In-situ Iron isotope measurements and mineral compositions from ODP Hole 206-1256D and IODP Hole 335-U1256D

In-situ Fe isotope measurements have been carried out to estimate the impact of the hydrothermal metamorphic overprint on the Fe isotopic composition of Fe-Ti-oxides and Fe-sulfides of the different lithologies of the drilled rocks from IODP Hole 1256D (eastern equatorial Pacific; 15 Ma crust formed at the East Pacific Rise). Most igneous rocks normally have a very restricted range in their 56Fe/54Fe ratio. In contrast, Fe isotope compositions of hot fluids (> 300 °C) from mid-ocean-ridge spreading centers define a narrow range that is shifted to lower delta 56Fe values by 0.2 per mil - 0.5 per mil as compared to igneous rocks. Therefore, it is expected that mineral phases that contain large amounts of Fe are especially affected by the interaction with a fluid that fractionates Fe isotopes during exsolution/precipitation of those minerals. We have used a femtosecond UV-Laser ablation system to determine mineral 56Fe/54Fe ratios of selected samples with a precision of < 0.1 per mil (2 sigma level) at micrometer-scale. We have found significant variations of the delta 56Fe (IRMM-014) values in the minerals between different samples as well as within samples and mineral grains. The overall observed scale of delta 56Fe (magnetite) in 1256D rocks ranges from - 0.12 to + 0.64 per mil, and of delta 56Fe (ilmenite) from - 0.77 to + 0.01 per mil. Pyrite in the lowermost sheeted dike section is clearly distinguishable from the other investigated lithological units, having positive delta 56Fe values between + 0.29 and + 0.56 per mil, whereas pyrite in the other samples has generally negative delta 56Fe values from - 1.10 to - 0.59 permil. One key observation is that the temperature dependent inter-mineral fractionations of Fe isotopes between magnetite and ilmenite are systematically shifted towards higher values when compared to theoretically expected values, while synthesized, well equilibrated magnetite-ilmenite pairs are compatible with the theoretical predictions. Theoretical considerations including beta-factors of different aqueous Fe-chlorides and Rayleigh-type fractionations in the presence of a hydrous, chlorine-bearing fluid can explain this observation. The disagreement between observed and theoretical equilibrium fractionation, the fact that magnetite, in contrast to ilmenite shows a slight downhole trend in the delta 56Fe values, and the observation of small scale heterogeneities within single mineral grains imply that a general re-equilibration of the magnetite-ilmenite pairs is overprinted by kinetic fractionation effects, caused by the interaction of magnetite/ilmenite with hydrothermal fluids penetrating the upper oceanic crust during cooling, or incomplete re-equilibration at low temperatures. Furthermore, the observation of significant small-scale variations in the 56Fe/54Fe ratios of single minerals in this study highlights the importance of high spatial-resolution-analyses of stable isotope ratios for further investigations.

Element abundances and Nd isotopic composition in boninite and tholeiite from DSDP Hole 60-458

Boninites are unusual high MgO-high SiO2 volcanic rocks found in several western Pacific island arcs. Their high Mg/(Mg + total Fe) (0.55-0.83) and compatible element contents (Ni = 70-450 ppm, Cr = 200-1800 ppm) indicate equilibration with mantle peridotite, but their low TiO2 contents (0.1-0.5%) indicate severe depletion of this source. K, Rb, Sr and Ba abundances in boninites are typical of primitive arc basalts, but ratios such as Ti/Zr and La/Yb are variable (Ti/Zr = 23-67, (La/Yb)e.f. = 0.6-4.7). Evidence for both enrichment and depletion of incompatible elements suggests that boninites are derived from refractory peridotite which has been metasomatically enriched in LREE, Zr, Sr, Ba and alkalis. Wide variations in 143Nd/144Nd (0.51262-0.51296) are correlated with La/Sm, Sm/Nd and Ti/Zr, which enables identification of components in the boninite source. Possible LREE depleted components have relative REE and Ti abundances like those in depleted peridotites and high 143Nd/144Nd ratios which reach MORB-like values. Possible LREE enriched components have relative REE abundances similar to those in metasomatized mantle peridotite nodules, and low 143Nd/144Nd ratios which indicate either sedimentary sources or mantle sources with recent to ancient LREE enrichment. Relative abundances of Ba and Sr in boninites decrease with increasing LREE enrichment and suggest a non-sedimentary source for the LREE enriched material. Enrichment in Ba, Sr and alkalis may result from a third component derived from subducted oceanic crust. Two models can account for the successive generation of boninites and arc tholeiites within a single area: 1) boninites can be derived from the peridotite residue of earlier arc tholeiite generation which is metasomatically enriched in LREE before boninite volcanism, or 2) arc tholeiites and boninites can be derived from a variably depleted peridotite source which has been pervasively enriched in LREE. Areas of fertile peridotite would yield tholeiites while refractory areas would yield boninites.

(Table 1) Physical properties of basalts from DSDP Holes 69-504B and 70-504B

A detailed study of physical properties was made on core samples from Deep Sea Drilling Project Hole 504B. The measured properties are density, porosity, sonic velocity, electrical resistivity, and fluid permeability. Basalts from this young oceanic crust have higher density and sonic velocity than the average DSDP basalts. Porosity (and temperature) dependences of physical properties are given by V = Vo - a-phi; roo = roo-0 exp(E*/RT)phi**-q; k = k0' phi**2q-1; where V is the sonic velocity (km/s), Vo = 6.45 (km/s), a = 0.111 (km/s %), phi is the porosity (%), roo is the electrical resistivity (ohm m), roo-0 = 0.002 (ohm m), E* = 2.7 (Kcal/mol) for fresh basalts, RT has its usual meaning, q = 1.67 ± 0.27, k is the permeability, k0' = (1 to about 10) x 10**-12 (cm**2). Porosity distribution in the crust in this area is estimated by combining the seismic velocity distribution and velocity-porosity relation. Because of the rapid decrease in porosity with depth, resistivity increases and permeability decreases rapidly with depth. The decreasing rate of permeability with increasing depth is approximately given by k(cm**2) = 2 x 10**-10 exp(-z (km)/0.3).

(Table 2) Chemical composition of quenched glasses from the Mohns and Knipovich Ridges

The aim of this project was a petrogeochemical study of igneous rocks in the areas of the Mohns and Knipovich Ridges, both being the northern extensions of the Mid-Atlantic Ridge (MAR), using data available for quenching glass samples collected during Cruises 36 and 38 of R/V Akademic Mstislav Keldysh and during Cruise 15 of R/V Professor Logachev. Results of igneous rock studying from the Mohns and Knipovich Ridges at the background of evolution of the total North Atlantic Province, which had been identified earlier from tectonic and geophysical data, showed that igneous rocks of the Knipovich Ridge can be ranked as shallow tholeiites, primary melts of which were relatively rich in Na and Si and poor in Fe. This type of magma is characteristic of colder regions of the oceanic lithosphere. Its occurrence in the Knipovich Ridge and its potential propagation up to the Gakkel Ridge suggest that igneous rocks of this region originated under conditions of passive spreading in contrast to the MAR region in vicinity of Iceland and Azores, where substantial contribution of hotter material of a rising plume contributed to formation of the oceanic crust. The North Atlantic Ocean is the youngest province in terms of ocean-floor opening. Geologically and geophysically it is one of well studied regions of the World Ocean. Nevertheless some basic key items of its origin still remain to be clarified. In 1975 Scatler et al. proved specifics of this region manifested in growth of the gravity field, and also in relative height of the ocean floor in the region of 33-70°N, which was associated by them with rise of the hotter mantle, as compared with common regions of the Mid-Atlantic Ridge. Later this view was confirmed by character of magmatism, which differed in depth of generation and by melting degree of the resulting primary magma. Uniqueness of the North Atlantic region was also proved by the fact that this region was marked by extensive geochemical anomalies associated with Azores, Iceland, and Jan Mayen. All of these data allow to consider the northern part of the MAR (north of 33°N) as an united global geotectonic province. The Mohns and Knipovich Ridges located north of Iceland locate at the northern end of this province. This is the least known region. Therefore, new data for ridge areas of 73-77°N are needed for more complete geologic history of the Arctic Basin. The aim of this study was to carry out a complex comparison of magmatism at the Mohns and Knipovich Ridges with magmatism at large segments of the MAR northern province and to reconstruct mechanisms of primary magma formation, as well as conditions of their fractionation. This paper was based on results of studying quenched glasses, which reflect evolution of melt in the course of its formation.