The role of contamination in the tightrope of Gran Canaria felsic magma differentiation

2020 ◽  
Author(s):  
Edgar Alejandro Cortes Calderon ◽  
Ben Ellis ◽  
Julia Neukampf ◽  
Chris Harris ◽  
Darren Mark ◽  
...  
Keyword(s):  
Oxygen Isotopes ◽  
Major And Trace Elements ◽  
Crustal Assimilation ◽  
Gran Canaria ◽  
Liquid Line ◽  
Ar Geochronology ◽  
Liquid Line Of Descent ◽  
Differentiation Trend ◽  
Alkali Feldspars

<p>Peralkaline magmatism is mostly sustained by extensive feldspar fractionation from mafic parents at shallow depths in intraplate settings. In this case, silica saturation is critical as it controls the differentiation trend that a peralkaline magma follows. SiO<sub>2</sub>-oversaturated parents fractionate towards rhyolites, and SiO<sub>2</sub>-undersaturated towards phonolitic compositions. The Miocene post-shield stage of Gran Canaria records both differentiation trends, which has previously been ascribed to changes in the mantle source. Such stage has been divided in the Mogan and Fataga Group based on silica saturation. Here, we propose that contamination plays a key role in the differentiation of Gran Canaria volcanics. This assumption is supported with new <sup>40</sup>Ar/<sup>39</sup>Ar geochronology, mineral, glass and juvenile clast chemistry (oxygen isotopes, major and trace elements) merged with a detailed stratigraphy. Two types of contaminants were identified, one being cogenetic feldspar-dominated cumulates and the second one being sediments within the island crust. We propose that barium-rich trachytic magmas with positive europium anomalies are linked to melting of the feldspar cumulates left after extensive fractional crystallisation. The chemistry of such trachytes does not follow a liquid line of descent and contains reverse-zoned alkali-feldspars. The shift in silica saturation took less than 1 Ma and is marked by an increase in peralkalinity from 0.9 to 1.5 and a decrease in oxygen isotopes ratio from 7.0 to 5.0 ‰. We interpret these observations as the consequence of maturation of the shallow magma reservoir towards less sediment contamination. Such assimilation of sediments is limited thermally, and compositionally because the system should remain alumina deficient. Crustal assimilation in Gran Canaria did not produce voluminous silicic melts by itself but allowed the deviation of the differentiation trend of a more primitive, initially SiO<sub>2</sub>-undersaturated magma. The tightrope of differentiation is represented by the thermal divide between the granite and phonolite minima (i.e. feldspar join in petrogeny’s residua system). Contamination by sediments produces a transient SiO<sub>2</sub>-oversaturated system (Mogan Group). Cogenetic assimilation of cumulates by thermal rejuvenation of the reservoir attracts the magma towards the thermal divide (ubiquitous during the peralkaline stage). Armouring against sediment assimilation through time relaxes the system back to the initial SiO<sub>2</sub>-undersaturated conditions (Fataga Group).</p>

scholarly journals A Liquid Line of Descent of the Jotunite (Hypersthene Monzodiorite) Suite

1998 ◽  
Vol 39 (3) ◽  
pp. 439-468 ◽  
Author(s):  
J. Vander Auwera ◽  
J. Longhi ◽  
J.-C. Duchesne
Keyword(s):  
Liquid Line ◽  
Liquid Line Of Descent

Bulk Liquid for the Skaergaard Intrusion and Its PGE-Au Mineralization: Composition, Correlation, Liquid Line of Descent, and Timing of Sulphide Saturation and Silicate–Silicate Immiscibility

2019 ◽  
Vol 60 (10) ◽  
pp. 1853-1880 ◽  
Author(s):  
Troels F D Nielsen ◽  
C Kent Brooks ◽  
Jakob K Keiding
Keyword(s):  
Precious Metal ◽  
Bulk Composition ◽  
Precious Metals ◽  
Bulk Liquid ◽  
Skaergaard Intrusion ◽  
Liquid Line ◽  
Multi Stage ◽  
Silicate Liquids ◽  
Crystallization Zone ◽  
Liquid Line Of Descent

Abstract Preferred and modelled bulk composition of the Skaergaard intrusion are compared to coeval basaltic compositions in East Greenland and found to relate to the second evolved cycle of Geikie Plateau Formation lavas and coeval Skaergaard-like dikes in major and trace element (Mg# ∼45, Ce/Nb ∼2·5, (Dy/Yb)N ∼1·35), and precious metal composition (Pd/Pt ∼3, Au/Pt ∼2) as well as in age (∼56 Ma). Successful comparisons of precious metal compositions only occur with Skaergaard models based on mass balance. The bulk liquid of the intrusion evolved along the liquid line of descent to immiscibility between Si- and Fe-rich silicate liquids after ∼90% of crystallization (F = ∼0·10) in agreement with experimental constraints. Immiscibility led to accumulation and fractionation of the Fe-rich silicate melt in the mushy floor of the intrusion and continued accumulation of granophyre component in the remaining bulk liquid. The composition of plagioclase in the precious metal mineralized gabbro and modelling of Pd/Pt and Au/Pt in first formed droplets of sulphide melt suggest that sulphide saturation was reached in interstitial melts in crystal mushes in the floor and roof and in bulk liquid with a composition equivalent to that of the bulk liquid at lower UZa times and after crystallization of 82–85% of the bulk liquid (F = 0·19–0·16). Prior to sulphide saturation in UZa type melt, the precious metals ratios of the bulk liquid were controlled by the loss of Pt relative to Pd and Au in agreement with the low empirical and experimental solubility of Pt of ∼9ppb compared to a much higher value for Pd and Au. The relative timing between sulphide saturation (F = ∼0·18) and immiscibility between silicate melts (F = ∼0·10) and modelled precious metal ratios underpin the proposed multi-stage model for the mineralization, advocating initial accumulation in the mushy floor of the magma chamber controlled by sulphide saturation in mush melts rather than bulk melt, followed by redistribution of precious metals in a macro-rhythmic succession of gabbroic layers of the upward migrating crystallization zone.

scholarly journals Equilibrium crystallization of massif-type anorthosite residual melts: a case study from the 1.64 Ga Ahvenisto complex, Southeastern Finland

2020 ◽  
Vol 175 (9) ◽  
Author(s):  
Riikka Fred ◽  
Aku Heinonen ◽  
Jussi S. Heinonen
Keyword(s):  
Magma Chambers ◽  
Equilibrium Crystallization ◽  
Liquid Line ◽  
Rock Types ◽  
Residual Magma ◽  
Icp Ms ◽  
Mineral Assemblages ◽  
Residual Melt ◽  
Liquid Line Of Descent

Abstract Fe–Ti–P-rich mafic to intermediate rocks (monzodiorites and oxide–apatite–gabbronorites, OAGNs) are found as small intrusions in most AMCG (anorthosite–magnerite–charnokite–granite) suites. The origin of the monzodioritic rocks is still debated, but in many studies, they are presumed to represent residual liquid compositions after fractionation of anorthositic cumulates. In the 1.64 Ga Ahvenisto complex, SE Finland, monzodioritic rocks occur as minor dike-like lenses closely associated with anorthositic rocks. We report new field, petrographic, and geochemical (XRF, ICP-MS, EMPA) data complemented with crystallization modeling (rhyolite-MELTS, MAGFRAC) for the monzodioritic rocks, apatite–oxide–gabbronorite, and olivine-bearing anorthositic rocks of the Ahvenisto complex. The presented evidence suggest that the monzodioritic rocks closely represent melt compositions while the apatite–oxide–gabbronorite and olivine-bearing anorthositic rocks are cumulates. The monzodioritic rocks seem to form a liquid line of descent (LLD) from primitive olivine monzodiorites to more evolved monzodiorites. Petrological modeling suggests that the interpreted LLD closely corresponds to a residual melt trend left after fractional crystallization (FC) and formation of the cumulate anorthositic rocks and minor apatite–oxide–gabbronorite in shallow magma chambers. Consequent equilibrium crystallization (EC) of separate monzodioritic residual magma batches can produce the observed mineral assemblages and the low Mg numbers measured from olivine (Fo25–45) and pyroxenes (En48–63, Mg#cpx 60–69). The monzodioritic rocks and apatite–oxide–gabbronorites show similar petrological and geochemical characteristics to corresponding rock types in other AMCG suites, and the model described in this study could be applicable to them as well.

Self-cannibalisation of an active cumulate system (Blumone complex, Adamello, Italy): Geochemical and experimental evidence

2020 ◽  
Author(s):  
Manuel Pimenta Silva ◽  
Peter Ulmer ◽  
Othmar Müntener
Keyword(s):  
Low Pressure ◽  
Experimental Results ◽  
Chemical Compositions ◽  
Calc Alkaline ◽  
Liquid Line ◽  
Mineral Phases ◽  
Peritectic Melting ◽  
Simple Mass ◽  
Mafic Magmas ◽  
Liquid Line Of Descent

<p>In the southern part of the Adamello Batholith (43-33 Ma; Schaltegger et al., 2019) in Northern Italy (Re di Castello superunit), we identified a multi-generational dyke suite with “exotic” chemical compositions intruding quartz-dioritic units surrounding a gabbroic complex. These dykes are characterised by SiO<sub>2</sub> contents between 43 and 46 wt.%, high Al<sub>2</sub>O<sub>3</sub> (20-21 wt.%), and low MgO and Ni (below 6.5 wt.% and 40 μg/g, respectively), displaying a nepheline-normative character. Furthermore, they exhibit positive Sr and Ba anomalies. These chemical features exclude a possible primitive character or derivation from a typical calc-alkaline liquid line of descent, as identified for the Adamello Massif (Ulmer et al, 2018). The primocrystic cargo of these dikes (clinopyroxene, anorthitic plagioclase, and low-Si, high-Na pargasitic amphibole) displays striking similarities with cumulate crystals of the contiguous Blumone amphibole gabbroic cumulate, inferring mechanical interaction of these exotic liquids with and/or derivation from the cumulate complex. Amphibole-plagioclase equilibration temperatures of the dikes (875 to 775ºC) are consistent with thermal equilibration with the surrounding quartz-dioritic mush. Sharp contacts and dyke fragmentation are also observed and are thermally congruent with the ductile-brittle transition of a quartz-dioritic to tonalitic mush (Marxer & Ulmer, 2019).</p><p>Simple mass balance calculations modelling of the peritectic melting of pargasitic amphibole and high-An plagioclase (major mineral phases of the contiguous amphibole gabbroic cumulates) with simultaneous crystallisation of low-Al clinopyroxene reveal that melt compositions similar to these dykes can be achieved with amphibole-plagioclase proportions ranging between 65:35 and 50:50. To verify if peritectic cumulate remelting represents a possible generation mechanism of these dykes we performed<span>  </span>experiments at 0.2 GPa.</p><p>Established phase equilibria of these dyke compositions reveal a lack of near-liquidus olivine, which is a rare phase in gabbroic complex. This is consistent with preliminary experimental results on cumulate melting, where olivine is also absent at high temperatures (> 1075ºC). These observations further disprove the petrogenesis of these liquids via a calc-alkaline liquid line of descent, where mafic magmas would be early saturated in olivine at low pressure further supporting their generation by local remelting of amphibole-plagioclase dominated mafic cumulates.Geochemical as well as experimental results both strongly point towards the petrogenesis of these nepheline-normative, high-Al, low-Mg picrobasalts by low pressure peritectic melting of a pargasite-anorthite cumulate assemblage in an active magmatic system.</p><p> </p><p>Marxer, F. & Ulmer, P. <em>Contrib Mineral Petr.</em> <strong>174(10)</strong>, 84 (2019).</p><p>Schaltegger, U. <em>et al. J Petrol. </em><strong>60(4)</strong>, 701-722 (2019).</p><p>Ulmer, P. <em>et al. J. Petrol.</em> <strong>59(1)</strong>, 11-58 (2018).</p>

scholarly journals The role of phase separation and related topography in the exceptional ice-nucleating ability of alkali feldspars

2017 ◽  
Vol 19 (46) ◽  
pp. 31186-31193 ◽  
Author(s):  
Thomas F. Whale ◽  
Mark A. Holden ◽  
Alexander N. Kulak ◽  
Yi-Yeoun Kim ◽  
Fiona C. Meldrum ◽  
...  
Keyword(s):  
Phase Separation ◽  
Better Than ◽  
Alkali Feldspars

Alkali feldspars which are phase separated into K- and Na-rich regions nucleate ice far better than those without phase separation.

scholarly journals The Central Atlantic Magmatic Province (CAMP) in Morocco

2019 ◽  
Vol 60 (5) ◽  
pp. 945-996 ◽  
Author(s):  
Andrea Marzoli ◽  
Hervé Bertrand ◽  
Nasrrddine Youbi ◽  
Sara Callegaro ◽  
Renaud Merle ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
Major And Trace Elements ◽  
Large Igneous Province ◽  
Published Data ◽  
Crustal Assimilation ◽  
The North ◽  
Pan African ◽  
Central Atlantic Magmatic Province ◽  
Intermediate Unit ◽  
Central Atlantic

Abstract The Central Atlantic Magmatic Province (CAMP) is a large igneous province (LIP) composed of basic dykes, sills, layered intrusions and lava flows emplaced before Pangea break-up and currently distributed on the four continents surrounding the Atlantic Ocean. One of the oldest, best preserved and most complete sub-provinces of the CAMP is located in Morocco. Geochemical, geochronologic, petrographic and magnetostratigraphic data obtained in previous studies allowed identification of four strato-chemical magmatic units, i.e. the Lower, Intermediate, Upper and Recurrent units. For this study, we completed a detailed sampling of the CAMP in Morocco, from the Anti Atlas in the south to the Meseta in the north. We provide a complete mineralogical, petrologic (major and trace elements on whole-rocks and minerals), geochronologic (40Ar/39Ar and U–Pb ages) and geochemical set of data (including Sr–Nd–Pb–Os isotope systematics) for basaltic and basaltic–andesitic lava flow piles and for their presumed feeder dykes and sills. Combined with field observations, these data suggest a very rapid (<0·3 Ma) emplacement of over 95% of the preserved magmatic rocks. In particular, new and previously published data for the Lower to Upper unit samples yielded indistinguishable 40Ar/39Ar (mean age = 201·2 ± 0·8 Ma) and U–Pb ages (201·57 ± 0·04 Ma), suggesting emplacement coincident with the main phase of the end-Triassic biotic turnover (c.201·5 to 201·3 Ma). Eruptions are suggested to have been pulsed with rates in excess of 10 km3/year during five main volcanic pulses, each pulse possibly lasting only a few centuries. Such high eruption rates reinforce the likelihood that CAMP magmatism triggered the end-Triassic climate change and mass extinction. Only the Recurrent unit may have been younger but by no more than 1 Ma. Whole-rock and mineral geochemistry constrain the petrogenesis of the CAMP basalts. The Moroccan magmas evolved in mid-crustal reservoirs (7–20 km deep) where most of the differentiation occurred. However, a previous stage of crystallization probably occurred at even greater depths. The four units cannot be linked by closed-system fractional crystallization processes, but require distinct parental magmas and/or distinct crustal assimilation processes. EC-AFC modeling shows that limited crustal assimilation (maximum c.5–8% assimilation of e.g. Eburnean or Pan-African granites) could explain some, but not all the observed geochemical variations. Intermediate unit magmas are apparently the most contaminated and may have been derived from parental magmas similar to the Upper basalts (as attested by indistinguishable trace element contents in the augites analysed for these units). Chemical differences between Central High Atlas and Middle Atlas samples in the Intermediate unit could be explained by distinct crustal contaminants (lower crustal rocks or Pan-African granites for the former and Eburnean granites for the latter). The CAMP units in Morocco are likely derived from 5–10% melting of enriched peridotite sources. The differences observed in REE ratios for the four units are attributed to variations in both source mineralogy and melting degree. In particular, the Lower basalts require a garnet peridotite source, while the Upper basalts were probably formed from a shallower melting region straddling the garnet–spinel transition. Recurrent basalts instead are relatively shallow-level melts generated mainly from spinel peridotites. Sr–Nd–Pb–Os isotopic ratios in the CAMP units from Morocco are similar to those of other CAMP sub-provinces and suggest a significant enrichment of the mantle-source regions by subducted crustal components. The enriched signature is attributed to involvement of about 5–10% recycled crustal materials introduced into an ambient depleted or PREMA-type mantle, while involvement of mantle-plume components like those sampled by present-day Central Atlantic Ocean Island Basalts (OIB, e.g. Cape Verde and Canary Islands) is not supported by the observed compositions. Only Recurrent basalts may possibly reflect a Central Atlantic plume-like signature similar to the Common or FOZO components.

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