、Low㊧rMantleMinemlAssociationsinDiamonds
fromS50Luiz，BraziI（受入教官1桂智男〉

B．HARTE1，J◎W．HARRls2，M．T．HuTcHlsoN皇，G．R．WATT㌦ND

M．C．WILD：NG肛、

豊De醐ment・fGe・1・gyan4Ge・physics，bniversity・fEdiぬburgh，King

Edinburgh EH93JW．
2DMsi。n。fEarεhSciences，Universiεy。fGlasg。w，LilybankGardピns，
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Abstract‑‑A suite of diamonds from the S o Luiz, alluvial deposit, Brazil, shows an unprecedentedabundd; nce of (Mg,Pe)O (ferropericlase‑magnesiowustite ‑ fPer) inclusions, which under normal

circumstances are most likely to be derived from the lower mantle. Furthermore, occasionai

coiourless inciusions in the same diamonds have compositions of (Mg,Fe)Si03 and CaSi03,
corresponding to those of the phases MgSi‑perovskite (MgSiPvk) and CaSi‑perovskite (CaSiPvk),
which are also expectegl to occur in the common peridotitic compositions of the lower mantle.

Aluminous phases in the same suite of diamonds inclLtde: ( l) a phase (TAPP) of garnet composition,(Mg,Fe)Al2SiJOla but with a distinct tetragonal crystal structure; ('‑) an Al‑rich MgSiPvk; (3) an Al203

phase.

Unlike the occasional and always magnesian fPer incl,usions reported in diamonds from otherlocalities, the Sao Luiz fPer have an extremely wide range of composition, with Pe' +/(Fe' ++hdg)

ranging from 0.14 to 0.62. In some of these fPer inclusions very fine scale exsoiution of amagnesiofe ite spinel phase has been detected. The CaSi03 are exceptionaily restricted in major‑minor element compositions, and are close to pure CaSi03. MgSiPvk inclusions have Al203 ranging
from 1.2 to 2.7wt%, except in one case where it is l0.0%. The Fe!+/(Fe2++Mg) of MgSiPvk ranges
from 0.02 to 0.05, so that Fe + is relatively distributed in favour 6f fPer as expected from experimental

work. However, Mossbauer spectroscopic determinations show MgSiPvk and TAPP to havesignificantly higher proportions of Fe3+ than. fPer, especially in the case of the high‑AI MgSiPvk andTAPP. TAPP grains have Pe2+/(Fe2*+Mg) of 0,03 to 0.06, with close to 3 Si cations in a 12 oxygen
formula unit, and thus no sign of majoritic solid solution. Associations of fPer, MgSiPvk and SiO.̲ inthe same diamonds give phase relationships consistent with experimental data for lower mantle

conditions. TAPP is unreported from experimental work.
Ion microprobe analysis of the inclusions for trace elements shows very low, generally lessthan chondritic, compositions for the fPer, MgSiPvk and TAPP. However, in CaSiPvk the REEcontents are very high (ca. 200,c chondrite), in agt'eement with previous experimental work for lower

mantle assemblages. A small positive Eu anomaly is seen in the CaSiPvk inciusions analysed, A‑ 164 ‑3culculated bulk pyrolite composition for the S o Luiz assemblages gives truce element bulk

composilions in between those of eslimarted primitive mantle undlverage OIB compositions. An ion

micropi'obe oxygen isotope analysis on one CaSiPvk incltision shOwS 6180 of ca. 6 permil, consistent
with expected mantle composiLions. For the diamonds containing the inclusions, bLilk 8tiC analyses

yield typical mantle valLtes,

Consideratiorf of the inclusion associations found in Lhe diamonds, in conjLmction with ultra‑
high pressure experitnentai d',rta, suggests that the natu,'al mineral assemblages have come from adepth t'ange of possibly less thtln 100ktns in the uppermost part of the lower mantle, with TAPP
largely taking the place normally assigned to garnet (majoritic) in the upper part of this depth range.

The possible stability range ofTAPP is probably vefy restricted, being t'eplaced with increasing depthby aluminotls MgSiPvk in not'mal ultrabasic‑basic compositions, on the basis of boL11 the inclusionand experimental evidence. Thus within the uppermost lower mantle thet'e is a shaliower Al‑poor

MgSiPvk mineral facies, and a deeper Al‑rich MgSiPvk mineral facies. The inclusions probably
became encapsulated in diamond in this depth range.
The lower mantle Sao Luiz inclusion suite suggests that some material has formed from othersources (protoliths) besides expected lower mantle ultraibasic/pyrolite compositions. The ELI anomaly
shown by the C.aSiPvk inclusions may indicate an o['iginal crustal source. On the other hand, Lhe very

wide range in Pe/(Fe+Mg) of the fPer, and in particttlar the high Fe compositions may result fromsome del lvaUon of matellal fiom the D" Iayer at Lhe mantle cole boundary. If these origins arecorrect, then the Sao Luiz da'taL may suggest a mixing of some materiatl of both crustal and D" origins

with pyrolitic mantie adjacent to lowel]upp r mantle boundary, which argues in favdur of layered

mantle convection.

‑ 165 ‑