The effects of serpentinisation and hydrothermal alteration on Mg isotopes in Mid-Atlantic Ridge peridotite

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Author(s): Wimpenny, Josh; Harvey, Jason; Yin, Qing-Zhu
Author Affiliation(s): Primary:
University of California, Davis, Department of Geology, Davis, CA, United States
University of Leeds, United Kingdom
Volume Title: Goldschmidt 2012 abstract volume
Source: Mineralogical Magazine, 76(6); Goldschmidt 2012, Montreal, QC, Canada, June 24-29, 2012. Publisher: Mineralogical Society, London, United Kingdom. ISSN: 0026-461X
Note: In English. 9 refs.
Summary: The magnesium isotope composition of the Earth's mantle, and bulk estimates for upper crustal rocks overlap with values obtained from chondritic meteorites, suggesting broad scale homogeneity of δ26Mg in bulk silicate Earth (∼ -0.2ppm), and that high temperature silicate differentiation does not affect Mg isotopic composition significantly. However, studies have shown that heterogeneities can be introduced during low temperature weathering processes. Secondary silicates such as shales can be significantly enriched in the heavy isotopes of Mg, so much so that this heavy isotopic signal has been recorded in primary crustal rocks such as granites. Consequently, Mg in seawater, which is sourced from continental weathering, has a δ 26Mg = -0.82 ± 0.01 that is significantly lighter than that of fresh mantle material and its derivatives. The aim of this study is to investigate seawater-mantle interaction, and to assess the consequences of peridotite alteration for the evolution of seawater composition. Abyssal peridotites recovered from Ocean Drilling Program Leg 209 display a wide range of bulk-rock compositions and have been demonstrated to have interacted with a number of low and high temperature fluids. We will investigate samples from two localities; Hole 1274a, north of the 15° 20' N Fracture Zone, has been variably serpentinized at low (c. 200 °C) temperatures. Hole 1268a, south of the 15° 20'N Fracture Zone, is located near to the Logatchev hydrothermal field and, in addition to extensive serpentinization, has also interacted with a higher temperature fluid (c.350 °C) with a low Mg/Si ratio resulting in abundant talc formation. There is a high abundance of serpentinized peridotite exposed at the seafloor at slow and ultra-slow spreading oceanic ridges. A significant proportion of Mg in seawater is removed at ridge settings, however studies have also shown that altered abyssal peridotites are relatively depleted in Mg suggesting their alteration could also be a source of Mg to the oceans. The results of isotopic analyses will help us understand whether the exchange of Mg between seawater and peridotite has the potential to influence the oceanic Mg composition, which at present has a δ26Mg value that is isotopically heavy relative to its continental source.
Year of Publication: 2012
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; Alkaline earth metals; Atlantic Ocean; Hydrothermal alteration; Igneous rocks; Isotope ratios; Isotopes; Leg 209; Magnesium; Metals; Metasomatism; Mg-26/Mg-24; Mid-Atlantic Ridge; North Atlantic; Ocean Drilling Program; Peridotites; Plutonic rocks; Serpentinization; Stable isotopes; Ultramafics
Coordinates: N144000 N154500 W0443000 W0470000
Record ID: 2014077638
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