Crustal Signature of δ^(13)C and Nitrogen Content in Microdiamonds from Erzgebirge, Germany: Ion Microprobe Studies

The study of δ13CCpDB （Pee Dee Belemnite） and nitrogen contents in 1 to 5-pm-diameter microdinmonds included in garnets from the qnartz-feldspathic gneisses （Erzgehirge, Germany） was performed in situ with the Nano-scale Secondary Ion Mass Spectrometer. The results revealed that there were two stages of diamond crystallization from a C-O-H supeeritical fluid rich in biogenic carbon and diverse minor elements of crustal origin. The δ13CCpDB Of the Erzgebirge diamond of the first stage falls in the range -17‰ to 19‰, with an average value of -17.8‰; the average content of nitrogen is 820 ppm. Diamonds of the second stage are characterized by δ13CPDB=-21.5‰ to -25.5‰, with an average value of -23.24‰; the average nitrogen content is non-homogeneously scattered from 740 ppm to 3 370 ppm among 6 diamonds situated in garnets within the same polished rock slide. Both diamond of the first stage and diamond of the second stage carbon reservoirs belong to biogenic matter, therefore confirming deep subduction of the continental crust sediments and their subsequent exhumation during the Variscan orogeny.

Foreword

The contributions to this Special Issue were called on the occasion of the upcoming birthday anni- versary of one of the outstanding scientists in field of mantle rheology and rock deformation, Prof. Zhenmin Jin, China University of Geosciences, to celebrate his scientific life achievements. The issue consists of a series of articles that highlight the versatilitv of Prof.

The Alpe Arami Story: Triumph of Data over Prejudice

The Alpe Arami garnet peridotite of the Southern Swiss Alps is associated with eclogites and included within quartzofeldspathic gneisses. Controversy has swirled around the depth of origin of this massif since the 1970s when application of the newly-developed technique of thermobarometry suggested a depth of last equilibration of greater than 120 kin. Such controversy accelerated in 1996 when we reported microstructural evidence of extensive precipitation of ilmenite and spinel from olivine and proposed a much greater depth of origin. Subsequent experiments showed that it was possible to dissolve the observed amount of TiO2 in olivine, but only at depths in excess of 300 kin, agreeing with the earlier proposal. In 1999 we added new, independent, evidence concerning exsolution of high-pressure clinoenstatite from diopside that in-and-of-itself required a depth of origin in excess of 250 km. Subsequently, we also added evidence from the surrounding eclogites of very high pressures and experimental evidence that the pyroxenes included in the amoeboid garnets of this rock had exsolved from a majoritic parent at perhaps even greater pressures. In refutation of the first two of these observations, suggestions were made that （i） we had made a serious error in our estimate of how much ilmenite was present in olivine （and therefore how much TiO2 had been dissolved in olivine）; （ii） the ilmenite had not been exsolved from olivine but former titanian clinohumite had been present and broke down to yield the ilmenite; （iii） the pyroxene exsolved from diopside had been high-temperature clinoenstatite. In all three of these cases, the alternatives offered were claimed to be accommodated at low pressures. Here we review the essence of this controversy and show that the only scenario that can explain all of the data is the one that we originally proposed; indeed, the more recent data have strongly supported that interpretation and pushed the minimum origin of the massif to depths approaching 400 km.