OXYGEN AND ARGON ISOTOPE RELATIONSHIPS IN SAUDI ARABIAN MANTLE XENOLITHS

A.I. Buikin¹, A.V. Ignatiev² and T.A. Velivetskaya²

¹ Vernadsky Institute for Geochemistry and Analytical Chemistry RAS, Moscow, Russia (buikin@geokhi.ru)

² Far East Geological Institute RAS, Vladivostok, Russia

Mantle-derived xenoliths transported to the Earth’s surface by ascending magmas preserve textural, chemical and isotopic informations about the physical and chemical evolution of their lithospheric source regions. In particular, the chemical and isotopic signatures of xenolith phases (minerals, glass, fluid inclusions) may help to constrain the nature and age of metasomatizing events. One of the most interesting and suitable objects in this respect is the subcontinental mantle beneath the Saudi-Arabian rift shoulder, which was subjected to intense metasomatism affecting mineral assemblages and also inducing cryptic metasomatic alterations that can be recognised in trace element patterns (Brueckner et al., 1988; Henjes-Kunst et al., 1990; Kurat et al. 1993; Blusztajn et al., 1995; Baker et al., 1998, 2002), including noble gases (Trieloff et al., 1997; Hopp et al., 2004, 2007).

Spinel-peridotite xenoliths of Cr-diopside group from Cenozoic volcanic fields of Arabian peninsula have documented – in different degree – two stages of mantle metasomatism [Henjes-Kunst et al. 1987, 1990]. Early metasomatism-1 led to the formation intra-granular Cr-Al-spinel, intra-granular Cr-pargasit (<2 vol. %), and inter-granular Ba-phlogopite (sample SA84-63). The second metasomatic event (metasomatism-2) resulted in formation of inter-granular amphibole and injection of inter-granular melt now consisting of patches of glass and phenocrysts of olivine, clinopyroxene, amphibole and spinel (sample SA84-128/3). The study of argon isotope composition have shown the presence mantle argon component in xenolith SA84-63 (metasomatism-1), while the xenolith SA84-128/3 (severely subjected to metasomatism-2) is characterized by close to atmospheric argon isotope composition, which points to a strong atmospheric contamination. Argon, chlorine and potassium relationships indicate that the most feasible contaminating agent was saline water introduced into the mantle source or intermediate magma chamber (Buikin 2005; Buikin et al. submitted).

In order to check the assumption about subsurface saline water introducing into the source of metasomatism-2 as well as to study the variations of δ¹⁸О values during mantle metasomatism we analyzed oxygen isotope composition in differently metasomatized mantle xenoliths from Saudi Arabian volcanic fields. Mineral separates of olivine, clino- and orthopyroxene and amphibole were picked up using a binocular. Oxygen was extracted by laser fluorination and isotope composition was determined on MAT-252 MS in FEGI RAS.

All the samples have δ¹⁸О characteristic of mantle minerals. One xenolith (SA84-38) has shown inter-mineral oxygen isotope equilibrium yielding an enstatit-forsterit isotope geothermometer temperature of 1030 ºС. Three other xenoliths have shown inter-mineral oxygen isotope disequilibrium. In these samples we observed a tendency to decreasing of δ¹⁸О values in olivines from xenoliths unaffected by metasomatism-2 to the xenolith most severely modified by this late metasomatic event. Similar tendency was observed in metasomatically overprinted xenoliths from Caapvaal Craton (Zhang et al. 2000). Moreover, as shown in figure 1, there is a correlation between argon and oxygen isotope composition in our unequilibrated samples: with decreasing (⁴⁰Ar/³⁶Ar)_max ratios from less contaminated by atmospheric argon xenolith (SA84-63) to the most contaminated sample (SA84-128/3), δ¹⁸О of olivines from these xenoliths is also dropping down. If change of oxygen isotope composition (as well as argon) occurred during interaction with agents of metasomatism-2, then decreasing of δ¹⁸О values should point to the lower δ¹⁸О of metasomatic fluid (or melt) in comparison to the rock. In this case our data at least not contradict to the earlier assumption of introducing a saline (i.e. sea or altered sea water) to the source of metasomatism-2. One should note that oxygen isotope composition of pyroxenes from unequilibrated xenoliths almost not vary, although there is some decrease of δ¹⁸О in Cpx from SA84-128/3.

One hornblende megacryst (SA84-42b) also suggests significant influence of a low ⁴⁰Ar/³⁶Ar component at the late stage of magmatic evolution. Its K, Cl, Ar inventory and argon isotope composition also support the idea of the presence of saline water as contaminating factor. We analyzed hydrogen isotope composition in this sample: δD = -54‰. This value is higher than typical  mantle one (~ -80‰), and could support our assumption about introducing of subsurface water into the source of late metasomatism.

The study was supported by the Program of the Department of Earth Sciences RAS №8 and Grant of President of Russian Federation for Young Scientists №МК-3013.2007.5.