic rocks. A rare 3-phase (ol + pl + cpx) cumulate resulted from cotectic fractionation of a low An/Di magma type. Olivine and plagioclase cumulates appear to be related to high An/Di magmas. Olivine accumulation has been monitored by comparison of olivine/bulk rock partitioning of Fe and Mg to experimental measurements of the equilibrium K_D value. A single extensive sub-axial magma chamber could not account for the observed chemical variation and would probably be dynamically unstable.     

Detecting trends in seagrass abundance using aerial photograph interpretation: Problems arising with the evolution of mapping methods

In the late 1970s and early 1980s an inventory of seagrass distribution in southeast Australia was made from multiscale aerial photographs using a simple analog mapping method, referred to here as theCamera lucida technique. Four priority estuaries were remapped in 2003, also using multi-scale aerial photographs, but using a geographic information system (GIS)-based analog mapping technique. We quantified the degree of error that arose when estimates of seagrass distribution were compared for the two techniques be ressessing. with GIS, the same aerial photographs used earlier for four estuaries: Port Hacking, Bermagui River, Merimbula Lake, and St Georges Basin.Camera lucida produced a larger estimate of total seagrass area than GIS in all four locations: 8% of total seagrass area or 12.9 ha in Port Hacking, 15% or 5 ha in Bermagui River, 20% or 38.0 ha in Merimbula Lake, and 243% or 502.2 ha in St Georges Basin. The consistent and moderate discrepancy in Port Hacking, Bermagui River, and Merimbula Lake was attributed mainly to differences betweenCamera lucida and GIS, such as the tendency ofCamera lucida to overestimate seagrass area by amalgamating disjunct seagrass patches into continuous meadows. The large discrepancy at St Georges Basin was attributed to interoperator error; i.e., the operators who mapped photographs taken in 1979 had widely differing interpretations of the aerial photographs. A 20-yr trend was calculated for each estuary using either theCamera lucida or revised GIS value as the start point. Perceived management status of the seagrass depended greatly on the initial data used. In St Georges Basin, a catastrophic decline (65%, 553.9 ha) occurred based on theCamera lucida-GIS comparison but only a small decline (14.7%, 51.8 ha) when all photos were GIS assessed, In view of the technical developments in habitat mapping, evaluation of change in seagrass abundance using remote sensing needs an assessment of intra-operator error, inter-operator error, and systemic error before maps produced by superseded technology are considered useful baselines.     

Trace Element and Isotope Geochemistry of the Volcanic Rocks of Bequia, Grenadine Islands, Lesser Antilles Arc: a Study of Subduction Enrichment and Intra-crustal Contamination

Pliocene volcanics on the island of Bequia comprise two interbeddedsuites of basalts and andesites. The isotopically homogeneoussuite (IHS) has a limited range of Sr—Nd—Pb isotopes(⁸⁷Sr/⁸⁶Sr 0.7040–0.7046, ¹⁴³ Nd/¹⁴⁴ Nd 0.5130 and ²⁰⁶Pb/²⁰⁴Pb 19.36–19.51), and mantle-like ¹⁸O values (5.5in clinopyroxene). The isotopically diverse suite (IDS) is characterizedby much wider ranges of radiogenic isotopes (⁸⁷ Sr/⁸⁶Sr 0.7048–0.7077,¹⁴³ Nd/¹⁴⁴ Nd 0.5128–0.5123 and ²⁰⁶ Pb/²⁰⁴ Pb 19.7–20.2),in which all of the Sr and Pb ratios are higher and Nd ratiosare lower than those of the IHS. The IDS is also characterizedby high ¹⁸ O values, up to 7.6 in clinopyroxene. The Sr andPb isotope ratios are too high, and the Nd isotope ratios aretoo low in the IDS for any of these lavas to be derived fromunmodified depleted mantle. Both suites are petrologically very similar and their majorelement compositions and phenocryst contents suggest that theywere formed largely by fractional crystallization of a hydroustholeiitic melt at pressures <3 kbar. The isotopic ratiosand enrichments in large ion lithophile elements (LILE), andto some extent light rare earth elements (LREE), as comparedwith mid-ocean ridge basalts (MORB), of the IHS lavas suggestthat they were derived from a depleted mantle source which hadbeen re-enriched by the addition of 1–4% of a subductioncomponent. This component probably comprised a mixture of dehydrationfluids, and perhaps minor siliceous melts, released from subductingsediments and mafic crust. The extreme isotopic ranges, largeenrichments in incompatible elements, more fractionated LREEpatterns and higher ¹⁸ O values of the IDS lavas are interpretedas resulting from 10–55% assimilation—fractionalcrystallization of sediments, derived from the Guyana Shield,which are present in the arc crust, by IHS type melts. KEY WORDS: trace elements; radiogenic isotopes; arc lavas; Lesser Antilles ^*Corresponding author.     

Facies analysis of the Trypali carbonate unit (Upper Triassic) in central-western Crete (Greece): an evaporite formation transformed into solution-collapse breccias

The Trypali carbonate unit (Upper Triassic), which crops out mainly in central‐western Crete, occurs between the parautochthonous series (Plattenkalk or Talea Ori‐Ida series, e.g. metamorphic Ionian series) and the Tripolis nappe (comprising the Tripolis carbonate series and including a basal Phyllite–Quartzite unit). It consists of interbedded dolomitic layers, represented principally by algally laminated peloidal mudstones, foraminiferal, peloidal and ooidal grainstones, as well as by fine‐grained detrital carbonate layers, in which coarse baroque dolomite crystals and dolomite nodules are dispersed. Baroque dolomite is present as pseudomorphs after evaporite crystals (nodules and rosettes), which grew penecontemporaneously by displacement and/or replacement of the host sediments (sabkha diagenesis). However, portions of the evaporites show evidence of resedimentation. Pre‐existing evaporites predominantly consisted of skeletal halite crystals that formed from fragmentation of pyramidal‐shaped hoppers, as well as of anhydrite nodules and rosettes (salt crusts). All microfacies are characteristic of peritidal depositional environments, such as sabkhas, tidal flats, shallow hypersaline lagoons, tidal bars and/or tidal channels. Along most horizons, the Trypali unit is strongly brecciated. These breccias are of solution‐collapse origin, forming after the removal of evaporite beds. Evaporite‐related diagenetic fabrics show that there was extensive dissolution and replacement of pre‐existing evaporites, which resulted in solution‐collapse of the carbonate beds. Evaporite replacement fabrics, including calcitized and silicified evaporite crystals, are present in cements in the carbonate breccias. Brecciation was a multistage process; it started in the Triassic, but was most active in the Tertiary, in association with uplift and ground‐water flow (telogenetic alteration). During late diagenesis, in zones of intense evaporite leaching and brecciation, solution‐collapse breccias were transformed to rauhwackes. The Trypali carbonate breccias (Trypali unit) are lithologically and texturally similar to the Triassic solution‐collapse breccias of the Ionian zone (continental Greece). The evaporites probably represent a major diapiric injection along the base of the parautochthonous series (metamorphic Ionian series) and also along the overthrust surface separating the parautochthonous series from the Tripolis nappe (Phyllite–Quartzite and Tripolis series). The injected evaporites were subsequently transformed into solution‐collapse breccias.     

Mossbauer detection of goethite (α-FeOOH) in coal and its potential as an indicator of coal oxidation

Mössbauer spectroscopy of three suites of oxidized coals shows that the transformation of pyrite to FeOOH correlates with other parameters of oxidation. As pyrite is very common in coals and the transformation to FeOOH is sensitive to small degrees of oxidation, the Mössbauer technique shows considerable promise as a means for the detection of coal oxidation.     

Relations between metamorphism and orogeny in a typical section of the Indian Himalayas

Summary A comprehensive geological and petrological investigation has been undertaken in an area of about 10 000 km² in the Indian Himalaya (S-Lahul, Himachal Pradesh). The development of mineral assemblages in metamorphic rocks of medium grade is considered to be a dominant Alpine event, although almost exclusively Paleozoic and Precambrian rocks have been involved. The Barrowian type of this metamorphism, ranging from the anchi- to the sillimanite zone, took place under the elavated T-gradient of about 4°C/100 m. It is suggested that normal geothermal conditions prevailed only in the outermost zone of this orogenic belt. In the Late Tertiary this metamorphic series has been moved as a huge nappe upon the Lower Himalaya. During this process a unique feature of reverse metamorphism has been formed. It can be shown that this feature was caused by a combination of metamorphism and very rapid tectonic movements.

---

Beziehungen zwischen Metamorphose und Orogenese in einem typischen Querschnitt des Indischen Himalaya

Zusammenfassung Ein etwa 10 000 km² großes Gebiet des Indischen Himalaya (S-Lahul, Himachal Pradesh) wurde einer umfassenden geologischen und petrographischen Bearbeitung unterzogen. Die Ausbildung der in den mittelgradig metamorphen Gesteinen vorliegenden Mineralparagenesen wird als ein dominant alpines Ereignis betrachtet, obwohl fast nur paläozoische und präkambrische Gesteine einbezogen wurden. Die Metamorphose selbst ist von Barrow-Typus und reicht von der Anchi- bis zur Sillimanitzone. Sie fand unter einem erhöhten Temperaturgradienten von etwa 4°C/100 m statt. Nur in der äußersten Zone des Orogens entsprach der Gradient etwa normalen geothermischen Verhältnissen. Im Spättertiär wurde diese alpin-metamorphe Serie in Form einer mächtigen Kristallindecke dem Niedrigen Himalaya aufgeschoben. Dabei wurde eine einzigartige Zone inverser Metamorphose ausgebildet. Es konnte gezeigt werden, daß diese inverse Metamorphose durch das Zusammenwirken von Metamorphose und sehr rasch ablaufender Tektonik entstand.

---

---

With 13 Figures

---

Dedicated to Prof. Dr. Dr. h. c.B. Sander on the occasion of his 90th birthday.     

Phosphorus oxynitride PON, a silica analogue: structure and compression of the cristobalite-like phase; P –T phase diagram

 The structure of the cristobalite-like polymorph of phosphorus oxynitride PON has been refined using neutron powder diffraction data. It is tetragonal, space group I&4macr;2d, Z=4. The four P–(O,N) distances are equal but the tetrahedron is compressed along c. In AX₂ or ABX₄ compounds, the tetragonal I&4macr;2d or I&4macr; structure is obtained when the average ratio of the cation to anion radius is below 1.186, whereas the tetragonal P4₁2₁2 or orthorhombic C222₁ structure is obtained at low temperatures for larger ratios. The cell parameters of this PON polymorph have been determined as a function of hydrostatic pressure by in situ angle dispersive X-ray powder diffraction in a diamond-anvil cell. Under truly hydrostatic pressure, a strong anisotropic behavior is observed with the c parameter being nearly incompressible. Very slight anisotropic stress strongly modifies the high-pressure behavior. According to the pressure-temperature conditions of treatment, three phases, cristobalite-, moganite-, and quartz-like, have been obtained by quenching experiments, and the P–T phase diagram of PON was derived. The high-pressure behavior of the α-quartz, moganite, and cristobalite-like polymorphs of PON and SiO₂ is discussed. Received: 7 August 2000 / Accepted: 21 January 2001     

The impact of water-rock interaction and vegetation on calcium isotope fractionation in soil- and stream waters of a small, forested catchment (the Strengbach case)

This study aims to constrain the factors controlling the calcium isotopic compositions in surface waters, especially the respective role of vegetation and water-rock interactions on Ca isotope fractionation in a continental forested ecosystem. The approach is to follow changes in space and time of the isotopic composition and concentration of Ca along its pathway through the hydro-geochemical reservoirs from atmospheric deposits to the outlet of the watershed via throughfalls, percolating soil solutions and springs. The study is focused on the Strengbach catchment, a small forested watershed located in the northeast of France in the Vosges mountains. The δ^44/40Ca values of springs, brooks and stream waters from the catchment are comparable to those of continental rivers and fluctuate between 0.17 and 0.87‰. Soil solutions, however, are significantly depleted in lighter isotopes (δ^44/40Ca: 1.00-1.47‰), whereas vegetation is strongly enriched (δ^44/40Ca: −0.48‰ to +0.19‰). These results highlight that vegetation is a major factor controlling the calcium isotopic composition of soil solutions, with depletion in “light” calcium in the soil solutions from deeper parts of the soil compartments due to preferential ⁴⁰Ca uptake by the plants rootsystem. However, mass balance calculations require the contribution of an additional Ca flux into the soil solutions most probably associated with water-rock interactions. The stream waters are marked by a seasonal variation of their δ^44/40Ca, with low δ^44/40Ca in winter and high δ^44/40Ca in spring, summer and autumn. For some springs, nourishing the streamlet, a decrease of the δ^44/40Ca value is observed when the discharge of the spring increases, with, in addition, a clear covariation between the δ^44/40Ca and corresponding H₄SiO₄ concentrations: high δ^44/40Ca values and low H₄SiO₄ concentrations at high discharge; low δ^44/40Ca values and high H₄SiO₄ concentrations at low discharge. These data imply that during dry periods and low water flow rate the source waters carry a Ca isotopic signature from alteration of soil minerals, whereas during wet periods and high flow rates admixture of significant quantities of ⁴⁰Ca depleted waters (vegetation induced signal) from uppermost soil horizons controls the isotopic composition of the source waters. This study clearly emphasizes the potential of Ca isotopes as tracers of biogeochemical processes at the water-rock-vegetation interface in a small forested catchment.     

U-Pb isotopic study of the gabbronorite–anorthosite drusite (coronite) body of Vorony Island (Kandalaksha Archipelago,the White Sea)

The Precambrian Belomorian mobile belt located between the Karelian craton and the Lapland–Umba granulite belt contains large amount of small rootless mafic–ultramafic intrusions, which are dispersed over a large area and distinguished as the Belomorian drusite (coronite) complex. U-Pb dating of magmatic zircon and metamorphic rutile from the drusite body on Vorony Island showed that it was crystallized at 2460 ± 11 Ma and metamorphosed at 1775 ± 45 Ma. Petrographic and geochemical data confirm that the parental magmas of the drusites belong to the siliceous high-magnesian (boninite-like) series, which also was responsible for the formation of large layered plutons in stable domains of the Baltic Shield.