Sr Isotopic Evolution Research Articles

The Indus river system (India-Pakistan): Major-ion chemistry, uranium and strontium isotopes

The Indus River is one of the large river systems draining the Himalaya. We report in this paper the major-ion chemistry, Sr and U isotope systematics of the Indus system, particularly its headwaters. The results show that: (1) on an average about a third of the cations in the waters can be from silicate weathering; however, most of the (Ca + Mg) is likely to be from the weathering of carbonates and evaporites; (2) the 87 Sr 86 Sr of the waters ranges between 0.7085 and 0.7595, the higher values (> 0.72) are typical of the tributaries draining the Precambrian granite/gneissic terrains. The 87 Sr 86 Sr of the Indus main channel, throughout its entire stretch, shows only minor variations, 0.7104–0.7116. The Indus transports ∼8.8 · 10 6 mol Sr to the Arabian Sea annually with a 87 Sr 86 Sr of 0.7111 if our results of Sr concentration and 87 Sr 86 Sr measured at Thatta can be considered typical of the Indus throughout the year; and (3) the U concentration in the Indus and its tributaries is generally high, 0.37–10.3 μg 1 −1. The source for the high uranium can be the weathering of granites, zones of uranium mineralisation and black shales. The Indus results when compared with our earlier data on the Ganga-Brahmaputra show that in all these three river systems carbonate weathering is the dominant source of (Ca + Mg) and HCO 3 and that U concentrations are high. In the case of 87 Sr 86 Sr , the Indus waters are less radiogenic; however, its tributaries draining the Precambrian granites/gneisses have 87 Sr 86 Sr in excess of 0.72, similar to that in the rivers of the Ganga system. The low 87 Sr 86 Sr of the Indus, 0.7111, suggests that its contribution to the Sr isotope evolution of oceans since the Cenozoic is less significant than that of the Ganga-Brahmaputra.

Isotope and chemical microsampling: Constraints on the history of an S-type rhyolite, San Vincenzo, Tuscany, Italy

Fine scale heterogeneities in strontium isotope ratios, 40Ar- 39Ar ages, and chemical composition have been determined for individual mineral grains and enclaves in the rhyolites of San Vincenzo, Italy. The rhyolites are peraluminous with phenocrysts of alkali feldspar, plagioclase, biotite, and cordierite and have previously been divided into two groups on the basis of whole rock major element chemistry. Group B lavas are distinguished from Group A in having higher MgO and CaO contents as well as containing two pyroxenes and chilled latite enclaves. The Group B lavas show textural evidence for disequilibrium, such as resorption and sieve zones in feldspars. Laser fusion 40Ar- 39Ar dating of individual alkali feldspar crystals suggests that the eruption of these rhyolites took place 4.38 ± 0.04 Ma, although small amounts of excess argon are present in subhedral and cloudy crystals. The groundmass in the Group A sample has a higher initial 87Sr 86Sr (0.725) than that of the Group B (0.713). The Sr isotope compositions of chemically and texturally characterized individual grains reveal marked disequilibrium between minerals and the host glass as well as within-grain zonation. The feldspars and coarse-grained biotites have initial 87Sr 86Sr that are intermediate between those of the Group A and B glasses, with alkali feldspar generally having higher initial 87Sr 86Sr ratios than plagioclase. Chilled latite enclaves, which are present in only the Group B lavas, have relatively low initial 87Sr 86Sr ratios in the range 0.7082–0.7088. The mineral chemistry and isotope data indicate that restite does not form a significant portion of the crystalline assemblage and, therefore, cannot be the source of the strontium isotope disequilibrium. Xenocrysts mixed into the rhyolite from a mafic magma were identified as clinopyroxene megacrysts, clinopyroxene-orthopyroxene clots, orthopyroxene-plagioclase clots, and plagioclase with extensive sieve textures. These grains all have relatively low initial 87Sr 86Sr ratios. Other grains that may be xenocrysts are cordierites with cores containing inclusions of biotite, spinel, and sillimanite, and alkali feldspars with excess 40Ar. The strontium isotope disequilibrium in phenocrysts of plagioclase, alkali feldspar, and biotite is attributed to a changing melt chemistry that was too rapid for strontium diffusive equilibration. A model for the San Vincenzo magma chamber is proposed in which the Sr isotope evolution of the Group B rhyolites is due to magma mixing of a crustal melt with latite magma containing unradiogenic strontium. The variable initial strontium isotope compositions of Group A phenocrysts may be due to assimilation or crystallization along the interface between mingled Group A and B magmas.

Strontium isotopic composition of Pliocene and Pleistocene molluscs from emerged marine deposits, North American Arctic

High-precision strontium (Sr) isotopic measurements were obtained for 53 Pliocene and Pleistocene molluscan shells from emerged marine deposits around the coasts of Arctic North America to test whether such data can be used for chronostratigraphic purposes. 87Sr/86Sr ratios from Sr isotopic measurements on many marine fossils from Arctic Ocean borderland sites are broadly consistent with their expected values based on independent age control and on a comparison with the Sr isotopic evolution of seawater recorded in deep-sea cores. All 87Sr/86Sr ratios measured for shells from Middle and Late Pleistocene deposits are consistent with expected values, but only 9 of 22 ratios in shells from older deposits are consistent with independent age estimates. Aberrant 87Sr/86Sr ratios are consistently higher than expected. At Nome, Alaska, and Baffin Island, Canada, all 87Sr/86Sr ratios are higher than expected. Because these shells were formed along the Pacific Ocean and Atlantic Ocean margins, respectively, their high 87Sr/86Sr ratios cannot be attributed to possible differences in the Sr isotopic evolution of the Arctic Ocean relative to that of remainder of the world's oceans. Radiogenic Sr from proximal river-water input, or leaching of detritus within the shell matrix, may have changed the 87Sr/86Sr ratios by as much as about 5 × 10−5, but these mechanisms cannot account for the very high 87Sr/86Sr values (from 20 × 10−5 to 200 × 10−5 higher than those of modern seawater) measured for some shells. Alteration by diagenetic fluids rich in radiogenic Sr is the most plausible explanation for the aberrant results. Diagenesis is recognized petrographically in the most altered shells by micritic overprinting of the original shell microstructure; in addition, one shell enriched in 87Sr from Baffin Island exhibits a broad range (170 × 10−5) of 87Sr/86Sr ratios across the shell, and an oxygen isotopic gradient (l.6‰) that is greater than the expected primary variability. Although our data suggest that Sr isotopic data from young Arctic molluscan fossils may offer a viable dating method, criteria for screening altered shells must be devised before the technique can be considered a reliable chronostratigraphic tool.

Formation waters from Mississippian-Pennsylvanian reservoirs, Illinois basin, USA: Chemical and isotopic constraints on evolution and migration

We have analyzed a suite of seventy-four formation-water samples from Mississippian and Pennsylvanian carbonate and siliciclastic strata in the Illinois basin for major, minor, and trace element concentrations and for strontium isotopic composition. A subset of these samples was also analyzed for boron isotopic composition. Data are used to interpret origin of salinity and chemical and Sr isotopic evolution of the brines and in comparison with a similar data set from an earlier study of basin formation waters from Silurian-Devonian reservoirs. Systematics of Cl-Br-Na show that present Mississippian-Pennsylvanian brine salinity can be explained by a combination of subaerial seawater evaporation short of halite saturation and subsurface dissolution of halite from an evaporite zone in the middle Mississippian St. Louis Limestone, along with extensive dilution by mixing with meteoric waters. Additional diagenetic modifications in the subsurface interpreted from cation/Br ratios include K depletion through interaction with clay minerals, Ca enrichment, and Mg depletion by dolomitization, and Sr enrichment through CaCO 3 recrystallization and dolomitization. Ste. Genevieve Limestone (middle Mississippian) formation waters show 87Sr 86Sr ratios in the range 0.70782–0.70900, whereas waters from the siliciclastic reservoirs are in the range 0.70900–0.71052. Inverse correlations between 87Sr 86Sr and B, Li, and Mg concentrations suggest that the brines acquired radiogenic 87Sr through interaction with siliciclastic minerals. Completely unsystematic relations between 87Sr 86Sr and 1/Sr are observed; Sr concentrations in Ste. Genevieve and Aux Vases (middle Mississippian) waters appear to be buffered by equilibrium with respect to SrSO 4. Although there are many similarities in their origin and evolution, these formation waters are distinguished from Silurian-Devonian brines in the basin by elevated Cl Br and Na Br ratios and by unsystematic Sr isotope relationships. Thus waters from these two major segments of the Illinois basin stratigraphic column form distinct geochemical regimes which are separated by the New Albany Shale Group (Devonian-Mississippian) regional aquitard. Geochemical evolution appears to have been influenced significantly by Paleozoic and Mesozoic hydrologic flow systems in the basin.

Use of Rb-Sr isotope data to constrain the time of deposition of Precambrian metasediments: an example from Hamborgerland, West Greenland

In high-grade metamorphic terrains it is often not possible to determine the relative age of metasedimentary units by field investigation. However, the time of deposition of the original sediment can be constrained by consideration of the Sr-isotopic evolution of the rocks on the scale of an outcrop. An outline of the method is given, and Rb-Sr data for high-grade (granulite facies) metasediments from HamborgerIand, West Greenland, are discussed as an example. Sm-Nd model age data indicate that these rocks were derived by erosion of a 3000–3200 Ma basement. Deposition took place not long before 2700 Ma ago, and closure of the Rb-Sr isotope system after high-grade metamorphism occurred at about 2600 Ma.

Sr isotope evolution of seawater: the role of tectonics

We use a high-resolution seawater Sr isotopic evolution curve for the last 100 m.y. in conjunction with modern riverine Sr flux measurements, and also geologic, tectonic and geochronological data, to make the case for a close relationship between seawater Sr isotopic composition and the India-Asia continental collision. Using a simple seawater Sr budget model we begin by showing that the Sr flux associated with alteration of seafloor basalts is too small and does not have the right time evolution to account for much of the seawater Sr isotopic curve of the last 100 m.y. The flux of dissolved Sr carried by rivers originating in the Himalaya-Tibet region on the other hand is presently a significant fraction of the global Sr budget. We calculate how this riverine flux would have had to change with time in order to match the observed seawater Sr isotopic curve and find that the riverine flux remains relatively constant prior to the collision of India with Asia but then increases very significantly after collision. We note that the period of most rapid change in seawater Sr isotopic ratio, from 20 Ma to 15 Ma, is also a period of exceptionally high erosion in parts of the Himalayas and the Tibetan Plateau. As further evidence that Sr derived from the collision of India with Asia plays a major role in the Sr isotopic evolution of seawater we show that the total amount erosion of the Himalaya-Tibetan Plateau since collision, which we calculate separately, represents a total amount of Sr that is very nearly the same as the cumulative amount required by the Sr isotopic change of seawater since collision. The relationship between erosion and riverine Sr flux allows us to use the Sr isotopic evolution of seawater to reconstruct a history of erosion since collision, and we find that the erosion rate accelerates with time since collision, with the present having the largest rate. When we apply the Sr budget model to the entire Phanerozoic using a new compilation of deformed continental area versus time we find that we can account for the large-scale structure of the seawater Sr isotopic curve, but fail to reproduce several local maxima and minima, especially in the period 100–300 Ma. The present high 87Sr/ 86Sr of seawater and similar highs in the Devonian and Cambrian do correlate with extensive deformation on the continents.

Strontium isotopes and rubidium in the Ganga-Brahmaputra river system: Weathering in the Himalaya, fluxes to the Bay of Bengal and contributions to the evolution of oceanic 87Sr/ 86Sr

The concentrations of Rb and Sr and 87Sr/ 86Sr isotopic ratios have been measured in the dissolved load of the Ganga-Brahmaputra (G-B) river system. The Ganga was sampled extensively from its source at Gangotri (in the Higher Himalaya) to Patna (on the alluvial plains). The Brahmaputra was sampled in its stretch in Assam, in India. The average Sr concentration in the Ganga (at Patna) is 1.2 μmol/l and that in the Brahmaputra (at Goalpara) is 0.73 μmol/l; the mean 87Sr/ 86Sr ratios are 0.7239 and 0.7192, respectively. The 87Sr/ 86Sr in the Ganga source waters (the Alaknanda, the Bhagirathi and their tributaries) range between 0.7300 and 0.7986, considerably higher than the global average runoff value of 0.7119. The high 87Sr/ 86Sr in the Ganga source waters result from the intense weathering of Precambrian granites and gneisses enriched in radiogenic Sr. The Sr isotope systematics of the Ganga waters is dominated by silicate weathering, whereas carbonate weathering plays a significant role in their major ion chemistry. The G-B system transports about 910 million moles of dissolved Sr annually to the Bay of Bengal, with an average 87Sr/ 86Sr of 0.7213. The Sr transported by the G-B system is about 2.7% of the global dissolved Sr flux to the oceans via rivers. Model calculations reveal that the G-B system has contributed significantly to the Sr isotope evolution of seawater during the past ∼ 20 Ma. The flux of Rb through the G-B system is ∼ 24 million moles per year. This is about 5% of the global river supply of Rb to the oceans, nearly twice the contribution of water via the G-B system to the oceans. Our study suggests that the marine geochemistries of Sr and Rb (and possibly U) may have been influenced considerably by the Himalayan orogeny.

Diagenesis and Sr isotopic evolution of seawater using data from DSDP 590B and 575

New Sr isotopic and concentration analyses of bulk carbonate and porewaters from a carbonate-rich Miocene section sampled at DSDP Site 575 are reported and used, along with our earlier data from a section of similar age at DSDP Site 590B, in a numerical model to determine for each site the rate of Sr exchange during diagenesis and the degree to which this amount of exchange has shifted the 87Sr 86Sr of the bulk carbonate from the ratio each increment of sediment had when first deposited. Despite the fact that the two sites have very different water depth (4536 m at 575 vs. 1299 m at 590B), different sedimentation rate ( ∼ 10 m/My at 575 vs. ∼ 40 m/My at 590B), and significantly different porewater Sr 2+ gradients, we find that the rate of Sr exchange as a function of sediment age is almost indistinguishable between the two sites. The rate of Sr exchange determined at the two sites is such that the resulting shift in bulk carbonate 87Sr 86Sr due to diagenesis is small compared to the total range of 87Sr 86Sr values measured, but large compared to the analytical uncertainty of the individual isotopic ratio measurements. By taking this shift into account we reconstruct the original 87Sr 86Sr of each increment of carbonate sampled, which when plotted as a function of age becomes our best estimate of the Sr isotopic evolution of seawater. Because Sr is very well mixed in the ocean, at any given time there is a single worldwide value of seawater 87Sr 86Sr . Therefore, if we are quantitatively accounting for the effect Sr exchange, we should find the same seawater evolution curve regardless of what DSDP Site is used. When we compare the observed bulk carbonate 87Sr 86Sr vs. age at the two sites they are seen to differ by amounts that are sometimes large compared to the analytical uncertainties of the measurements. However, when these data are corrected for the post-depositional Sr isotopic shifts predicted by our diagenetic model, we find almost perfect agreement. This agreement suggests that we have made a realistic determination of the rate of Sr exchange and its consequences in terms of shifting the 87Sr 86Sr of the bulk carbonate, and more importantly, that Sr isotopes can be used to correlate marine sediments with an accuracy comparable to the very small analytical uncertainties of modern isotopic measurements.

Lower crustal evolution under central Arizona: Sr, Nd and Pb isotopic and geochemical evidence from the mafic xenoliths of Camp Creek

Oligocene potassic volcanism at the Camp Creek area, Arizona has sampled the lower crust by bringing mafic granulite, amphibolite and garnet-bearing pyroxenite (eclogite) xenoliths to the surface. Xenolith geobarometry records metamorphic pressures of approximately 10 kbar and temperatures of 650–900°C. Whole-rock Sr and Nd isotopic compositions correlate roughly with xenolith metamorphic mineral assemblages: higher temperature eclogites have 87Sr/ 86Sr ranging from 0.7066 to 0.7080 and ε Nd of −2 to −5.8 while lower temperature amphibolites have 87Sr/ 86Sr of 0.7074 to 0.7082 and ε Nd of −6 to −9. Sr and Nd isotopic compositions of mineral separates reflect, in some xenoliths, intermineral diffusion up to the time of latite eruption and limited exchange with the host latite. Pb isotopic compositions of the xenoliths show little mineralogical dependence and overlap the field for normal MORB. These data suggest that lower-crustal metamorphism has significantly affected the Sr and Nd isotope evolution of the eclogites but has had little effect on their Pb isotope systematics. For xenoliths closer to igneous bulk compositions (i.e. not enriched in metamorphic garnet and pyroxene), Nd depleted mantle model ages range from 1.2 to 1.9 Ga, in agreement with the Pb Pb array “age” of 1.1 ± 0.6Ga. This is interpreted as dating the separation of the lower crust from the mantle during the Proterozoic crust-building event in this area. Similar Nd model ages for nearby Cenozoic and Mesozoic granitoids suggest that the type of mafic lower crust represented by the Camp Creek xenoliths may be a suitable source for younger plutonics in the area.

Constraints and interpretation of 87Sr/86Sr ratios in Cenozoic dolomites

Replacement dolomitization by seawater has been modeled in order to quantify the Sr‐isotope signature in Cenozoic dolomites as a function of precursor mineralogy and 87Sr/86Sr ratio, reaction stoichiometry and 87Sr/86Sr ratio of the dolomitizing fluids. High Sr carbonates, such as aragonite, may introduce a significant precursor memory into an otherwise seawater dominated Sr‐isotope signature if small quantities of seawater per unit volume of precursor carbonate are involved. Dolomitization of low Sr carbonates (i.e. low‐Mg calcite) are shown to create an isotopic signature indistinguishable from that of the seawater involved in the reaction. Therefore, by comparison with the Sr‐isotope evolution curve of seawater, the‐ 87Sr/86Sr ratios of the dolomites can be used to record the oldest possible age of dolomitization and the youngest age of deposition.The implications for this approach have been applied to data obtained from a dolomitized core from Little Bahama Bank, Bahamas. Two periods of dolomitization are recognized, one in the early Late Miocene involving Middle Miocene or older rocks, and a second one around 2.4 Ma ago affecting early Pliocene carbonates.

Numerical models for diagenesis and the Neogene Sr isotopic evolution of seawater from DSDP Site 590B

A numerical model for the diagenetic exchange of Sr between carbonates and their pore fluids during sedimentation and compaction has been developed. The model has been applied to data from DSDP Site 590B in order to assess the accuracy with which the Sr isotope record in the carbonate sediment reflects that of seawater. The most important process affecting the Sr in the solid carbonate is exchange with the pore fluid due to solution-reprecipitation, but the concentration or isotopic composition of Sr in the solid itself gives little or no information as to the magnitude of this exchange. The key to determining the rate of exchange is the pore fluid, where the variations of Sr 2+ and 87Sr/ 86Sr with depth are very sensitive indicators. The logical structure of applying the model to data from DSDP 590B is to find by successive iteration an ocean history (i.e., the initial 87Sr/ 86Sr and Sr concentration of each increment of carbonate deposited) and a rate of Sr exchange between pore water and solid carbonate such that the model matches the present Sr concentration and 87Sr/ 86Sr of both pore water and solid carbonate. Once all the data are matched, the model provides an estimate of the rate of Sr exchange due to solution-reprecipitation and the evolution of 87Sr/ 86Sr in seawater over the past 20 million years. For DSDP 590B we find that solution-reprecipitation decreases rapidly with depth, from a near surface value of about 10% per million years to about 1% per million years below 200 m. This rate of exchange of Sr results in the carbonates of DSDP 590B preserving an accurate record of the Sr isotopic evolution of the ocean over the past 5 million years, but for ages greater than 5 million years the 87Sr/ 86Sr ratio of the carbonate is systematically displaced from that of the seawater in which it was deposited. The maximum difference is of order 5 × 10 −5.

Trace element and Sr and Nd isotope geochemistry of peridotite xenoliths from the Eifel (West Germany) and their bearing on the evolution of the subcontinental lithosphere

Peridotite xenoliths from the Eifel can be divided into incompatible element-depleted and -enriched members. The depleted group is restricted to dry lherzolites whereas the enriched group encompasses dry harzburgites, dry websterite and amphibole and/or phlogopite-bearing peridotites. Isotopically the depleted group is very diverse with 143Nd/ 144Nd ranging from ∼ 0.51302 to 0.51355 and 87Sr/ 86Sr from ∼ 0.7041 to 0.7019, thus occupying a field larger than expected for oceanic-type subcontinental mantle. These xenoliths are derived from a mantle which appears to have diverged from a bulk-earth Nd and Sr isotopic evolution path ∼ 2 Ga ago as a consequence of partial melting. The combination of high 143Nd/ 144Nd with high 87Sr/ 86Sr in some members of the depleted-xenoliths suite is likely to be the result of incipient reaction with incompatible element-enriched fluids in the mantle. In the enriched group such reactions have proceeded further and erased any pre-enrichment isotope memory resulting in a smaller isotopic diversity ( 143Nd/ 144Nd∼ 0.51256–0.51273, 87Sr/ 86Sr∼ 0.7044–0.7032). An evaluation of Sm Hf and Yb Hf relationships suggests that the amphibole-bearing lherzolites and harzburgites acquired their high enrichment of light rare earth elements by fluid infiltration into previously depleted peridotite rather than by silicate melt-induced metasomatism. Upper mantle composed of such metasomatized peridotites does not represent a potential source for the basanites and nephelinites from the Eifel. The isotopic and chemical diversity of the subcontinental lithospheric part of the mantle may result from it having remained isolated from the convecting mantle for times > 1 Ga.

A quantitative approach to trace element and Sr isotope evolution in the Adamello batholith (northern Italy)

A development of De Paolo's mathematical procedure (1981) for magmatic AFC (Assimilation-Fractional Crystallization) processes is discussed with respect to both trace element and Sr isotopic ratio behaviours during the genesis and evolution of Adamello batholith (northern Italy). Resolution of a two equation-system (one relative to 87Sr/86Sr ratio variation in a magma generated by an AFC process, the other to its trace element content variations) gives the F (mass of magma at time t/mass of initial magma) and D (bulk partition coefficient) values, by which one can deduce the r (rate of assimilation/rate of crystallization) value during each step of magmatic evolution. This quantitative approach suggests that: 1) there was a common precursor magma for all the Adamello granitoids, with a Mg-rich tholeiitic composition; 2) each intrusive unit appears to have been generated by different extents of AFC; 3) the trace element distribution in the magma seems essentially influenced by mineral fractionation, rather than by the composition of the assimilated crustal material.

Detailed record of the Neogene Sr isotopic evolution of seawater from DSDP Site 590B

A detailed study of strontium isotope variations in Neogene marine carbonate sediments from Deep Sea Drilling Project Site 590B, using techniques that allow the 87Sr/86Sr ratio to be determined to better than ±0.00001, gives a high-resolution record of the Sr isotopic evolution of seawater. The data show that the rate of change of the marine 87Sr/86Sr ratio has varied significantly even on time scales as short as 1 m.y. Periods of particularly rapid growth appear to follow major marine regressions and probably reflect an increase in the delivery of radiogenic Sr from the continents coupled with a decreased submarine carbonate dissolution rate (greater carbonate compensation depth). Periods of relatively slowly changing 87Sr/86Sr follow major marine transgressions. On the basis of correlations with the marine oxygen isotope record and the times of major continental glacier growth, it is inferred that the effects of sea-level variations are modified by climatic factors that affect the intensity of continental weathering and runoff. The effects of sea-floor generation rate variations are not discernible for the Neogene. The maximum attainable stratigraphic resolution using Sr isotopes is between 0.1 and 2 m.y. for this time period.

Strontium isotopic chronostratigraphy and correlation of the Miocene Monterey Formation in the Ventura and Santa Maria basins of California

Research Article| June 01, 1986 Strontium isotopic chronostratigraphy and correlation of the Miocene Monterey Formation in the Ventura and Santa Maria basins of California Richard W. Hurst Richard W. Hurst 1Department of Geology, California State University, Los Angeles, California 90032 Search for other works by this author on: GSW Google Scholar Author and Article Information Richard W. Hurst 1Department of Geology, California State University, Los Angeles, California 90032 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1986) 14 (6): 459–462. https://doi.org/10.1130/0091-7613(1986)14<459:SICACO>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Richard W. Hurst; Strontium isotopic chronostratigraphy and correlation of the Miocene Monterey Formation in the Ventura and Santa Maria basins of California. Geology 1986;; 14 (6): 459–462. doi: https://doi.org/10.1130/0091-7613(1986)14<459:SICACO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Sr isotopic composition of authigenic marine carbonates can be used effectively with the known Sr isotopic evolution of seawater to decipher chronostratigraphic relationships and correlate the Monterey Formation. The Sr isotopic chronostratigraphy of both the Oakview and Mussel Rock sections agrees with published biostratigraphic ages. Correlations of lithologic members between the two sections are also in accord with the published lithologic and biostratigraphic correlations. Time and stratigraphic resolutions of Sr isotopic chronostratigraphy are dependent upon the slope of the seawater Sr evolution curve, sedimentation rates, inherent Sr isotopic variability, and analytical precisions. On the average, individual analyses yield time resolutions of 0.5 Ma and stratigraphic resolutions of 10–70 m of section. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Earth degassing, mantle metasomatism, and isotopic evolution of the mantle

Excess 3 He in mantle-derived rocks relative to atmospheric abundance shows that degassing of Earth is a continuing process. The importance of chemical changes induced by fluids from the deep mantle on the oceanic and continental lithosphere has been recognized increasingly in recent years. I propose that the parent-daughter element fractionation caused by such fluids dominates the Sr, Nd, and Pb isotope evolution of the mantle in the second half of Earth9s history. A simple two-layered mantle model based on such a premise seems to be consistent with the present knowledge of the Sr, Nd, Pb, and He isotopic characteristics of mantle-derived rocks. The model predicts two populations of intraplate oceanic-island volcanism: “primary” fluid-induced volcanism and “secondary” volcanism caused by remelting of ancient metasomatized domains. The model implies a new interpretation of the Nd-Sr isotopic covariations in oceanic basalts.

Precambrian sial-sima relations: evidence for earth expansion

It is suggested that the early and middle Proterozoic crustal record is not amenable to explanations in terms of present-day Earth surface dimensions. With few exceptions, lithostratigraphic, geochemical and isotopic data for the 2.6−1.0 aeons time span indicate intrasialic crustal environments — with little evidence for the existence of contemporaneous sima and operation of two-stage mantle melting processes. Unlike Archaean history, which was dominated by sima-sial transformation processes, the majority of cratonic and mobile Proterozoic domains show evidence for sialic basement. Principal examples for this observation are brought from the Australian, South African and Canadian shields. The relations between Proterozoic mobile belts and the Archaean crust allow a three-fold classification in terms of intercratonic, marginal and external mobile belts. Marginal mobile belts flanked on one side by Archaean crust display some features analogous to Alpine geosynclines but fail to yield evidence for simatic crust. In external mobile belts, whose relation to the Archaean crust is large unknown, geochemical and isotopic evidence commonly suggests an underlying sialic substratum. Ophiolites are unknown and ocean-floor or arc-trench type volcanic assemblages are rare, though not unknown, in these terrains. With few exceptions, Proterozoic palaeomagnetic data suggest that little horizontal relative movement has occurred between cratons across intercratonic belts. The essentially intact nature of the early and middle Proterozoic sialic crust suggests that, had simatic crust existed — on an Earth of present-day surface area — it must have occupied a vast, hemispheric regime. It is calculated that sea-floor spreading, subduction and partial melting processes within and along the margins of such oceanic regime should have given rise to at least 500 · 10 6 km 3 of sima-derived materials, for which no record is observed. The Sr isotopic evolution of carbonates and K/Na ratios in clastic sediments argues against extensive distribution of simatic crust and its fusion products during 2.6−0.6 aeons ago. Unless all evidence for sima was inexplicably eliminated, a global sial must have existed during the early and middle Proterozoic. However, considerations of the Proterozoic sial budget do not allow a global sial of continental thickness on an Earth of present-day dimensions. Nor is a thin sialic crust consistent with the evidence, as its subsequent accretion into thicker crust is disallowed by palaeomagnetic data and as evidence from mineral PT indicators suggests continental thicknesses of the Archaean sial. The enigma regarding the nature of the unrecorded two-thirds to three-quarters of the Proterozoic crust on a globe of present-day dimensions is explained had Earth's surface area grown with time — in agreement with the expanding Earth hypothesis. The increasing importance of ophiolites and calc-alkaline suites in several regions about 1.0 aeon ago may correspond to onset of expansion and related horizontal plate tectonics.

Ar degassing and the origin of the sialic crust

The degassing of radiogenic Ar 40 is defined as coherent if only the Ar 40 associated with parent K is degassed as K is transferred from the mantle to crust. Coherency predicts, for a 4.55 b.y. Earth, a sialic crust with 2.50 per cent K, using only the Ar content of the atmosphere and present crust (from a Hurley and Rand, 1969, age distribution). This is a maximum limit to K content of the sialic crust if the age of the Earth is no younger than 4.55 b.y. A K content of the sialic crust of 1.9 per cent (Holland and Lambert, 1972) implies an efficiency ( E) less than 100 per cent for K transfer from oceanic basalt to sialic crust in subduction zones and/or some non-coherent (preferential) degassing of Ar from the mantle. K, Ar coherence for mantle differentiation to crust is supported however, by the agreement of the predicted oceanic He flux and radiogenic He-Ar ratios of volcanic gases with the observed limits if the best estimate of K, U, Th influx rates at oceanic ridges is used. Assuming K, Ar coherence, various sea-floor spreading rates as functions of time, and limiting K contents of the sialic crust, computed models give E and the portion of the sialic crust derived from melting oceanic basalt in subduction zones. Except for models with very high spreading rates in the Precambrian, they also predict that a significant part of the sialic crust was derived from vertical differentiation of the mantle, presumably early in Earth history. The results are in accord with Armstrong's model of an early sialic crust that is recycled to give a Hurley-type age pattern with the proviso that the ‘ vertical’ sial K υis formed early in Earth history for models with a high K υcomponent. The coherent K, Ar models with preferred estimates of input parameters are also consistent with a limited mixing model (only old and new sial are equilibrated) for Sr isotopic evolution and the probable average Sr 87 Sr 86 ratio now of the sialic crust.

Computer simulation of Pb and Sr isotope evolution of the Earth's crust and upper mantle

Isotope evolution in a differentiated (crust and upper mantle) and chemically heterogeneous Earth has been computed for a model with isotopic exchange between crust and mantle at an exponentially decreasing rate. To simulate the effects of subduction, cells of crust and mantle are selected at random, their contents mixed and then redistributed into the cells from which they came—to give new chemically heterogeneous but momentarily isotopically homogeneous systems. Daughter isotopes in each cell grow according to equations appropriate for closed chemical systems. Rock age distributions and isotopic data created by the computer calculation mimic nature. Pb isotope data changes through geologic time are illustrated to demonstrate that two-stage interpretations applied to Pb data for rocks with complex histories may be misleading. The intercept of the Pb ore growth curve and regressions fit to Pb data gives minimum values only for the duration of heterogeneous U Pb systems, not the time when heterogeneous distribution first occurred. An intercept derived time of about 3 b.y., in the Pb system is shown to be quantitatively compatible with an average Rb Sr age of crustal rocks of 1.5 b.y. and with a constant degree of chemical heterogeneity for all of Earth history.

A model for the evolution of strontium and lead isotopes in a dynamic Earth

Contrasting interpretations of existing models of Sr and Pb isotope evolution can be eliminated with a model in which crustal material is recycled through the mantle. In this model the earth's crust and upper mantle (above approximately 500 km depth) are in a steady‐state system, and the volumes and bulk compositions of ocean, continent, and mantle have been nearly constant for at least the last 2.5 b.y. and probably for most of the earth's history.Sialic material is continuously eroded from continents into ocean basins and, as a consequence of this process, is isotopically homogenized. In continental‐margin orogenic belts and island arcs, the ocean basin, rise, and trench sediments are dragged into the mantle. Isotopic equilibration between sialic and simatic material takes place within the mantle, and the sialic material is returned to the continents or island arcs as juvenile‐appearing volcanics, thus completing the geochemical cycle. Most of the radioactive parent isotopes reside within the continental sial, whereas the mantle remains depleted and unable to sustain its observed isotope evolution.With this model it is possible to explain Pb isotope evidence of widespread ancient continents and common Pb evolution in a system which appears to have a very uniform U/Pb and Th/U ratio, even though most of the U and Th are highly enriched in the heterogeneous sialic crust. At the same time the model provides an explanation for Sr isotope evidence of continual addition of material to continents. Sr isotope evolution is dominated by the reservoir of Sr in the mantle; in contrast, Pb isotope evolution is dominated by isotopic mixing during erosion and sedimentation.The apparent differences in the evolutions of Sr and Pb isotopes are due to differing responses to various parts of the steady‐state cycle as a consequence of the differences in parent to daughter ratios in the sialic crust as compared with the upper mantle and in the degree of enrichment of parent and daughter products in the crust. Identical mathematical models may be used to describe the evolution of both isotope systems.