HR: 1330h
AN: V13B-01 [Abstracts]
TI: New Paradigms and Intractable Problems: A new Approach to Source Rock Characterization
AU: * Gargi, S P
EM: sgargi@yahoo.com
AF: Myers University, 112 Prospect Ave, Cleveland, OH 44115 United States
AB:
Problems relating to the negative correlation between Sr and Nd isotopic ratios of recent basalts, initial isotopic ratio for the bulk Earth, excess radiogenic lead and a few others could be resolved if it is believed that all radiogenic isotopes are fractionated with respect to the non-radiogenic isotopes. First, a new approach to the source rock characterization is
outlined here. It is based on the fact that there is a perfect correlation between Rb/Sr vs. ((Rb/Sr) /
(87Rb/86Sr)) (termed WAR). The line formed by this relation is named here MUOX, whose significance is that for a
rock of any age derived from an undepleted reservoir, its intercept at the Rb/Sr value of 0.031 remains constant, i.e.
0.34572097 (termed BEWRUB). In deriving the above relation, the following parameters were presumed: IBE 4.55 Ga ago:
0.69877; age of the Earth: 4.55 Ga; NBE: 0.09536; λ 87Rb: 1.408E-11/yr. The various symbols used in the
text denote the following: I is the 87Sr/86Sr ratio; BE is the bulk Earth; N is the 87Rb/86Sr ratio 4.55
Ga ago; R is the source rock; λ is the decay constant; and ME is the time of magma extraction. The other relations
that have been worked out are as follows: (1) The relation between wRb vs. dSr defines a slope (named RUH) of
29.697285 and an intercept of -0.0000000041; where wRb = BEWRUB - SOWRUB, and SOWRUB is the predicted value of the MUOX
Intercept at the Rb/Sr value of 0.031; and dSr = (IR(ME) - IBE(ME)). (2) The relation between MUOX Intercept
(MI) vs. IR(ME) defines a slope (termed PUCI) of -29.69176 and an intercept of 10.96976744. (3) The relation between the MUOX slope vs. the age of a rock (in Ma years) defines a slope (named Gargi) of -705465.433 and an intercept of 8.668. (4)
The relation between the age of a rock (in Ma years) vs. 1/E defines a slope (termed Raman) of -0.0000001265 and an intercept of 0.000579232; where E = P / WRb; P is (NR / NBE - 1) * (100); and WRb = (wRb / BEWRUB) * (100).
This relation can be used to find the NR of a source rock. The above relations when applied to the data from basaltic
achondrites (Papanasstassiou, D.A. et al. 1969) yield the following results: wRb: -0.00000275; IR(ME) (PUCI):
0.69897705; IR(ME) (RUH): 0.69897659; Age (Gargi): 4.34 Ga (4.39 Ga if λ of 1.39E-11/yr is used); NR
(Raman): 0.07097; Source Reservoir Depletion: 26%. The close match of these results with the actual data, strongly suggests
that the presumed parameters are valid. The apparently depleted nature of the achondrites is not because of their depleted
source reservoir, but because of the fractionation of 87Sr isotopes. It is likely that similar depletion in the early
basic melt fractions may have resulted in the residual mantle to be enriched. The reason that the negative correlation
between Sr and Nd isotopic ratios is not observed in older rocks, is because the production of new radiogenic isotopes
obliterates the effect of fractionation. To prove fractionation, other examples from the literature will be discussed.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1025 Composition of the mantle
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1099 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly