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Noble Gases in Abyssal Peridotite, Gabbro and Basalt

Abyssal Peidotite Figure 2

Click image to enlarge. 3He/4He measured in peridotite, gabbro and basalt from 14°-16°N along the Mid-Atlantic Ridge. All gas extractions to date have been by in vacuo crushing. Bars show ±2 standard errors (for the basalt glasses the analytical uncertainties are smaller than the data points except where shown).

This project is addressing two main questions using 3He/4He, 40Ar/36Ar, 4He/40Ar* and CO2-He-Ar concentrations in oceanic peridotite, gabbro and basalt:

  • To what extent is the geochemical variability in basalt along ocean ridges due to small scale heterogeneity (e.g., veins) in the underlying mantle and to what extent is it due to regional (>100 km) and segment-scale (>10 km) heterogeneity?
  • To what extent is the geochemical variability of upper mantle peridotite and gabbro related to its history of melt extraction, and to what extent is it caused by retention of low degree partial melts? The study is focused on 3 sample suites: 1) peridotite and gabbro from 14°-16°N along the Mid-Atlantic Ridge, recovered during ODP Leg 209, plus basalt glasses from the adjacent ridge axis; 2) peridotite and gabbro from the Kane oceanic core complex adjacent to the MAR at 23.5°N; and 3) peridotite and basalt from the ultra-slow spreading Gakkel Ridge, Arctic Ocean.

Leg 209 of the Ocean Drilling Program drilled upper mantle peridotite and lower crustal gabbroic rocks from a series of sites located to the north and south of the large transform fault along the Mid-Atlantic Ridge (MAR) at ~15°20' N. Volcanic rocks are thin or absent in much of the region, making upper mantle and lower crustal rocks accessible for detailed structural and chemical analysis. Initial results show a generally good agreement between the basalt value at any location along the ridge axis and the value measured in nearby gabbro or ultramafic rock (see figure). A few peridotites and gabbros extend to lower 3He/4He, especially when the He content is low. The large uncertainty in the harzburgite analyses reflects the very low He content in the highly serpentinized rocks from Hole 1274A, where only two of the five samples analyzed so far had 3He above the mass spectrometer detection limit, despite their relatively large sample sizes (up to 1 gram of separated bastite). Notably, a single amphibole crystal at Site 1271 (brown amphibole gabbro) is very rich in [He] and has 3He/4He identical to the nearest basalt. Rocks and minerals with [He] > 10 nccSTP/g (plus a troctolite with [He]=5 ncc/g and 3He/4He=8 RA at Site 1275) show good agreement with the nearby basalts. Overall these initial data from the MAR suggest that any small scale heterogeneity due to veining (if it is present in the upper mantle of this region) does not strongly control the He isotope signal. More work is underway using different types of gas extraction, such as step heating/fusion.

Helium isotope in basalts from the Gakkel Ridge show the presence of a geochemical boundary in the mantle beneath the mid-section of the Gakkel Ridge, in agreement with earlier results for Pb, Nd and Sr isotopes. Fresh harzburgites from the Sparsely Magmatic Zone (SMZ) along the Gakkel Ridge have 3He/4He between 9 and 9.5 RA, overlapping with upper limit for basalts from nearby. Analyses of additional peridotites are currently underway, including samples having a range of mineral chemistry (fertile and depleted) and lithology (harzburgite and lherzolite).

References

Goldstein, S. L., G. Soffer, C. H. Langmuir, K. A. Lehnert,. D. W. Graham and P. J. Michael (2008) Origin of a "southern hemisphere" geochemical signature in the Arctic upper mantle. Nature 453, 89-94. PDF

Graham, D. W., J. E. Lupton, S. L. Goldstein, C. H. Langmuir and P. J. Michael (2009) Helium Isotope Variations in Basalts Along Gakkel Ridge and Heterogeneity of the Arctic Upper Mantle. EOS, abstract, Trans. AGU. PDF

Projects

Mid-Ocean Ridges and Upper Mantle Processes

Mantle Plumes and Geodynamics of Earth's Interior