Fibrous diamonds encapsulate pristine metasomatic high-density fluids (HDFs) of silicic, carbonatitic and saline compositions. In general, the trace element patterns are similar in all HDFs. The REEs are fractionated and most samples show variable negative anomalies of Sr, Ti, Zr, Hf and Y relative to the corresponding REE. Larger diversity exists in the highly incompatible elements (Cs-La), where two patterns are distinguished: one is mostly flat with no significant anomalies and shows a moderate decrease of concentrations with decreasing ionic radius ('Bench'); the other ('Table') has elevated Ba, U, Th and LREE and depleted Nb, Ta, K, Rb and Cs. The two can be best distinguished by the ratios (Nb, Rb)/(La, Pr, U, Th). The similar signature of incompatible elements in diamonds-forming fluids from various mantle localities, well separated in space and time, the persistence of 'Table' and 'Bench' patterns in HDFs of such diverse major-element compositions and the uniform carbon isotopic composition of fibrous diamonds is fascinating. We seek to explain these features, which require collection from a large source region and/or interaction of the HDFs with a relatively large volume of mantle rocks. The Nb/(Th, U, La) ratios in HDFs with 'Bench' patterns fall within a narrow range and are similar to MORB/OIB/PM values. This smooth pattern can be approximated by very small degree of melting of a source with PM trace element concentrations. In HDFs with the more fractionated 'Table' patterns the above ratios deviate from the strict MORB/OIB/PM range and decrease by approximately 2 orders of magnitude with decreasing Nb content. The patterns can be closely produced by low-degree partial melting of a metasomatised continental lithosphere with carbonate, phlogopite and rutile as accessory phases. Searching for a possible relation between the two patterns, we examined fractional crystallization and percolation. Major elements allow only <20% phlogopite removal from HDFs with 'Bench' patterns, not enough to produce the high depletion in alkalis of the 'Table' patterns. Percolation of a melt with a 'Bench' pattern through SCLM rocks with the above accessory phases does lead to evolution of a 'Table' pattern, with minimal changes to the major elements.