Pressure-induced incipient amorphization of α-quartz and transition to coesite in an eclogite from Antarctica: a first record and some consequences

Monocrystalline quartz inclusions in garnet and omphacite from various eclogite samples from the Lanterman Range (Northern Victoria Land, Antarctica) have been investigated by cathodoluminescence (CL), Raman spectroscopy and imaging, and in situ X‐ray (XR) microdiffraction using the synchrotron. A few inclusions, with a clear‐to‐opalescent lustre, show ‘anomalous’ Raman spectra characterized by weak α‐quartz modes, the broadening of the main α‐quartz peak at 465 cm^?1, and additional vibrations at 480–485, 520–523 and 608 cm^?1. CL and Raman imaging indicate that this ‘anomalous’α‐quartz occurs as relicts within ordinary α‐quartz, and that it was preserved in the internal parts of small quartz inclusions. XR diffraction circular patterns display irregular and broad α‐quartz spots, some of which show an anomalous d‐spacing tightening of ～2%. They also show some very weak, hazy clouds that have d‐spacing compatible with coesite but not with α‐quartz. Raman spectrometry and XR microdiffraction characterize the anomalies with respect to α‐quartz as (i) a pressure‐induced disordering and incipient amorphization, mainly revealed by the 480–485 and 608‐cm^?1 Raman bands, together with (ii) a lattice densification, evidenced by d‐spacing tightening; (iii) the cryptic development of coesite, 520–523 cm^?1 being the main Raman peak of coesite and (iv) Brazil micro‐twinning. This ‘anomalous’α‐quartz represents the first example of pressure‐induced incipient amorphization of a metastable phase in a crustal rock. This issue is really surprising because pressure‐induced amorphization of metastable α‐quartz, observed in impactites and known to occur between 15 and 32 GPa during ultrahigh‐pressure (UHP) experiments at room temperature, is in principle irrelevant under normal geological P–T conditions. A shock (due to a seism?) or a local overpressure at the inclusion scale (due to expansion mismatch between quartz and its host mineral) seem the only geological mechanisms that can produce such incipient amorphization in crustal rocks. This discovery throws new light on the modality of the quartz‐coesite transition and on the pressure regimes (non‐lithostatic v. lithostatic) during high‐pressure/UHP metamorphism. In particular, incipient amorphization of quartz could favour the quartz‐coesite transition, or allow the growth of metastable coesite, as already experimentally observed.     

U-Th Disequilibrium and Rb-Sr Age Constraints on the Magmatic Evolution of Peralkaline Rhyolites from Kenya

Mildly peralkaline rhyolites of the Olkaria Volcanic Complex,located in the Kenyan sector of the East African rift valley,have low Sr concentrations and elevated Rb/Sr ratios (Sr 1·3–2ppm; ⁸⁷Rb/⁸⁶Sr = 748–1769) that potentially allow theresolution of time differences on the order of 1 ka by conventionalSr isotope determination. Because of their young eruption ages(     

Crustal Influences in the Petrogenesis of the Naivasha Basalt--Comendite Complex: Combined Trace Element and Sr-Nd-Pb Isotope Constraints

The bimodal Naivasha complex (central Kenya) comprises 2 suitesof transitional basalts and 7 chemostratigraphic groups of comendites.The early basalt series (EBS) predates the Group 1 comenditeswith the later series (LBS) erupted between Groups 5 and 6.Basalts from both suites are notable for their relatively radiogenic²⁰⁷Pb/²⁰⁴Pb isotope ratios which are higher than in the majorityof ocean island basalt (OIB, Zindler & Hart, 1986), and⁸⁷Sr/⁸⁶Sr ratios more radiogenic than basalts from northernKenya. Both basalt suites exhibit systematic trace element andisotopic variations which appear related to greater assimilationof Proterozoic amphibolite facies crust by the chemically moreevolved rock types. Their mantle source regions show evidenceof residual plagioclase and have a ‘Dupal’-likeOIB trace element and Pb-Sr-Nd isotope signature (Hart, 1984).A contribution from the sub-continental lithosphere is proposedin basalt genesis. The seven comendite groups have distinct trace element and isotopesystematics. Hydration of comendite glass causes significantchanges in Sr and Pb isotope ratios. In terms of their Sr-Ndisotope relationships the unaltered comendites could be derivedfrom the basalts by an assimilation-fractional crystallization(AFC) process dominated by the fractional crystallization offeldspars. However, the Pb systematics clearly demonstrate thatthe basalts and comendites are not part of a cogenetic suite.Chemical variations within individual comendite groups are predominantlythe result of fractional crystallization of the observed phenocrystassemblages (i.e. alkali feldspar dominated) and minor crustalinteraction. The majority of the chemical and isotopic differencesbetween Groups 1–7 cannot be explained by fractional crystallizationand appear to represent crustal melts derived from close tothe interface between Pan African basement and the overlyingMiocene-Holocene volcanoclastic rocks, at approximately 6 kmdepth (KRISP working group, 1987). Halogens play a fundamental role in the petrogenesis of thecomendites (Cl+F<1?7 per cent) permitting small degree meltsof low viscosity to be extracted from the crust and causingthe breakdown of minor phases e.g, zircon. These factors explainthe extreme enrichment of certain incompatible trace elements(Zr<2500, Nb<700) in the comendites and coupled with theretention of zircon in the source of the halogen poor comendites(Group 1<0?6 per cent Cl+F) result in notable fractionationamong the HFSE (Zr/Nb 1?5–5?5). Halogens may be concentratedin the source region from the surrounding crust by the presentlyactive hydrothermal system. Each of the chemostratigraphic comenditegroups is chemically distinct, implying that partial meltingof the heterogeneous crust is on a limited scale and that noextensive magma chambers exist beneath Naivasha.     

Age, origin and significance of a new middle MIS 3 tephra horizon identified within a long-core sequence from Les Echets, France

A new tephra has been identified within a long core (EC 3) sequence recovered from Les Echets, near Lyon, France. This visible tephra was discovered as part of a high resolution multiproxy re-investigation of the Les Echets sequence. Independent chronological information suggests that the tephra is c . 2 000–45 000 years old, and geochemical analysis indicates that it is of basanitic composition. The latter suggests a possible origin in the Eifel; however, as yet, no other volcanic events or deposits can be correlated to the Les Echets tephra. New sedimentological and chronological data are presented indicating that the tephra falls within an interval that most likely correlates with Dansgaard–Oeschger events 12-9. Thus, this tephra could potentially be an important middle MIS 3 marker horizon in central Europe if it can be traced in other palaeorecords.     

Late Proterozoic High-pressure granulite facies meta-morphism in the north-east Ox inlier, north-west Ireland

Abstract High-pressure granulite-facies gneisses in the NE Ox inlier in NW Ireland have undergone extensive Caledonian retrogression. In the local area of Slishwood, however, reworking was negligible and the gneisses (psammites, semipelites, pelites, metabasites and ultramafites) preserve evidence of P–T changes at high grade which mainly post-date pre-Caledonian polyphase deformation. Temperatures reached 850–900°C (based on garnet-clinopyroxene geothermometry and the presence of mesoperthite) during and after decompression from earlier eclogite-facies conditions (inferred from textural evidence of plagioclase release in sieve-textured augite). Subsequent cooling at high pressure is inferred from the unequivocal replacement of sillimanite by kyanite.
A Sm–Nd mineral isochron (gt–cpx–plag–WR) of 605 ± 37 Ma is taken to date a point on the cooling path, and confirms the hitherto suspected pre-Caledonian age of the high-grade metamorphism. Geochemical and Sm–Nd isotopic data indicate that the protoliths were probably late Proterozoic arkosic sediments and tholeiites. Following metamorphism they apparently came to reside near the base of the crust where they slowly cooled. The eventual exhumation of these gneisses is attributed to Caledonian crustal imbrication, followed by rapid isostatic recovery.     

Interlobate ice-sheet dynamics during the Last Glacial Maximum at Whitburn Bay, County Durham, England

This research reconstructs ice-sheet processes operating during the Late Devensian in northeast England. The article assesses the lithostratigraphy of the Devensian glacial tills of Whitburn Bay, eastern County Durham, and presents the first detailed analysis of petrological, geochemical and biostratigraphical data to reconstruct lithostratigraphy, provenance and iceflow pathways. Two Devensian tractions tills (the Blackhall and Horden tills) are separated by a boulder pavement, pointing to a switch in ice-bed conditions and the production of a melt-out lag prior to deposition of the upper traction till, the Horden Till. The Blackhall Till contains Magnesian Limestone, Carboniferous Limestone, Whin Sill dolerite and Old Red Sandstone, suggesting a northwesterly source, probably from the Midland Valley and the Southern Uplands. The Horden Till contains erratics and heavy minerals derived from crystalline bedrock sources in the Cheviot Hills and northeast Scotland. Within the Horden Till are numerous sand, clay and gravel-filled canals incised downwards into the diamicton which are attributed to a low-energy, distributed, subglacial canal drainage system. Coupled with hydro-fractures and the boulder pavement, this suggests that a partially decoupled, fast-flowing ice stream deposited the Horden Till. The uphill, landward direction of ice movement indicates that the ice stream was confined in the North Sea Basin, possibly by the presence of Scandinavian Ice.     

Kiglapait Mineralogy I: Apatite, Biotite, and Volatiles

Electron microprobe analyses show that Upper Zone apatites inthe Kiglapait intrusion are fluorine rich and contain minorchlorine and hydroxyl (calculated). Apatite from the Outer BorderZone has a higher Cl content. The refractive indices of UZ apatites have the following ranges: = 1.6345–1.6379, = 1.6326–1.6352, and B = 0.0020–0.0028.The birefringence is low for apatites with these refractiveindices. Some Outer and Upper Border Zone apatites have higherindices of refraction and normal birefringence. Fractional crystallization of the basaltic Kiglapait magma producedcumulus apatite beginning at the 94 per cent solidified levelwhen P₂O₅ reached saturation in the liquid. The amounts of P₂O₅and modal apatite decreased gradually from the 94 per cent tothe 99.99 per cent solidified level as the liquid was depletedin P₂O₅. F and Cl appear to be equally partitioned between theliquid and apatite because no fractionation trends are notedbetween the two halogens. There is a slight decrease in thecalculated ratio OH/F in apatite which suggests possible depletionof OH in the liquid with fractionation. Kiglapait apatites appear to be stoichiometric, based on microprobechemistry, refractive index, and unit cell dimensions. However,infrared absorption analyses show no detectable water, whichleaves approximately 11 per cent of the monovalent anion siteunaccounted for. Anion deficiencies in apatites from low-H₂Oenvironments may be explained either by substitution of O forF, or domains of tetracalcium phosphate. Non-cumulus biotite occurs in minor quantities in the intrusion.Electron microprobe analyses of Upper Zone biotites show thatthey contain an average (by weight) pf 0.4 per cent F, 0.07per cent Cl, and 4.0 per cent H₂O (calculated). The volatile chemistry of the Kiglapait intrusion is calculatedfrom apatite and biotite chemistry. The intrusion contains anestimated 900 ppm P₂O₅, 166 ppm F, and 12 ppm Cl. There is amaximum of 68 ppm H₂O using calculated H₂O from microprobe data,or a minimum of 8 ppm using H₂O from infrared analysis. It isproposed that the anhydrous basaltic Kiglapait magma was a secondpartial melt of amphibole-bearing mantle rock.     

THE ANALYSIS AND DESIGN OF TWO-DIMENSIONAL FILTERS FOR TWO-DIMENSIONAL DATA *

Some of the methods such as regional removal and second derivative calculations which can be used to outline anomalies on potential data maps can be thought of as a filtering operation. The analysis and design of such two-dimensional filters by means of direct and inverse two-dimensional Fourier transforms have been considered. An analysis of several published sets of second derivative coefficient sets indicates that, in general, they are not a good approximation to the theoretical second derivative filter. Alternate methods of designing regional removal and second derivative filters are discussed. The properties of various two-dimensional filters are further illustrated by means of maps obtained from the convolution of several of these filters with a set of observed field data. These maps show the large changes in anomaly shape which can result from the inclusion or rejection of various wavelength components.     

Intra-Grain Sr Isotope Evidence for Crystal Recycling and Multiple Magma Reservoirs in the Recent Activity of Stromboli Volcano, Southern Italy

Over the last several hundred years, Stromboli has been characterizedby steady-state Strombolian activity. The volcanic productsare dominated by degassed and highly porphyritic (HP-magma)black scoria bombs, lapilli and lava flows of basaltic shoshoniticcomposition. Periodically (about one to three events per year),more energetic explosive eruptions also eject light colouredvolatile-rich pumices with low phenocryst content (LP-magma)that have more mafic compositions than the HP-magma. An in situmajor and trace element and Sr isotope microanalysis study ispresented on four samples chosen to characterize the differentmodes of activity at Stromboli: a lava flow (1985–1986effusive event), a scoria bomb from the ‘normal’present-day activity of Stromboli (April 1984), and a scoriaand coeval pumice sample from a recent more explosive eruption(September 1996). Plagioclase (An_62–90) and clinopyroxene(Mg-number between 0·69 and 0·91) phenocrystsin all samples record marked major element variations. Largeand comparable Sr isotope variations have been detected in plagioclaseand clinopyroxene. HP-magma crystals have resorbed cores, witheither high ⁸⁷Sr/⁸⁶Sr (0·70635–0·70630)or low ⁸⁷Sr/⁸⁶Sr (0·70614–0·70608); thelatter values are similar to the values of the outer cores.Mineral rims and glassy groundmasses generally have intermediate⁸⁷Sr/⁸⁶Sr (0·70628–0·70613). Similarly,mineral growth zones with three groups of ⁸⁷Sr/⁸⁶Sr values characterizeminerals from the LP-pumice, with the lowest values presentin mineral rims and groundmass glass. These results define amixing process between HP- and LP-magmas, plus crystallizationof clinopyroxene, plagioclase and olivine, occurring in a shallowmagma reservoir that feeds the present-day magmatic activityof Stromboli. An important observation is the presence of athird component (high ⁸⁷Sr/⁸⁶Sr in mineral cores) consideredto represent a pre-AD 1900 cumulus crystal mush reservoir situatedjust below the shallow magma chamber. These cumulus phases areincorporated by the LP-magma arriving from depth and transportedinto the shallow reservoir. A rapid decrease of ⁸⁷Sr/⁸⁶Sr inthe replenishing LP-magma immediately prior to eruption of theAD 1985 lava flow is associated with an increased volume ofLP-magma in the shallow magma chamber. The HP-magma in the shallowreservoir is not fully degassed when it interacts with the LP-magma,making efficient mixing possible that ultimately produces awell overturned homogeneous magma. Further degassing and crystallizationoccur at shallower levels as the HP-magma moves through a conduitto the surface. KEY WORDS: isotopic microsampling; mineral recycling; mixing; Sr isotope disequilibria; Stromboli