Abstract The growth and recycling of continental crust has resulted in the chemical and thermal modification of Earth’s mantle, hydrosphere, atmosphere, and biosphere for ∼4.0 b.y. However, knowledge of the protolith that gave rise to the first continents and whether the environment of formation was a subduction zone still remains unknown. Here, tonalite melts are formed in high P-T experiments in which primitive oceanic plateau starting material is used as an analogue for Eoarchean (3.6–4.0 Ga) oceanic crust generated at early spreading centers. The tonalites are produced at 1.6–2.2 GPa and 900–950 °C and are mixed with slab-derived aqueous fluids to generate melts that have compositions identical to that of Eoarchean continental crust. Our data support the idea that the first continents formed at ca. 4 Ga and subsequently, through the subduction and partial melting of ∼30–45-km-thick Eoarchean oceanic crust, modified Earth’s mantle and Eoarchean environments and ecosystems.
This thesis consists of a review of experimental studies of sulphur solubility in mafic
magmas and the stability of magmatic sulphides, a theoretical examination of the
thermodynamics of C-O-S-H fluids to high temperature and pressure and three
experimental studies examining the role of sulphur under both volatile absent and
volatile saturated conditions.
Data concerning the sulphur solubility in mafic melts comes from experimental
studies on natural basalts and from metallurgical sources concerned with the
equilibrium between silicate slags and liquid metals. All of the metallurgical studies and
most of those involving natural compositions have been conducted at one atmosphere
and have identified the iron activity and the ambient oxygen and sulphur fugacities as
being the major factors controlling the sulphur solubility in a silicate melt. Studies at
pressures greater than one atmosphere are few and have examined only a few
compositions under relatively unconstrained sulphur and oxygen fugacities. Therefore
there is a lack of sulphur solubility data at high pressures and the partitioning of
sulphur between solid, melt and fluid phases remains an unknown of potential
significance.
Recent models for the genesis of MORB have emphasized the non-primary
nature of the majority of MORB glass compositions. The compositions of a range of
sulphide saturated potential MORB parent picritic liquids in equilibrium with either
lherzolitic or harzburgitic assemblages have been determined. The liquids produced are
similar to those generated from a similar but sulphur free source composition (pyrolite)
indicating that the presence of sulphur does not have the same fluxing effect as either
CO2 or H20.
Whereas volatile-poor conditions is appropriate for the modelling of MORB,
xenolithic material from the upper mantle frequently shows evidence of fluid
activity. A thermodynamic model for super-critical C-O-S-H fluids at high
temperature and pressure has been derived. Available P-V-T data for sulphur
bearing species has been combined with data for C-O-H species to calculate
fugacity coefficients by means of a modified Redlich-Kwong equation of state.
Using fugacity coefficients for multicomponent fluids the species distribution\as I
been determined as a function of the intensive variables P, T, f02 and fS2. The
results show that reduced fluids (f02≤1W) may contain a significant sulphur
fraction as H2S at moderate fS2 (IT+1) whereas more oxidized fluids (f02≥GC0)
by comparison contain little sulphur even at high fS2. SO2 is not a significant fluid
component in the range of f02's thought to characterize the upper mantle redox
range (IWCH4). By
analysis of the quenched fluid from the capsule, the relationship between the
parent fluid and the daughter fluid inclusions may be examined. Laser Raman micro
analysis of inclusions formed from a variety of pressure/temperature combinations
show compositional variation between individual inclusions and between inclusions
and the parent fluid showing that the assumption of isochemical entrapment may not
be valid for fluid inclusions formed under mantle conditions. Inclusions formed
also show development of daughter crystals indicating appreciable cation solubility
in the fluid phase at the run conditions. Infra-red spectrometry of the host olivine
shows development of structural O-H during the experiments revealing a possible
'sink' for inclusion hydrogen; a model for post entrapment chemical change in the
fluid phase is devised which has implications for the origin of CO2 rich inclusions
in mantle xenoliths.'
In the second series of experiments the same oxygen/sulphur fluid buffer
technique has been used to explore the fluid saturated phase relations for the system
pyrolite-C-0-S-H. The condition of melting (solidus) appears to coincide with the
disappearance of a hydrous phase (amphibole and/or phlogopite) and occurs midway
between the reduced C-0-H solidus as determined by Taylor and Green(1988)
and the water saturated solidus. Fractured natural olivine crystals were incorporated
into the capsule assembly and the inclusions produced show fluids dominated by
CH4-H20 mixtures in agreement with the analysis of the capsule gasses by capsule
piercing/mass spectrometry. In addition to these volatile phases substantial amounts
of daughter crystals are observed in the fluid inclusions from conditions below the
'solidus' indicating significant cation solubility in supercritical CH4-H20 mixtures.
At higher temperatures this solid component increases continuously in abundance
until true melt inclusions are obtained at conditions above the solidus. These
sulphide/silicate compositional relationships are examined and a model derived
which at a given pressure and temperature relates the compositions of olivine,
orthopyroxene and sulphide to the ambient sulphur and oxygen fugacities. Such a
reaction may be used as an oxygen/sulphur fugacity sensor if an independent means
of estimating the temperature and pressure is available.
Quaternary volcanic centres north of the Bitlis-Zagros suture in Turkey, Iran and the Caucasus represent both volcanic hazards and potential or actual geothermal energy resources. Such challenges and opportunities cannot be fully quantified without understanding these volcanoes’ petrogenesis, geochronology and magmatic, tectonic or other eruption triggers. In this preliminary study, we discuss the age and geology of the Karkar monogenetic volcanic field in Syunik, SE Armenia. The ~70 km2 field is close to Armenia’s only geothermal energy test drilling site. Fissure-fed trachybasaltic andesite to trachyandesite lavas erupted on a trans-tensional segment of the Syunik branch of the Pambak-Sevan-Syunik Fault, where previous studies suggested a Holocene age for the youngest eruptions. Here, high-resolution duplicate 40Ar/39Ar dating of 7 groundmass separates provided composite plateau or inverse isochron ages ranging from 6 ± 3 ka and 8 ± 3 ka to 332 ± 9 ka (2). Each lava flow displays petrographic and whole rock geochemical patterns consistent with melting of subduction-modified lithospheric mantle and extensive evolution within the crust involving fractional crystallisation and mixing of magma batches. Data confirm that volcanic activity in Syunik and also Vardenis provinces overlapped with Palaeolithic to Bronze Age human occupation and remains a minor lava inundation hazard. Further geochemical work will allow constraint of the depth and timescales of magma storage. Both Karkar and the area around Porak volcano, which lies 35 km N of Karkar on the Syunik Fault, might be considered for future geothermal energy developments.
Scottish “Newer” Granites record the evolution of the Caledonides resulting from Iapetus subduction and slab breakoff during the Silurian-Devonian Scandian Orogeny, but relationships between geodynamics, petrogenesis and emplacement are incomplete. Laser ablation U-Pb results from magmatic zircons at the Cluanie Pluton (Northern Highlands) identify clusters of concordant Silurian data points. A cluster with a weighted mean 206Pb/238U age of 431.6 ± 1.3 Ma (2sigma confidence interval, n = 6) records emplacement whilst older points (clustered at 441.8 ± 2.3 Ma, n = 9) record deep crustal hot zone magmatism prior to ascent. The Cluanie Pluton, and its neighbour the ~428 Ma Clunes tonalite, have adakite-like high Na, Sr/Y, La/Yb and low Mg, Ni and Cr characteristics, and lack mafic facies common in other “Newer Granites”. These geochemical signatures indicate the tapping of batches of homogenised, evolved magma from the deeper crust. The emplacement age of the Cluanie Pluton confirms volumetrically modest subduction-related magmatism occurred beneath the Northern Highlands before slab breakoff, probably as a result of crustal thickening during the ~450 Ma Grampian 2 event. Extensive new in-situ geochemical-geochronological studies for this terrane may further substantiate the deep crustal hot zone model and the association between Caledonian magmatism and potentially metallogenesis. The term “Newer Granites” is outdated as it ignores the demonstrated relationships between magmatism, Scandian orogenesis and slab breakoff. Hence, “Caledonian intrusions” would be a more appropriate generic term to cover those bodies related to either Iapetus subduction or to slab breakoff.
Extensional tectonics and incipient rifting on the north side of the Iapetus suture were associated with eruption of (mainly) mildly alkaline olivine basalts. Initially in the Tournaisian (Southern Uplands Terrane), magmatic activity migrated northwards producing the Garleton Hills Volcanic Formation (GHVF) across an anomalous sector of the Southern Uplands. The latter was followed by resumption of volcanism in the Midland Valley Terrane, yielding the Arthur's Seat Volcanic Formation. Later larger-scale activity generated the Clyde Plateau Volcanic Formation (CPVF) and the Kintyre lavas on the Grampian Highlands Terrane. Comparable volcanic successions occur in Limerick, Ireland. This short-lived ( c. 30 myr) phase was unique in the magmatic history of the Phanerozoic of the British Isles in which mildly alkaline basaltic magmatism locally led to trachytic differentiates. The Bangly Member of the GHVF represents the largest area occupied by such silicic rocks. The most widespread lavas and intrusions are silica-saturated/oversaturated trachytes for which new whole-rock and isotopic data are presented. Previously unrecognized ignimbrites are described. Sparse data from the fiamme suggest that the magma responsible for the repetitive ignimbrite eruptions was a highly fluid rhyolite. The Bangly Member probably represents the remains of a central-type volcano, the details of which are enigmatic.
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