scholarly journals Zircon survival in shallow asthenosphere and deep lithosphere

2020 ◽  
Vol 105 (11) ◽  
pp. 1662-1671
Author(s):  
Anastassia Y. Borisova ◽  
Ilya N. Bindeman ◽  
Michael J. Toplis ◽  
Nail R. Zagrtdenov ◽  
Jérémy Guignard ◽  
...  
Keyword(s):  
Tholeiitic Basalt ◽  
Alkali Basalts ◽  
Mid Ocean Ridge ◽  
Basaltic Melt ◽  
Fast Transport ◽  
Basalt Composition ◽  
Oceanic Asthenosphere ◽  
Ocean Ridge ◽  
Low Pressures

Abstract Zircon (ZrSiO4) is the most frequently used geochronometer of terrestrial and extraterrestrial processes. To shed light on question of zircon survival in the Earth's shallow asthenosphere, high-temperature experiments of zircon dissolution in natural mid-ocean ridge basaltic (MORB) and synthetic haplobasaltic melts have been performed at temperatures of 1250–1300 °C and pressures from 0.1 MPa to 0.7 GPa. Zirconium measurements were made in situ by electron probe microanalyses (EPMA) at high current. Taking into account secondary fluorescence effects in zircon-glass pairs during EPMA, a zirconium diffusion coefficient of 2.87E-08 cm2/s was determined at 1300 °C and 0.5 GPa. When applied to the question of zircon survival in asthenospheric melts of tholeiitic basalt composition, the data are used to infer that typical 100 mm zircon crystals dissolve rapidly (~10 h) and congruently upon reaction with basaltic melt at pressures of 0.2–0.7 GPa. We observed incongruent (to crystal ZrO2 and SiO2 in melt) dissolution of zircon in natural mid-ocean ridge the basaltic melt at low pressures <0.2 GPa and in the haplobasaltic melt at 0.7 GPa pressure. Our experimental data raise questions about the origin of zircon crystals in mafic and ultramafic rocks, in particular, in shallow oceanic asthenosphere and deep lithosphere, as well as the meaning of the zircon-based ages estimated from these minerals. The origin of zircon in shallow (ultra-) mafic chambers is likely related to the crystallization of intercumulus liquid. Large zircon megacrysts in kimberlites, peridotites, alkali basalts, and carbonatite magmas suggest fast transport and short interaction durations between zircon and melt. The origin of zircon megacrysts is likely related to metasomatic addition of Zr into the mantle as an episode of mantle melting should eliminate them on geologically short timescales.

Carbon solubility in Mid-Ocean Ridge basaltic melt at low pressures (250–1950 bar)

1997 ◽  
Vol 138 (1-2) ◽  
pp. 81-92 ◽  
Author(s):  
Nathalie Jendrzejewski ◽  
Thomas W. Trull ◽  
Françoise Pineau ◽  
Marc Javoy
Keyword(s):  
Carbon Solubility ◽  
Mid Ocean Ridge ◽  
Basaltic Melt ◽  
Ocean Ridge ◽  
Low Pressures

Platinum-group element geochemistry of intraplate basalts from the Aleppo Plateau, NW Syria

2012 ◽  
Vol 150 (3) ◽  
pp. 497-508 ◽  
Author(s):  
GEORGE S.-K. MA ◽  
JOHN MALPAS ◽  
JIAN-FENG GAO ◽  
KUO-LUNG WANG ◽  
LIANG QI ◽  
...  
Keyword(s):  
Partial Melting ◽  
Middle Miocene ◽  
Primitive Mantle ◽  
Element Geochemistry ◽  
Alkali Basalts ◽  
Melt Extraction ◽  
Mid Ocean Ridge ◽  
Order Of Magnitude ◽  
Platinum Group ◽  
Ocean Ridge

AbstractEarly–Middle Miocene intraplate basalts from the Aleppo Plateau, NW Syria have been analysed for their platinum-group elements (PGEs). They contain extremely low PGE abundances, comparable with most alkali basalts, such as those from Hawaii, and mid-ocean ridge basalts. The low abundances, together with high Pd/Ir, Pt/Ir, Ni/Ir, Cu/Pd, Y/Pt and Cu/Zr are consistent with sulphide fractionation, which likely occurred during partial melting and melt extraction within the mantle. Some of the basalts are too depleted in PGEs to be explained solely by partial melting of a primitive mantle-like source. Such ultra-low PGE abundances, however, are possible if the source contains some mafic lithologies. Many of the basalts also exhibit suprachondritic Pd/Pt ratios of up to an order of magnitude higher than primitive mantle and chondrite, an increase too high to be attributable to fractionation of spinel and silicate minerals alone. The elevated Pd/Pt, associated with a decrease in Pt but not Ir and Ru, are also inconsistent with removal of Pt-bearing PGE minerals or alloys, which should have concurrently lowered Pt, Ir and Ru. In contrast, melting of a metasomatized source comprising sulphides whose Pt and to a lesser extent Rh were selectively mobilized through interaction with silicate melts, may provide an explanation.

scholarly journals Lithogeochemistry of the Mid-Ocean Ridge Basalts near the Fossil Ridge of the Southwest Sub-Basin, South China Sea

Minerals ◽  
2020 ◽  
Vol 10 (5) ◽  
pp. 465 ◽  
Author(s):  
Kai Sun ◽  
Tao Wu ◽  
Xuesong Liu ◽  
Xue-Gang Chen ◽  
Chun-Feng Li
Keyword(s):  
South China Sea ◽  
Partial Melting ◽  
Fractional Crystallization ◽  
South China ◽  
Spreading Rate ◽  
Tholeiitic Basalt ◽  
Spinel Peridotite ◽  
China Sea ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

Mid-ocean ridge basalts (MORB) in the South China Sea (SCS) record deep crust-mantle processes during seafloor spreading. We conducted a petrological and geochemical study on the MORBs obtained from the southwest sub-basin of the SCS at site U1433 and U1434 of the International Ocean Discovery Program (IODP) Expedition 349. Results show that MORBs at IODP site U1433 and U1434 are unaffected by seawater alteration, and all U1433 and the bulk of U1434 rocks belong to the sub-alkaline low-potassium tholeiitic basalt series. Samples collected from site U1433 and U1434 are enriched mid-ocean ridge basalts (E-MORBs), and the U1434 basalts are more enriched in incompatible elements than the U1433 samples. The SCS MORBs have mainly undergone the fractional crystallization of olivine, accompanied by the relatively weak fractional crystallization of plagioclase and clinopyroxene during magma evolution. The magma of both sites might be mainly produced by the high-degree partial melting of spinel peridotite at low pressures. The degree of partial melting at site U1434 was lower than at U1433, ascribed to the relatively lower spreading rate. The magmatic source of the southwest sub-basin basalts may be contaminated by lower continental crust and contributed by recycled oceanic crust component during the opening of the SCS.

Differentiation and source of the Nipissing Diabase intrusions, Ontario, Canada

1993 ◽  
Vol 30 (6) ◽  
pp. 1123-1140 ◽  
Author(s):  
P. C. Lightfoot ◽  
H. de Souza ◽  
W. Doherty
Keyword(s):  
Trace Element ◽  
Crustal Contamination ◽  
Primitive Mantle ◽  
Magma Source ◽  
Magma Type ◽  
Ni Content ◽  
Chemical Signatures ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

Major and trace element data are presented for 2.2 Ga Proterozoic diabase sills from across the Nipissing magmatic province of Ontario. In situ differentiation of the magma coupled with assimilation of Huronian Supergroup roof sediments is responsible for the variation in composition between quartz diabase and granophyric diabase seen within many of the differentiated intrusions. Uniform trace element and isotope ratio signatures, such as La/Sm (2.8 – 3.7) and εNdCHUR (−2.7 to −5.9) characterize chilled margins and undifferentiated quartz diabases. These chemical signatures suggest the existence of a single magma source that was parental to intrusions throughout the magmatic province; this magma has higher La/Sm and lower Ti/Y than primitive mantle and is displaced towards the composition of shales. Most chilled diabases and quartz diabases have a similar Mg# (0.64 and 0.60) and Ni content (98 and 127 ppm), and it is argued that the magma differentiated at depth and was emplaced as a uniform low-Mg magma. The Wanapitei intrusion and Kukagami Lake sill are an exception in that although the quartz diabase has La/Sm similar to the Nipissing magma type, which suggests that they came from the same source, the Mg# (0.68–0.71) and Ni content (130–141 ppm) are higher, which may suggest that they are either slightly more primitive examples of the normal Nipissing magma or that cumulus hypersthene has been resorbed. The light rare earth element enriched signature of the Nipissing magmas was perhaps introduced from the continental crust as the magma migrated from the mantle to the surface, but a remarkably constant and large amount (>20%) of crustal contamination would be required. An addition of 1 –3% shale to the source of a transitional mid-ocean ridge basalt type magma can broadly reproduce the compositional features of the Nipissing magma type. The source characteristics were perhaps imparted during subduction accompanying the terminal Kenoran orogeny.

CONTINENTAL MARGIN TECTONICS AND THE EVOLUTION OF SOUTH EAST AUSTRALIA

1971 ◽  
Vol 11 (1) ◽  
pp. 75 ◽  
Author(s):  
J. R. Griffiths
Keyword(s):  
Continental Margin ◽  
Rift Valley ◽  
Structural Evolution ◽  
Initial Crack ◽  
Alkali Basalts ◽  
Australian Plate ◽  
Differential Movement ◽  
Mid Ocean Ridge ◽  
Regional Tectonic ◽  
Ocean Ridge

Following recent advances in geotectonics, a new approach can be applied to the study of the development of continental margins.A continental margin begins to form as an older continental craton breaks up. The initial crack develops into a rift valley, which becomes filled with thick clastic and volcanic deposits. As separation continues a new mid-ocean ridge is formed, and the two plates begin to drift apart more rapidly. At this stage the structural evolution of the margins is virtually complete, and marine sediments are deposited unconformably across the fault troughs.The continental fragments in the south west Pacific can be reassembled as a part of the ancient continent of Gondwanaland. Gondwanaland began to break up in the mid-Jurassic. A rift valley developed along the line of the present southern coast of Australia, through the Otway Basin. Two subsidiary tensional splays gave rise to the Elliston and Robe-Penola Troughs. Clastic sediments stripped from the cratonic highlands, and alkali basalts, occur in the rift grabens. Faulting and deposition continued throughout the Lower Cretaceous. About mid-Cretaceous a marine transgression from the west entered the subdividing rift valley. In the Eocene a new mid-ocean ridge formed and the Australian and Antarctic plates began to separate more rapidly. After this, quiet marine sedimentation occurred on the continental shelf and slope.The Bass and Gippsland Basins began to develop in the Cretaceous as differential movement occurred between the main Australian plate and a partially detached Tasmanian sub-plate. In the Upper Cretaceous the Gippsland Basin became open towards the evolving Tasman Sea, as New Zealand detached. The Tasmanian sub-plate ceased fo exist after the Eocene, becoming firmly fixed to the Australian plate. Later readjustments have occurred giving rise to further limited movements, mainly in the Gippsland Basin.The integration of detailed geological work and a regional tectonic analysis has been successfully applied to south east Australia and it is probable that a similar approach would yield fruitful results applied elsewhere.

Proterozoic mantle under Quesnellia: variably reset Rb–Sr mineral isochrons in ultramafic nodules carried up in Cenozoic volcanic vents of the southern Omineca Belt

1991 ◽  
Vol 28 (8) ◽  
pp. 1239-1253 ◽  
Author(s):  
Min Sun ◽  
Richard Lee Armstrong ◽  
R. J. Maxwell
Keyword(s):  
British Columbia ◽  
Upper Mantle ◽  
Isotopic Exchange ◽  
Mantle Lithosphere ◽  
Linear Arrays ◽  
Mid Ocean Ridge ◽  
Interstitial Material ◽  
Host Rocks ◽  
Ocean Ridge

Handpicked, acid-washed diopside, orthopyroxene, and olivine separates from 14 lherzolite nodules from four localities in southern British Columbia have been analyzed for 87Rb/86Sr and 87Sr/86Sr. The host lavas, of late Cenozoic to Pleistocene age, are nepheline-normative alkali basalt and basanite with Sr and Nd isotopic ratios (0.7025–0.7029; 0.51310–0.51326) close to those of mid-ocean ridge basalt. The present-day Sr isotopic compositions of nodule diopsides (9–239 ppm Sr) are 0.7021–0.7041, similar to those of the host rocks. Orthopyroxene and olivine, however, have 87Rb/86Sr ratios of 0.1 to > 1.0 and much more radiogenic Sr (87Sr/86Sr commonly 0.708–0.720), and in several cases the three minerals form linear arrays in 87Rb/86Sr versus 87Sr/86Sr plots. If interpreted as mineral isochrons, these give dates from 99 to 1989 Ma; many cluster around 440–770 Ma. Four nodules from two localities (West Kettle River and Big Timothy Mountain) give dates of 1201–1989 Ma. Maximum dates for the other localities (Jacques Lake and Lassie Lake) are 600–650 Ma.87Rb/86Sr ratios for acid leaches of crushed and sieved nodule material are generally < 0.706, and 87Sr/86Sr ratios for the host rocks are < 0.703, so radiogenic interstitial material or host contamination cannot explain the high ratios observed in the low-Sr minerals. The acid leaches lie both above and below mineral isochrons, virtually never exactly on the isochrons, indicating that secondary material, not cogenetic with the nodules, is present along grain boundaries. Discordant mineral points lie as expected for partially reset old isochrons, with olivine most easily reequilibrated by Sr isotopic exchange with less radiogenic minerals, orthopyroxene next in changeability, and clinopyroxene least modified.The nodules cannot be cognate or related in any way to recent magma genesis. The radiogenic Sr in low-Sr minerals is due to in situ decay of Rb and thus the nodules are Proterozoic upper mantle, some of an age similar to the 1.7–2.3 Ga granitic basement that tectonically underlies southwestern Quesnellia in the same region containing the nodule-bearing vents. The 440–770 Ma isochrons are speculatively correlated with Late Proterozoic to early Paleozoic craton-margin-forming rift event(s). Pre-Phanerozoic mantle lithosphere extends as far west as the Okanagan Valley in southern British Columbia.

Mantle heterogeneity beneath oceanic islands: some inferences from isotopes

1993 ◽  
Vol 342 (1663) ◽  
pp. 91-103 ◽  
Keyword(s):  
Alternative Hypothesis ◽  
Melting Zone ◽  
Oceanic Islands ◽  
Alkali Basalts ◽  
Mauna Loa ◽  
Mid Ocean Ridge ◽  
Mantle Sources ◽  
Hawaiian Plume ◽  
Mantle Helium ◽  
Ocean Ridge

Radiogenic isotopes in oceanic basalts are extremely useful as tracers of long-lived heterogeneities in the Earth's mantle. Helium isotopes provide unique information in that high 3 He / 4 He ratios are indicative of relatively undegassed mantle reservoirs (i.e. mantle with high time-integrated 3 He/(Th + U) ratios). An alternative hypothesis is that high 3 He / 4 He ratios may have been produced by ancient melting events, if the solid/melt partition coefficient (K d ) for He is greater than that for Th and U (i.e. yielding relatively high He/(Th + U) in the residue of melting). However, the distribution of helium within basaltic phenocrysts, and olivine/glass helium partitioning within mid-ocean ridge basalts, suggest that helium behaves as an incompatible element during melting (K d (olivine/glass) < 0.0055), which strongly supports the hypothesis that high 3 He / 4 He ratios are derived from undegassed mantle reservoirs. Isotopic measurements of He, Sr, and Pb in Hawaiian volcanoes lavas demonstrate that the mantle sources have changed on extremely short timescales, between 100 and 10 000 years before present. The preferred explanation for these variations is that they represent heterogeneities within the Hawaiian mantle plume, combined with late stage melting in the lithosphere for post shield alkali basalts. Helium isotopic data from Kilauea, Hualalai and Mauna Loa suggest that the plume is presently located beneath Kilauea (and Loihi seamount), and constrain the melting zone of the Hawaiian plume to be less than 40 km in radius.

Geochemical, 40Ar/39Ar age, and isotopic data for crustal rocks of the Neyriz ophiolite, Iran

2006 ◽  
Vol 43 (1) ◽  
pp. 57-70 ◽  
Author(s):  
Hassan A Babaie ◽  
Abbed Babaei ◽  
A Mohamad Ghazi ◽  
Mohsen Arvin
Keyword(s):  
Trace Element ◽  
Accretionary Prism ◽  
Tholeiitic Basalt ◽  
Isotopic Data ◽  
Crustal Rocks ◽  
Mid Ocean Ridge ◽  
Subduction Erosion ◽  
Southwest Iran ◽  
Ocean Ridge ◽  
Low K

Trace-element data (including the rare-earth elements) in the crustal sequence of the Neotethyan Neyriz ophiolite in southwest Iran indicate normal mid-ocean ridge basalt (N-MORB) or island-arc tholeiite chemistry for the Tang-e Hana basalt. The data suggest that the Tang-e Hana rhyodacite, basalt, plagiogranite, and gabbro derived from a low-K tholeiitic parent magma. Trace-element distributions in amphibolite clasts, in the sole detachment of the ophiolite south of Lake Neyriz, correlate well with distributions in basalt clasts in the mélange and in the Tang-e Hana basalt. These trace elements suggest that the amphibolite originated from metamorphism and deformation of a tholeiitic basalt protolith. New 40Ar/39Ar incremental heating plateau ages from two hornblende plagiogranite specimens, in the crustal sequence in Tang-e Hana, are 92.07 ± 1.69 and 93.19 ± 2.48 Ma. Isotopic data for five Tang-e Hana basalts yield εNd values of +7.8 and +7.9, and 87Sr/86Sr values of 0.70368 to 0.70476. The isotopic compositions, ophiolite tectonostratigraphy, and correlation of the 40Ar/39Ar cooling (plagiogranite) and deformation (amphibolite) ages suggest emplacement of the Neyriz ophiolite either into an accretionary prism, through offscraping and subduction erosion, and (or) formation in a supra-subduction zone environment, around 82–96 Ma. Progressive accretion probably led to the development of a fore-arc basin and deposition of Upper Cretaceous – Eocene fore-arc and arc-derived sediments on the ophiolite.

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