Seawater alteration effects on REE, K, Rb, Cs, Sr, U, Th, Pb and Sr-Nd-Pb isotope systematics of Mid-Ocean Ridge Basalt.

Abstract

A detailed isotopic and trace element acid-leaching study of "less-altered" and "more-altered" splits of a crystalline MORB ( 12 Ma) and a glassy ( 15 Ma) MORB from NE Pacific has been carried out by collecting two successive acid-leached fractions and residues left after leaching, and analyzing for Sr, Nd and Pb isotopes and related elements. During oxidation of the crystalline basalt, the inferred whole-rock isotope ratios show only a small increase for Sr, a very small insignificant decrease for Nd, but very large increase for Pb. Relative to "fresh" basaltic glass, the whole-rock isotope ratios for palagonitized glass show a very large increase for Sr, a very small decrease for Nd, and small increase for Pb.Most trace elements and their ratios in both types of samples and their splits also show very signifi cant changes related to seawater interaction. During oxidation of the crystalline MORB, the whole-rock Rb / Sr and U / Pb increase by -75% and -500%, whereas Sm / Nd, Th / Pb and Th / U decrease by -4%, --30% and --90% respectively. On the other hand during palagonitization of the basaltic glass, the whole-rock Rb/ Sr increases by 275%, whereas Th / Pb and Th/ U both decrease by -60%. For the first acid-leached fractions, all elements including the REE show high enrichment factors; the highest values are for U and Pb in the crystalline MORB. A viable mechanism to explain these data may be an elemental and isotopic exchange (under seawater-dominated conditions; modified seawater/rock ratios --10-400) between basalt and seawater that had interacted earlier with overlying sediments.

Genetic implications of new Sr and Nd isotopic data of the intrusive rocks from the Laramide Arc in Northern Sonora, Mexico.

“…The sample shows very low CaO (0.11%) and high K 2 O (6.4%) with relatively high loss on ignition (LOI-2.6%). The value of initial 87 Sr/ 86 Sr in sample 7 as well as of sample 3 are somewhat higher compared to other samples reported from the area (Damon et al, 1983;Housh and McDowell, 2005; see also Table 2), differences could be caused by hydrothermal alteration as it has been observed in oceanic basalts and in the volcanic geothermal field of Los Azufres (Verma, 1992;Verma et al, 2005). The isotopic values of Sr and Nd from La Caridad and Cananea are very similar suggesting the same origin for the magmas in both districts.…”

“…New isotopic data on hydrothermally mineralized rocks as for the Maria and La Caridad mines indicate that the isotopic compositions may change with respect to the fresh rock values, as it has been documented by other authors (Verma, 1992;Verma et al, 2005). This is important for future interpretations taking into account that many of the published isotopic data come from mineralized areas (Damon et al, 1983;SansoresBolivar and Wayne, 1977;Mead et al, 1988).…”

Elemental changes and alteration recorded by basaltic drill core samples recovered from in situ temperatures up to 345°C in the active, seawater‐recharged Reykjanes geothermal system, Iceland

“…The isotopic variations are useful for identifying mantle heterogeneities, but they cannot be used to distinguish the compositional protoliths (e.g., TEE or TED) of the altered rocks for the purpose of constraining the chemical exchange between the basalts and hydrothermal fluids, as discussed below. It has also been suggested that these isotope systems may be modified during seawater‐basalt interaction [ Verma , ].…”

“…Alteration of oceanic crust by seawater is a critical planetary‐scale geochemical process that affects seawater composition [ Hart , ; Elderfield et al ., ; Humphris and Thompson , ]. Drill core samples recovered by the Deep Sea Drilling Project (DSDP), Ocean Drilling Program (ODP), and Integrated Ocean Drilling Program (IODP), and sections of sheeted dikes tectonically uplifted and exposed on the East Pacific Rise at Hess Deep and Pito Deep have been essential for estimating the role of hydrothermal element fluxes between oceanic crust and the oceans in global geochemical cycles [ Staudigel et al ., , ; Alt et al ., ; Verma , ; Mottl and Wheat , ; Staudigel et al ., ; Gillis , ; Alt and Teagle , ; Hart et al ., ; Laverne et al ., ; Wheat et al ., ; Bach et al ., ; Teagle et al ., ; Heft et al ., ; Alt et al ., ; Dekkers et al ., ].…”

Alteration at the ultramafic‐hosted Logatchev hydrothermal field: Constraints from trace element and Sr‐O isotope data

“…Similarly, low temperature alteration is also reflected by the addition of U and Pb to the LHF samples. Such increased U and Pb concentrations are known from oxidatively altered rocks of the ocean crust penetrated by ODP drill hole 504B [ Bach et al , 2003] and seawater‐altered basalts [ Verma , 1992].…”