Hilary Downes - Profile on Academia.edu (original) (raw)

Papers by Hilary Downes

Επιστημονική Επετηρίδα του Τμήματος Γεωλογίας (Α.Π.Θ.), 2010

This paper presents an interpretation of results of a long-lasting scientific investigation of th... more This paper presents an interpretation of results of a long-lasting scientific investigation of the Herzegovina karst using the methods of Regionalized Variable Theory, i.e.

Geochemical Evidence for Mantle Enrichment and Lower Crustal Assimilation in Orogenic Volcanics from Monte Arcuentu, Southern Sardinia: Implications for Geodynamics and Evolution of the Western Mediterranean

AGU Fall Meeting Abstracts, Dec 1, 2016

Garnet Granulite Xenoliths from the Northern Baltic Shield: the Lower Crust of a Paleoproterozoic LIP

Ninth Annual V. M. Goldschmidt Conference, Aug 1, 1999

International Kimberlite Conference Extended Abstracts: 1998, Feb 14, 2019

Metasomatic interactions in the lithospheric mantle beneath the Massif Central, France: Characterization of metasomatic processes by trace element and redox features of spinel peridotites

EGUGA, May 1, 2014

Thermobarometry of diamond inclusions: Mantle structure and evolution beneath Archean cratons and mobile belts worldwide

Geosystems and Geoenvironment

Varieties of eclogites and their positions in the cratonic mantle lithosphere revealed by Jd-Di thermobarometry and trace element geochemistry

Multistage Metasomatized Mantle Beneath Alakit: Evidence from Mantle Xenoliths from Komsomolskaya Kimberlite Pipe, Yakutia

SSRN Electronic Journal, 2021

Deep mantle roots of the Zarnitsa kimberlite pipe, Siberian craton, Russia: Evidence for multistage polybaric interaction with mantle melts

Journal of Asian Earth Sciences, 2021

Abstract Zarnitsa kimberlite pipe in Central Yakutia contains pyrope garnets with Cr2O3 ranging f... more Abstract Zarnitsa kimberlite pipe in Central Yakutia contains pyrope garnets with Cr2O3 ranging from 9 to 19.3 wt% derived from the asthenospheric mantle. They show mostly S-shaped, inflected rare earth element (REE) patterns for dunitic and harzburgitic, lherzolitic and harzburgitic varieties and all are rich in high field strength elements (HFSE) due to reaction with protokimberlite melts. Lithospheric garnets ( 50 mWm−2) at 6–9 GPa, and a stepped HT geotherm with heated pyroxenite lenses at four levels from 6.5 to 3.5 GPa. Parental melts calculated with KDs suggest that augites and high-Cr garnets in the lithosphere base reacted with essentially carbonatitic melts while garnets from lower pressure show subduction peaks in U, Ba and Pb. The roots of the Zarnitsa pipe served to transfer large portions of deep (>9 GPa) protokimberlite melts to the lithosphere. Smaller diamonds were dissolved due to the elevated oxidation state but in peripheral zones large diamonds could grow.

Sr, Nd and Pb isotopic evidence for the lower crustal origin of Hercynian granodiorites and monzogranites, Massif Central, France (vol 136, pg 99, 1997)

A re-interpretation of the pyroclastic rocks marginal to the Eastern Red Hills around Kilchrist, Isle of Skye, N.W Scotland

Reconstructions of lithospheric mantle beneath Aldan shield based on deep-seated xenocrysts from lamprophyres of Chompolo field, Russia

Journal of Earth System Science, 2021

Lamprophyric (lamproitic) diatremes in Chompolo field, NE Aldan shield, Siberia dated 132 Ma cont... more Lamprophyric (lamproitic) diatremes in Chompolo field, NE Aldan shield, Siberia dated 132 Ma contain abundant mantle xenocrysts studied by EPMA and LA-ICP-MS. The deepest rocks ~6.5 GPa determined with thermobarometry are phlogopite eclogites. Thermobarometry on pyrope garnets, Cr-diopside and chromites yield a relatively low-temperature geotherm of 35 mW/m2 at the base to 55 mW/m2 at the Moho, while orthopyroxenes give 55 mW/m2. Amphiboles vary from magmatic Cr-hornblendes to metasomatic richterites. Trace element patterns for the host monchiquite are inclined with HFSE minima and are common for all Aldan lamprophyres but there are also more strongly inclined lamproitic varieties with ten times higher LREE contents. The geochemistry of Cr-diopsides, pyrope garnets and phlogopites reveal ancient multistage metasomatism of an originally depleted mantle keel related to both continental subduction and mantle plume activity. The lithospheric mantle is composed of harzburgites, roughly layered with rare dunites. Garnets appear widely at depths from 5.5 to 4.5 GPa. Trace element patterns show four groups from dunites to pyroxenites, related to low degree melting of hydrous and carbonatite melts. Clinopyroxenes reflect various melting degrees and variable trace element contents. The high-temperature orthopyroxene-based branches of the geotherm resulted from the percolation of hydrous plume-related melts. The high temperature 90 mW/m2 geotherm from 2.5 GPa to the Moho reflects fractionation of Fe-rich plume-related siliceous melts. Eclogites and metasomatites melting at the lithosphere base produce lamprophyres rich in LREE (100/C1), LILE, Zr and Ta–Nb minima. The Mg-lamproites are more LREE-enriched to 1000/C1.

International Geology Review, 2020

Wyoming Craton Mantle Lithosphere: Reconstructions Based on Xenocrysts from Sloan and Kelsey Lake Kimberlites

Proceedings of 10th International Kimberlite Conference, 2013

ABSTRACT The structure of the lithospheric mantle of the Wyoming craton beneath two Paleozoic kim... more ABSTRACT The structure of the lithospheric mantle of the Wyoming craton beneath two Paleozoic kimberlite pipes, Sloan and Kelsey Lake 1 in Colorado, was reconstructed using single-grain thermobarometry for a large data set ([2,600 EPMA analyses of xenocrysts and mineral intergrowths). Pyrope compositions from both pipes relate to the lherzolitic field (up to 14 wt% Cr2O3) with a few deviations in CaO to harzburgitic field for KL-1 garnets. Clinopyroxene variations (Cr-diopsides and omphacites) from the Sloan pipe show similarities with those from Daldyn kimberlites, Yakutia, and from kimberlites in the central part of the Slave craton, while KL-1 Cpx resemble those from the Alakit kimberlites in Yakutia that sample metasomatized peridotites. LAM ICP analyses recalculated to parental melts for clinopyroxenes from Sloan resemble contaminated protokimberlite melts, while clinopyroxenes from KL-1 show metasomatism by subduction fluids. Melts calculated from garnets from both pipes show peaks for Ba, Sr and U, and HFSE troughs, typical of subduction-related melts. Parental melts calculated for ilmenites from Sloan suggest derivation from highly differentiated melts, or melting of Ilm-bearing metasomatites, while those from Kelsey Lake do not display extreme HFSE enrichment. Three P-Fe# (where Fe# = Fe/(Fe ? Mg) in atomic units) trends within the mantle lithosphere beneath Sloan have been obtained using monomineral thermobarometry. At the base, the trends reveal melt metasomatized (possibly sheared) peridotites (Fe# = 13–15 %), refertilized peridotites (Fe# = 10–11 %) and primary mantle peridotites (Fe# = 7–9 %). Anomalous heating was found at depths equivalent to 4.0 and 3.0–2.0 GPa. The mantle section beneath KL-1 is widely metasomatized with several stages of refertilization with dispersed Ilm–Cpx trends. The step-like subadibatic heating in the mantle column beneath the Sloan pipe is strong in the base and the middle part and weaker within 2–3 GPa. Heating beneath the KL-1 pipe is more evident in the base and middle part from 7.0 to 3.0 GPa.

Using of clinopyroxene thermobarometry for the eclogites and omphacite diamond inclusions of Yakutian and worldwide kimberlites

Contributions to Mineralogy and Petrology, Jun 25, 2021

Mount Etna in NE Sicily occupies an unusual tectonic position in the convergence zone between the... more Mount Etna in NE Sicily occupies an unusual tectonic position in the convergence zone between the African and Eurasian plates, near the Quaternary subduction-related Aeolian arc and above the down-going Ionian oceanic slab. Magmatic evolution broadly involves a transition from an early tholeiitic phase (~ 500 ka) to the current alkaline phase. Most geochemical investigations have focussed on either historic (> 130-years old) or recent (< 130-years old) eruptions of Mt. Etna or on the ancient basal lavas (ca. 500 ka). In this study, we have analysed and modelled the petrogenesis of alkalic lavas from the southern wall of the Valle del Bove, which represent a time span of Mt. Etna's prehistoric magmatic activity from ~ 85 to ~ 4 ka. They exhibit geochemical variations that distinguish them as six separate lithostratigraphic and volcanic units. Isotopic data ( 143 Nd/ 144 Nd = 0.51283-0.51291; 87 Sr/ 86 Sr = 0.70332-0.70363; 176 Hf/ 177 Hf = 0.28288-0.28298; 206 Pb/ 204 Pb = 19.76-20.03) indicate changes in the magma source during the ~ 80 kyr of activity that do not follow the previously observed temporal trend. The oldest analysed Valle del Bove unit (Salifizio-1) erupted basaltic trachyandesites with variations in 143 Nd/ 144 Nd and 87 Sr/ 86 Sr ratios indicating a magma source remarkably similar to that of recent Etna eruptions, while four of the five subsequent units have isotopic compositions resembling those of historic Etna magmas. All five magma batches are considered to be derived from melting of a mixture of spinel lherzolite and pyroxenite (± garnet). In contrast, the sixth unit, the main Piano Provenzana formation (~ 42-30 ka), includes the most evolved trachyandesitic lavas (58-62 wt% SiO 2 ) and exhibits notably lower 176 Hf/ 177 Hf, 143 Nd/ 144 Nd, and 206 Pb/ 204 Pb ratios than the other prehistoric Valle del Bove units. This isotopic signature has not yet been observed in any other samples from Mt. Etna and we suggest that the parental melts of the trachyandesites were derived predominantly from ancient pyroxenite in the mantle source of Etna.

Lower crustal granulite xenoliths from the Arkhangelsk kimberlite pipes: petrological, geochemical and geophysical results

Lithos, Mar 1, 2000

ABSTRACT

Przegląd Geologiczny, 1997

$Research paper thumbnail of A new compositional estimate for refractory lower continental crust with implications for the first terrestrial Pb-isotope paradox$

A new compositional estimate for refractory lower continental crust with implications for the first terrestrial Pb-isotope paradox

Lithos, 2023

Mineralogy and Petrology, 1996

Ultramafic xenoliths from the basaltic tuff of Kapfenstein (Graz Basin) are mainly protogranular ... more Ultramafic xenoliths from the basaltic tuff of Kapfenstein (Graz Basin) are mainly protogranular and protogranular-to-porphyroclastic spinel lherzolites. In addition to these common rocks, a coarse-grained spinel harzburgite and a heterogeneous spinel peridotite consisting of a clinopyroxene-poor harzburgite and a clinopyroxene-rich lherzolite have been found. Interstitial amphiboles (pargasite and kaersutite) are present in most xenoliths. The magmatic activity which brought these xenoliths to the surface is part of the Plio-Pleistocene extension-related alkali basalt volcanism which took place in the Carpatho-Pannonian Region, post-dating the subduction-related calc-alkaline volcanism which formed the Inner Carpathian arc. Mineralogical and bulk geochemical data for Kapfenstein xenoliths are similar to those for other sub-continental shallow mantle xenolith suites. Correlation diagrams for major and trace elements indicate that their chemical variations are related to different degrees of partial melting due to the extraction of marie melts. Rare earth element (REE) patterns of separated clinopyroxenes from the xenoliths are generally light rare earth element (LREE)-depleted, although LREE-enriched and more complex patterns have also been found. 87Sr/S6Sr and l~3Nd/14~Nd isotopic ratios of separated clinopyroxenes vary between 0.70193 and 0.70330 (Ssr = -36.4 to -17.0) and 0.512824 and 0.513589 (eNd ~-4.2 to 18.1), respectively. The highest 87Sr/86Sr and lowest a43Nd/144Nd ratios occur in those samples which have the most refractory character and also have the most unusual REE patterns. REE determined by ion microprobe on separated interstitial Die ultramafischen Xenolithe aus dem Basaltuff von Kapfenstein (Grazer Becken) bestehen groBteils aus protogranularem und porphyroklastischem Spinell-Lherzolith. Abgesehen yon diesen weitverbreiteten Gesteinen wurden auch ein grobkSringer Spinell-Harzburgit und ein heterogener Spinell-Peridotit, der aus einem Klinopyroxen-armen Harzburgit und einem Klinopyroxen-reichen Lherzolith besteht, gefunden. Interstitielle Amphibole (Pargasit und Kaersutit) sind in den meisten Xenolithen vorhanden. Die magmatischen Prozesse, die diese Xenolithe an die Erdoberfl~che bef6rdeten, stehen mit dem mit Extension assoziierten Alkali-Basalt-Vulkanismus, der wfihrend des Pliozfin/ Pleistoz~ins dem durch Subduktion ausgel6sten Kalk-alkalischen Vulkanismus des inneren Karpathenbogens folgte, in Zusammenhang. Die mineralogischen und geochemischen Vollanalysen der Kapfensteiner Xenolithe sind mit den Daten yon anderen Gruppen von subkontinentalen Xenolithen des oberen Mantels vergleichbar. Korrelationsdiagramme fiir Haupt-und Spurenelemente zeigen, dab die chemische Variation auf verschiedene Grade von Teilschmelzbildung zurfickzuffihren ist. Die Verteilungsmuster der Seltenen Erden in Klinopyroxenen der Xenolithe sind generall LREE-verarmt, aber LREE angereichte und kompliziertere Verteilungsmuster kommen vor. Die STSr/S6Sr und 143Nd/l~4Nd Isotopenverh~iltnisse der Klinopyroxene schwanken zwischen 0.70193 und 0.70330 (esr = yon -36.4 bis -17.0), beziechungsweise zwischen 0.512854 und 0.513589 (eNd =von --4.2 bis 18.1). Die h6chsten SVSr/86Sr und niedrigsten 143Nd/144Nd Isotopenverh~iltnisse treten in den Proben mit dem st~rksten refrakt~ren Charakter und den ungew6hnlichsten REE-Verteilungsmustern auf. Die mittels Ionensonde ermittelten Gehalte an REE der interstitiellen Amphibole deuten ein Gleichgewicht mit dem Wirt-Peridotit an, obwohl auch Anzeichen von gest6rtem Gleichgewicht, unter anderem extreme Anreicherungen an LREE, beobachtet wurden. Ein Vergleich der Kapfenstein-Xenolithe mit denen aus dem 6stlichen Transsylvanischen Becken zeigt bemerkenswerte strukturelle und einige geochemische und isotopengeochemische Ahnlichkeiten. Beide Gegenden befinden sich am Rande des Erdmanteldiapirs, im Zentrum des Pannonischen Beckens. Die Deformationen und Verarmungsund Anreicherungsprozesse, die in beiden Gebieten stattfanden, sind vergleichbar. Die an LREE angereicherten interstitiellen Amphibole und das Auftreten yon diskordanten Amphibolg~ingen legen nahe, dab der obere verarmte lithosph~irische Mantel unter dem Grazer Becken yon, mit den mafischen alkalischen Grund-Magmen verwandten, metasomatischen Prozessen erfal3t worden ist, und nicht yon mit Subduktion assoziierten Flfissigkeiten oder Schmelzen.