TY - JOUR
T1 - Highly unradiogenic lead isotope ratios from the horoman peridotite in japan
AU - Malaviarachchi, Sanjeewa P.K.
AU - Makishima, Akio
AU - Tanimoto, Masaaki
AU - Kuritani, Takeshi
AU - Nakamura, Eizo
N1 - Funding Information:
Whole-rock Pb isotope ratios of the massive peridotites exhibit a large variation (Fig. 2a,b). However, importantly, the average difference between the unleached (double-spike thermal ionization mass spectrometer) and leached (double-spike multi-collector inductively coupled plasma mass spectrometer) samples is 0.01%, indicating the extreme freshness of the samples without significant grain-boundary enrichments (see Supplementary Information, Table S3). The plagioclase lherzolites without enrichment of Cs, Rb and Ba have unradiogenic compositions that fall below the ‘normal’ DMM endmember4–6 region (hereafter referred to as the ‘highly unradiogenic’ signature). The more refractory spinel lherzolites and harzburgites have more enriched/radiogenic isotopic compositions. They exhibit a mixing relationship extending from the highly unradiogenic to the global subducting sediments20 (GLOSS) field. Contribution of metasomatic fluids derived from the sediments, possibly adjacent subducting Hidaka meta-sediments, therefore, is discernible. Transport of material via fluids from the sediments overlying refractory peridotite is supported by the trace-element patterns with significant enrichment in alkaline elements, alkaline earth elements and LREEs (Fig. 1a). Both Sm–Nd and Lu–Hf isotope systematics of the massive plagioclase lherzolites define isochron ages of ∼1 Gyr (see Supplementary Information, Fig. S2). Previous estimates of whole-rock Sm–Nd (ref. 17) and Re–Os (ref. 18) dates also yield ages close to our estimates, implying that chemical depletion of these residues may have occurred at ∼1 Gyr ago. Many of our samples show higher 143Nd/144Nd and 176Hf/177Hf than the normal DMM (Fig. 2e,f), consistent with their low Pb isotope ratios, reflecting sub-ridge mantle isotopic depletion with preservation of original old depleted mantle character. Unradiogenic Os isotope data of the Horoman peridotites18 also support our interpretations.
PY - 2008/12
Y1 - 2008/12
N2 - Basalts at mid-ocean ridges are generated by partial melting of the Earth's upper mantle. As a result of this process, the upper mantle has become depleted over time in elements that are preferentially removed by melting. Although mid-ocean-ridge basalts have traditionally been thought to reflect the chemical composition of such depleted mantle, recent work has revealed the existence of domains in the upper mantle that are apparently not sampled by the basalts. Here we present the lead (Pb), neodymium (Nd) and hafnium (Hf) isotope compositions of peridotites from the Horoman orogenic massif in Japan, which is considered to represent the residues of melting of the upper mantle. These peridotites exhibit the lowest Pb isotope ratios reported from any known mantle material, along with high Nd and Hf isotope ratios. These data suggest that chemical depletion of the peridotites occurred around a billion years ago, and that they represent ancient mantle domains that have escaped convective stirring and homogenization. We suggest that such domains if abundant in the mantle may constitute a hitherto unrecognized reservoir with highly unradiogeniclead.
AB - Basalts at mid-ocean ridges are generated by partial melting of the Earth's upper mantle. As a result of this process, the upper mantle has become depleted over time in elements that are preferentially removed by melting. Although mid-ocean-ridge basalts have traditionally been thought to reflect the chemical composition of such depleted mantle, recent work has revealed the existence of domains in the upper mantle that are apparently not sampled by the basalts. Here we present the lead (Pb), neodymium (Nd) and hafnium (Hf) isotope compositions of peridotites from the Horoman orogenic massif in Japan, which is considered to represent the residues of melting of the upper mantle. These peridotites exhibit the lowest Pb isotope ratios reported from any known mantle material, along with high Nd and Hf isotope ratios. These data suggest that chemical depletion of the peridotites occurred around a billion years ago, and that they represent ancient mantle domains that have escaped convective stirring and homogenization. We suggest that such domains if abundant in the mantle may constitute a hitherto unrecognized reservoir with highly unradiogeniclead.
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U2 - 10.1038/ngeo363
DO - 10.1038/ngeo363
M3 - Article
AN - SCOPUS:65349150622
SN - 1752-0894
VL - 1
SP - 859
EP - 863
JO - Nature Geoscience
JF - Nature Geoscience
IS - 12
ER -