University:
Widener University
Course:
CHEM-255 | Organic Chemistry I
Academic year:

2023
Views:

420

Pages:

12
Author:

Gawrychuao

187Os Enrichment in Some Plumes: Evidence for CoreMantle Interaction? Re, Os geochemistry… Re and Os are highly siderophile elements; ca. 99% of the Earth’s budget of Re and Os resides in the metallic core Re/Os and 187Os/188Os ratios of the bulk core similar to those of the bulk Earth and chondrites During partial melting/crystallization Re is partitioned preferentially into the melt while Os is retained by the residue Decay equations Æ 187Os + β− t1/2 =42 billion years 190Pt Æ 186Os + 4He t1/2 =666 billion years 187Os/188Os=(187Os/188Os) +187Re/188Os*(eλt-1) i 190Pt/188Os=(186Os/188Os) +190Pt/188Os*(eλt-1) i 187Re 187Os ¾ ¾ ¾ enrichment in OIBs OIBs have elevated 187Os/188Os ratios compared with Chondrites and MORBs, indicating a longterm increase in the Re/Os ratio Recycling of oceanic crust can only explain some OIBs with 187Os enrichment Incorporation of the outer core material into the plume -- An alternate hypothesis Keep in mind … There are several processes that can attribute to the enrichment (contaminated by aged oceanic crust..) But in this paper only discuss one potential process: chemical interaction between the outer core and lower mantle and plume derivation from CMB The crystallization trends in small asteroidal cores are analogs for the Earth Core Crystallization of inner Core Assuming inner core formed very early kRe=15, kOs=19 (187Os/188Os)i=0.096 (187Os/188Os)t=0.1288 Upper mantle: (187Os/188Os)t=0.1271 (187Re/188Os)i 0.1288=0.096 + 0.4224*(eλt-1) Re=0.263 ppm Os=3 ppm (1-l)=5.5% 0.4229 Increasing Re/Os ratio C C Re l Re 0 =l k Re −1 Re/Os=0.263/3=0.088 Re l Os l C C Re 0 Os 0 C = C ×l 0.119/1.08=0.1098 k Re − k Os ( λt ) 0.1372 = 0.096 + 0.5303 × e − 1 t = 4.56 × 109 λ = 1.64 ×10 −11 Crystallization of inner core increased Re/Os ratio (therefore 187Os/188Os ratio) and decreased Re, Os concentrations (but still hundreds times higher than lower mantle) The outer core vs the mantle Outer core: Re=119 ppb Os=1080 ppb Upper Mantle: Re=0.25 ppb Os=3.3 ppb Even if only 1% of outer core material is added into plume, Os concentration is dominated by the core component Crystallization of inner core also increased Pt/Os ratio (therefore 186Os/188Os ratio) Enrichment of 186Os and 187Os is expected if core material is involved in the plume Conclusions The ascent of plume from the core-mantle boundary is a possible mechanism for 187Os enrichment in some plumes Pt-Os system 186 188 ( Os/ Os) Pt-Os system can be used to distinguish core mantle interactions and mantle-crust mixing since Pt and Os are not as enriched as Re in the oceanic crust and most sediments relative to the mantle