Thompson, R. N. and Ottley, C. J. and Smith, P. M. and Pearson, D. G. and Dickin, A. P. and Morrison, M. A. and Leat, P. T. and Gibson, S. A. (2005) Source of the Quaternary Alkalic Basalts, Picrites and Basanites of the Potrillo Volcanic Field, New Mexico, USA: Lithosphere or Convecting Mantle? Journal of Petrology, 46 (8). pp. 1603-1643. ISSN 0022-3530 http://resolver.caltech.edu/CaltechAUTHORS:20110825-143658896
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The <80 ka basalts–basanites of the Potrillo Volcanic Field (PVF) form scattered scoria cones, lava flows and maars adjacent to the New Mexico–Mexico border. MgO ranges up to 12·5%; lavas with MgO < 10·7% have fractionated both olivine and clinopyroxene. Cumulate fragments are common in the lavas, as are subhedral megacrysts of aluminous clinopyroxene (with pleonaste inclusions) and kaersutitic amphibole. REE modelling indicates that these megacrysts could be in equilibrium with the PVF melts at ~1·6–1·7 GPa pressure. The lavas fall into two geochemical groups: the Main Series (85% of lavas) have major- and trace-element abundances and ratios closely resembling those of worldwide ocean-island alkali basalts and basanites (OIB); the Low-K Series (15%) differ principally by having relatively low K_2O and Rb contents. Otherwise, they are chemically indistinguishable from the Main Series lavas. Sr- and Nd-isotopic ratios in the two series are identical and vary by scarcely more than analytical error, averaging ^(87)Sr/^(86)Sr = 0·70308 (SD = 0·00004) and ^(143)Nd/^(144)Nd = 0·512952 (SD=0·000025). Such compositions would be expected if both series originated from the same mantle source, with Low-K melts generated when amphibole remained in the residuum. Three PVF lavas have very low Os contents (<14 ppt) and appear to have become contaminated by crustal Os. One Main Series picrite has 209 ppt Os and has a γ_(Os) value of +13·6, typical for OIB. This contrasts with published ^(187)Os/^(188)Os ratios for Kilbourne Hole peridotite mantle xenoliths, which give mostly negative γ_(Os) values and show that Proterozoic lithospheric mantle forms a thick Mechanical Boundary Layer (MBL) that extends to ~70 km depth beneath the PVF area. The calculated mean primary magma, in equilibrium with Fo_(89), has Na_2O and FeO contents that give a lherzolite decompression melting trajectory from 2·8 GPa (95 km depth) to 2·2 GPa (70 km depth). Inverse modelling of REE abundances in Main Series Mg-rich lavas is successful for a model invoking decompression melting of convecting sub-lithospheric lherzolite mantle (ϵNd = 6·4; T_p ~ 1400°C) between 90 and 70 km. Nevertheless, such a one-stage model cannot account for the genesis of the Low-K Series because amphibole would not be stable within convecting mantle at T_f ~ 1400°C. These magmas can only be accommodated by a three-stage model that envisages a Thermal Boundary Layer (TBL) freezing conductively onto the ~70 km base of the Proterozoic MBL during the ~20 Myr tectonomagmatic quiescence before PVF eruptions. As it grew, this was veined by hydrous small-fraction melts from below. The geologically recent arrival of hotter-than-ambient (T_p ~ 1400°C) convecting mantle beneath the Potrillo area re-melted the TBL and caused the magmatism.
|Additional Information:||© The Author 2005. Published by Oxford University Press. Received September 15, 2003. Accepted February 15, 2005. First published online: April 22, 2005. Supplementary data for this paper are available on Journal of Petrology online. The fieldwork and INAA were funded by NERC Research Grant GR3/5299. Agents of the US Border Patrol and Potrillo ranchers helped greatly with advice on access and other issues. The microprobe analyses were made at Manchester University, using the NERC/MEMF instruments. Mike Henderson enabled this and Dave Plant gave excellent technical support. The majority of XRF analyses were made at Birmingham University, with assistance from Graham Hendry. Our particular thanks go to Nick Marsh for undertaking some final XRF major-element analyses at Leicester University. Paul Asimow, Steve Foley, Vicky Hards, Dimitri Ionov, Dan McKenzie, Glen Milne, Geoff Nowell, Terry Plank, Dave Sales and Bill Seager, together with Glen Izett, Peter Lipman, Dallas Peck and Ren Thompson (USGS), gave us all manner of invaluable advice and assistance. The perceptive comments of Terry Plank, Peter Reiners, Julia Shaw, Marjorie Wilson and an anonymous referee improved the manuscript immensely.|
|Subject Keywords:||western USA; picrites; Sr-Nd-Os isotopes; petrogenetic modelling; thermal boundary layer thermal boundary layer|
|Official Citation:||R.N. THOMPSON, C.J. OTTLEY, P.M. SMITH, D. G. PEARSON, A.P. DICKIN, M.A. MORRISON, P.T. LEAT, and S.A. GIBSON Source of the Quaternary Alkalic Basalts, Picrites and Basanites of the Potrillo Volcanic Field, New Mexico, USA: Lithosphere or Convecting Mantle? J. Petrology (August 2005) 46(8): 1603-1643 doi:10.1093/petrology/egi028|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||25 Aug 2011 22:20|
|Last Modified:||25 Aug 2011 22:20|
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