Agents of low temperature ocean crust alteration

¹⁸O and⁸⁷Sr/⁸⁶Sr isotopic data from smectites, calcites, and whole rocks, together with published isotopic age determinations, alkali element concentration data and petrographic observations suggest a sequential model of ocean floor alteration. The early stage lasts about 3 m.y. and is characterized by palagonite and smectite formation, and solutions with a large basaltic component, increasing with temperature which varies from 15° to 80° C at DSDP site 418A. Most carbonates are depositedafter this stage from solutions with a negligible basaltic Sr component and temperatures of 15° to 40° C. Water of seawater Sr and O isotopic composition is shown to percolate to at least 500 m into the basaltic basement. No evidence was found for continuing exchange of strontium or oxygen after 3 m.y.Lamont-Doherty Geological Observatory Contribution number 3199     

The Shabogamo Intrusive Suite,Labrador: Sr and Nd isotopic evidence for contaminated mafic magmas in the Proterozoic

The Shabogamo Intrusive Suite comprises numerous bodies of variably metamorphosed gabbro which intrude Archean and Proterozoic sequences at the junction of the Superior, Churchill, and Grenville structural provinces in western Labrador. Combined Sm-Nd and Rb-Sr systematics in two bodies, ranging from unmetamorphosed to lightly metamorphosed, document a crystallization age of about 1375 m.y., and suggest that both bodies crystallized from magmas with similar Nd and Sr isotopic compositions. This age is in accordance with the existence of a regional magmatic event in the Churchill Province at approximately 1400 m.y.Rb-Sr systematics in two bodies of amphibolite-grade gabbro suggest a regional metamorphic event at about 950 m.y., corresponding to the waning stages of Grenville activity. Sm-Nd systematics in these high-grade bodies are affected to a much lesser degree than Rb-Sr.Initial ratios for¹⁴³Nd/¹⁴⁴Nd and⁸⁷Sr/⁸⁶Sr are lower and higher, respectively, than bulk earth values at 1375 m.y. Both these displacements are in the direction of older crustal material at 1375 m.y., and a model is proposed to produce the Shabogamo magma by mixing a mantle-derived magma with a partial melt of crustal rocks (approximately 4: 1 by volume). Young volcanic rocks with anomalous Nd and Sr isotopic ratios, which have previously been taken as evidence for “enriched” mantle, may be interpreted similarly.     

The recognition of mid-Cretaceous sea-level changes by means of Foraminifera

Detailed microfaunal investigations of the mid-Cretaceous (Albian-Santonian) successions of S. E. England have allowed the recognition of a major rise in sea level during the Cenomanian, that reached a maximum in the Early Turonian. The water depth model presented in this account has also been compared to successions from environments ranging from marginal marine, to open shelf, and finally deep ocean, in an attempt to confirm its validity.     

High pressure metamorphism in the Scourian of NW Scotland: Evidence from garnet granulites

Ultramafic and mafic granulites from Archaean gneisses in N.W. Scotland (the Scourian) show evidence of two periods of granulite facies mineral growth. The first produced a high pressure clinopyroxene +garnet±plagioclase assemblage at an estimatedP-T of 12–15 kb and 1,000° C. Uplift of the complex caused partial breakdown of the garnet by reaction with clinopyroxene to produce orthopyroxene +plagioclase ±spinel±amphibole symplectites, at an estimatedP-T of 10–14 kb and 800°–900° C. Garnet stability is shown to depend on both whole-rock Fe/Mg ratios and onP-T conditions. The pressures imply crustal thicknesses in the Archaean of least 35–45 km.     

On the theory of the East African Low Level Jet Stream

Several theoretical models for the East African Low Level Jet Stream are described. They all share the notion that the northward advection of planetary vorticity across the equator, coupled with the presence of a north-south mountain barrier, leads to the formation of a low-level western boundary current (akin to the Gulf-Stream) along the equatorial east coast of Africa. They differ in the manner in which the planetary vorticity advection is balanced to obtain a quasi-steady state. A purely inertial model predicts the correct cross-stream scale of the jet, but does not reproduce the observed inner shear layer which reduces the jet velocity to zero inland near the highlands. The lateral friction model can produce a realistic jet profile if the horizontal eddy viscosity (appearing as a free parameter) is chosen appropriately. However this solution shows a recirculation, i.e., northerly flow, off the coast that has not yet been observed. Finally, a model that includes bottom friction over variable topography also can give realistic jet profiles. If one accepts that the mountains, the Beta effect, and some form of inertial or frictional acceleration act together to produce the cross-equatorial low level jet stream, then one can formulate the types of observations needed to distinguish between the various theories.     

The geochemistry and evolution of early precambrian mantle

Seven high-purity cumulate clinopyroxenes from 2.7 b.y. maficultramafic rock associations from the Abitibi belt, Superior Province, Canada, have been analyzed for major elements and K, Rb, Cs, Ba, Sr and ⁸⁷Sr/⁸⁶Sr ratio. Attempts to reconstruct the trace element patterns of the original parent magmas were partially successful; Sr contents (140 ppm), K/Rb (470) and K/Ba (16) ratios are similar to those of modern low-K island arc tholeiites. K/Cs ratios (2700) are significantly lower than island arc tholeiites (17,000) or oceanic island and oceanic ridge basalts (> 30,000); the presentday mantle seems to be more depleted in Cs than in Archean times. Initial Sr isotope ratios of the 7 Archean clinopyroxenes average 0.70114±13(2σ) with relatively little variation; this value is in good agreement with initial ratios published for felsic and mafic rocks of the same age, though the latter show much larger variations and uncertainties. The pyroxene Sr isotope data, in conjunction with data for rocks of other ages, defines the following simple model for mantle evolution:

1. starting with primordial Sr, a short period of relatively rapid ⁸⁷Sr/⁸⁶Sr growth, followed by Rb depletion;
2. a period between ≧ 3.5 b.y. and ～ 1.7 b.y. when closed-system Sr isotope evolution occurred (with Rb/Sr ～ 0.023);
3. development of large-scale Rb/Sr heterogeneities in the mantle at ～ 1.7 b.y., leading to a present-day mantle with ⁸⁷Sr/⁸⁶Sr ranging from 0.7023 to 0.7065 and Rb/Sr ranging from ～ 0 to 0.065.

     

Local contributions to the M2 lunar geomagnetic variations in the South West of England

By comparing the M₂ lunar geomagnetic variations of 12.42 h period at Sidmouth and Exeter, it is shown that local concentrations of the electric currents, induced by sea tides in the seas and oceans around the U.K., can give a significant contribution to the M₂ variations at the two observatories.     

Extinct I129 in C3 chondrites

Eight C3 chondrites were examined by the I¹²⁹Xe¹²⁹ dating method, to see whether their IXe “ages” (better, initial $\text{I}{}^{129}\text{I}{}^{127}$ratios ≡ R₀) correlate with any other properties. The R₀'s range from 1.60 × 10^?4 to 1.09 × 10^?4, corresponding to IXe ages from 2.0 Myr before to 6.7 Myr after Murchison magnetite. Three C3O's (Lancé, Felix, Ornans) have essentially indistinguishable R₀'s of (1.41 ± 0.13) to (1.17 ± 0.10) × 10^?4; the fourth C3O, Warrenton, is undatable owing to homogenization of radiogenic and trapped Xe.Four C3V's show a distinct spread: Vigarano and Grosnaja are highest [R₀ = (1.60 ± 0.07) and (1.57 ± 0.14) × 10^?4], Mokoia is intermediate, and Kaba is lowest [R₀ = (1.38 ± 0.06) and (1.09 ± 0.10) × 10^?4]. Literature values for Allende place it near Kaba. These R₀'s correlate inversely with 4 other properties: I-, Br-, and Cd-content, and olivine composition, both percent mean deviation (PMD) and proportion of iron-poor olivine grains (≤2% fayalite).It is difficult to accept the ～9 Myr spread in R₀ as a true age, reflecting either nebular or parent-body processes. This time span is more than an order of magnitude longer than the lifetime of the solar nebula inferred from astronomical evidence. Nor does the degree of thermal metamorphism, which is slight for C3's anyway, correlate with R₀. A more plausible interpretation is that the variations in R₀ reflect mainly isotopic heterogeneity of iodine. The simplest model that accounts for the correlations with R₀ involves mixing of two iodine components in the solar nebula, associated with gas and grains, respectively. The second, of lower $\text{I}{}^{129}\text{I}{}^{127}$ ratio, predominated at later times and thus became enriched in late-formed meteorites, along with other volatiles such as Cd and Br. The low Fe content and large PMD of olivine may reflect either less metamorphism owing to shallow location in the parent body, or greater reduction of Fe²⁺ during chondrule formation.     

Nickel partitioning between olivine and silicate melt

Partitioning of Ni between olivine and silicate melt has been determined for compositions in the system Fo-Ab-An (1 atm) for temperatures ranging from 1250°C to 1450°C. Nickel concentrations were determined by electron microprobe; concentration levels in the liquids ranged from 0.1% to 0.5%. Platinum capsules or Pt wire loops were used as containers. Equilibrium was evaluated from kinetic considerations and by variation of run parameters; it was documented in one case by a bracketed reversal. No evidence was found for a dependence of the partition coefficient D (Ni in olivine/Ni in liquid) on Ni concentration. D is strongly dependent on melt composition, varying linearly with (1/MgO) at constant temperature. The intrinsic temperature dependence of D is small; the apparent temperature dependence reported in previous studies is largely related to the variation of melt composition with temperature. Our D values determined in the simple system Fo-An-Ab agree well with those reported by Leeman for natural (Fe-bearing) basalt systems. Overall variation of D in our system (and in natural basalts) can be expressed by the regression: D = (124/MgO) ? 0.9Our data are used to evaluate published Ni-MgO relationships in natural basalt series from Kilauea, Crozet, Cape Verde and Baffin Bay. A combination of olivine accumulation and fractional crystallization processes are sufficient to model these series. Using our data, unique “parental” liquids can be specified for each of these series; the MgO content of these liquids varies from 6% to 13%. Basalts with MgO contents greater than these “parental” liquids must be accumulative. The linear Ni-MgO trends, high absolute Ni concentrations, and large spread of Ni contents for the high-MgO basalts argue convincingly against their being “primary” liquids. Models such as those of O'Hara [6,13] and Clarke [24], based on the assertion of primary high-MgO liquids, must therefore be re-evaluated.Because of the high Si/O ratio and low MgO content of island arc andesites, the Ni partition coefficient D may be quite high. Therefore, the relatively low Ni content of such andesites may not be an argument against their derivation as direct partial melts of the mantle.