The ~ 3700 Ma Inner Arc Group of the Isua supracrustal belt (Greenland) contains a 10 km long strip of ultramafic schists with two ≤ 1 km long meta-dunite lenses, preserving relict olivine + antigorite + titano-clinohumite and titano-chondrodite ~ 2.6 GPa ultra-high-pressure (UHP) assemblages. There are two distinct relict meta-peridotite variants: The southern lens ‘A’ variant is dominated by an aggregate of Fo_91-92 olivine with δ¹⁸O_VSMOW of + 5.4 ± 0.1‰ (95% confidence) in which are rare small clinopyroxene inclusions, plus rare interstitial accessory chrome spinel partly altered to magnetite. The olivine grains are bounded by serpentine domains, which are intergrown with the olivine margins with a marginally more iron-rich composition of Fo_90.5. The northern lens ‘B’ contains coarser-grained Fo_96-98 olivine which encloses magnetite as clusters and trails, some arranged with 60°-120° conjugate intersections. The Fo_96-98 olivine has δ¹⁸O_VSMOW of + 3.2 ± 0.2‰ and is in equilibrium with high-Al antigorite and the accessory UHP Ti-rich minerals.

We interpret that the Fo_90-92 versus the Fo_96-98 olivine assemblage variants are caused by a varying degree of fluid influx during ca. 3700 Ma serpentinisation, prior to peak UHP metamorphism. The Fo_90-92 variant was a rock-dominated system with a postulated early serpentine + ferroan brucite alteration assemblage, with abundant relict mantle-like δ¹⁸O_VSMOW + 5.4‰ olivine. The Fo_96-98 variant was a fluid-dominated system, where obliteration of the protolith olivine formed serpentine + magnetite + brucite ± magnesite. During prograde UHP metamorphism consumption of brucite by reaction with serpentine gave rise to a lower δ¹⁸O_VSMOW Fo_96-98 olivine + serpentine assemblage. Both varieties have similar bulk chemistry, interpreted as depleted mantle that was enriched in LILE and LREE by a melt or fluid, prior to varying degrees of serpentinisation and superimposed UHP metamorphism.

In Phanerozoic abyssal peridotites similar textural, chemical and isotopic variations are the result of rock versus fluid-dominated serpentinisation and variable alteration temperatures. Analogously, the Fo_96-98 Isua peridotites are interpreted as mantle peridotite entirely serpentinised by high temperature seawater in a fluid-dominated environment and then metamorphosed under UHP conditions at a convergent plate boundary. This indicates lateral lithosphere motions facilitating hydrosphere - mantle communication early in Earth’s history.

Isua 上地壳带（格陵兰）的 ~ 3700 Ma 内弧群包含一条 10 公里长的超镁铁质片岩带，带有两个≤ 1 公里长的变质纯英岩透镜体，保存着残存的橄榄石 + 叶蛇纹石 + 钛斜辉石和钛球粒陨石 ~ 2.6 GPa 超高压 (UHP) 组合。有两种不同的残余变橄榄岩变体：南部透镜体“A”变体由 Fo _91-92橄榄石的聚集体支配，δ ¹⁸ O _VSMOW为 + 5.4 ± 0.1‰（95% 置信度），其中罕见的小单斜辉石夹杂物，加上罕见的间隙附件铬尖晶石，部分转变为磁铁矿。橄榄石颗粒以蛇纹石域为界，这些域与橄榄石边缘共生，Fo 的铁含量略高_90.5。北部的透镜“ B”包含粒度较粗的Fo _96-98橄榄石，其将磁铁矿围成团簇和尾迹，有些以60°-120°共轭相交点排列。Fo _96-98橄榄石的 δ ¹⁸ O _VSMOW为 + 3.2 ± 0.2‰，与高铝叶蛇纹石和附属的 UHP 富钛矿物处于平衡状态。

我们解释说，Fo _90-92与 Fo _96-98橄榄石组合变体是由大约 10 年期间不同程度的流体流入引起的。3700 Ma 蛇纹石化，在峰值 UHP 变质之前。对FO _90-92变体是一个岩石主导系统与假定早期蛇形+水镁石含铁组合改变，具有丰富的遗地幔状δ ¹⁸ ö _VSMOW  + 5.4‰橄榄石。Fo _96-98变体是一种以流体为主的系统，在该系统中，原生质橄榄石的消灭形成了蛇纹石+磁铁矿+水镁石±菱镁矿。在顺行超高压变质过程中，水镁石与蛇纹石反应消耗产生较低的 δ ¹⁸ O _VSMOWFo _96-98橄榄石 + 蛇纹石组合。这两个品种具有相似的整体化学性质，被解释为在不同程度的蛇纹石化和叠加超高压变质之前，通过熔体或流体富含 LILE 和 LREE 的贫化地幔。

在显生宙深海橄榄岩中，类似的结构、化学和同位素变化是岩石与流体主导的蛇纹石化和变化的蚀变温度的结果。类似地，Fo _96-98 Isua 橄榄岩被解释为在以流体为主的环境中完全被高温海水蛇纹石化，然后在超高压条件下在会聚板块边界处变质的地幔橄榄岩。这表明横向岩石圈运动促进了地球历史早期的水圈-地幔通信。