Consistent olivine Mg# in cratonic mantle reflects Archean mantle melting to the exhaustion of orthopyroxene

Abstract

Shallow (garnet-free), depleted cratonic mantle, occurring as xenoliths in kimberlites and alkaline basaltic lavas, has a high Mg# (100 × Mg/(Mg + Fe) > 92) and is poor in Al and Ca compared to off-cratonic mantle. Here we compile data for many suites of shallow cratonic mantle xenoliths worldwide, and demonstrate a remarkably small range in their olivine Mg#, with an average of ~92.8. Via comparison with data for experimental melting of mantle peridotite compositions, we explain consistent olivine Mg# as the result of mantle melting and melt extraction to the point of orthopyroxene exhaustion, leaving a nearly monomineralic olivine residue.

Widespread refertilization of cratonic and circum-cratonic lithospheric mantle

“…Archean SCLM is distinctive in containing significant proportions of refractory harzburgites with minor lherzolites (Herzberg, 2004) and highly refractory dunites (Bernstein et al, 2006(Bernstein et al, , 2007, marking the most significant difference between Archean SCLM and that beneath younger terranes (Griffin et al, 1999a). The compositional variations significantly contribute to the difference in the density of SCLM of different ages.…”

Trace-element abundances in the shallow lithospheric mantle of the North Atlantic Craton margin: Implications for melting and metasomatism beneath Northern Scotland

“…The western part of the NAC in Greenland hosts abundant late Proterozoic ultramafic lamprophyre dykes and sills and kimberlites, which contain mantle xenoliths (c.f., Bizzarro and Stevenson, 2003;Tappe et al, 2006;Bernstein et al, 2007;Tappe et al, 2007;Wittig et al, 2010 and references therein). Xenolith/xenocryst suites comprise (variably) serpentinized ultra-refractory dunites, harzburgites, eclogites, wehrlites and minor lherzolites (e.g.…”

Dunite xenoliths and olivine xenocrysts in gabbro from Taihang Mountains: Characteristics of Mesozoic lithospheric mantle in Central China

“…Dunites may form via three different processes: accumulation from mafic magma (Zhang et al, 2005), a high degree of melting of the mantle (Bernstein et al, 2007), or melt-peridotite interaction (Garrido et al, 2007;Kelemen et al, 1998;Kelemen, 1990;Quick, 1981).…”

“…In the second case, olivine from dunite xenoliths is expected to have a higher Mg # than olivine from cratonic harzburgite and lherzolite xenoliths, which are the products of a low degree of melting and that yield a melt-depletion trend in terms of major element composition (Bernstein et al, 2007). The Mg # of olivine in the Xi'anli dunite xenoliths (average Mg # =90) is lower than that of Archean and Paleoproterozoic harzburgite and lherzolite xenoliths (average Mg # =92.3-92.5) from the Fushan intrusion (central NCC) and Tietonggou intrusion (eastern NCC) (Xu W L et al, , 2008a.…”