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Minerals ◽  
2022 ◽  
Vol 12 (1) ◽  
pp. 92
Author(s):  
Olga Kiseleva ◽  
Pavel Serov ◽  
Evgenia Airiyants ◽  
Aleksey Travin ◽  
Dmitriy Belyanin ◽  
...  

We report the first radiogenic Nd-Sr isotope data in the magmatic rocks island-arc ophiolite assemblage from the middle branch of the East Sayan ophiolite complexes to better constrain geodynamic processes in this segment of the CAOB in southern central Siberia. The magmatic rocks belong to the following geochemical types: (1) Ensimatic island-arc boninites; (2) island-arc assemblage; (3) enriched basalts of mid-ocean ridges; and (4) oceanic island-like basalts. The boninites have a positive value εNd (T), which is generated from a depleted mantle source (N-MORB). The island-arc assemblage has negative or slightly positive εNd (T) and was formed from an enriched mantle source due to the subduction of terrigenous rocks. The source of the terrigenous material was most likely the rocks of the Archean TTG (Trondhjemite Tonalite Granodiorite) complex of the Gargan block. Isotopic ratios for E-MOR and OIB-like basalts are characterized by positive or slightly negative values of εNd (T). The mafic dike, which crosscut ophiolite rocks, corresponds to OIB-like basalts. The values of εNd (T), measured 87Sr/86Sr and I (Sr), in the mafic dike correspond to the EM I mantle source. The E-MOR and OIB-like basalts appear to be formed in late-stage asthenospheric mantle melting via the decompression melting processes. The obtained isotope geochemical data for the E-MOR and OIB-like basalts probably indicate the mixing of island-arc melts with asthenospheric melts. We undertook 40Ar/39Ar dating of the mafic dike, which crosscut the ophiolite unit. The mafic dike has a whole-rock 40Ar/39Ar weighted mean plateau age of 799 ± 11 Ma. The dating constrains the minimum age of the ophiolite and island-arc magmatism in the region.


2022 ◽  
Vol 9 ◽  
Author(s):  
G. F. Cooper ◽  
E. C. Inglis

Lavas produced at subduction zones represent the integration of both source heterogeneity and an array of crustal processes, such as: differentiation; mixing; homogenisation; assimilation. Therefore, unravelling the relative contribution of the sub-arc mantle source versus these crustal processes is difficult when using the amalgamated end products in isolation. In contrast, plutonic xenoliths provide a complementary record of the deeper roots of the magmatic plumbing system and provide a unique record of the true chemical diversity of arc crust. Here, we present the δ56Fe record from well characterised plutonic xenoliths from two distinct volcanic centres in the Lesser Antilles volcanic arc–the islands of Martinique and Statia. The primary objective of this study is to test if the Fe isotope systematics of arc lavas are controlled by sub-arc mantle inputs or during subsequent differentiation processes during a magma’s journey through volcanic arc crust. The Fe isotopic record, coupled to petrology, trace element chemistry and radiogenic isotopes of plutonic xenoliths from the two islands reveal a hidden crustal reservoir of heavy Fe that previously hasn’t been considered. Iron isotopes are decoupled from radiogenic isotopes, suggesting that crustal and/or sediment assimilation does not control the Fe systematics of arc magmas. In contrast to arc lavas, the cumulates from both islands record MORB-like δ56Fe values. In Statia, δ56Fe decreases with major and trace element indicators of differentiation (SiO2, Na2O + K2O, Eu/Eu*, Dy/Yb), consistent with fractionating mineral assemblages along a line of liquid descent. In Martinique, δ56Fe shows no clear relationship with most indicators of differentiation (apart from Dy/Yb), suggesting that the δ56Fe signature of the plutonic xenoliths has been overprinted by later stage processes, such as percolating reactive melts. Together, these data suggest that magmatic processes within the sub-arc crust overprint any source variation of the sub-arc mantle and that a light Fe source is not a requirement to produce the light Fe isotopic compositions recorded in volcanic arc lavas. Therefore, whenever possible, the complimentary plutonic record should be considered in isotopic studies to understand the relative control of the mantle source versus magmatic processes in the crust.


2022 ◽  
Vol 368 ◽  
pp. 106480
Author(s):  
Basem Zoheir ◽  
Aliaa Diab ◽  
Petros Koutsovitis ◽  
Tamer Abu Alam ◽  
Mark Feigenson ◽  
...  

2022 ◽  
Vol 578 ◽  
pp. 117307
Author(s):  
Matthew J. Comeau ◽  
Michael Becken ◽  
Alexander V. Grayver ◽  
Johannes S. Käufl ◽  
Alexey V. Kuvshinov

2021 ◽  
Vol 177 (1) ◽  
Author(s):  
Maryjo Brounce ◽  
Edward Stolper ◽  
John Eiler

AbstractGlasses quenched from relatively undegassed ocean island magmas erupted from volcanoes at Iceland, Hawaii, the Canary Islands, and Erebus have elevated Fe3+/∑Fe ratios compared to glasses quenched from mid-ocean ridge basalts. This has been ascribed to elevated fO2 of their mantle sources, plausibly due to subducted, oxidized near-surface-derived components in their mantle sources. The basaltic magmas from Reunion Island in the Indian ocean have Sr–Nd-Hf-Pb-Os isotopic compositions suggesting that their mantle sources contain little or no subducted near-surface materials and contain the C/FOZO/PREMA mantle component. To constrain the fO2 of the C/FOZO/PREMA mantle component and test the link between oxidized OIB and recycled surface-derived materials in their sources, we measured major and volatile element abundances and Fe3+/∑Fe ratios of naturally glassy, olivine-hosted melt inclusions from Piton de La Fournaise volcano, La Reunion. We conclude that the fO2 of the mantle source of these Reunion lavas is lower than of the mantle sources of primitive, undegassed magmas from Hawaii, Iceland, the Canary Islands, and Mt. Erebus, and indistinguishable from that of the Indian-ocean upper mantle. This finding is consistent with previous suggestions that the source of Reunion lavas (and the C/FOZO/PREMA mantle component) contains little or no recycled materials and with the suggestion that recycled oxidized materials contribute to the high fO2 of some other OIBs, especially those from incompatible-element-enriched mantle sources. Simple mixing models between oxidized melts of EM1 and HIMU components and relatively reduced melts of DMM can explain the isotopic compositions and Fe3+/∑Fe ratios of lavas from Hawaii, Iceland, the Canary Islands, and Mount Erebus; this model can be tested by study of additional OIB magmas, including those rich in the EM2 component.


2021 ◽  
Author(s):  
Thomas Gernon ◽  
Stephen Jones ◽  
Sascha Brune ◽  
Thea Hincks ◽  
Anne Glerum ◽  
...  

Abstract Diamonds are erupted at Earth’s surface in volatile-rich magmas called kimberlites1,2,3. These enigmatic magmas, originating from depths exceeding 150 kilometres in Earth’s mantle1, occur in stable cratons and in pulses broadly synchronous with supercontinent cyclicity4. Whether their mobilization is driven by mantle plumes5 or mechanical weakening of cratonic lithosphere4,6 remains unclear. Here we show that most kimberlites spanning the past billion years erupted approximately 25 million years after the onset of continental fragmentation, suggesting an association with rifting processes. Our dynamic models show that physically steep lithosphere-asthenosphere boundaries formed during terminal rifting (necking) generate convective instabilities in the asthenosphere that slowly migrate many hundreds of kilometres inboard of the rift, causing destabilization of cratonic mantle keel tens of kilometres thick. Displaced lithosphere is replaced by hot, upwelling asthenosphere in the return flow, causing partial melting of carbonated mantle and variable assimilation of lithospheric material. The resulting small-volume kimberlite magmas ascend rapidly and adiabatically, exsolving amounts of carbon dioxide (CO2) that are consistent with independent constraints7. Our model reconciles diagnostic kimberlite features including association with cratons and geochemical characteristics that implicate a common asthenospheric mantle source contaminated by cratonic lithosphere8. Together, these results provide a quantitative and mechanistic link between kimberlite episodicity and supercontinent cycles via progressive disruption of cratonic keels.


2021 ◽  
Vol 19 ◽  
pp. 1-23
Author(s):  
Esteban Mellado ◽  
Mercè Corbella ◽  
Didac Navarro ◽  
Andrew Kylander

Post-collisional mafic dykes crosscut the Paleozoic metamorphic basement and late-Variscan plutons in Les Guilleries massif (Catalan Coastal Ranges, NE Iberia). The predominance of mafic phenocrysts, porphyritic texture, abundant amphibole, high MgO and volatile content, together with crustal-like trace-element patterns indicate that the dykes correspond to calc-alkaline lamprophyres, mainly spessartites. Their enrichment in LILE, HFSE and REE and initial Sr-Nd isotopic compositions (87Sr/86Sri between 0.70851 and 0.71127, epsilon Ndi between -5.23 and -4.63) are consistent with an enriched subcontinental lithospheric mantle source. U-Pb ages of matrix titanite crystals yield concordia ages of 262±7Ma, congruent with crosscutting relationships. Postmagmatic processes are evidenced by intense chloritization and albitization of the lamprophyres, together with systematic variations of Na2O vs SiO2, K2O, CaO, Ba, Rb, Cs, Pb, Sr, Tl, and Zn, and possibly the removal of F. The geochemical and geochronological data support an orogenic geochemical affinity, in accordance with the transitional tectonic regime between Variscan compression/transpression and post-collisional transtension/extension, related to the fragmentation of Pangea and thinning of the lithosphere. The lamprophyre dykes studied could represent the youngest pulse of Variscan orogenic magmatism and, therefore, mark its end in NE Iberia before the onset of the generalized Triassic extension.


2021 ◽  
Author(s):  
◽  
Jacob Leath

<p>The southern Kermadec Arc – Havre Trough (SKAHT) is an intra-oceanic arc – back-arc system where the Pacific plate is subducting beneath the Australian plate. The Kermadec volcanic arc front consists of 33 volcanic centres, four of which host hydrothermal mineralization (Brothers, Haungaroa, Rumble II West, and Clark) such as volcanogenic massive sulfide (VMS) deposits, which are characterised by high concentrations of base and precious metals (e.g., Au, Cu, Zn, Pb). The sources of these metals are strongly tied to the metal contents within underlying magmatic rocks and associated magmatic systems with which the hydrothermal fluids interact. Understanding the sources, movements, and accumulation of metals associated with porphyry copper and exhalative base metal deposits within a subduction – arc setting remains limited.  This study reports major, trace, and volatile element contents in basaltic groundmass glasses and olivine-hosted melt inclusions from lavas from four locations within the arc – back-arc setting of the SKAHT. The focus is on understanding the controls on base metal (Pb, Cu, Zn, Mo, V) contents in the magmas. The sample locations, Rumble III and Rumble II West volcanoes, and back-arc Basins D and I, form an arc-perpendicular transect extending from arc front into the back-arc. The analysed melt inclusion and groundmass glasses are all basalt to basaltic andesite in composition, with back-arc basin samples more mafic than arc front volcano samples. The magmatic evolution of the melts is primarily controlled by crystal fractionation of olivine + pyroxene + plagioclase. All glasses have undergone variable degassing, indicated by an absence of detectable CO₂ and curvilinear decreases in S contents with increasing SiO₂. Of the volatile phases analysed, only Cl appears unaffected by degassing.  Distinct compositional differences are apparent between arc front and back-arc melts. The arc front magmas formed from higher degrees of melting of a less fertile mantle source and are more enriched in trace elements then the back-arc magmas due to greater additions of slab-derived aqueous fluids to their source. Magmas from a single arc front volcano (Rumble II West) incorporate melts that have tapped variably enriched sources, indicating heterogeneity of the mantle at small scales. Significant variation in mantle composition, however, is also apparent laterally along strike of the arc. Rumble III volcano and Basin I lie on an arc-perpendicular transect south of Rumble II West volcano and Basin D. Their greater enrichment in trace elements and higher concentrations of base metals than Rumble II West and Basin D lavas can be attributed to higher fluxes of subduction derived components.  Base metals (Cu, Zn, Pb, Mo, and V) are variably enriched in the SKAHT melts compared with typical mid-ocean ridge basalts with relative enrichments in the order Pb >> Cu > Mo, V > Zn. All metals appear to be affected by mantle metasomatism related to slab-derived fluids, either directly from slab components introduced to the mantle source (e.g., Pb) or through mobilisation of metals within the ambient mantle wedge. The apparently compatible behaviour of Zn, Cu, and V in the mantle means that these elements may be enriched in arc front magmas relative to back-arc magmas by higher degrees of partial melting and/or melting of more depleted sources.  All base metals behave incompatibly in the magma during crystal fractionation between 48 – 56 wt.% SiO₂. Lead and Cu concentrations, however, begin to level out from ~ 52 wt.% SiO₂ suggesting some subsequent loss to fractionating volatile phases as metal sulfide complexes. Rumble III samples show a decrease in metal concentration (Pb, Cu, V), from melt inclusions to groundmass glasses, suggestive of more significant loss associated with sulfur degassing.  Although other factors such as heat generation, hydrothermal flow, fault systems, and magma venting are key in the development of VMS deposits, this study shows that variations in subduction parameters can significantly affect metal concentrations in arc magmas that may host hydrothermal systems, and hence the amount of metals available to be scavenged into the deposits.</p>


2021 ◽  
Author(s):  
◽  
Jacob Leath

<p>The southern Kermadec Arc – Havre Trough (SKAHT) is an intra-oceanic arc – back-arc system where the Pacific plate is subducting beneath the Australian plate. The Kermadec volcanic arc front consists of 33 volcanic centres, four of which host hydrothermal mineralization (Brothers, Haungaroa, Rumble II West, and Clark) such as volcanogenic massive sulfide (VMS) deposits, which are characterised by high concentrations of base and precious metals (e.g., Au, Cu, Zn, Pb). The sources of these metals are strongly tied to the metal contents within underlying magmatic rocks and associated magmatic systems with which the hydrothermal fluids interact. Understanding the sources, movements, and accumulation of metals associated with porphyry copper and exhalative base metal deposits within a subduction – arc setting remains limited.  This study reports major, trace, and volatile element contents in basaltic groundmass glasses and olivine-hosted melt inclusions from lavas from four locations within the arc – back-arc setting of the SKAHT. The focus is on understanding the controls on base metal (Pb, Cu, Zn, Mo, V) contents in the magmas. The sample locations, Rumble III and Rumble II West volcanoes, and back-arc Basins D and I, form an arc-perpendicular transect extending from arc front into the back-arc. The analysed melt inclusion and groundmass glasses are all basalt to basaltic andesite in composition, with back-arc basin samples more mafic than arc front volcano samples. The magmatic evolution of the melts is primarily controlled by crystal fractionation of olivine + pyroxene + plagioclase. All glasses have undergone variable degassing, indicated by an absence of detectable CO₂ and curvilinear decreases in S contents with increasing SiO₂. Of the volatile phases analysed, only Cl appears unaffected by degassing.  Distinct compositional differences are apparent between arc front and back-arc melts. The arc front magmas formed from higher degrees of melting of a less fertile mantle source and are more enriched in trace elements then the back-arc magmas due to greater additions of slab-derived aqueous fluids to their source. Magmas from a single arc front volcano (Rumble II West) incorporate melts that have tapped variably enriched sources, indicating heterogeneity of the mantle at small scales. Significant variation in mantle composition, however, is also apparent laterally along strike of the arc. Rumble III volcano and Basin I lie on an arc-perpendicular transect south of Rumble II West volcano and Basin D. Their greater enrichment in trace elements and higher concentrations of base metals than Rumble II West and Basin D lavas can be attributed to higher fluxes of subduction derived components.  Base metals (Cu, Zn, Pb, Mo, and V) are variably enriched in the SKAHT melts compared with typical mid-ocean ridge basalts with relative enrichments in the order Pb >> Cu > Mo, V > Zn. All metals appear to be affected by mantle metasomatism related to slab-derived fluids, either directly from slab components introduced to the mantle source (e.g., Pb) or through mobilisation of metals within the ambient mantle wedge. The apparently compatible behaviour of Zn, Cu, and V in the mantle means that these elements may be enriched in arc front magmas relative to back-arc magmas by higher degrees of partial melting and/or melting of more depleted sources.  All base metals behave incompatibly in the magma during crystal fractionation between 48 – 56 wt.% SiO₂. Lead and Cu concentrations, however, begin to level out from ~ 52 wt.% SiO₂ suggesting some subsequent loss to fractionating volatile phases as metal sulfide complexes. Rumble III samples show a decrease in metal concentration (Pb, Cu, V), from melt inclusions to groundmass glasses, suggestive of more significant loss associated with sulfur degassing.  Although other factors such as heat generation, hydrothermal flow, fault systems, and magma venting are key in the development of VMS deposits, this study shows that variations in subduction parameters can significantly affect metal concentrations in arc magmas that may host hydrothermal systems, and hence the amount of metals available to be scavenged into the deposits.</p>


Minerals ◽  
2021 ◽  
Vol 11 (11) ◽  
pp. 1277
Author(s):  
Zhengxin Yin ◽  
Weiping Wang ◽  
Liang Chen ◽  
Zhengyuan Li ◽  
Qiang Liu ◽  
...  

We present geological, bulk-rock geochemical and Sr–Nd–Hf isotopic data for mafic rocks from the West Philippine Basin (WPB). These mafic rocks comprise pillow basalts characterized by a vesicular structure. The mid-ocean ridge basalt (MORB)-normalized trace element patterns of basalts from the study area display depletions in Nb. In addition, the chondrite-normalized lanthanide patterns of basalts from the WPB are characterized by significant depletions in the light lanthanides and nearly flat Eu to Lu segments. The investigated rocks have initial 87Sr/86Sr ratios (87Sr/86Sr(i)) of 0.703339–0.703455 and high εNd(t) values (8.0 to 8.7). Furthermore, basalts from the WPB have 176Hf/177Hf ratios that range from 0.28318 to 0.28321 and high εHf(t) from 15.2 to 16.3. Semi-quantitative modeling demonstrates that the parental melts of basalts from the study area were derived by ~20% adiabatic decompression melting of a rising spinel-bearing peridotite source. The Sr–Nd–Hf isotopic compositions of basalts from the WPB indicate that their parental magmas were derived from an upper mantle reservoir possessing the so-called Indian-type isotopic anomaly. Interpretation of the isotopic data suggests that the inferred mantle source was most likely influenced by minor inputs of a sediment melt derived from a downgoing lithospheric slab. Collectively, the petrographic and geochemical characteristics of basalts from the study area are analogous to those of mafic rocks with a back-arc basin (BAB)-like affinity. As such, the petrogenesis of basalts from the WPB can be linked to upwelling of an Indian-type mantle source due to lithospheric slab subduction that was followed by back-arc spreading.


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