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.

scholarly journals A subduction influence on ocean ridge basalts outside the Pacific subduction shield

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
A. Y. Yang ◽  
C. H. Langmuir ◽  
Y. Cai ◽  
P. Michael ◽  
S. L. Goldstein ◽  
...  
Keyword(s):  
Upper Mantle ◽  
The Arctic ◽  
Mantle Flow ◽  
Mantle Heterogeneity ◽  
Gakkel Ridge ◽  
The Pacific ◽  
Mid Ocean Ridge ◽  
The Pacific Ocean ◽  
Back Arc ◽  
Ocean Ridge

AbstractThe plate tectonic cycle produces chemically distinct mid-ocean ridge basalts and arc volcanics, with the latter enriched in elements such as Ba, Rb, Th, Sr and Pb and depleted in Nb owing to the water-rich flux from the subducted slab. Basalts from back-arc basins, with intermediate compositions, show that such a slab flux can be transported behind the volcanic front of the arc and incorporated into mantle flow. Hence it is puzzling why melts of subduction-modified mantle have rarely been recognized in mid-ocean ridge basalts. Here we report the first mid-ocean ridge basalt samples with distinct arc signatures, akin to back-arc basin basalts, from the Arctic Gakkel Ridge. A new high precision dataset for 576 Gakkel samples suggests a pervasive subduction influence in this region. This influence can also be identified in Atlantic and Indian mid-ocean ridge basalts but is nearly absent in Pacific mid-ocean ridge basalts. Such a hemispheric-scale upper mantle heterogeneity reflects subduction modification of the asthenospheric mantle which is incorporated into mantle flow, and whose geographical distribution is controlled dominantly by a “subduction shield” that has surrounded the Pacific Ocean for 180 Myr. Simple modeling suggests that a slab flux equivalent to ~13% of the output at arcs is incorporated into the convecting upper mantle.

Sea-floor evolution: rare-earth evidence

1971 ◽  
Vol 268 (1192) ◽  
pp. 663-706 ◽  
Keyword(s):  
Rare Earth ◽  
Mantle Source ◽  
Upper Mantle ◽  
Gulf Of Aden ◽  
Low Velocity ◽  
East Pacific ◽  
Mid Ocean Ridge ◽  
Low Velocity Layer ◽  
Ocean Ridge ◽  
Velocity Layer

A systematic survey of rare-earth (r.e.) abundances in submarine tholeiitic basalts along mid-oceanic ridges has been made by neutron activation analysis. The r.e. fractionation patterns are remarkably uniform along each mid-oceanic ridge and from one ridge to another (Juan de Fuca Ridge, East Pacific and Chile Rise, Pacific-Antarctic, Mid-Indian and Carlsberg Ridge, Gulf of Aden, Red Sea Trough and Reykjanes Ridge). The patterns are all depleted in light r.e. except for three samples (Gulf of Aden and Mid-Indian Ridge) which are unfractionated relative to chondrites. They contrast markedly with tholeiitic plateau basalt which are shown to be related to the early volcanic phases associated with continental drift. Tholeiitic plateau basalts are light r.e. enriched as are most continental rocks. Mid-ocean ridge basalts are also distinguishable from spatially related oceanic shield volcanoes of tholeiitic composition (Red Sea Trough-Jebel Teir Is., East Pacific Rise-Culpepper Island). Thus on a r.e. basis there are tholeiites within tholeiites. The r.e. difference between mid-ocean ridge tholeiites and tholeiitic plateau basalts can be related to distinct thermal and tectonic régimes and consequently magmatic modes and rates of intrusions from the low velocity layer in the upper mantle. The difference between continental and oceanic volcanism appears to be triggered by: (1) presence or absence of a moving continental lithosphere over the low velocity layer, and (2) whether or not major rifts tap the low velocity layer through the lithosphere. Fractional crystallization during ascent of melts before eruption at the ridge crest does not affect appreciably the relative r.e. patterns. R.e. in mid-ocean ridge basalts appear to intrinsically reflect their distribution in the upper mantle source, i.e. the low velocity layer. Based on secondary order r.e. variation of mid-ocean ridge basalts: (1) If fractional crystallization is invoked for the small r.e. variations, up to approximately 50 % extraction of olivine and Ca-poor orthopyroxene in various combinations can be tolerated. However, only limited amount of plagioclase or Ca-rich clinopyroxene can be extracted, the former because of its effect on the abundance of Eu abundance and the latter because of its effect on the [La/Sm] e.f. ratio, alternatively. (2) If partial melting during ascent is invoked, and a minimum of 10% melting is assumed, the permissible degree of melting of originally a lherzolite upper mantle may vary between 10 and 30% . It is not possible to establish readily to what extent these two processes have been operative as they cannot be distinguished on the basis of r.e. data only. However, there is evidence indicating that both have been operative and are responsible for the small r.e. variations observed in mid-ocean ridge basalts. An attempt to correlate second order r.e. variations along or across mid-oceanic ridges with spreading rate, age, or distance from ridge crests has been made but the results are inconclusive. No r.e. secular variation of the oceanic crust is apparent. R.e. average ridge to ridge variations are attributed to small lateral inhomogeneities of the source of basalts in the low velocity layer, and to a certain extent, to its past history. The remarkable r.e. uniformity of mid-oceanic ridge tholeiites requires a unique and simple volcanic process to be operative. It calls for upward migration of melt or slush from a relatively homogeneous source in the mantle—the low velocity layer, followed by further partial melting during ascent. The model, although consistent with geophysics, may have to be reconciled with some evidence from experimental petrology. Models for r.e. composition of the upper mantle source of ridge basalt, formation of layers 2 and 3, and the moho-discontinuity, are also presented.

Age and radiogenic isotopic systematics of the Borden carbonatite complex, Ontario, Canada

1987 ◽  
Vol 24 (1) ◽  
pp. 24-30 ◽  
Author(s):  
Keith Bell ◽  
John Blenkinsop ◽  
S. T. Kwon ◽  
G. R. Tilton ◽  
R. P. Sage
Keyword(s):  
Upper Mantle ◽  
Volcanic Rocks ◽  
Crustal Contamination ◽  
Growth Curves ◽  
Carbonatite Complex ◽  
Northern Ontario ◽  
Mantle Origin ◽  
Mid Ocean Ridge ◽  
Ocean Ridge ◽  
Isotope Correlation

Rb–Sr and U–Pb data from the Borden complex of northern Ontario, a carbonatite associated with the Kapuskasing Structural Zone, indicate a mid-Proterozoic age. A 207Pb/206Pb age of 1872 ± 13 Ma is interpreted as the emplacement age of this body, grouping it with other ca. 1900 Ma complexes that are the oldest known carbonatites associated with the Kapuskasing structure. A 206Pb–238U age of 1894 ± 29 Ma agrees with the Pb–Pb age but has a high mean square of weighted deviates (MSWD) of 42. A Rb–Sr apatite–carbonate–mica whole-rock isochron date of 1807 ± 13 Ma probably indicates later resetting of the Rb–Sr system.An εSr(T) value of −6.2 ± 0.5 (87Sr/86Sr = 0.70184 ± 0.00003) and an εNd(T) value of +2.8 ± 0.4 for Borden indicate derivation of the Sr and Nd from a source with a time-integrated depletion in the large-ion lithophile (LIL) elements. These closely resemble the ε values for Sr and Nd from the Cargill and Spanish River complexes, two other 1900 Ma plutons. The estimated initial 207Pb/204Pb and 206Pb/204Pb ratios from Borden calcites plot significantly below growth curves for average continental crust in isotope correlation diagrams, a pattern similar to those found in mid-ocean ridge basalts (MORB) and most ocean-island volcanic rocks, again suggesting a source depleted in LIL elements. The combined Nd and Sr, and probably Pb, data strongly favour a mantle origin for the Borden complex with little or no crustal contamination and support the model of Bell et al. that many carbonatites intruded into the Canadian Shield were derived from an ancient, LIL-depleted subcontinental upper mantle.

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.

scholarly journals Basalt derived from highly refractory mantle sources during early Izu-Bonin-Mariana arc development

2020 ◽  
Author(s):  
He Li ◽  
Richard Arculus ◽  
Osamu Ishizuka ◽  
Rosemary Hickey-Vargas ◽  
Gene Yogodzinski ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Philippine Sea Plate ◽  
Island Arcs ◽  
Element Abundances ◽  
Mid Ocean Ridge ◽  
Mantle Sources ◽  
Backarc Basins ◽  
Mariana Arc ◽  
Ocean Ridge ◽  
Aluminous Spinel

Abstract The character of magmatism associated with the early stages of subduction zone and island arc development is unlike that of mature systems, being dominated in the Izu-Bonon-Mariana (IBM) case by low-Ti-K tholeiitic basalts and boninites. Basalts recovered by coring the basement of the Amami Sankaku Basin (ASB), located west of the oldest remnant arc of the IBM system (Kyushu-Palau Ridge; KPR), were erupted at ~49 Ma, about 3 million years after subduction inception. The chain of stratovolcanoes defined by the KPR is superimposed on this basement. The basalts were sourced from upper mantle similar to that tapped following subduction inception, and represented by forearc basalt (FAB) dated at ~52-51 Ma. The mantle sources of the ASB basalt basement were more depleted by prior melt extraction than those involved in the vast majority of mid-ocean ridge (MOR) basalt generation. The ASB basalts are low-Ti-K, aluminous spinel-olivine-plagioclase-clinopyroxene-bearing tholeiites. We show this primary mineralogy is collectively distinct compared to basalts of MOR, backarc basins of the Philippine Sea Plate, forearc, or mature island arcs. In combination with bulk compositional (major and trace element abundances plus radiogenic isotope characteristics) data for the ASB basalts, we infer the upper mantle involved was hot (~1400oC), reduced, and refractory peridotite. For a few million years following subduction initiation, a broad region of mantle upwelling accompanied by partial melting prevailed. The ASB basalts were transferred rapidly from moderate pressures (1-2 GPa), preserving a mineralogy established at sub-crustal conditions, and experienced little of recharge-mix-tap-fractionate regimes typical of MOR or mature arcs.

scholarly journals Secular oxidation of Archean upper mantle caused by increasing crustal recycling

2021 ◽  
Author(s):  
Lei Gao ◽  
Shuwen Liu ◽  
Peter Cawood ◽  
Jintuan Wang ◽  
Guozheng Sun ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Redox State ◽  
Incompatible Element ◽  
Volcanic Rocks ◽  
Crustal Recycling ◽  
Incompatible Elements ◽  
Mid Ocean Ridge ◽  
Redox Evolution ◽  
Ocean Ridge ◽  
Plate Subduction

Abstract The redox evolution of Archean mantle impacted Earth differentiation, mantle melting and the nature of chemical equilibrium between mantle, ocean and atmosphere of the early Earth. However, how and why it varies with time remain controversial. Archean mantle-derived volcanic rocks, especially basalts are ideal lithologies for reconstructing the mantle redox state. Here we show that the ~3.8-2.5 Ga basalts from fourteen cratons are subdivided geochemically into two groups, B-1, showing incompatible element depleted and modern mid-ocean ridge basalt-like features ((Nb/La)PM ≥ 0.75) and B-2 ((Nb/La)PM < 0.75), characterized by modern island arc basalt-like features. Our updated V-Ti redox proxy indicates the Archean upper mantle was more reducing than today, and that there was a significant redox heterogeneity between ambient and modified mantle presumably related to crustal recycling, perhaps via plate subduction, as shown by B-1 and B-2 magmas, respectively. The oxygen fugacity of modified mantle exhibits a ~1.5-2.0 log units increase over ~3.8-2.5 Ga, whereas the ambient mantle becomes more and more heterogeneous with respect to redox, apart from a significant increase at ~2.7 Ga. These findings are coincident with the increase in the proportions of crustal recycling-related lithologies with associated enrichment of associated incompatible elements (e.g., Th/Nb), indicating that increasing recycling played a crucial role on the secular oxidation of Archean upper mantle.

Amphibole in a spinel lherzolite xenolith: evidence for volatiles and partial melting in the upper mantle beneath southern British Columbia

1984 ◽  
Vol 21 (9) ◽  
pp. 1067-1072 ◽  
Author(s):  
Mark Brearley ◽  
Christopher M. Scarfe
Keyword(s):  
British Columbia ◽  
Partial Melting ◽  
Upper Mantle ◽  
Stability Field ◽  
Spinel Lherzolite ◽  
Ultramafic Xenolith ◽  
Metasomatic Fluid ◽  
Small Grains ◽  
First Time

Pargasitic amphibole has been observed for the first time in an ultramafic xenolith from British Columbia. The xenolith is a chrome diopside-bearing spinel lherzolite trapped within an alkali basaltic lava flow at Lightning Peak, near Vernon, British Columbia. Amphibole (<5%) occurs within the xenolith as small grains, interstitial between other xenolith mineral phases, and always shows evidence of melting. Microprobe analyses of the amphibole reveal that it is a pargasite rich in MgO (MgO = 17.1–17.7 wt.%; Mg/(Mg + Fe2+) = 0.89) and CaO (10.4–10.7 wt.%). Textural and chemical evidence suggests that the pargasite is in equilibrium with the other phases in spinel lherzolite. The pargasite probably crystallized within the spinel stability field of the upper mantle from a volatile-rich metasomatic fluid that was produced by dehydration of subducted material. Melting in the amphibole may have been caused by one of three processes: superheating by the host alkali basalt, decompression as the magma ascended, or by in situ partial melting within the upper mantle. The partial melting of amphibole-bearing spinel lherzolite provides a possible mechanism for the generation of late Cenozoic alkalic magmas of the Intermontane Belt of British Columbia.

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