yc led ▁into ▁the ▁ocean ▁cr ust . ▁Hel ium - 3 , ▁an ▁is ot ope ▁that ▁accompan ies ▁vol can ism ▁from ▁the ▁mant le , ▁is ▁em itted ▁by ▁hydro ther mal ▁v ents ▁and ▁can ▁be ▁detected ▁in ▁pl umes ▁within ▁the ▁ocean . ▁ ▁Fast ▁spread ing ▁rates ▁will ▁expand ▁the ▁mid - o cean ▁r idge ▁causing ▁bas alt ▁re actions ▁with ▁se aw ater ▁to ▁happen ▁more ▁rapidly . ▁The ▁mag nes ium / cal ci um ▁ratio ▁will ▁be ▁lower ▁because ▁more ▁mag nes ium ▁ ions ▁are ▁being ▁removed ▁from ▁se aw ater ▁and ▁consum ed ▁by ▁the ▁rock , ▁and ▁more ▁calci um ▁ ions ▁are ▁being ▁removed ▁from ▁the ▁rock ▁and ▁released ▁to ▁se aw ater . ▁H ydro ther mal ▁activity ▁at ▁r idge ▁cr est ▁is ▁efficient ▁in ▁removing ▁mag nes ium . ▁A ▁lower ▁M g / Ca ▁ratio ▁fav ors ▁the ▁precip itation ▁of ▁low - M g ▁cal cite ▁polym orph s ▁of ▁calci um ▁carbon ate ▁( cal cite ▁se as ). ▁ ▁S low ▁spread ing ▁at ▁mid - o cean ▁rid ges ▁has ▁the ▁opposite ▁effect ▁and ▁will ▁result ▁in ▁a ▁higher ▁M g / Ca ▁ratio ▁favor ing ▁the ▁precip itation ▁of ▁a rag on ite ▁and ▁high - M g ▁cal cite ▁polym orph s ▁of ▁calci um ▁carbon ate ▁( ar agon ite ▁se as ). ▁ ▁Exper iments ▁show ▁that ▁most ▁modern ▁high - M g ▁cal cite ▁organ isms