Composition of minerals and rocks from the Markov Deep, Central Atlantic

The paper presents materials on composition and texture of weakly serpentinized ultrabasic rocks from the western and eastern walls of the Markov Deep (5°30.6'-5°32.4'N) in the rift valley of the Mid-Atlantic Ridge. Predominant harzburgites with protogranular and porphyroclastic textures contain two major generations of minerals: the first generation composes the bulk of rocks and consists of Ol_89.8-90.4 + En_90.2-90.8 + Di_91.8 + Chr (Cr#32.3-36.6, Mg#67.2-70.0), while the second generation composes very thin branching veinlets and consists of PlAn_32-47 + Ol_74.3-77.1 + Opx_55.7-71.9 + Cpx_67.5 + Amph_53.7-74.2 + Ilm. Syndeformational olivine neoblasts in recrystallization zones are highly magnesian. Concentrations and covariations of major elements in harzburgites indicate that these rocks are depleted in mantle residues (high Mg# of minerals and whole-rock samples and low in CaO, Al2O3, and TiO2) that are significantly enriched in trace HFSE and REE (Zr, Hf, Y, LREE, and all REE). Mineralogy and geochemistry of harzburgites were formed by interaction of mantle residues with hydrous, strongly fractionated melts that impregnated them. Mineral composition of veinlets in harzburgites and mineralogical-geochemical characteristics of related plagiogranites and gabbronorites suggest that these plagiogranites were produced by melt residuals after crystallization of gabbronorites. Modern characteristics of harzburgites were shaped by the following processes: (i) partial melting of mantle material simultaneously with its subsolidus deformations, (ii) brittle-plastic deformations associated with cataclastic flow and recrystallization, and (iii) melt percolation along zones of maximal stress relief and interaction of this melt with magnesian mantle residue.

Major and trace elements and minerals at DSDP Hole 79-544A

Fifty m of basement rocks underlying 185 m of Neogene and Mesozoic sediments were drilled seaward of the Mazagan Slope about 100 km west of Casablanca during Leg 79. These rocks are metagranites with mylonitic textures consisting dominantly of quartz, plagioclase, and potassium feldspar. Chemically, they are strongly peraluminous. This along with the absence of hornblende suggest that these rocks are similar to the S-type granites. Petrographic and chemical data suggest the possible existence of a former weathering surface on top of the Mazagan metagranite.

Lithofacies, chronostratigraphy, minerals and diagenetic silica facies at DSDP Leg 80 Holes

X-ray powder diffraction and optical and scanning-electron microscope analyses of sediment samples taken from four sites drilled in the Goban Spur area of the northeast Atlantic show variable diagenetic silicification of sediments at several stratigraphic horizons. The results are as follows: 1. The silicified sediments are middle Eocene at Site 548, Paleocene to lower Albian at Site 549, upper to lower Paleocene at Site 550, and lower Turanian at Site 551. 2. There are three types of these silicified sediments: nodular type in carbonate-rich host sediments, bedded type in clayey host sediments, and a type transitional between the other two. 3. Silica diagenesis is considered to progress as follows: dissolution of siliceous fossils; precipitation of opal CT in pore spaces and transformation of biogenic silica (opal A) to opal CT, development of opal CT cement; chalcedonic quartz precipitation in pore spaces and replacement of foraminiferal tests by chalcedonic quartz; and finally, transformation of opal CT to quartz, and cementation. But the strong influence of host-sediment types on diagenetic silica fades is recognized. Bedded-type silicified sediments in a clayey environment indicate a lower grade of silica diagenesis. Only very weak chalcedonic quartz formation is recognized, and there is no opal CT cementation, even in Lower Cretaceous bedded-type clayey silicified sediments. 4. The rf(101) spacing of opal CT shows two distinct trends of ordering or decrease with burial depth; one is a rapid change, in the case of nodular silicified sediments, and the other is a more gentle shift, found in bedded silicified sediments. 5. Diagenetic silica facies of the nodular type develop as irregular concentric zones around some nodule nuclei. Also, quartz-chert nodule formation occurs at rather shallower horizons, and is discordant with the trend of decreasing d(101) spacing in opal CT. 6. Silicified sediments at Site 551 are shallower than at the other sites. The diagenetic silica facies suggest the probable erosion of 300 m or more of sediment at this site. 7. The zeolites clinoptilolite and phillipsite were found in the sediment samples recovered on Leg 80. Clinoptilolite occurs from the shallower levels to the deepest horizons of diagenetically silicified zones, suggesting that clinoptilolite formation is related to diagenesis of biogenic silica. Phillipsite at Site 551 (Section 551-5-2) may originate from volcanogenie material.

Major, trace and rare earth elements and minerals at DSDP Legs 69-70 Holes

We obtained major and trace element data on 113 samples from basalts drilled during DSDP Legs 69 and 70 in the Costa Rica Rift area. The majority have major and trace element characteristics typical of ocean-ridge tholeiities. Most of the basalts are relatively MgO rich (MgO > 8 wt.%) and have Mg values (MgO/MgO + 0.85FeO x 100) of about 53, characteristics that clearly indicate that the various magmas underwent only a small amount of crystal fractionation before being erupted onto the seafloor. According to their normative mineralogies, the rocks are olivine tholeiites. A few samples plot close to the diopside-hypersthene join of the projected basalt tetrahedron. Except for basalts from two thin intervals in Hole 504B, which differ significantly from all the other basalts of the hole, practically no chemical downhole variation could be established. In the two exceptional intervals, both TiO2 and P2O5 contents are markedly enriched among the major oxides. The trace elements in these intervals are distinguished by relatively high contents of magmatophile elements and have flat to enriched chondrite-normalized distribution patterns of light rare earth elements (LREE). Most of the rocks outside these intervals are strongly depleted in large-ionlithophile (LIL) elements and LREE. We offer no satisfactory hypothesis for the origin of these basalts at this time. They might have originated within pockets of mantle materials that were more primitive than the LIL-element-depleted magmas that were the source of the other basalts. A significant change with depth in the type of alteration occurs in the 561 meters of basalt cored in Hole 504B. According to the behavior of such alteration-sensitive species as K2O, H2O-, CO2, S, Tl, and the iron oxidation ratio, the alteration is oxidative in the upper part and nonoxidative or even reducing in the lower part. The oxidative alteration may have resulted from low temperature basalt/seawater interaction, whereas hydrothermal solutions may be responsible for the nonoxidative alteration.

Grain entrainment and transport rates of magnetic minerals of coastal sediment samples from the Bay of Plenty, New Zealand

Heavy (magnetic & non-magnetic) minerals are found concentrated by natural processes in many fluvial, estuarine, coastal and shelf environments with a potential to form economic placer deposits. Understanding the processes of heavy mineral transport and enrichment is prerequisite to interpret sediment magnetic properties in terms of hydro- and sediment dynamics. In this study, we combine rock magnetic and sedimentological laboratory measurements with numerical 3D discrete element models to investigate differential grain entrainment and transport rates of magnetic minerals in a range of coastal environments (riverbed, mouth, estuary, beach and near-shore). We analyzed grain-size distributions of representative bulk samples and their magnetic mineral fractions to relate grain-size modes to respective transport modes (traction, saltation, suspension). Rock magnetic measurements showed that distribution shapes, population sizes and grain-size offsets of bulk and magnetic mineral fractions hold information on the transport conditions and enrichment process in each depositional environment. A downstream decrease in magnetite grain size and an increase in magnetite concentration was observed from riverine source to marine sink environments. Lower flow velocities permit differential settling of light and heavy mineral grains creating heavy mineral enriched zones in estuary settings, while lighter minerals are washed out further into the sea. Numerical model results showed that higher heavy mineral concentrations in the bed increased the erosion rate and enhancing heavy mineral enrichment. In beach environments where sediments contained light and heavy mineral grains of equivalent grain sizes, the bed was found to be more stable with negligible amount of erosion compared to other bed compositions. Heavy mineral transport rates calculated for four different bed compositions showed that increasing heavy mineral content in the bed decreased the transport rate. There is always a lag in transport between light and heavy minerals which increases with higher heavy mineral concentration in all tested bed compositions. The results of laboratory experiments were validated by numerical models and showed good agreement. We demonstrate that the presented approach bears the potential to investigate heavy mineral enrichment processes in a wide range of sedimentary settings.