Chemistry of the Lower Sheeted Dike Complex, Hole 504B (Leg 148): Influence of Magmatic Differentiation and Hydrothermal Alteration

Abstract

We present major-and trace-element data on the lower sheeted dike complex (SDC) drilled during ODP Leg 148 in Hole 504B. Similar to previously studied rocks from the upper sections of Hole 504B, the rocks recovered during Leg 148 are mildly to moderately evolved mid-ocean ridge basalts (MORB) ranging in their MgO concentrations from 7.8 to 10.0 wt%. The rocks exhibit the same sort of depletion as the lavas of Hole 504B, characterized by very low incompatible element abundances (e.g., TiO 2 = 0.54-1.03 wt%, Zr = 29-53 ppm, and Ce = 2.2-5.5 ppm). Chondrite-normalized La/Sm and Nb/Zr ratios are markedly lower than average N-MORB, suggesting a highly depleted mantle source. Fractional crystallization of plagioclase, olivine, ± clinopyroxene can account for most of the chemical variability, whereby the degree of fractionation is less than 50%. The chemical effects of alteration are more difficult to distinguish because they can sometimes have the same consequences as magmatic differentiation. Formation of chlorite, amphibole, albite-oligoclase, and titanite replacing groundmass and primary minerals are typical for all samples. The results of this type of alteration are slight increases of MgO and H 2 O and decreases of CaO and SiO 2 . TiO 2 , MnO, Zr, and the REEs appear to slightly decrease with increasing extents of alteration. Halos and patches of intensive alteration are common in the lower SDC. The alteration halos show perplexing chemical trends with no clear relationship to the degree of alteration. The alteration patches are significantly enriched in H 2 O and highly depleted in Cu (Zn) and S. This make the patches in the lower SDC possible sources for base-metal deposits in the oceanic crust. The patches are also significantly depleted in REE, Zr, Nb, Ti, and exhibit positive Eu anomalies. Theoretical considerations and mass-balance calculations indicate that this behavior is probably not only the consequence of intensive alteration but requires primary chemical differences between the patches and the neighboring diabases. It remains unclear how far such differences could be related to magmatic differentiation processes within the dikes. The puzzling question of the nature of these alteration patches deserves further attention.

Lithology, mineralogy, and geochemistry of the lower ocean crust, ODP Hole 735B, Southwest Indian Ridge

“…Further details on analytical accuracy and precision of the data are given in Bach et al (1996) and Zuleger et al (1996Zuleger et al ( , 1995.…”

Pumpellyite from the oceanic crust, DSDP/ODP Hole 504B

“…Bulk rock compositions of Lithologic Unit 290 rocks have been analysed by XRF (Shipboard Scientific Party, 1993;Bach et al, 1996) and ICP-MS (Bach et al, 1996), and are listed in Table 1. The rocks of Lithologic Unit 290 are moderately evolved MORBs (MgO = 8.37-9.15 wt.%, Fe203* -8.77-10.7, Ni = 86 144 ppm, Cr = 265-332 ppm), and depleted in incompatible elements (e.g.…”

Methods

“…Certified international rock reference materials (see below) were used to test analytical accuracy and reproducibility of the obtained data. ROA-3, a pyroxenite from the Ronda ultramafic massif (Remaïdi, 1993), Sample 305-U1309D-171R-4, 18-30 cm, a gabbro sampled during Expedition 305, and BAS-140, a shipboard diabase standard (Sparks and Zuleger, 1995;Bach et al, 1996), were used as secondary standards during ultramafic rock, gabbro, and basalt analyses, respectively.…”