Mid-ocean Ridge Basalt Source Mantle Research Articles

Deep mantle origin of kimberlite magmas revealed by neon isotopes

We measured noble gases in olivines extracted from alteration‐free samples of the Udachnaya‐East kimberlite pipe, eastern Siberia. Neon isotope ratios indicate that the kimberlite has a less nucleogenic Ne signature than a typical mid‐ocean ridge basalt (MORB) source, which is consistent with the less radiogenic 40Ar/36Ar ratio compared to MORB and subcontinental lithospheric mantle (SCLM) sources. Based on He‐Ne systematics, the 3He/4He ratio of the plume source of the Udachnaya kimberlite pipe would be higher than 15 RA with the dominant He signature of the magma acquired by assimilation of SCLM and/or crustal material during magma ascent. Our results indicate that the source of the Udachnaya kimberlite has similar noble gas characteristics to those of ocean island basalts (OIBs). Its origin is therefore deeper than that of the MORB source mantle, and probably is the same as the OIB‐source mantle.

Deep mantle origin of kimberlite magmas revealed by neon isotopes

[1] We measured noble gases in olivines extracted from alteration-free samples of the Udachnaya-East kimberlite pipe, eastern Siberia. Neon isotope ratios indicate that the kimberlite has a less nucleogenic Ne signature than a typical mid-ocean ridge basalt (MORB) source, which is consistent with the less radiogenic 40Ar/36Ar ratio compared to MORB and subcontinental lithospheric mantle (SCLM) sources. Based on He-Ne systematics, the 3He/4He ratio of the plume source of the Udachnaya kimberlite pipe would be higher than 15 RA with the dominant He signature of the magma acquired by assimilation of SCLM and/or crustal material during magma ascent. Our results indicate that the source of the Udachnaya kimberlite has similar noble gas characteristics to those of ocean island basalts (OIBs). Its origin is therefore deeper than that of the MORB source mantle, and probably is the same as the OIB-source mantle.

Petrology of Alarcon Rise lavas, Gulf of California: Nascent intracontinental ocean crust

The Alarcon Basin in the southern Gulf of California originated by intracontinental rifting of the southwestern margin of North America and has evolved into an intercontinental ocean basin by 3.7 m.y. of seafloor spreading. Lava samples collected on the axial ridge along Alarcon Rise, two of the many near‐axis seamounts on the northwest flank of the rise, and an abyssal hill on 0.6 Ma crust on the southeast flank were examined petrographically and analyzed for major and trace element and He‐Sr‐Nd‐Pb isotope composition. Most samples are typical mid‐ocean ridge basalt (MORB), but rocks at the axial ridge site closest to continent are more differentiated. Rocks from the axial ridge and abyssal hill are genetically related through separate episodes of fractional crystallization. Seamount samples are relatively more primitive than seafloor samples and may have been formed by smaller degree of melting of the same mantle source as the seafloor lavas. Tamayo transform fault has no apparent effect on the geochemistry of axial ridge lavas erupted close to it; this is contrary to the postulated transform fault effect. Alarcon Basin differs from the Salton Trough and Guaymas Basin in that its youngest seafloor shows no evidence for contamination with continental crust or a subduction component. We propose that Pacific MORB source mantle underlies the Gulf of California because the western North American margin has overridden the Pacific suboceanic mantle. Miocene rifting and Pliocene spreading in Alarcon Basin have created a window allowing the most recent MORB melts to rise uncontaminated to the surface.

Helium in the Archean komatiites revisited: significantly high 3He/ 4He ratios revealed by fractional crushing gas extraction

In order to provide constraints on 3He/ 4He ratios in the Archean mantle source, we have analyzed helium isotopic compositions in 2.7 Ga old Archean komatiites from the Abitibi green stone belt, Ontario, Canada. Two spinifex-textured komatiites yielded significantly high 3He/ 4He ratios of about 30 Ra (where Ra denotes the atmospheric 3He/ 4He ratio) in fractions released by sequential crushing. These results are the first confirmation of the occurrence of high 3He/ 4He ratios in Archean komatiites after the intriguing finding by Richard et al. [Science 273 (1996) 93–95] in komatiites from a nearby locality, Alexo. We also found that the crystal structure of the komatiites was significantly enriched in a radiogenic component ( 4He) and that this 4He was actually degassed by crushing gas extraction, indicating that the nominal 3He/ 4He ratios measured by crushing are lower limits for the 3He/ 4He ratio of the intrinsic component. By constraining the release behavior of radiogenic 4He by crushing, we have estimated the initial 3He/ 4He ratio of the inclusion-trapped component to be 73.0 +7.8 −5.5 Ra. A mantle source with such a high 3He/ 4He ratio at 2.7 Ga, if evolved in a closed system, would have a present-day 3He/ 4He ratio of 46–60 Ra, indicating that the komatiites from Munro have trapped their helium from a mantle reservoir with a very high 3He/ 4He ratio in the context of the present-day value. However, whether or not such a source can be considered as equivalent to the primitive mantle source (such that sampled at hotspots) is highly model-dependent. If a closed system evolution model is assumed, helium in the Munro komatiites is not likely to be derived from the mid-ocean ridge basalt (MORB) source-like reservoir. However, the notion that the komatiites may be derived from a depleted reservoir in terms of trace elemental and isotopic geochemistry might require an alternative view for the 3He/ 4He evolution in ancient mantle reservoirs, as has been demonstrated by a recent model calculation by Seta et al. [Earth Planet. Sci. Lett. 188 (2001) 211–219] in which the 3He/ 4He ratios in the MORB mantle source could have been as high as those in the primitive (less degassed) mantle source in the Archean.

Depleted Iceland mantle plume geochemical signature: Artifact of multicomponent mixing?

The geochemical variations in basalts from Iceland and the Reykjanes Ridge have for a long time been attributed to mixing between the depleted mid‐ocean ridge basalt (MORB) asthenosphere source and the enriched Iceland mantle plume (Schilling, 1973; Hart et al., 1973; Sun et al., 1975). In contrast, the occurrence of some Iceland basalts with relatively depleted incompatible trace element ratios and low Pb and Sr isotope ratios compared to normal MORB (NMORB) have been interpreted to indicate that the Iceland mantle plume transports both geochemically enriched and depleted material from the lower mantle into the upper mantle and that the depleted MORB asthenosphere source does not contribute significantly to the generation of the Iceland basalts (Thirlwall et al., 1994; Thirlwall, 1995; Hards et al., 1995; Kerr, 1995; Kerr et al., 1995; Fitton et al., 1997; Hardarson and Fitton, 1997; Hardarson et al., 1997; Kempton et al., 1998, 1999; Nowell et al., 1998). As a follow‐up to this controversy, we show on the basis of new Nb, Zr, Y, La, and Sm elemental abundances and Hf isotope ratios reported here, and published trace element and isotope data for Iceland and the North Atlantic MOR (50°–79°N), that the apparent distinction between Iceland basalts and MORB can readily be accounted for by a three‐component mixing model involving two incompatible trace element enriched components, one with relatively high, the other with low 206Pb/204Pb ratios, and the usual surrounding depleted MORB mantle source as the third component. The radiogenic 206Pb‐rich component represents the hot Iceland mantle plume source, while the enriched but low‐206Pb/204Pb EM1‐like component most likely represents entrained subcontinental lithospheric material embedded in the North Atlantic depleted MORB source. The incompatible element and isotope ratio variations do not require a depleted Iceland plume (DIP) component, nor do they exclude the presence of a MORB depleted asthenosphere source component for some of the Iceland basalts, particularly in recent time.

Noble gases in the Cameroon line and the He, Ne, and Ar isotopic compositions of high μ (HIMU) mantle

Ultramafic xenoliths, basaltic lavas, and CO2 gases from the Cameroon line volcanic chain provide the first characterization of combined He, Ne, and Ar isotopes in a high‐time‐integrated 238U/204Pb = μ (HIMU) magmatic system. Helium isotopic compositions typically range from 5.0 to 6.7 Ra, with an average of 6.3. These values are indistinguishable from the 3He/4He of other HIMU locales (Austral Islands, St. Helena). Neon isotopic compositions for xenoliths and CO2 gases are mid‐ocean ridge basalt (MORB)‐like, with a maximum 20Ne/22Ne of 11.87 and 21Ne/22Ne of 0.0508. Argon isotopic compositions in silicates range from atmospheric to 40Ar/36Ar = 4910±430 (crushing) and up to 16,300±1000 (single grain, laser step heating). The correlation between 20Ne/22Ne and 40Ar/36Ar in CO2 gases suggests a minimum 40Ar/36Ar = 1650±30 for the mantle‐derived component. Uniform 3He/4He in silicates and in CO2 fluids across both the continental and oceanic sectors of the Cameroon line argues strongly for a negligible lithospheric contribution to noble gas isotopic compositions. This inference is supported by high 238U/3He in lherzolites, indicating that noble gases in these samples must have been recently introduced (<50,000 years ago) to the sample, most likely from the host magma. Ocean crust recycling models of mixing between MORB source regions and highly radiogenic slabs cannot produce the observed He and Ne isotopic compositions. Isolation and aging of MORB source mantle can generate the isotopic compositions but require extreme 3He/22Ne fractionation. Involvement of plume‐derived gases, consistent with the lithophile element isotopic compositions, alleviates the need for strong 3He/22Ne fractionation. Closed‐system aging of plume‐derived heterogeneities can reproduce the data with minimum 3He/22Ne fractionation at reasonable 238U/3He ratios. However, diffusive exchange of He and to a lesser extent Ne between aged MORB source and aged plume veins could explain the occurrence of low 3He/4He compositions in all HIMU centers and the apparent low time‐integrated 3He/22Ne of the Cameroon line.

Re [sbnd]Os isotopes in orogenic peridotite massifs in the Eastern Alps, Austria

The Re Os and Sm Nd isotopic systematics of polymetamorphic massif peridotite bodies, which have been described as ophiolites, are examined. The serpentinized mantle rocks from different tectonic units of the Eastern Alps reveal that Os isotopes are robust and provide information that field studies, petrographic and major and trace element studies do not. Presumed Paleozoic (e.g., Kraubath and Hochgrössen) and Mesozoic (e.g., Reckner Complex) bodies have uniform 187Os/ 188Os ratios that are slightly higher than, but within the range of, abyssal peridotites, which are presumed to represent the depleted mid-ocean ridge basalt source mantle (DMM). One serpentinite from the Penninic unit yields 187Os/ 188OS much lower than DMM. This mantle rock must have had a source in a long-term Re-depleted reservoir of the upper mantle. Although our understanding of the Re Os isotope system of upper mantle reservoirs is still limited, this system can be used to help constrain the tectonic evolution of diverse terrains.

Cross-arc geochemical variations in volcanic fields in Honduras C.A.: progressive changes in source with distance from the volcanic front

A geochemical traverse across Honduras reveals the heterogeneity of the mantle underneath Central America. Alkali basalts from Lake Yojoa (170 km behind the front) have low 87Sr/86Sr but high La/Yb, and elevated incompatible trace element abundances, consistent with derivation from a normal mid-ocean ridge basalt source mantle via low degrees of melting. These lavas lack evidence for an enriched source thought to be intermingled with normal mid-ocean ridge basalt source mantle beneath most of Central America. The amplitude of the subducted slab signature decreases smoothly with distance from the volcanic front. Lavas from Zacate Grande, the area nearest to the volcanic front (17 km behind the arc), display large ion lithophile element enrichment and high field strength element depletion indicating the involvement of subducted material in magma genesis. Components of subducted material are not evident in lavas from Lake Yojoa, the area furthest from the arc. Basalts and basaltic andesites from Tegucigalpa, 102 km behind the volcanic front, are geochemically intermediate between those of Lake Yojoa and Zacate Grande. The lavas from Tegucigalpa show a decreased influence of the subduction component, and are affected by assimilation-fractional crystallization processes at shallow depths. The gradual decrease in the subducted component from the volcanic front to Zacate Grande, Tegucigalpa and finally Lake Yojoa contrasts with the abrupt decrease documented for southeast Guatemala, the only other area in Central America where a cross-arc transect has been studied.

Hydration and dehydration of oceanic crust controls Pb evolution in the mantle

The position of the Pb isotopic compositions of mid-ocean ridge basalts (MORB) to the right of the geochron has long puzzled geochemists. Uranium is more incompatible than Pb during mantle melting, and the mantle source of MORB, being depleted in incompatible elements, should have low Pb isotope ratios and plot to the left of the geochron. This has been called the “lead paradox”. MORB have a second peculiar characteristic: Pb concentrations are drastically depleted relative to other elements of similar compatibility such as Ce or Nd. We suggest that both characteristics can be explained by preferential mobility of Pb during hydrothermal alteration of the oceanic crust associated with sea-floor spreading, and subsequent dehydration during subduction. A portion of this Pb migrates into the mantle wedge and is then added to the continental crust via arc magmas. Parts of the Pb-depleted recycled oceanic crust are stored at some deep level in the mantle and eventually become the source of ocean island basalts, the rest mixes into the mantle to become the MORB source. This model is evaluated quantitatively and special attention is given to the evolution of the Ce/Pb ratio in the depleted mantle from the beginning of Earth history to the present.

Geochemistry of Cenozoic basaltic and silicic magmas in the central portion of the Loei–Phetchabun volcanic belt, Lop Buri, Thailand

Cenozoic volcanic rocks outcrop in the central portion of the Loei–Phetchabun volcanic belt in central Thailand in the Lop Buri area. The volcanic rocks range in composition from basalt to high-silica rhyolite. In general, the volcanic rocks decrease in age from south to north. The oldest rocks studied are 55–57 Ma rhyolites that are isotopically and geochemically distinct from younger (13–24 Ma) rhyolites that occur farther north. Intermediate rocks (andesite and dacite) are less voluminous than rhyolite. Basalt occurs in the central and northern parts of the area and ranges in composition from olivine tholeiites to nepheline normative alkali basalts. The isotopic, major, and trace element compositions of the andesites, dacites, and younger rhyolites are consistent with an origin for these rocks by variable degrees of partial melting of metabasaltic crustal rocks, themselves derived from a depleted mantle source at approximately 530 ± 100 Ma. The apparent extent of partial melting of metabasalt increases from rhyolite to andesite. The isotopic and trace element systematics of the basalts are consistent with a refertilized depleted mantle source with characteristics of a mixture of normal mid-ocean ridge basalt source mantle and enriched mantle II type mantle.

The H 2O content of basalt glasses from Southwest Pacific back-arc basins

The H 2O content of 35 glasses from Southwest Pacific back-arc basins (Lau, North Fiji, Woodlark and Manus) have been determined by infrared spectroscopy. On a plot of K 2O vs. H 2O the glass data define two distinct trends characterized by different slopes. Trend I, with a slope (K 2O/H 2O) of 0.25, can be explained by addition of a subduction-related component with K 2O/H 2O = 0.25 to a depleted mid-ocean ridge basalt mantle source (N- or D-MORB-like). Trend II, which coincides with the N- to E-MORB compositional spectrum, can be produced by addition of a non-subduction component, possibly an alkaline magma with K 2O/H 2O∼ 1.5, to the same depleted mantle source. The K 2O/TiO 2 and K/Nb values of E-MORB and back-arc basin basalts (BABB) of Trend II suggest that the enriched component involved in their genesis is not derived from a typical ocean island basalt (OIB, e.g. Hawaiian) mantle source. Our data show that the entire spectrum of BABB compositions can be explained by different degrees of mixing of a mantle source of either D-, N- or E-MORB composition with the subduction-related component, characterized by a K 2O/H 2O value of 0.25. Different BABB types correlate with tectonic setting. Samples from the Trend II are associated with relatively stable spreading ridges, whereas those affected by the subduction-related component are always associated with more complex tectonic settings, or come from young or incipient back-arc basins. Pronounced E-MORB affinities of mantle sources are demonstrated only for samples from the Lau, North Fiji and Scotia Sea basins. The most H 2O enriched BABB of Trend I partly overlap in terms of H 2O and K 2O content and H 2O/TiO 2 and K 2O/TiO 2 values with island arc tholeiites. This suggests involvement of similar subduction-related components in the genesis of these two magma types. Because a larger database is now available, the K 2O/H 2O vs. TiO 2 tectonic discriminan diagram of Muenow et al. [2] appears to be less useful than when originally proposed. The very low K 2O/H 2O value ( < 0.05) of the H 2O-bearing phase involved in boninite genesis implies that it maybe a fluid derived from the subducted slab. The significantly higher K 2O/H 2O value (0.25) of the subduction-related component involved in petrogenesis of BABB and some arc tholeiites indicates that it was a melt, rather than a fluid. This K 2O/H 2>O value (0.25) is also of some interest, as the same value occurs in depleted MORB.

Southwestern limits of Indian Ocean Ridge Mantle and the origin of low 206Pb/204Pb mid‐ocean ridge basalt: Isotope systematics of the central Southwest Indian Ridge (17°–50°E)

Basalts from the Southwest Indian Ridge reflect a gradual, irregular isotopic transition in the MORB (mid‐ocean ridge basalt) source mantle between typical Indian Ocean‐type compositions on the east and Atlantic‐like ones on the west. A probable southwestern limit to the huge Indian Ocean isotopic domain is indicated by incompatible‐element‐depleted MORBs from 17° to 26°E, which possess essentially North Atlantic‐ or Pacific‐type signatures. Superimposed on the regional along‐axis gradient are at least three localized types of isotopically distinct, incompatible‐element‐enriched basalts. One characterizes the ridge between 36° and 39°E, directly north of the proposed Marion hotspot, and appears to be caused by mixing between hotspot and high ∈Nd, normal MORB mantle; oceanic island products of the hotspot itself exhibit a very restricted range of isotopic values (e.g., 206Pb/204Pb = 18.5–18.6) which are more MORB‐like than those of other Indian Ocean islands. Between 39° and 41°E, high Ba/Nb lavas with unusually low 206Pb/204Pb (16.87–17.44) and ∈Nd (−4 to +3) are dominant; these compositions are not only unlike those of the Marion (or any other) hotspot but also are unique among MORBs globally. Incompatible‐elementenriched lavas in the vicinity of the Indomed Fracture Zone (∼46°E) differ isotopically from those at 39°–41°E, 36°–39°E, and both the Marion and Crozet hotspots. Thus, no simple model of ridgeward flow of plume mantle can explain the presence or distribution of all the incompatible‐element‐enriched MORBs on the central Southwest Indian Ridge. The upper mantle at 39°–41°E, in particular, may contain stranded continental lithosphere, thermally eroded from Indo‐Madagascar in the middle Cretaceous. Alternatively, the composition of the; Marion hotspot must be grossly heterogeneous in space and/or time, and one of its intrinsic components must have substantially lower 206Pb/204Pb than yet measured for any hotspot. The origin of the broadly similar but much less extreme isotopic signatures of MORBs throughout most of the Indian Ocean could be related to the initiation of the Marion, Kerguelen, and Crozet hotspots, which together may have formed a more than 4400‐km‐long band of juxtaposed plume heads beneath the nearly stationary lithosphere of prebreakup Gondwana.

Evaluation of rare gas data in relation to oceanic magmas

Summary The isotopic compositions of the rare gases measured in oceanic rock samples present stimulating but incomplete information concerning the time and extent of magma degassing processes. Isotopic abundances of gases trapped from suboceanic mantle source regions are related to the depleted/undepleted mantle concepts, while atmospheric or crustal components form an important part of the data set. A reasonable story can be presented for the rare gas component from normal mid-ocean ridge basalt mantle source regions, but oceanic islands and island arc basalts result from more complex sources; the rare gas evidence is not yet sufficiently free of ambiguities and experimental uncertainties to provide a firm constraint to various models.

Radiogenic rare gases and the evolutionary history of the depleted mantle

U, Th/He, K/Ar, and U/Xe ages of the depleted mantle source region of oceanic basalts are much less than the age of the earth, showing clearly that the midocean ridge basalt mantle source was not completely degassed and decoupled from the atmosphere early in earth history. Rather, extensive degassing has continued up to at least the last few hundred million years. A stable, layered mantle is not suggested by these data.