An alternative new approach to the old Pb paradoxes P. R. Castillo Scripps Institution of...
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Transcript of An alternative new approach to the old Pb paradoxes P. R. Castillo Scripps Institution of...
An alternative new approach to the old Pb paradoxes
P. R. CastilloScripps Institution of OceanographyUniversity of California, San Diego
La Jolla, CA 92093-0212U.S.A.
Gold2015:abs:1251
Oceanic basalts have radiogenic Pb isotopic ratios
Increase in Pb isotopes a function of: 238U 206Pb 235U 207Pb 232Th 208Pb
Thus, major concerns on the concentrations of U, Th and Pb in the mantle
Allegre (2008)
The Pb paradoxes
1st : long time-integrated high U/Pb
2nd : long time-integrated low Th/U
3rd : constant (’canonical’) Ce/Pb & Nb/U
The Pb paradoxes
1st : long time-integrated high U/Pb
2nd : long time-integrated low Th/U
3rd : constant (’canonical’) Ce/Pb & Nb/U
Proposed significant solutions (~40 yrs):
- lose Pb o into core - Allegre et al. (1982)
o into cont. lithosphere/crust – Zartman & Haines (1988) Chauvel et al. (1992)
o into sulfide – Hart & Gaetani (2006)
o from early depleted reservoir (EDR)– Jackson et al. (2010)
- increase U relative to ThTatsumoto (1978); Galer and O’Nions (1985); Elliot et al.
(1999)
- two major ways:
o mantle re-homogenization – Hofmann et al. (1986)
o changing Kd’s for Ce or Pb – Simms & DePaolo (1997) Hart & Gaetani (2006)
2nd Pb paradox• Conventional approach – Th/U (or k = 232Th/238U) lower than BSE
2nd Pb paradox• Conventional approach – Th/U (or k = 232Th/238U) lower than BSE
e.g.,Tatsumoto (1978) Galer and O’Nions (1985) Elliot et al. (1999)
• But it can also be expressed as - U/Th ( or 1/ – k non-conventional) higher than BSE
i.e., long time-integrated
high U/Th
• Thus, 1st and 2nd paradoxes can be solved through long time-integrated U enrichment !
long time-integrated enrichment in U
1st : long time-integrated high U/Pb
2nd : long time-integrated low Th/U
3rd : constant (’canonical’) Ce/Pb & Nb/U
Important implications:
o simultaneous solution to 1st and 2nd paradoxes
o produces Pb* - hence, radiogenic Pb isotopes
o inconsistent with proposed solutions to 3rd paradox (conservation of mass !)
o for MORB at least, Th/U is ‘constant’
2nd Pb paradox
• Conventional approach – Th/U (or k = 232Th/238U) lower than BSE (= 3.88)
Th/U of MORB (at ~3.1)
“remarkably homogeneous”
(Elliot et al., 1999)
2nd Pb paradox
• Conventional approach – Th/U (or k = 232Th/238U) lower than BSE (= 3.88)
Th/U of MORB (at ~3.1) “remarkably homogeneous” (Elliot et al., 1999)
• Later studies -Th/U of (ALL) MORB
Arevalo & McDonough (2010) 2.87 +/- 1.35Jenner & O’Neil (2012) 3.16 +/- 0.60Gale et al. (2013) 3.16 +/- 0.11
• Thus, Th/U of MORB is also “constant”
• (Th/U of OIB is only between 3.16 and 3.88 !)
If Ce/Pb, Nb/U and Th/U constant (in MORB, at least)
(Ratio of constants is also constant)
• K1 = (Ce/Pb) / (Th/U)
= (U/Pb) * ( Ce/Th)
• K2 = (Ce/Pb) / (Nb/U)
= (U/Pb) * (Ce/Nb)
• K3 = (Th/U) / (Nb/U)
= (Th/Nb)
Trivial ? = Yes, but important because these also show close relationships among Pb paradoxes
More relevant question = why are Ce/Pb, Nb/U, Th/U, Th/Nb constant?
Castillo (submitted)
Basic principle – two component mixing in a binary element plot generates a line, y = mx + b
- Binary mixing line is special when b = 0, making y/x = m (= constant)
- in Ce vs. Nb plot (MORB – Gale et al.,
2013), mixing between enriched OIB and DMM generates a line with b ~ 0, hence Ce/Pb ‘constant’
Lucky ? – perhaps…..
OIB (Willbold & Stracke, 2010)
DMM (Workman & Hart, 2005)
Castillo (submitted)
Nb vs. U, Th vs. Nb (& Th vs. U) plots of MORB (Gale et al., 2013)
• 1) mixing OIB + DMM also generates binary mixing lines with b ~ 0 in Nb vs. U, Th vs. Nb (& U vs Th) plots
• Other methods: 2) by finding average ratios (b = 0)
3) Least-squares method (b ~0)
• All methods produce the ~same (+/- errors) constant (‘canonical’) ratios
OIB (Willbold & Stracke, 2010)
DMM (Workman & Hart, 2005)
OIB (Willbold & Stracke, 2010)
DMM (Workman & Hart, 2005)
Castillo (submitted)
Summary and conclusions
• The radiogenic Pb isotopes of oceanic basalts create the Pb paradoxes – many excellent solutions proposed, but mainly individualized
• Paradoxes are inter-related, comprising a “system of equations” that should be solved altogether or simultaneously as solution to each equation should also be consistent to solutions to other equations
• Systems of equations require linear or non-linear solutions. Pb paradoxes can be simply solved through linear, binary mixing solutions
Castillo (submitted)
A conceptual model
MORB: binary mixing
(enriched melt + DMM)
OIB: binary mixing
(end-members + FOZO)
Modified after Castillo (2015)Castillo (submitted)
subduction of a small amount of marine limestone (natural HIMU) is required
some limestone are being subducted and not being consumed by arc magmatism