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Castillo, PR, Macisaac C, Perry S, Veizer J.  2018.  Marine carbonates in the mantle source of oceanic basalts: Pb isotopic constraints. Scientific Reports. 8   10.1038/s41598-018-33178-4   AbstractWebsite

For almost fifty years, geochemists have been interpreting the clues from Pb isotopic ratios concerning mantle composition and evolution separately. The Pb isotopes of ocean island basalts (OIB) indicate that their mantle source is heterogeneous, most likely due to the presence of end-components derived from recycled crust and sediment. Some OIB have unusually high Pb-206/Pb-204 coming from one of the end-components with a long time-integrated high U-238/Pb-204 or mu (HIMU). Most OIB and many mid-ocean ridge basalts (MORB) also have high Pb-206/Pb-204, indicating a HIMU-like source. Moreover, measured Th-232/U-238 (kappa) for most MORB are lower than those deduced from their Pb-208/Pb-204 and Pb-206/Pb-204. Such high mu and low kappa features of oceanic basalts are inconsistent with the known geochemical behavior of U, Pb and Th and temporal evolution of the mantle; these have been respectively termed the 1st and 2nd Pb paradox. Here we show that subducted marine carbonates can be a source for HIMU and a solution to the Pb paradoxes. The results are consistent with the predictions of the marine carbonate recycling hypothesis that posits the Pb isotopes of oceanic basalts indicate a common origin and/or magma generation process.

Castillo, PR.  2016.  A proposed new approach and unified solution to old Pb paradoxes. Lithos. 252:32-40.   10.1016/j.lithos.2016.02.015   AbstractWebsite

One of the most remarkable features of many and, perhaps, all oceanic basalts is that their Pb isotopic ratios ((206)pb/(204)pb, Pb-207/Pb-204 and (208)pb/Pb-204) are too radiogenic to be coming from the undifferentiated mantle or bulk silicate Earth. This has created three major concerns in the behavior of U, Th and Pb in the Earth's mantle that have been termed the Pb paradoxes. These are the unexpectedly long time-integrated high U/Pb (1st paradox), long time-integrated low Th/U (2nd paradox) and constant Ce/Pb and Nb/U (3rd paradox) in the mantle sources of oceanic basalts. The origins of such unexpected ratios have been the object of intense studies that produced several highly significant, but generally individualized results during the last four decades. Detailed analysis of available data shows that the paradoxes are closely interrelated as they all pertain to the mantle and have many common characteristic features. Thus, the Pb paradoxes constitute a system of equations that must be solved all together as each solution must satisfy every equation in the system. For example, compositional data for the voluminous mid-ocean ridge basalts (MORE) show that the 1st and 2nd paradoxes exhibit a long time-integrated enrichment of U and the Th/U and Nb/Th ratios are also constant. A single solution to simultaneously explain the paradoxes in MORE is possible if recycled materials with variable enrichments in incompatible trace elements, particularly U and its daughter Pb* plus Nb, Ce, and Th are added to or mixed with the depleted upper mantle. Significantly, a similar binary mixing solution has been proposed for the Pb paradoxes in ocean island basalts. (c) 2016 Elsevier B.V. All rights reserved.

Hilton, DR, Gronvold K, Macpherson CG, Castillo PR.  1999.  Extreme He-3/He-4 ratios in northwest Iceland: constraining the common component in mantle plumes. Earth and Planetary Science Letters. 173:53-60.   10.1016/s0012-821x(99)00215-0   AbstractWebsite

Olivine and clinopyroxene phenocrysts contained in late Tertiary basalts from Selardalur, northwest Iceland, carry volatiles with the highest helium isotope ratio yet reported for any mantle plume. He-3/He-4 ratios measured on three different samples and extracted by stepped crushing in vacuo fall consistently similar to 37 R-A (R-A = air He-3/He-4) - significantly higher than previously reported values for Iceland or Loihi Seamount (see K.A. Farley, E. Neroda [Annu. Rev. Earth Planet. Sci. 26 (1998) 189-218]). The Sr, Nd and Ph isotopic composition of the same sample places it towards the center of the mantle tetrahedron of Hart et al. (S.R. Hart, E.H. Hauri, L.A. Oschmann, J.A. Whitehead [Science 256 (1992) 517-520]) - in exactly the region predicted for the common mantle endmember based on the convergence of a number of pseudo-linear arrays of ocean island basalts worldwide (E.H. Hauri, J.A. Whitehead, S.R. Hart [J. Geophys. Res. 99 (1994) 24275-24300]). This observation implies that Selardalur may represent the best estimate available to date of the He-Sr-Nd-Pb isotopic composition of the 5th mantle component common to many mantle plumes. (C) 1999 Elsevier Science B.V. All rights reserved.

Castillo, PR, Floyd PA, France-Lanord C.  1992.  Isotope geochemistry of Leg 129 basalts; implications for the origin of the widespread Cretaceous volcanic events in the Pacific. Proceedings of the Ocean Drilling Program, Scientific Results. 129:405-413., College Station, TX, United States (USA): Texas A & M University, Ocean Drilling Program, College Station, TX   10.2973/   AbstractWebsite
Castillo, PR, Carlson RW, Batiza R.  1991.  Origin of Nauru Basin Igneous Complex - SR, ND and PB Isotope and REE Constraints. Earth and Planetary Science Letters. 103:200-213.   10.1016/0012-821x(91)90161-a   AbstractWebsite

The tholeiitic basalts and microdolerites that comprise the Cretaceous igneous complex in the Nauru Basin in the western equatorial Pacific have moderate ranges in initial Sr-87/Sr-86 (0.70347-0.70356), initial Nd-147/N-144(0.51278-0.51287), and measured Pb-206/Pb-204 (18.52-19.15), Pb-207/Pb-204 (15.48-15.66) and Pb-208/Pb-204 (38.28-38.81). These isotopic ratios overlap with those of both oceanic island basalts (OIB) and South Atlantic and Indian mid-ocean ridge basalts (MORB). However, the petrography, mineralogy, and bulk rock chemistry of the igneous complex are more similar to MORB than to OIB. Also, the rare earth element contents of Nauru Basin igneous rocks are uniformly depleted in light elements (La/Sm(ch) < 1) indicative of a mantle source compositionally similar to that of MORB. These results suggest that the igneous complex is the top of the original ocean crust in the Nauru Basin, and that the notion that the crust must be 15 to 35 m.y. older based on simple extrapolation and identification of the M-sequence magnetic lineations [1,2] may be invalid because of a more complicated tectonic setting. The igneous complex most probably was extruded from an ocean ridge system located near the anomalously hot, volcanically active, and isotopically distinct region in the south central Pacific which has been in existence since approximately 120 Ma.

Castillo, P, Batiza R, Vanko D, Malavassi E, Barquero J, Fernandez E.  1988.  Anomalously young volcanoes on old hot-spot traces; I, Geology and petrology of Cocos Island; with Suppl. Data 88-18. Geological Society of America Bulletin. 100:1400-1414., Boulder, CO, United States (USA): Geological Society of America (GSA), Boulder, CO   10.1130/0016-7606(1988)100<1400:ayvooh>;2   AbstractWebsite

Cocos Island is the summit of a seamount on the aseismic Cocos Ridge, a proposed trace of the Galapagos hot spot. The island lies on a portion of the ridge that is middle Miocene in age, but K/Ar and paleomagnetic dates indicate that Cocos is only about 2 m.y. old. Cocos thus offers a rare opportunity for an on-land study of seamount volcanism superimposed on an early hot-spot volcanism.Cocos Island was built in three major stages that define three lithostratigraphic units: (1) shield-building, (2) explosive volcanism, and (3) post-explosive volcanism stages. All Cocos rocks belong to the typical oceanic island alkali basalt-to-trachyte series and have fairly homogeneous Sr ( (super 87) Sr/ (super 86) Sr = 0.70299-0.70308), Nd ( (super 143) Nd/ (super 144) Nd = 0.512952-0.513001), and Pb ( (super 206) Pb/ (super 204) Pb = 19.214-19.251; (super 207) Pb/ (super 204) Pb =15.553-15.596; (super 206) Pb/ (super 204) Pb = 38.899-39.036) isotopic ratios. The Cocos rock series was generated by fractional crystallization of olivine, clinopyroxene, plagioclase, ilmenite, and apatite from similar alkali basalt parental magmas. Sr, Nd, and Pb isotopic ratios indicate that Cocos and Galapagos volcanic rocks may have come from a common, although heterogeneous, mantle reservoir, and this implies that the young Cocos volcano is still a part of the Galapagos hot-spot signal.

Castillo, P.  1988.  The Dupal anomaly as a trace of the upwelling lower mantle. Nature (London). 336:667-670., London, United Kingdom (GBR): Macmillan Journals, London   10.1038/336667a0   AbstractWebsite

The Dupal anomaly records Sr and Pb isotopic anomalies in lavas from oceanic islands in the S Atlantic and Indian oceans. It is shown that the two Dupal anomaly maxima appear to be directly associated with two large-scale regions of low seismic velocity in the lower mantle, and are correlated with the loci of active hotspots. This suggests that large-scale structural features in the lower mantle produce geochemical characteristics on the surface, and may place constraints on the chemical structure of the mantle and on mantle convection.