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.     

River Warren boulders, Minnesota, USA: catastrophic paleoflow indicators in the southern spillway of glacial Lake Agassiz

Boulders resting on meltwater-sculpted and striated-granite bedrock near the head of the southern outlet spillway of glacial Lake Agassiz are used to generate paleodischarge calculations. The rounded nature of many boulders suggests fluvial transport and a corestone origin. The distribution of boulders in clusters and linear trains records the interaction of clasts during transport and deposition. The geomorphology of the spillway with streamlined erosional remnant hills is characteristic of other large flood spillways. Using the Manning equation and a variety of empirical equations to determine paleovelocity, preferred discharges between 0.364 and 0.102 Sv are calculated. These discharges agree well with flood discharges using modeling methodologies, and most likely represent ephemeral and catastrophic flood events linked to either episodic incision at the outlet or the result of rapid inputs of meltwater to Lake Agassiz.     

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.     

The Burnwell,Kentucky, low iron oxide chondrite fall: Description,classification and origin

Abstract— The Burnwell, Kentucky, meteorite fell as a single stone on 1990 September 4. The Burnwell meteorite has lower Fa in olivine (15.8 mol%), Fs in orthopyroxene (13.4 mol%), Co in kamacite (0.36 wt%), FeO from bulk chemical analysis (9.43 wt%), and Δ¹⁷O (0.51 ± 0.02%), and higher Fe, Ni, Co metal (19.75 wt% from bulk wet chemical analysis) than observed in H chondrites. The Burnwell meteorite plots on extensions of H-L-LL chondrite trends for each of these properties towards more reducing compositions than in H chondrites. Extensions of this trend have been previously suggested in the case of other low-FeO chondrites or silicate inclusions in the HE iron Netschaëvo, but interpretation of the evidence in these meteorites is complicated by terrestrial weathering, chemical disequilibrium or reduction. In contrast, the Burn-well meteorite is an equilibrated fall that exhibits no evidence for reduction. As such, it provides the first definitive evidence for extension of the H-L-LL ordinary chondrite trend beyond typical H values towards more reducing compositions.     

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.     

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.