ALBERT

Description: The variations in major elements and isotope composition (87Sr/86Sr, delta18O, deltaD) of interstitial waters in Leg 104 sediments is most probably caused by the alteration of volcanic matter. A reaction scheme where volcanic glass reacts with pore-water magnesium and potassium to form trioctahedral smectite, phillipsite, and chert is proposed. Model calculations demonstrate that the pore waters may evolve their negative 6180 signatures without recourse to unreasonably large amounts of volcanic detritus or external sources.

Description: Forty-five volcanic ash horizons cored at Sites 642, 643, and 644 on the Vøring Plateau and ranging in age from early Miocene to Pleistocene, are discussed in terms of their magmatic features as well as their diagenetic evolution. Most of these layers, some centimeters thick, are mainly made of fresh rhyolitic glass. Twenty-five percent of the ash layers, however, contain variable amounts of more basic glass shards, ranging in composition from Mg-rich tholeiites to icelandites through Mg-poor basalts, ferrobasalts, and tholeiitic andesites, and are commonly associated with rhyolitic shards. Many chemically heterogeneous ash layers show bimodal acidic (rhyolites to icelandites) - basic (Mg-rich basalts to ferrobasalts) frequency distributions of the glass shards; intermediate compositions are not simple mixtures between acidic and basic endmembers. We suggest these ash layers result from the ejection of the upper (rhyolitic) to intermediate (ferrobasaltic) levels of density-stratified magma chambers intruded by ascending basaltic magmas, as exemplified by several Quaternary Icelandic explosive eruptions. The overall characteristics of the chemical trends of glass shards from heterogeneous ash layers are typically tholeiitic, with a strong increase of total iron and TiO2 at the level of intermediate compositions. Major and trace element data on bulk ash layers indicate that all the tephra of bimodal composition, as well as most of Neogene rhyolitic ash levels (low-K type), belong to LREE-enriched tholeiitic series. However, some Miocene rhyolitic ash levels (high-K type) show distinct geochemical features and are probably derived from other sources. The volcanic glass alteration patterns have no relationship with the ages of the deposits, and are different for acid and basic glasses. The most common alteration process leads to the formation of iron-beidellite-type smectites through loss of Si and alkalis from silicic glasses and loss of Si, Mg, Fe from basic glasses. Another kind of alteration, observed in lower Miocene ash layers, leads to glauconite formation through the development of iron-rich smectites. The frequency of ash layer occurrence with time indicates two apparent maxima of volcanic activity, an early one at the early-middle Miocene boundary (16 to 14 Ma) and another one in the late Miocene (8 to 7 Ma). Iceland is the most likely source for all the chemically heterogeneous ash layers as well as the low-K rhyolitic ash levels belonging to these two major episodes. Other subaerial sources, possibly at rifted margins (East Greenland, Jan Mayen Ridge), are required for the high-K rhyolitic ash horizons.

Description: Subaerially erupted tholeiites at Hole 642E were never exposed to the high-temperature seawater circulation and alteration conditions that are found at subaqueous ridges. Alteration of Site 642 rocks is therefore the product of the interaction of rocks and fluids at low temperatures. The alteration mineralogy can thus be used to provide information on the geochemical effects of low temperature circulation of seawater. Rubidium-strontium systematics of leached and unleached tholeiites and underlying, continentally-derived dacites reflect interactions with seawater in fractures and vesicular flow tops. The secondary mineral assemblage in the tholeiites consists mainly of smectite, accompanied in a few flows by the assemblage celadonite + calcite (+/- native Cu). Textural relationships suggest that smectites formed early and that celadonite + calcite, which are at least in part cogenetic, formed later than and partially at the expense of smectite. Smectite precipitation occurred under variable, but generally low, water/rock conditions. The smectites contain much lower concentrations of alkali elements than has been reported in seafloor basalts, and sequentially leached fractions of smectite contain Sr that has not achieved isotopic equilibrium. 87Sr/86Sr results of the leaching experiments suggest that Sr was mostly derived from seawater during early periods of smectite precipitation. The basalt-like 87Sr/86Sr of the most readily exchangeable fraction seems to suggest a late period of exposure to very low water /rock. Smectite formation may have primarily occurred in the interval between the nearly 58-Ma age given by the lower series dacites and the 54.5 +/- 0.2 Ma model age given by a celadonite from the top of the tholeiitic section. The 54.5 +/- 0.2 Ma Rb-Sr model age may be recording the timing of foundering of the Voring Plateau. Celadonites precipitated in flows below the top of the tholeiitic section define a Rb-Sr isochron with a slope corresponding to an age of 24.3 +/- 0.4 Ma. This isochron may be reflecting mixing effects due to long-term chemical interaction between seawater and basalts, in which case the age provides only a minimum for the timing of late alteration. Alternatively, inferrential arguments can be made that the 24.3 +/- 0.4 isochron age reflects the timing of the late Oligocene-early Miocene erosional event that affected the Norwegian-Greenland Sea. Correlation of 87Sr/86Sr and 1/Sr in calcites results in a two-component mixing model for late alteration products. One end-member of the mixing trend is Eocene or younger seawater. Strontium from the nonradiogenic endmember can not, however, have been derived directly from the basalts. Rather, the data suggest that Sr in the calcites is a mixture of Sr derived from seawater and from pre-existing smectites. For Site 642, the reaction involved can be generalized as smectite + seawater ++ celadonite + calcite. The geochemical effects of this reaction include net gains of K and CO2 by the secondary mineral assemblage. The gross similarity of the reactions involved in late, low-temperature alteration at Site 642 to those observed in other sea floor basalts suggests that the transfer of K and C02 to the crust during low-temperature seawater-ocean crust interactions may be significant in calculations of global fluxes.

Description: Native Cu occurs in amygdules, fractures and groundmass of tholeiites from Ocean Drilling Program Site 642 on the Vøring Plateau. Similar occurrences have been reported in other tholeiites of the early Tertiary North Atlantic Volcanic Province drilled at Deep Sea Drilling Project Sites 342 on the Vøring Plateau and 553 on the Rockall Plateau. The flows containing the native Cu have distinctive alteration patterns characterized by the combination of reddened flow tops, distinctive pastel coloration of the upper parts of the flows, relative abundance of celadonite, and the presence of native Cu. These associations suggest that subaerial weathering and subsequent seawater-basalt interaction are related to the occurrence of native Cu. An additional factor may be the increase in compatibility of Cu in silicates and Fe- Ti oxides that may accompany sub-solidus oxidation of basaltic flows. Native Cu occurrences in Site 642 tholeiites have some striking similarities to the large native Cu deposits in the Precambrian basalts of the Keweenaw Peninsula, Michigan, that are suggestive of similar mineralization processes.

Description: We have attempted to date several samples from Site 642 using a combination of Sr isotope stratigraphy and Rb-Sr dating of glauconite. A carbonate shell fragment from Sample 104-642B-22H-6, 70-73 cm gives a Sr isotope stratigraphy date of 17.3 +/- 1.0 Ma, which agrees well with available biostratigraphic and paleomagnetic data. Glauconites from a nearby sample (104-642B-23H-1, 66-69 cm) give a similar date. One carbonate shell fragment and two fish teeth samples from Core 104-642D-12X give concordant Sr isotope stratigraphy ages of about 37 Ma (latest Eocene). Rb-Sr glauconite analyses from one of the samples, while showing some substrate contamination, also support an Eocene age. Our results are in conflict with Miocene palynomorph dates from Core 104-642D-12X. As specific reworking of fish teeth and carbonate macrofossils (and also glauconite) from 37 Ma old sediments into three different samples in Core 104-642D-12X is most unlikely, we view the 37-Ma date as the depositional age of the core.