Composition and metasomatic history of the upper mantle beneath South-Central Texas: evidence from spinel peridotite xenoliths, Knippa, Texas
Abstract
Mantle xenoliths from a ~85Ma nephelinite plug in the Knippa Quarry, Texas provide new insights into the thermal and compositional structure of the lithospheric mantle beneath the southcentral USA. The xenoliths range in size from 2-12 cm, and comprise spinel-bearing lherzolite, harzburgite and cpx-rich lherzolite with predominantly coarse, equigranular texture. The peridotites have mineral compositions with mantle-like Mg# (Mg/Mg+Fe) for olivine (89-91.6), orthopyroxene (89.3-92.3) and clinopyroxene (90.4-93.4). Cr# (Cr/(Cr+Al)) in spinel ranges from 0.14-0.21 for lherzolite and 0.25-0.36 for harzburgite. In terms of mineral chemistry and texture, Knippa xenoliths can be classified as Type I peridotites (Frey and Prinz, 1978, EPSL). Spinel Cr# (Hellebrand et al., 2001, Nature), shows that the lherzolites were formed by 5-9% melt depletion and the harzburgite formed by 11-14% melt extraction. That the Knippa xenoliths were derived from a, moderately depleted mantle source is also indicated by strongly positive ɛ(Nd) whole-rock value of +11.6±3.6. An errorchron age of 816± 220 Ma is calculated from whole-rock Sm/Nd isotopic data; this is most consistent with an interpretation that this age reflects cooling below the closure temperature of Sm/Nd re-equilibration a few hundred M.y. after the end of ~ 1.1 Grenville igneous and metamorphic events in the region. Calculated oxygen fugacities based on the equations of Ballhaus et al., 1991, Contrib to Mineral and Petrol, indicate equilibration of the xenoliths at (-2.6 to 0.61) log units with respect to Fayalite-Magnetite-Quartz (FMQ) buffer and there is no difference in ΔlogfO 2 values between fertile lherzolites and depleted harzburgites. Temperatures of equilibration range from 900 to 1016°C for TCa-opx (Brey and Kohler 1990, J Petrol ). Constrained by the mineral phase relations we infer that these xenoliths define a depth range of 40-70 km (top of the mantle lithosphere). Apart from that, enrichment and depletion in variety of REE and trace elements in the peridotites indicate that the melt extraction event was followed by different degrees of enrichment by a range of metasomatic media. Enrichment in fluid mobile element (La) and depletion in fluid immobile element (Nb) suggest that the sub-Knippa mantle interacted with fluids from subduction zone setting.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V51B1671R
- Keywords:
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- 1025 GEOCHEMISTRY / Composition of the mantle