We report mineral
chemistry, whole-rock major element compositions, and trace element analyses on
Hole 735B samples drilled and selected during Leg 176. We discuss these data,
together with Leg 176 shipboard data and Leg 118 sample data from the
literature, in terms of primary igneous petrogenesis. Despite mineral
compositional variation in a given sample, major constituent minerals in Hole
735B gabbroic rocks display good chemical equilibrium as shown by significant
correlations among Mg# (= Mg/[Mg + Fe2+]) of olivine, clinopyroxene,
and orthopyroxene and An (=Ca/[Ca + Na]) of plagioclase. This indicates that the
mineral assemblages olivine + plagioclase in troctolite, plagioclase +
clinopyroxene in gabbro, plagioclases + clinopyroxene + olivine in olivine
gabbro, and plagioclase + clinopyroxene + olivine + orthopyroxene in
gabbronorite, and so on, have all coprecipitated from their respective parental
melts. Fe-Ti oxides (ilmenite and titanomagnetite), which are ubiquitous in most
of these rocks, are not in chemical equilibrium with olivine, clinopyroxene, and
plagioclase, but precipitated later at lower temperatures. Disseminated oxides
in some samples may have precipitated from trapped Fe-Ti-rich melts. Oxides that
concentrate along shear bands/zones may mark zones of melt coalescence/transport
expelled from the cumulate sequence as a result of compaction or filter
pressing. Bulk Hole 735B is of cumulate composition. The most primitive olivine,
with Fo = 0.842, in Hole 735B suggests that the most primitive melt parental to
Hole 735B lithologies must have Mg#
0.637, which is significantly less than Mg# = 0.714 of bulk Hole 735B. This
suggests that a significant mass fraction of more evolved products is needed to
balance the high Mg# of the bulk hole. Calculations show that 25%-45% of average
Eastern Atlantis II Fracture Zone basalt is needed to combine with 55%-75% of
bulk Hole 735B rocks to give a melt of Mg#
0.637, parental to the most primitive Hole 735B cumulate. On the other hand, the
parental melt with Mg#
0.637 is far too evolved to be in equilibrium with residual mantle olivine of Fo
> 0.89. Therefore, a significant mass fraction of more primitive cumulate
(e.g., high Mg# dunite and troctolite) is yet to be sampled. This hidden
cumulate could well be deep in the lower crust or simply in the mantle section.
We favor the latter because of the thickened cold thermal boundary layer atop
the mantle beneath slow-spreading ridges, where cooling and crystallization of
ascending mantle melts is inevitable. These observations and data interpretation
require reconsideration of the popular concept of primary mantle melts and
relationships among the extent of mantle melting, melt production, and the
composition and thickness of igneous crust.
1Niu, Y., Gilmore, T., Mackie, S., Greig, A., and Bach, W., 2002. Mineral chemistry, whole-rock compositions, and petrogenesis of Leg 176 gabbros: data and discussion. In Natland, J.H., Dick, H.J.B., Miller, D.J., and Von Herzen, R.P. (Eds.), Proc. ODP, Sci. Results, 176 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/176_SR/chap_08/chap_08.htm>. [Cited YYYY-MM-DD]
2Department of Earth Sciences, The University of Queensland, Brisbane QLD 4072, Australia. Correspondence author: NiuY@Cardiff.ac.uk
3Present address: Department of Earth Sciences, Cardiff University, Cardiff CF10 3YE, United Kingdom.
4Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole MA 02543, USA.
Initial
receipt: 12 August 2000
Acceptance: 4 January 2002
Web publication: 30 April 2002
Ms 176SR-011